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  • 1.
    Baidar, Binaya
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nicolle, Jonathan
    Institut de Recherche D’Hydro-Québec.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effects of runner change on the Winter-Kennedy flow measurement method: A numerical study2020In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 153, p. 975-984Article in journal (Refereed)
    Abstract [en]

    The Winter-Kennedy (WK) method is a popular choice to estimate the relative flow rates, and thus the expected improvement in the efficiency of a low head turbine after its refurbishment. Runner refurbishment is a common way to improve the plant’s efficiency. However, a previous experiment on a model turbine reported deviations between the WK coefficients obtained from two different runners ‒ suggesting a deviation between the estimated and actual improvement in the efficiency. Without formal proof, the deviation was attributed to flow changes in the spiral casing. This paper presents a numerical investigation of the effects of a runner change on the WK method. For this purpose, unsteady Reynolds-averaged Navier-Stokes equations (URANS) simulations of a turbine model with two different runners were conducted. The runner’s impact on the average flow conditions upstream and its subsequent effect on the WK coefficients were studied. The study shows the dependence of the WK coefficients to the runner ‒ with a maximum deviation on the coefficient up to 0.7%. The larger deviations were observed in regions prone to strong secondary flow. Following a radial and circumferential sensitivity study, a suitable location to minimize the effects of runner change on the WK method is reported.

  • 2.
    Sotoudeh, Nahale
    et al.
    Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran.
    Maddahian, Reza
    Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Investigation of Rotating Vortex Rope formation during load variation in a Francis turbine draft tube2020In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 151, p. 238-254Article in journal (Refereed)
    Abstract [en]

    Rotating Vortex Rope (RVR) has been a matter of focus for years due to the major effects on hydraulic turbine’s efficiency. The exact procedure of RVR formation is still vague. The present research focuses on the dynamics of the RVR formation during the load variation employing transient numerical simulations. Two different geometries including the full geometry and the reduced one, which consists of one stay vane, two guide vanes, one runner blade, one splitter blade and full draft tube, are considered. In order to capture the transient swirling flow features inside the draft tube, the Shear Stress Transport-Scale Adaptive Simulation (SST-SAS) model is utilized to approximate the turbulent stresses. The pressure results inside the draft tube agree well with the experimental measurements. Moreover, the velocity results show the central low-axial-velocity and high-tangential-velocity region in the draft tube properly. The flow structure is visualized using λ2 criterion. The dynamic of RVR and the physics behind the RVR formation are investigated during the load variation. The results indicate four flow regimes with different characteristics during RVR formation. The first flow regime is a stable swirling structure occurring at Best Efficiency Point (BEP). The second flow regime occurs at the beginning of the load variation where signs of flow instabilities appear. These instabilities are temporary and washed down by the upstream flow. Expanding the instabilities and creating the vortical structures in the draft tube are the important flow features in the third flow regime. The fourth flow regime is the presence of a developed rotating rope occurring at the Part Load (PL) condition. The flow regimes differ according to the size and shape of the stalled region during load rejection inside the draft tube cone. They also reveal that despite some shortcomings, the reduced model is reliable to simulate the RVR transient formation. The full geometry simulations could be also applicable for practical problems provided that the modified time step is slightly greater than the main blade rotational angle is used.

  • 3.
    Baidar, Binaya
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nicolle, Jonathan
    Institut de recherche d’Hydro-Québec (IREQ), Varennes, QC J3X 1S1, Canada.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee, Uttarakhand 247667, India.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical Study of the Winter–Kennedy Flow Measurement Method in Transient Flows2020In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 13, no 6, article id 1310Article in journal (Refereed)
    Abstract [en]

    This paper explores the possibility of using the Winter–Kennedy (WK) method for transient flow rate measurement in hydraulic turbines. Computational fluid dynamic (CFD) analysis of a numerical model of an axial turbine was carried out for accelerating and decelerating flows. Those were obtained by linearly opening and closing of the guide vanes, respectively, while retaining the inlet pressure constant during the simulations. The behavior of several WK configurations on a cross-sectional plane and along the azimuthal direction of the spiral casing was studied during the transients. The study showed that there are certain WK configurations that are more stable than others. The physical mechanism behind the stability (or instability) of the WK method during transients is presented. Using the steady WK coefficient obtained at the best efficiency point (BEP), the WK method could estimate the transient flow rate with a deviation of about 7.5% and 3.5%, for accelerating and decelerating flow, respectively.

  • 4.
    Goyal, Rahul
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee, India.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee, India.
    Synchronized PIV and pressure measurements on a model Francis turbine during start-up2020In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 58, no 1, p. 70-86Article in journal (Refereed)
    Abstract [en]

    This paper presents the experiments performed on a high head model Francis turbine during start-up. Synchronized time dependent pressure and velocity measurements were performed to investigate the instabilities in the turbine. A total of four steady state operating points, namely synchronous load, part load, best efficiency point, and high load are considered to perform the turbine start-up. The runner angular speed was observed to increase almost exponentially during the guide vane positions from completely closed to no load condition. The frequency of wave propagation due to the interaction between runner blades and guide vanes was observed to follow the trend of increase of runner angular speed. A vortex rope frequency was captured in the draft tube during synchronous load to part load of the start-up. Two different mechanisms, namely, the development of stagnation point and the available recirculation regions were observed to cause the formation of vortex rope in the draft tube.

  • 5.
    Saemi, Simindokht
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. School of Mechanical Engineering, College of Engineering, University of Tehran.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Energy and Process Engineering, Water Power Laboratory, Norwegian University of Science and Technology.
    Nourbakhsh, Ahmad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Computation of two- and three-dimensional water hammer flows2019In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 57, no 3, p. 386-404Article in journal (Refereed)
    Abstract [en]

    This paper investigates water hammer flows using two- and three-dimensional (2D and 3D) numerical simulations. The unsteady Reynolds-averaged Navier–Stokes (URANS) equations in conjunction with the k–ω SST turbulence model are employed for the computations. The valve modelling approach is used for 3D simulations, with superior agreement with the experiments. Similar predictions are obtained by 2D simulations and the flow rate reduction curve obtained from the 3D computations. The asymmetric flow patterns induced by the valve are confined within approximately one pipe diameter upstream of the valve. The contributions of inertia and pressure gradient terms are dominant at the instance of pressure wave passage, leading to abrupt changes in the fluid flow parameters. However, the effects of inertia and viscous shear stress terms are significant after the pressure wave passage, resulting in the flow tendency to approach a new steady condition. The viscous and turbulent dissipations are dominant close to and away from the wall, respectively.

  • 6.
    Jonsson, Pontus
    et al.
    Vattenfall AB, Luleå, Sweden.
    Dunca, Georgiana
    Polytechnic University of Bucharest, Bucharest, Romania.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Development of the pressure-time method as a relative method2019In: IOP Conference Series: Earth and Environment, Institute of Physics (IOP), 2019, Vol. 240, article id 022003Conference paper (Refereed)
    Abstract [en]

    The pressure-time method is an absolute method commonly used for flow rate measurements in hydropower plants. The method determines the flow rate by measuring the differential pressure and estimating the losses between two sections in the penstock during a closure of the guide vanes. The method has limitations according to the IEC41 standard, which make it difficult to use at low head hydropower plants. The relative method called Winter-Kennedy is usually used on low head machines to determine the step-up efficiency between the old and refurbished runner. However, due to differences of the flow field in the spiral casing induce by both runners, the Winter Kennedy method might not allow estimate the flow rate similarly and thus the correct step-up efficiency. In cases where the absolute pressure-time method cannot be used because of waterway geometry limitations, the method might be used as a relative method by measuring the pressure difference between the free surface and a section in the penstock or even a point in the spiral casing without knowing the exact geometry, i.e., pipe factor. Such measurements may be simple to perform as most of the spiral casings have pressure taps for Winter-Kennedy measurements. Furthermore, the viscous losses do not need to be accurately determined if they are handled similarly before and after the refurbishment. The pressure-time method may thus become an alternative to the Winter-Kennedy method. The present paper consists in the experimental analysis of the pressure-time method accuracy used as a relative method. The experiments are performed on Porjus U9, a Kaplan prototype turbine operated under a head of 55 m generating 10 MW at full load. The flow rate is evaluated based on pressure-time measurements with different friction models and considering or not the compressibility effect. The accuracy of the flow rate evaluation method is compared using an 8-path transit-time flow rate measurement device as reference.

  • 7.
    Saemi, Simindokht
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Sundström, Joel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Iran.
    Evaluation of transient effects in the pressure-time method2019In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 68, article id 101581Article in journal (Refereed)
    Abstract [en]

    The pressure-time is a method for measuring the flow rate in closed conduits and is typically used in hydropower applications. The scope of the present paper is to examine the flow physics in the pressure-time method using experimental measurements and two-dimensional numerical simulations. The Unsteady Reynolds-averaged Navier–Stokes (URANS) equations and the low-Re k-ω SST turbulence model are employed for the simulations. The contributions of inertia, pressure gradient, viscous and turbulent shear stresses are investigated in the flow during a pressure-time measurement. It is shown that away from the wall and at the first times, the turbulent shear stress balances with the pressure gradient. By increasing the time, the inertia effect becomes dominant and balances with the pressure gradient and turbulent shear stress. Close to the wall, both viscous and turbulent shear stresses are the dominant terms which are decreasing by increasing the time. It is also shown that the prediction of the friction losses can be improved by modeling the dependence of the friction factor on the dimensionless parameter instead of the Reynolds number.

  • 8.
    Soltani Dehkharqani, Arash
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Engström, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Experimental Investigation of a 10 MW Prototype Kaplan Turbine during Start-Up Operation2019In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, no 23, article id 4852Article in journal (Refereed)
    Abstract [en]

    An increase in the start/stop cycles of hydraulic turbines due to the penetration of intermittent renewable energy sources is important. Hydraulic instabilities that occur in hydraulic turbines during start/stops may cause structural issues in the turbine components. High-stress fluctuations on the runner blades are expected during start-ups due to the unsteady pressure loading on the runner blades. This paper presents experiments performed on a 10 MW prototype Kaplan turbine at the Porjus Hydropower Center during a start-up cycle. Synchronized unsteady pressure and strain measurements on a runner blade and axial, bending (in two directions) and torsion strain measurements on the shaft were performed. In addition, the general parameters of the turbine (e.g., rotational speed, guide vane opening and runner blade angle) were acquired. Low-frequency fluctuations (0–15 Hz) were observed in the pressure data on the runner blade after opening the guide vanes from the completely closed position. A higher strain value was observed on the strain gauges installed on the runner blade near the hub (200–500 μm/m ) compared to the ones near the shroud at the leading and trailing edge. The strain fluctuation level on the shaft decreased after loading the generator by further opening the guide vanes. Higher fluctuations were observed in the torsion strain compared to axial and bending strain. In addition, the torsion strain peak-to-peak value reached 12 times its corresponding value at 61% guide vane opening.

  • 9.
    Goyal, Rahul
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Grenoble-INP/CNRS/UJF-Grenoble 1, Grenoble, France.
    Gandhi, BK
    Indian institute of Technology, Roorkee, India.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Experimental Investigation of a High Head Francis Turbine Model During Shutdown Operation2019In: IOP Conference Series: Earth and Environment, Institute of Physics (IOP), 2019, Vol. 240, article id 022028Conference paper (Refereed)
    Abstract [en]

    Increased penetration of intermittent energy resources disturbs the power grid network. The frequency band of the power grid is normally controlled by automatic opening and closing of the guide vanes of hydraulic turbines. This has increased the number of shutdown cycles as compared to the defined ones for the normal operation of turbines. Turbine shutdown induced a significantly higher level of pressure fluctuations and unsteadiness in the flow field, decreasing its expected life. This paper presents experiments performed on a high head model Francis turbine during shutdown. The pressure and 2D Particle Image Velocimetry (PIV) measurements were performed to investigate the pressure fluctuations and flow instabilities in the turbine. The pressure sensors were mounted in the draft tube cone and vaneless space to measure the instantaneous pressure fluctuations. In the present study, the initial high load operating condition was considered to perform the turbine shutdown. The data were logged at the sampling frequency of 40 Hz and 5 kHz for PIV and pressure measurements, respectively. Time-resolved velocity and pressure data are presented in this paper to show the pressure fluctuations and causes of generation of unsteady flow in the draft tube.  

  • 10.
    Soltani Dehkharqani, Arash
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Engström, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Fluid added polar inertia and damping for the torsional vibration of a Kaplan turbine model runner considering multiple perturbations2019In: IOP Conference Series: Earth and Environmental Science, Institute of Physics (IOP), 2019, Vol. 240, article id 062007Conference paper (Refereed)
    Abstract [en]

    A water turbine runner is exposed to several perturbation sources with differentfrequencies, phases, and amplitudes both at the design and off-design operations. Rotor-statorinteraction, cavitation, rotating vortex rope, and blade trailing edge vortices are examples of suchperturbations which can disturb the runner. The rotor dynamic coefficients require beingdetermined to perform a reliable dynamic analysis. Fluid added inertia, damping, and stiffnesshave previously been investigated for individual perturbation frequencies for the torsionalvibration of a Kaplan turbine model runner. However, a number of perturbation sources mostlytake place simultaneously and alter the dynamics of the runner. Soltani et al. [1] have evaluatedthe torsional added parameters for a Kaplan turbine runner using numerical simulationsconsidering single perturbation frequency. In the present work, the fluid added parameters areassessed in the presence of multiple perturbation sources. A similar methodology is used. Asingle-degree-of-freedom (SDOF) model for the dynamic model and unsteady ReynoldsaveragedNavier–Stokes approach for the flow simulations are assumed. Perturbations withdifferent frequencies are applied to the rotational speed of the runner to determine the fluid addedparameters for the torsional vibration. A number of previously investigated frequencies arechosen and their combinations are investigated. In addition, two different phase shifts areconsidered between the applied perturbations to study the effect of phase. Two more test caseswith higher perturbation amplitude are also conducted to investigate its influence on the fluidadded inertia and damping. The results are compared with the previous study and the interactionof multiple perturbations on the added parameters is investigated.

  • 11.
    Sotoudeh, Nahaleh
    et al.
    Tarbiat Modares University, Iran.
    Maddahian, Reza
    Tarbiat Modares University, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Formation of Rotating Vortex Rope in the Francis-99 Draft Tube2019In: IOP Conference Series: Earth and Environment, Institute of Physics (IOP), 2019, Vol. 240, article id 022017Conference paper (Refereed)
    Abstract [en]

    The aim of this research is to understand the mechanism(s) of RVR formation duringthe changes in operating condition from the Best Efficiency Point (BEP) to Part Load (PL). AComputational Fluid Dynamic (CFD) methodology by the means of ANSYS-CFX is applied ona reduced high head Francis turbine model. The reduced model consists of one stay vane, twoguide vanes, one runner blade, one splitter and a full draft tube. Numerical simulation is firstperformed at BEP as well as PL to ensure the appropriate employment of turbulence models andboundary conditions. In the second step, the inlet boundary conditions are changed linearly fromBEP to PL in order to achieve the transient conditions inside the draft tube. The initial conditionof the second step is the converged BEP result. The transient simulation is continued until theRVR is fully developed in the draft tube at part load condition. The numerical results for BEP,PL and BEP to PL are in a good agreement with the experimental data. The effect of the RVR isconsidered from two aspects. The first one is the frequency, and the amplitude of the pressurepulsations induced by the RVR in the draft tube. The second one is the velocity field in the drafttube which is investigated over time during load rejection. Moreover, the flow structure isvisualized using the λ2 criterion. The mechanism(s) of RVR formation and damping is accuratelyinvestigated by the presented approach. Furthermore, the results provide a better understandingof the physics behind the RVR formation. The obtained results aim to design an effective RVRcontrolling approach.

  • 12.
    Sundström, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Saemi, Simindokht
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Raisee, M.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Improved frictional modeling for the pressure-time method2019In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 69, article id 101604Article in journal (Refereed)
    Abstract [en]

    The pressure-time method is classified as a primary method for measuring discharge in hydraulic machinery. The uncertainty in the discharge determined using the pressure-time method is typically around ±1.5 %; however, despite dating back almost one hundred years in time, there still exists potential to reduce this uncertainty. In this paper, an improvement of the pressure-time method is suggested by implementing a novel formulation to model the frictional losses arising in the evaluation procedure. By analyzing previously obtained data from CFD, laboratory and full-scale pressure-time measurements it is shown that the new friction model improves the accuracy of the flow rate calculation by approximately 0.1–0.2% points, compared to currently utilized friction models. Despite being a small absolute improvement, the new friction model presents an important development of the pressure-time method because the relative improvement is significant.

  • 13.
    Sundström, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Saemi, Simindokht
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Raisee, Mehrdad
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Improved frictional modelling for the pressure-time method2019In: Flow measurement and instrumentationArticle in journal (Refereed)
    Abstract [en]

    The pressure-time method is classified as a primary method for measuring discharge in hydraulic machinery. The uncertainty in the discharge determined using the pressuretime method is typically around±1.5%; however, despite dating back almost one hundred years in time, there still exists potential to reduce this uncertainty. In this paper, an improvement of the pressure-time method is suggested by implementing a novel formulation to model the frictional losses arising in the evaluation procedure. By analyzing previously obtained data from CFD, laboratory and full-scale pressure-time measurements it is shown that the new friction model improves the accuracy of the flow rate calculation by approximately 0.1-0.2 percentage points, compared to currently utilized friction models. Despite being a small absolute improvement, the new friction model presents an important development of the pressure-time method because the relative improvement is significant.

  • 14.
    Iovanel, Raluca Gabriela
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. University Politehnica of Bucharest, Romania.
    Bucur, D M
    University Politehnica of Bucharest, Romania.
    Dunca, G
    University Politehnica of Bucharest, Romania.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical analysis of a Kaplan turbine model during transient operation2019In: IOP Conference Series: Earth and Environmental Science, Institute of Physics (IOP), 2019, Vol. 240, article id 022046Conference paper (Refereed)
    Abstract [en]

    Hydropower plants are currently being intensively employed for electrical grid regulation. As a consequence, the frequency of start/stops and load variations is considerably increasing, leading to the operation of hydraulic turbines under improper conditions. During the last years, studies have focused on Francis turbines. The present paper aims to investigate a Kaplan turbine model. The flow through the turbine is modelled during transient operation, from the best efficiency point to a part load operating point, using a moving mesh for the guide vane displacement. The simulations are validated against experimental velocity profiles. A time step sensitivity analysis is performed in order to determine the optimum discretization time. The possibility of using large time steps is explored. The numerically simulated unsteady pressure pulsations on the runner blades are analysed. The influence of the inlet boundary conditions on the accuracy of numerical simulations is studied. The results show that a linear flow rate variation defined during the guide vane closure leads to an overestimation of the turbine head compared to the experimental value due to an overestimation of losses. The second type of boundary conditions, a constant total pressure, results in an underestimation of the flow rate compared to the experimental value due again to an overestimation of the losses.

  • 15.
    Maddahian, Reza
    et al.
    Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Bucur, Diana M.
    Hydraulic Machinery & Environmental Engineering Dept, Polytechnic University of Bucharest, Bucharest, Romania.
    Numerical investigation of entrapped air pockets on pressure surges and flow structure in a pipe2019In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079Article in journal (Refereed)
    Abstract [en]

    This research presents a numerical investigation of two-phase flow during the expulsion of entrapped air in a non-confined pipe. A modified version of the volume of fluid (VOF) approach is employed considering the effect of compressibility in the liquid. A modification is introduced to the original approach relating the density changes in the liquid to the pressure changes using the fluid bulk modulus. The bulk modulus is also modified to consider the pipe elasticity, the air bubble entrainment and the two-phase flow regime in a pipe. Fluid–structure interaction (FSI) code is developed and used to calculate the motion of the downstream orifice wall during the impact of the water column on the pipe end wall. The numerical results of the pressure variation agree well with experimental data. The two-phase flow structure and physics behind the pressure waves are investigated. The numerical results show that to capture the amplitude and time interval of the pressure surges, the effect of FSI should be considered during the expulsion of the entrapped air. Additionally, the effects of initial air pocket size, supply pressure and orifice size on the pressure increase are investigated. The developed VOF-FSI approach can be employed as a numerical tool to investigate the transient flow during pipe filling.

  • 16.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Tabatabaei, Narges
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Numerical Investigation of the Aeroelastic Behavior of a Wind Turbine with Iced Blades2019In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, no 12, article id 2422Article in journal (Refereed)
    Abstract [en]

    Wind turbines installed in cold-climate regions are prone to the risks of ice accumulation which affects their aeroelastic behavior. The studies carried out on this topic so far considered icing in a few sections of the blade, mostly located in the outer part of the blade, and their influence on the loads and power production of the turbine are only analyzed. The knowledge about the influence of icing in different locations of the blade and asymmetrical icing of the blades on loads, power, and vibration behavior of the turbine is still not matured. To improve this knowledge, multiple simulation cases are needed to run with different ice accumulations on the blade considering structural and aerodynamic property changes due to ice. Such simulations can be easily run by automating the ice shape creation on aerofoil sections and two-dimensional (2-D) Computational Fluid Dynamics (CFD) analysis of those sections. The current work proposes such methodology and it is illustrated on the National Renewable Energy Laboratory (NREL) 5 MW baseline wind turbine model. The influence of symmetrical icing in different locations of the blade and asymmetrical icing of the blade assembly is analyzed on the turbine’s dynamic behavior using the aeroelastic computer-aided engineering tool FAST. The outer third of the blade produces about 50% of the turbine’s total power and severe icing in this part of the blade reduces power output and aeroelastic damping of the blade’s flapwise vibration modes. The increase in blade mass due to ice reduces its natural frequencies which can be extracted from the vibration responses of the turbine operating under turbulent wind conditions. Symmetrical icing of the blades reduces loads acting on the turbine components, whereas asymmetrical icing of the blades induces loads and vibrations in the tower, hub, and nacelle assembly at a frequency synchronous to rotational speed of the turbine.

  • 17.
    Baidar, Binaya
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nicolle, Jonathan
    Institut de recherche d'Hydro-Québec, Varennes, QC, Canada.
    Gandhi, Bhupendra K
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, India.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical study of the Winter-Kennedy method for relative transient flow rate measurement2019In: IOP Conference Series: Earth and Environment, ISSN 1755-1307, E-ISSN 1755-1315, Vol. 405, no 1, article id 012022Article in journal (Refereed)
    Abstract [en]

    The Winter-Kennedy (WK) method is used to estimate relative flow rate using the differential pressure between two taps located at a radial section of a spiral casing (SC). It is widely used in index testing, for double regulated turbines optimization and sometimes for continuous discharge measurement in low head plants. This paper explores the possibility of using the WK method for relative transient flow rate measurements. A numerical model of a Kaplan model turbine from the penstock to the distributor has been developed. Unsteady RANS simulations with k-ω SST turbulence model are performed. Previously conducted experiments on the model turbine are used to validate the numerical results. In the simulations, the guide vanes (GVs) are closed from 26.5°, the best efficient point (BEP), to about 5° opening angle. Two azimuthal locations of the SC and four different WK configurations at each location are considered. The variation of the WK coefficients with time are investigated and compared to the ones at several stationary GV angles. The results showed a difference between the WK coefficients obtained at transient and stationary operations. However, there may be a possibility of using the WK method during transients by locating the pressure taps in appropriate locations for an acceptable variation of the WK coefficient from its BEP value.

    The research has been funded by Swedish Hydropower Centre (SVC).

  • 18.
    Baidar, Binaya
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nicolle, Jonathan
    Institut de recherche d’Hydro-Québec, Varennes, QC, Canada.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, India.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sensitivity of the Winter-Kennedy method to different guide vane openings on an axial machine2019In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 68, article id 101585Article in journal (Refereed)
    Abstract [en]

    This work studies the effects of guide vane openings (GVOs) on the Winter-Kennedy (WK) flow measurement method using CFD. The dependence of the WK coefficient with GVOs and its physical mechanism are presented. Although the WK method is reported to be sensitive to different factors including GVO, it is still unclear to which extent the GVO can be changed without modifying the WK coefficient significantly and the mechanism leading to such modification, if any. A numerical model of a Kaplan model turbine with a semi-spiral casing is developed and used to such purpose. Previously conducted experiments on the model turbine are used to validate the numerical results. The magnitude and behavior of the secondary flow are investigated together with the WK coefficients. The GVO is found to have an impact on the WK method, and the impact increases with the GVOs as the flow structure change. A suitable location to minimize the impact of the GVO is suggested. Furthermore, the theoretical WK constant with a suitable location and configuration are also presented; this can be useful in the absence of the measured WK coefficient.

  • 19.
    Baidar, Binaya
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nicolle, Jonathan
    Institut de recherche d'Hydro-Québec.
    Gandhi, Bhupendra K.
    Indian Institute of Technology Roorkee.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sensitivity of the Winter-Kennedy method to inlet and runner blade angle change on a Kaplan turbine2019In: IOP Conference Series: Earth and Environmental Science, Institute of Physics (IOP), 2019, Vol. 240, article id 022038Conference paper (Refereed)
    Abstract [en]

    The Winter-Kennedy (WK) method is a widely used index testing approach, which provides a relative or index value of the discharge that can allow to determine the on-cam relationship between blade and guide vane angles for Kaplan turbines. However, some discrepancies were noticed in previous studies using the WK approach. In this paper, a numerical model of a Kaplan model turbine is used to study the effects of upstream and downstream flow conditions on the WK coefficients. Experiment on the model turbine is used to validate unsteady CFD calculations. The CFD results show that the inflow condition affects the pressure distribution inside the spiral case and hence the WK results. The WK coefficients fluctuate with high amplitude - suggesting to use a larger sampling time for on-site measurement as well. The study also concludes that to limit the impact of a change in runner blade angle on the coefficients, the more suitable WK locations are at the beginning of the spiral case with the inner pressure tap placed between stay vanes on the top wall.

  • 20.
    Iovanel, Raluca Gabriela
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. University Politehnica of Bucharest, Romania.
    Bucur, Diana-Maria
    University Politehnica of Bucharest, Romania.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 1 - Velocity Field2019In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 12, no 5, p. 1449-1461Article in journal (Refereed)
    Abstract [en]

    This paper investigates the accuracy of Reynolds-averaged Navier-Stokes (RANS) turbulence modelling applied to complex industrial applications. In the context of the increasing instability of the energy market, hydropower plants are frequently working at off-design parameters. Such operation conditions have a strong impact on the efficiency and life span of hydraulic turbines. Therefore, research is currently focused on improving the design and increasing the operating range of the turbines. Numerical simulations represent an accessible and cost efficient alternative to model testing. The presented test case is the Porjus U9 Kaplan turbine model operated at best efficiency point (BEP). Both steady and unsteady numerical simulations are carried out using different turbulence models: k-epsilon, RNG k-epsilon and k-omega Shear Stress Transport (SST). The curvature correction method applied to the SST turbulence model is also evaluated showing nearly no sensitivity to the different values of the production correction coefficient Cscale. The simulations are validated against measurements performed in the turbine runner and draft tube. The numerical results are in good agreement with the experimental time-dependent velocity profiles. The advantages and limitations of RANS modelling are discussed. The most accurate results were provided by the simulations using the k-epsilon and the SST-CC turbulence models but very small differences were obtained between the different tested models. The precision of the numerical simulations decreased towards the outlet of the computational domain. In a companion paper, the pressure profiles obtained numerically are investigated and compared to experimental data.

  • 21.
    Iovanel, Raluca Gabriela
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. University Politehnica of Bucharest, Romania.
    Dunca, Georgiana
    University Politehnica of Bucharest, Romania.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 2 - Pressure Fluctuations2019In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 12, no 5, p. 1463-1473Article in journal (Refereed)
    Abstract [en]

    The aim of the paper is to investigate the limitations of unsteady Reynolds-averaged Navier-Stokes (RANS) simulations of the flow in an axial turbine. The study is focused on modelling the pressure pulsations monitored on the runner blades. The scanned blade geometry renders the meshing process more difficult. As the pressure monitor points are defined on the blade surface the simulation relies on the wall functions to capture the flow and the pressure oscillations. In addition to the classical turbulence models, a curvature correction model is evaluated aiming to better capture the rotating flow near curved, concave wall boundaries. Given the limitations of Reynolds-averaged Navier-Stokes models to predict pressure fluctuations, the Scale Adaptive Simulation-Shear Stress Transport (SAS-SST) turbulence model is employed as well. The considered test case is the Porjus U9, a Kaplan turbine model, for which pressure measurements are available in the rotating and stationary frames of reference. The simulations are validated against time-dependent experimental data. Despite the frequencies of the pressure fluctuations recorded on the runner blades being accurately captured, the amplitudes are considerably underestimated. All turbulence models estimate the correct mean wall pressure recovery coefficient in the upper part of the draft tube.

  • 22.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Uncertainty Quantification of Aerodynamic Icing Losses in Wind Turbine With Polynomial Chaos Expansion2019In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 141, no 5, article id 051210Article in journal (Refereed)
    Abstract [en]

    Icing of wind turbine blades poses a challenge for the wind power industry in cold cli-mate wind farms. It can lead to production losses of more than 10%of the annual energyproduction. Knowledge of how the production is affected by icing is of importance. Com-plicating this reality is the fact that even a small amount of uncertainty in the shape ofthe accreted ice may result in a large amount of uncertainty in the aerodynamic perform-ance metrics. This paper presents a numerical approach using the technique of polyno-mial chaos expansion (PCE) to quantify icing uncertainty faster than traditionalmethods. Time-dependent bi-dimensional Reynolds-averaged Navier–Stokes computa-tional fluid dynamics (RANS-CFD) simulations are considered to evaluate the aerody-namic characteristics at the chosen sample points. The boundary conditions are based onthree-dimensional simulations of the rotor. This approach is applied to the NREL 5 MWreference wind turbine allowing to estimate the power loss range due to the leading-edgeglaze ice, considering a radial section near the tip. The probability distribution functionof the power loss is also assessed. The results of the section are nondimensionalized andassumed valid for the other radial sections. A correlation is found allowing to model theload loss with respect to the glaze ice horn height, as well as the corresponding probabil-ity distribution. Considering an equal chance for any of the ice profiles, load loss is esti-mated to be lower than 6.5%for the entire blade in half of the icing cases, while it couldbe roughly 4–6 times in the most severe icings.

  • 23.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gantasala, Sudhakar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Wind Turbine Aerodynamic Modeling in Icing Condition: Three-Dimensional RANS-CFD Versus Blade Element Momentum Method2019In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 141, no 7, article id 071201Article in journal (Refereed)
    Abstract [en]

    Icing limits the performance of wind turbines in cold climates. The prediction of the aerodynamic performance losses and their distribution due to ice accretion is essential. Blade element momentum (BEM) is the basis of blade structural studies. The accuracy and limitations of this method in icing condition are assessed in the present study. To this purpose, a computational study on the aerodynamic performance of the full-scale NREL 5 MW rotor is performed. Three-dimensional (3D) steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for both clean and iced blade, as well as BEM calculations using two-dimensional (2D) computational fluid dynamics (CFD) sectional airfoil data. The total power calculated by the BEM method is in close agreement with the 3D CFD results for the clean blade. There is a 4% deviation, while it is underestimated by 28% for the iced one. The load distribution along the clean blade span differs between both methods. Load loss due to the ice, predicted by 3D CFD, is 32% in extracted power and the main loss occurs at the regions where the ice horn height exceeds 8% of the chord length.

  • 24.
    Soltani Dehkharqani, Arash
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Engström, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    A Review of Available Methods for the Assessment of Fluid Added Mass, Damping, and Stiffness With an Emphasis on Hydraulic Turbines2018In: Applied Mechanics Review, ISSN 0003-6900, E-ISSN 1088-8535, Vol. 70, no 5, article id 050801Article in journal (Refereed)
    Abstract [en]

    Fluid added mass, damping, and stiffness are highly relevant parameters to consider when evaluating the dynamic response of a submerged structure in a fluid. The prediction of these parameters for hydraulic turbines has been approached relatively recently. Complex fluid-structure analyses including three-dimensional flow and the need for experiments during operation are the main challenges for the numerical and experimental approaches, respectively. The main objective of this review is to address the impact of different parameters, for example, flow velocity, cavitation, nearby solid structure, and rotational speed on the fluid added mass and damping of Kaplan/Propeller and Francis turbine runners. The fluid added stiffness is also discussed in the last section of the paper. Although studies related to hydraulic turbines are the main objective of this paper, the literature on hydrofoils is also taken into consideration to provide valuable information on topics such as individual runner blades. In this literature survey, the analytical, numerical, and experimental approaches used to determine fluid added parameters are discussed, and the pros and the cons of each method are addressed.

  • 25.
    Salehi, Saeed
    et al.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Water Power Laboratory, Norwegian University of Science and Technology, Trondheim, Norway.
    Nourbakhsh, Ahmad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
    An efficient multifidelity ℓ1-minimization method for sparse polynomial chaos2018In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 334, p. 183-207Article in journal (Refereed)
    Abstract [en]

    The Polynomial Chaos Expansion (PCE) methodology is widely used for uncertainty quantification of stochastic problems. The computational cost of PCE increases exponentially with the number of input uncertain variables (known as curse of dimensionality). Therefore, use of PCE for uncertainty quantification of industrial applications with large number of uncertain variables is challenging. In this paper, a novel methodology is presented for efficient uncertainty quantification of stochastic problems with large number of input random variables. The proposed method is based on PCE with combination of ℓ1-minimization and multifidelity methods. The developed method employs the ℓ1-minimization method to recover important coefficients of PCE using low-fidelity computations. The low-fidelity evaluations should be accurate enough to capture the physical trends well. After that the multifidelity PCE method is utilized to correct a subset of recovered coefficients using high-fidelity computations. A threshold parameter is defined in order to select the subset of recovered coefficients to be corrected. Two challenging analytical and CFD test cases namely, the Ackley function and the transonic RAE2822 airfoil with combined operational and geometrical uncertainties are considered to examine the performance of the methodology. It is shown that the proposed method can reproduce accurate results with much lower computational cost than the classical full Polynomial Chaos (PC), and ℓ1-minimization methods. It is observed that for the considered examples, the present method can achieve comparable accuracy with respect to the full PC and the ℓ1-minimization methods with significantly lower number of samples.

  • 26.
    Sundström, L.R. Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Energy and Process Engineering, Norwegian University of Science and Technology.
    Characteristics of the wall shear stress in pulsating wall-bounded turbulent flows2018In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 96, p. 257-265Article in journal (Refereed)
    Abstract [en]

    A pulsating turbulent pipe flow has been investigated experimentally using hot-film anemometry and particle image velocimetry. Particularly, a `paradoxical phenomenon' that is known to occur for a range of forcing frequencies and time-averaged Reynolds numbers has been investigated in detail. The paradoxical phenomenon is that the oscillating component of the wall shear stress exhibits a smaller amplitude in a turbulent flow compared to in a laminar flow exposed to the same oscillation in the pressure gradient. In here, the phenomenon is explained by splitting the response of the wall shear stress into one contribution resulting from the imposed pressure gradient , and a second contribution resulting from the oscillating Reynolds shear stress, At the conditions of maximum reduction of the wall shear stress amplitude,  and  are nearly 136 degrees out of phase. The contributions are thus interfering destructively, this being the ultimate reason for the reduced amplitude. It is also shown that the level of reduction is dependent on the imposed forcing amplitude, this in turn residing from a dependence of the time-development of the oscillating Reynolds shear stress on the forcing amplitude.

  • 27. Salehi, Saeed
    et al.
    Raisee, Mehrdad
    University of Tehran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nourbakhsh, Ahmad
    University of Tehran.
    Development of an efficient multifidelity non-intrusive uncertainty quantification method2018In: Evolutionary and Deterministic Methods for Design Optimization and Control With Applications to Industrial and Societal Problems / [ed] E. Andrés-Pérez, L.M. González, J. Periaux, N. Gauger, D. Quagliarella, K. Giannakoglou, Springer, 2018, p. 483-497Chapter in book (Refereed)
    Abstract [en]

    Most engineering problems contain a large number of input random variables, and thus their polynomial chaos expansion (PCE) suffers from the curse of dimensionality. This issue can be tackled if the polynomial chaos representation is sparse. In the present paper a novel methodology is presented based on combination of -minimization and multifidelity methods. The proposed method employ the - minimization method to recover important coefficients of PCE using low-fidelity computations. The developed method is applied on a stochastic CFD problem and the results are presented. The transonic RAE2822 airfoil with combined operational and geometrical uncertainties is considered as a test case to examine the performance of the proposed methodology. It is shown that the new method can reproduce accurate results with much lower computational cost than the classical full Polynomial Choas (PC), and - minimization methods. It is observed that the present method is almost 15–20 times faster than the full PC method and 3–4 times faster than the classical -minimization method.

  • 28.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Trivedi, Chirag
    Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim.
    Investigation of the numerical methodology of a model wind turbine simulation2018In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 11, no 3, p. 527-544Article in journal (Refereed)
    Abstract [en]

    The present work aims to investigate different methodologies for the numerical simulation of an upwind three-bladed wind turbine; which is supposed to be a base model to simulate icing in cold climate windmills. That is a model wind turbine for which wind tunnel tests have been completed at the Norwegian University of Science and Technology (NTNU). Using the assumption of axisymmetry, one-third of rotor has been modeled and periodic boundaries applied to include the effects of other blades. Then the full rotor was studied with transient simulation. To take in the effects of wind turbine wakes, the wind tunnel entrance and exit have been considered 4 and 5 diameters upstream and downstream of the rotor plane, respectively. Furthermore, the effects of tower and nacelle are included in a full-scale transient model of the wind tunnel. Structured hexa mesh has been created and the mesh is refined up to y+=1 in order to resolve the boundary layer. The simulations were performed using standard k-e, Shear Stress Transport (SST) model and a sophisticated model Scale-Adaptive Simulation (SAS)-SST to investigate the capability of turbulence models at design and off-design conditions The performance parameters, i.e., the loads coefficients and the wake behind the rotor were selected to analyze the flow over the wind turbine. The study was conducted at both design and offdesign speeds. The near wake profiles resulted from the transient simulation match well with the experiments at all the speed ranges. For the wake development modelling at high TSR, the present simulation needs to be improved, while at low and moderate TSR the results match with the experiments at far wake too. The agreement between the measurements and CFD is better for the power coefficient than for the thrust coefficient

  • 29.
    Sundström, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Laminar similarities between accelerating and decelerating turbulent flows2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 71, p. 13-26Article in journal (Refereed)
    Abstract [en]

    An experimental study of a pipe flow ramping monotonically between two turbulent states has been undertaken. Ensemble-averaged mean and turbulent flow quantities obtained from two-component particle image velocimetry and hot-film anemometry measurements have been presented. It is shown that the initial developments of the mean and turbulence quantities in linearly as well as impulsively accelerating and decelerating flows are similar. Specifically, the mean perturbation velocity (defined as the surplus/deficit from the initial value) can be described using self-similar expressions. The duration of this initial stage, when normalized appropriately, is shown to be approximately invariant of the type of transient imposed on the bulk flow. Data from studies of linearly accelerating and decelerating flows as well as impulsively accelerating and decelerating flows have been used to validate the results, covering four orders of magnitude of the dimensionless parameter . The highest initial Reynolds number (31,000) is, however, relatively low thus requiring further studies at high Reynolds numbers to assure the universality of the results. We have also shown that the time-development of the mean and turbulent quantities between an accelerating and a decelerating flow looses their similarity as the transient proceeds beyond the initial stage. The departure was explained by the time-evolvement of the production of turbulence kinetic energy, which exhibit differences between the two types of transients.

  • 30.
    Saemi, Simindokht
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. University of Tehran, Iran.
    Raisee, Mehrdad
    University of Tehran, Iran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nourbakhsh, Ahmad
    University of Tehran, Iran .
    Numerical Investigation of the Pressure-Time Method Considering Pipe with Variable Cross Section2018In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 140, no 10, article id 101401Article in journal (Refereed)
    Abstract [en]

    A common method to calculate the flow rate and consequently hydraulic efficiency in hydropower plants is the pressure-time method. In the present work, the pressure-time method is studied numerically by three-dimensional (3D) simulations and considering the change in the pipe cross section (a contraction). Four different contraction angles are selected for the investigations. The unsteady Reynolds-averaged Navier-Stokes (URANS) equations and the low-Reynolds k-ω shear stress transport (SST) turbulence model are used to simulate the turbulent flow. The flow physics in the presence of the contraction, and during the deceleration period, is studied. The flow rate is calculated considering all the losses: wall shear stress, normal stresses, and also flux of momentum in the flow. The importance of each term is evaluated showing that the flux of momentum plays a most important role in the flow rate estimation while the viscous losses term is the second important factor. To extend the viscous losses calculations applicability to real systems, the quasi-steady friction approach is employed. The results showed that considering all the losses, the increase in the contraction angle does not influence the calculated errors significantly. However, the use of the quasi-steady friction factor introduces a larger error, and the results are reliable approximately up to a contraction angle of Θ = 10 deg. The reason imparts to the formation of a local recirculation zone upstream and inside the contraction, which appears earlier as the contraction angle increases. This feature cannot be captured by the quasi-steady friction models, which are derived based on the fully developed flow assumption.

  • 31.
    Goyal, Rahul
    et al.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee.
    Trivedi, Chirag
    Norwegian University of Science and Technology, Trondheim, .
    Gandhi, Bhupendra K.
    Indian Institute of Technology Roorkee.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical Simulation and Validation of a High Head Model Francis Turbine at Part Load Operating Condition2018In: Journal of The Institution of Engineers (India): Series C, ISSN 2250-0545, Vol. 99, no 5, p. 557-570Article in journal (Refereed)
    Abstract [en]

    Hydraulic turbines are operated over an extended operating range to meet the real time electricity demand. Turbines operated at part load have flow parameters not matching the designed ones. This results in unstable flow conditions in the runner and draft tube developing low frequency and high amplitude pressure pulsations. The unsteady pressure pulsations affect the dynamic stability of the turbine and cause additional fatigue. The work presented in this paper discusses the flow field investigation of a high head model Francis turbine at part load: 50% of the rated load. Numerical simulation of the complete turbine has been performed. Unsteady pressure pulsations in the vaneless space, runner, and draft tube are investigated and validated with available experimental data. Detailed analysis of the rotor stator interaction and draft tube flow field are performed and discussed. The analysis shows the presence of a rotating vortex rope in the draft tube at the frequency of 0.3 times of the runner rotational frequency. The frequency of the vortex rope precession, which causes severe fluctuations and vibrations in the draft tube, is predicted within 3.9% of the experimental measured value. The vortex rope results pressure pulsations propagating in the system whose frequency is also perceive in the runner and upstream the runner.

  • 32.
    Baidar, Binaya
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nicolle, Jonathan
    Institut de recherche d'Hydro-Québec, Canada.
    Trivedi, Chirag
    Norwegian University of Science and Technology, NTNU, Norway.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical study of the Winter-Kennedy method: a sensitivity analysis2018In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 140, no 5, article id 051103Article in journal (Refereed)
    Abstract [en]

    The Winter-Kennedy (WK) method is commonly used in relative discharge measurement and to quantify efficiency step-up in hydropower refurbishment projects. The method utilizes the differential pressure between two taps located at a radial section of a spiral case, which is related to the discharge with the help of a coefficient and an exponent. Nearly a century old and widely used, the method has shown some discrepancies when the same coefficient is used after a plant upgrade. The reasons are often attributed to local flow changes. To study the change in flow behavior and its impact on the coefficient, a numerical model of a semi-spiral case (SC) has been developed and the numerical results are compared with experimental results. The simulations of the SC have been performed with different inlet boundary conditions. Comparison between an analytical formulation with the computational fluid dynamics (CFD) results shows that the flow inside an SC is highly three-dimensional (3D). The magnitude of the secondary flow is a function of the inlet boundary conditions. The secondary flow affects the vortex flow distribution and hence the coefficients. For the SC considered in this study, the most stable WK configurations are located toward the bottom from θ =30deg to 45deg after the curve of the SC begins, and on the top between two stay vanes.

  • 33.
    Salehi, Saeed
    et al.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nourbakhsh, Ahmad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    On the flow field and performance of a centrifugal pump under operational and geometrical uncertainties2018In: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 61, p. 540-560Article in journal (Refereed)
    Abstract [en]

    Uncertainties are present in most engineering applications such as turbomachines. The performance of turbomachinery can be highly affected by the presence of uncertainties and these effects should be considered in the design procedure. The present study is the first work to quantify the impact of various uncertainties in a hydraulic machine. The paper is concerned with the combined effects of geometrical and operational uncertainties on the flow field and performance of a low specific-speed centrifugal pump. The uncertainty analysis is performed for three different flow rates, i.e., Q/Qd=0.825,1.0 and 1.1175. The volumetric flow rate, rotational speed and blade geometry of the pump are assumed to be stochastic with uniform Probability Distribution Functions (PDFs). The randomness in the blade geometry, due to the manufacturing tolerances, is imposed through two Karhunen-Loève (KL) expansions. The eigenvalues of covariance kernel of KL expansions rapidly decay due to the large correlation lengths and thus a few terms are used in the truncated KL expansion to describe the blade geometrical uncertainties. The uncertainties are propagated in the flow field and performance of the pump using the Non-Intrusive Polynomial Chaos (NIPC) method. The respective effects of assumed uncertain variables on the quantities of interest are assessed using Sobol’ indices. The uncertain operational and geometrical conditions have significant influences on the flow field and performance of the pump. It is observed that while variation of the pump head coefficient under operational and geometrical uncertainties is significant, the pump efficiency shows a robust behavior under assumed uncertain conditions.

  • 34.
    Sundström, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway.
    On the Similarity of Pulsating and Accelerating Turbulent Pipe Flows2018In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 100, no 2, p. 417-436Article in journal (Refereed)
    Abstract [en]

    The near-wall region of an unsteady turbulent pipe flow has been investigated experimentally using hot-film anemometry and two-component particle image velocimetry. The imposed unsteadiness has been pulsating, i.e., when a non-zero mean turbulent flow is perturbed by sinusoidal oscillations, and near-uniformly accelerating in which the mean flow ramped monotonically between two turbulent states. Previous studies of accelerating flows have shown that the time evolution between the two turbulent states occurs in three stages. The first stage is associated with a minimal response of the Reynolds shear stress and the ensemble-averaged mean flow evolves essentially akin to a laminar flow undergoing the same change in flow rate. During the second stage, the turbulence responds rapidly to the new flow conditions set by the acceleration and the laminar-like behavior rapidly disappears. During the final stage, the flow adapts to the conditions set by the final Reynolds number. In here, it is shown that the time-development of the ensemble-averaged wall shear stress and turbulence during the accelerating phase of a pulsating flow bears marked similarity to the first two stages of time-development exhibited by a near-uniformly accelerating flow. The stage-like time-development is observed even for a very low forcing frequency; ω+=ων/u¯τ2=0.00073" role="presentation">ω+=ων/u¯¯¯2τ=0.00073 (or equivalently, ls+=2/ω+=52" role="presentation">l+s=2/ω+−−−−√=52), at an amplitude of pulsation of 0.5. Some previous studies have considered the flow to be quasi-steady at ls+=52" role="presentation">l+s=52; however, the forcing amplitude has been smaller in those studies. The importance of the forcing amplitude is reinforced by the time-development of the ensemble-averaged turbulence field. For, the near-wall response of the Reynolds stresses showed a dependence on the amplitude of pulsation. Thus, it appears to exist a need to seek alternative similarity parameters, taking the amplitude of pulsation into account, if the response of different flow quantities in a pulsating flow are to be classified correctly.

  • 35.
    Goyal, Rahul
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Indian Institute of Technology, Department of Mechanics & Industrial Engineering, Roorkee .
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee .
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    PIV measurements in Francis turbine: A review and application to transient operations2018In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 81, no 2, p. 2976-2991Article in journal (Refereed)
    Abstract [en]

    Penetration of solar and wind energy into the grid network has raised the concern for grid stability which is generally balanced by operating the hydropower plants over a wide range. This results in several issues, such as rotor-stator interaction (RSI), vortex breakdown, rotating vortex rope (RVR), pressure shocks, vibration, and noise which may lead to failure. Particle Image Velocimetry (PIV) has been used to understand several physical mechanisms in the flow at various operating conditions. A non-negligible uncertainty may arise in the measurements due to calibration, abbreviation, and distortion of the light. Various parameters such as laser sheet thickness, particle type, particle size, particle density, camera resolution, image size and number of images may affect the quality of the measurements. In the present work, a review of PIV measurements performed in hydraulic turbines, mainly Francis, has been carried out. The objective is to develop an experimental set up to perform steady and transient measurements on a model Francis turbine. A maximum deviation of 1.8% in absolute velocity is estimated in the present study as compared to 2–3% reported in the previously performed measurements on Francis turbines. The repeatability of transient measurements is also investigated by extracting two velocity points on a PIV plane

  • 36.
    Sundström, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    The self-similarity of wall-bounded temporally accelerating turbulent flows2018In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 19, no 1, p. 49-60Article in journal (Refereed)
    Abstract [en]

    From the study of viscous flow it is known that certain time-dependent laminar problems, such as the impulsively started flat plate and the diffusion of a vortex sheet, possess self-similar solutions. Previous studies of turbulent channel and pipe flows accelerating between two steady states have shown that the flow field evolves in three distinct stages. Furthermore, recent direct numerical simulations have shown that the perturbation velocity, i.e. the surplus velocity from the initial value, in an impulsively accelerating turbulent channel and pipe flow also possesses a self-similar distribution during the initial stage. In here, these results are developed analytically and it is shown that accelerating flows in which the centreline velocity develops as Uc(t) = U0(t/t0)m will possess a self-similar velocity distribution during the initial stage. The displacement thickness of the perturbation velocity is shown to be dependent only on the type of acceleration, and not on the initial Reynolds number, the acceleration rate or the change in Reynolds number. The derived formulas are verified with good agreement against measurements performed in a linearly accelerating turbulent pipe flow and with data from channel flow simulations.

  • 37.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Trivedi, Chirag
    Department of Energy and Process Engineering, Faculty of Engineering, Norwegian University of Science and Technology (NTNU), Trondheim.
    Time-Dependent Effects of Glaze Ice on the Aerodynamic Characteristics of an Airfoil2018In: International Journal of Rotating Machinery, ISSN 1023-621X, E-ISSN 1542-3034, Vol. 2018, article id 2981739Article in journal (Refereed)
    Abstract [en]

    The main objective of this study is to estimate the dynamic loads acting over a glaze-iced airfoil. This work studies the performance of unsteady Reynolds-averaged Navier-Stokes (URANS) simulations in predicting the oscillations over an iced airfoil. The structure and size of time-averaged vortices are compared to measurements. Furthermore, the accuracy of a two-equation eddy viscosity turbulence model, the shear stress transport (SST) model, is investigated in the case of the dynamic load analysis over a glaze-iced airfoil. The computational fluid dynamic analysis was conducted to investigate the effect of critical ice accretions on a 0.610 m chord NACA 0011 airfoil. Leading edge glaze ice accretion was simulated with flat plates (spoiler-ice) extending along the span of the blade. Aerodynamic performance coefficients and pressure profiles were calculated and validated for the Reynolds number of 1.83 × 106. Furthermore, turbulent separation bubbles were studied. The numerical results confirm both time-dependent phenomena observed in previous similar measurements: (1) low-frequency mode, with a Strouhal number Sth≈0,013–0.02, and (2) higher frequency mode with a Strouhal number StL≈0,059–0.69. The higher frequency motion has the same characteristics as the shedding mode and the lower frequency motion has the flapping mode characteristics

  • 38.
    Goyal, Rahul
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Indian Institute of Technology Roorkee.
    Gandhi, Bhupendra K.
    Indian Institute of Technology Roorkee.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Transient Pressure Measurements in the Vaneless Space of a Francis Turbine during Load Acceptances from Minimum Load2018In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 1042, article id 012009Article in journal (Refereed)
    Abstract [en]

    Increased penetration of solar and the wind impels the designers of the hydroelectric power generation unit to provide more flexibility in operation for the stability of the grid. The power generating unit includes turbine which needs to sustain sudden change in its operating conditions. Thus, the hydraulic turbine experiences more transients per day which result in chronic problems such as fatigue to the runner, instrument malfunctioning, vibrations, wear and tear etc. This paper describes experiments performed on a high model (1.5:1) Francis turbine for load acceptances from the minimum load. The experiments presented in the paper are the part of Francis-99 workshop which aims to determine the performance of numerical models in simulations of model Francis turbine under steady and transient operating conditions. The aim of the paper is to present the transient pressure variation in the vaneless space of a Francis turbine where high-frequency pulsations are normally expected. For this, two pressure sensors, VL1 and VL2, are mounted at the vaneless space, one near the beginning of the spiral casing and the other before the end of the spiral casing. Both are used to capture the unsteady pressure field developed in the space between guide vanes and runner inlet. The time-resolved pressure signals are analyzed and presented during the transient to observe the pressure variation and dominant frequencies of pulsations.

  • 39.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Raisee, Mehrdad
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Uncertainty quantification of iced airfoil  on wind Turbine Using polynomial chaos expansion2018In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994Article in journal (Refereed)
  • 40.
    Safari, Alaleh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel Jose
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Water Power Laboratory , Norwegian University of Science and Technology .
    Viscoelasticity and shear-thinning effects on bio-polymer solution and suspended particle behaviours under oscillatory curve Couette flow conditions2018In: Biosurface and Biotribology, ISSN 2405-4518, Vol. 4, no 1, p. 1-17Article in journal (Refereed)
    Abstract [en]

    Formation of wear particles within total hip replacement is one of the main causes of its failure. In addition to improving the lubrication and wear resistance of materials used as bearing surfaces, understanding of wear particle distribution patterns within lubricants inside an implant gap could be used to improve design parameters and implants’ lifespan. In this study, the behaviours of biolubricants (with compositions similar to human joint synovial fluid) and suspended particles were investigated by micro-particle image velocimetry in curved mini channels under oscillatory Couette flow conditions. The studied biolubricants had shear-thinning viscoelastic characteristics. The authors found that increasing shear-thinning, elasticity or motion frequency levels did not affect the trend behaviours of biolubricant flows due to the low strain values of the experimental conditions applied. However, suspended particles formed strings along flow directions and exhibited cross-stream migration to channel walls. Motion frequency, fluid shear thinning and elasticity characteristics and channel dimensions strongly affected particle behaviours.

  • 41.
    Goyal, Rahul
    et al.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology.
    Characteristics of Synchronous and Asynchronous modes of fluctuations in Francis turbine draft tube during load variation2017In: International Journal of Fluid Machinery and Systems, ISSN 1882-9554, E-ISSN 1882-9554, Vol. 10, no 2, p. 164-175Article in journal (Refereed)
    Abstract [en]

    Francis turbines are often operated over a wide load range due to high flexibility in electricity demand and penetration of other renewable energies. This has raised significant concerns about the existing designing criteria. Hydraulic turbines are not designed to withstand large dynamic pressure loadings on the stationary and rotating parts during such conditions. Previous investigations on transient operating conditions of turbine were mainly focused on the pressure fluctuations due to the rotor-stator interaction. This study characterizes the synchronous and asynchronous pressure and velocity fluctuations due to rotor-stator interaction and rotating vortex rope during load variation, i.e. best efficiency point to part load and vice versa. The measurements were performed on the Francis-99 test case. The repeatability of the measurements was estimated by providing similar movement to guide vanes twenty times for both load rejection and load acceptance operations. Synchronized two dimensional particle image velocimetry and pressure measurements were performed to investigate the dominant frequencies of fluctuations, vortex rope formation, and modes (rotating and plunging) of the rotating vortex rope. The time of appearance and disappearance of rotating and plunging modes of vortex rope was investigated simultaneously in the pressure and velocity data. The asynchronous mode was observed to dominate over the synchronous mode in both velocity and pressure measurements.

  • 42.
    Salehi, Saeed
    et al.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Computation of Developing Turbulent Flow through a Straight Asymmetric Diffuser with Moderate Adverse Pressure Gradient2017In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 10, no 4, p. 1029-1043Article in journal (Refereed)
    Abstract [en]

    In this paper, numerical investigation of three-dimensional, developing turbulent flow, subjected to a moderate adverse pressure gradient, has been investigated using various turbulence models, namely: the low-Re k -ε, the SST k - ω, the v2 - f and a variant of Reynolds stress model. The results are compared with the detailed velocity and pressure measurements. Since the inlet condition is uncertain, a study was first performed to investigate the sensitivity of the results to the inlet boundary condition. The results showed the importance of including the contraction effects. It is seen that the developing flow inside the straight duct, is highly sensitive to the inlet boundary condition. The comparisons indicate that all turbulence models are able to predict a correct trend for the centerline velocity and pressure recovery inside the straight duct and diffuser but the low-Re k -ε and RSM turbulence models yield more realistic results. The SST k - ω model largely overpredicts the centerline velocity and boundary layer thickness in the straight duct. The comparisons of the numerical results also revealed that the RSM model, due to its anisotropic formulation, is able to reproduce the secondary flows. As expected, the RSM model demonstrates the best performance in prediction of the flow field and pressure recovery in the asymmetric diffuser.

  • 43.
    Safari, Alaleh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Espanol, Montserrat
    Biomaterials, Biomechanics and Tissue Engineering Group, Department of Materials Science and Metallurgy, Technical University of Catalonia.
    Ginebra, Maria-Pau
    Biomaterials, Biomechanics and Tissue Engineering Group, Dept. of Materials Science and Metallurgy, Technical University of Catalonia.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Water Power Laboratory, Norwegian University of Science and Technology.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Effect of dynamic loading versus static loading on the frictional behavior of a UHMWPE pin in artificial biolubricants2017In: Biosurface and Biotribology, ISSN 2405-4518, Vol. 3, no 1, p. 35-44Article in journal (Refereed)
    Abstract [en]

    To obtain reliable results from in vitro measurements on the tribological behavior of joint implant materials, the parameters of the measurements must simulate in vivo conditions. Although the nature of the load in human joints is dynamic, most of the studies using simple pin-on-disk tribometers were performed with a constant load. The current study focused on investigating the effect of dynamic loading in comparison with static loading in the tribological behavior of ultra-high-molecular-weight polyethylene (UHMWPE) sliding against a cobalt chromium molybdenum (CoCrMo) counter surface with different lubricants, where the effects of hyaluronic acid (HA) and protein content in the lubricants were also investigated. The results suggested that although the dynamic loading did not affect the friction evolution for any of the lubricants, the friction value decreased for the lubricants that did not contain HA. The results showed that higher protein content in the lubricant increased the friction coefficient, however, it provided the highest protection against wear for sliding surfaces.

  • 44.
    Amiri, Kaveh
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Mulu, Berhanu G.
    Vattenfall Research and Development, Älvkarleby.
    Raisee, Mehrdad
    Mechanical Engineering Department, University of Tehran.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effects of upstream flow conditions on runner pressure fluctuations2017In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 10, no 4, p. 1045-1059Article in journal (Refereed)
    Abstract [en]

    The rotor-stator interaction and the corresponding pressure fluctuations represent one of the sources of pressure and load fluctuations on the rotating parts of rotating machineries. The high-Reynolds flow is subject to rotation in the comparably large vaneless space of axial turbines, causing wake interaction and wake dissipation in this region. This increases the level of flow complexity in this region. This study examined the effect of the flow condition entering the spiral casing on the flow condition within the distributor and the runner and the physical source of pressure fluctuations exerted on the runner of a Kaplan turbine model. Simulations were performed within the water supply system, including the upstream tank, penstock, and the Francis turbines, the level of entering the spiral casing; the results were compared with laser Doppler anemometry (LDA) results. The results were considered as the inlet boundary condition for simulation of the turbine model from the spiral inlet to the draft tube outlet to investigate the flow condition within the distributor and the runner. The CFD simulations showed that the water supply system induces inhomogeneity to the velocity distribution at the spiral inlet. However, the flow condition does not affect the pressure fluctuations exerted on the runner blades due to the rotor-stator interactions. Moreover, the dominant frequencies exerted on the runner blades were accurately approximated although the amplitudes of the fluctuations were underestimated.

  • 45.
    Salehi, Saeed
    et al.
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Raisee, Mehrdad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Nourbakhsh, Ahmad
    Hydraulic Machinery Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran.
    Efficient uncertainty quantification of stochastic CFD problems using sparse polynomial chaos and compressed sensing2017In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 154, p. 296-321Article in journal (Refereed)
    Abstract [en]

    Most engineering problems contain a large number of input random variables, and thus their Polynomial Chaos Expansion (PCE) suffers from the curse of dimensionality. This issue can be tackled if the polynomial chaos representation is sparse. In the current study, the compressed sensing theory is employed to reconstruct the sparse representation of polynomial chaos expansion of challenging stochastic problems. The sparse recovery problem is solved using the Orthogonal Matching Pursuit (OMP). The Leave-One-Out (LOO) cross-validation is employed for the estimation of truncation error in the OMP procedure. In contrast to previous studies, which mainly focused on the random variables with uniform or Gaussian distributions, this paper applies the ℓ1-minimization technique to arbitrarily distributed random variables. The orthogonal polynomials are constructed using the Gram–Schmidt orthogonalization method. Two challenging analytical test functions namely, Isighami and corner-peak, and three CFD problems namely, the two-dimensional heat diffusion problem with stochastic thermal diffusivity, the transonic RAE2822 airfoil with operational and geometrical uncertainties and the fully-developed turbulent channel flow with random turbulence model coefficients are considered to examine the performance of the methodology. The numerical results of the developed method are compared with the results of Monte Carlo (MC) simulation and regression-based polynomial chaos expansion. It is demonstrated that the ℓ1-minimization method can be successfully applied to arbitrarily distributed uncertainties. Results show that the method can reproduce the sparse PCE with much lower computational time than the classical full Polynomial Chaos (PC) method. For the problems considered in the current study, for the same accuracy, the number of required samples for the sparse PC representation is significantly reduced.

  • 46.
    Goyal, Rahul
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology.
    Experimental study of mitigation of a spiral vortex breakdown at high Reynolds number under an adverse pressure gradient2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 9, article id 104104Article in journal (Refereed)
    Abstract [en]

    The flow in the off-design operation of a Francis turbine may lead to the formation of spiral vortex breakdowns in the draft tube, a diffuser installed after the runner. The spiral vortex breakdown, also named a vortex rope, may induce several low-frequency fluctuations leading to structural vibrations and a reduction in the overall efficiency of the turbine. In the present study, synchronized particle image velocimetry, pressure, and turbine flow parameter (Q, H, α, and T) measurements have been carried out in the draft tube cone of a high head model Francis turbine. The transient operating condition from the part load to the best efficiency point was selected to investigate the mitigation of the vortex rope in the draft tube cone. The experiments were performed 20 times to assess the significance of the results. A precession frequency of 1.61 Hz [i.e., 0.29 times the runner rotational frequency (Rheingans frequency)] is observed in the draft tube cone. The frequency is captured in both pressure and velocity data with its harmonics. The accelerating flow condition at the center of the cone with a guide vane opening is observed to diminish the spiral form of the vortex breakdown in the quasi-stagnant region. This further mitigates the stagnant part of the cone with a highly dominated axial flow condition of the turbine at the best efficiency point. The disappearance of the stagnant region is the most important state in the present case, which mitigates the spiral vortex breakdown of the cone at high Reynolds numbers. In contrast to a typical transition, a new type of transition from wake to jet is observed during the mitigation of the breakdown. The obtained 2D instantaneous velocity fields demonstrate the disappearance region of shear layers and stagnation in the cone. The results also demonstrate the existence of high axial velocity gradients in an elbow draft tube cone.

  • 47.
    Trivedi, Chirag
    et al.
    Waterpower Laboratory, Norwegian University of Science and Technology.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Fluid-structure interactions in Francis turbines: A perspective review2017In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 68, no 1, p. 87-101Article in journal (Refereed)
    Abstract [en]

    Competitive electricity prices and reduced profit margins have forced hydraulic turbines to operate under critical conditions. The demand for extended operating ranges and the high efficiency of the turbine runners have forced manufacturers to produce lightweight runners. A turbine runner sometimes experiences resonance when a forced (flow-induced) excitation frequency approaches the runner’s natural frequency, resulting in failure. The cost of structural failure after commissioning is prohibitive. To attain a reliable and safe runner design, understanding of the structural response to flow-induced excitations is important. High amplitude pressure pulsations cause fatigue loading of the blades, which develop cracks over time. The amplitudes are dependent on the flow conditions, type of turbine and stator/rotor vane combinations. The structural response is dependent on the material properties, flow-induced damping and natural frequencies. Moreover, in a hydraulic turbine, changes in flow velocity from less than 1 m s−1 to over 40 m s−1 create challenges in predicting the response.

    The main objective of this article is to review the studies conducted on fluid-structure interactions within hydraulic turbines. Several aspects are reviewed, such as flow-induced excitation, added mass effect, hydrodynamic damping, and blade flutter. Both experimental and numerical studies are discussed in this article. This review also discusses the consequences of an increased number of transient cycles, such as load variation, start-stop and total load rejection, on the turbines and the fatigue loading. Finally, an attempt is made to highlight the important requirements for prospective fluid-structure analysis to fill current gaps in the literature.

  • 48.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Trivedi, Chirag
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Dahlhaug, Ole Gunnar
    Department of Energy and Process Engineering, Water Power Laboratory, Norwegian University of Science and Technology, Trondheim, Norwegian University of Science and Technology (NTNU), Trondheim.
    Nielsen, Torbjørn Kristian
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Francis-99 Workshop 2: transient operation of Francis turbines2017In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 782, article id 11001Article in journal (Other academic)
  • 49.
    Grecu, Ionuț Stelian
    et al.
    Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest.
    Bucur, Diana Maria
    Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest.
    Dunca, Georgiana
    Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest.
    Panaitescu, Valeriu Nicolae
    Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Norwegian University of Science and Technology Trondheim.
    Implementation of the standard wall function in numerical computation software2017Conference paper (Refereed)
    Abstract [en]

    The paper presents the validation of the implementation made by the user of the standard wall function for the standard k-ε turbulence model against the built-in standard wall function for the standard k-ε turbulence mode, in numerical computation software (ANSYS Fluent). A comparison was made between the results of two flow simulations: the first case consisted in using the built-in standard wall function and in the second case the implemented standard wall function was considered. The numerical simulations were made for the water flow inside a 3D diffuser. The results showed that both the wall functions used had a similar influence on the simulated flow.

  • 50.
    Bucur, Diana Maria
    et al.
    University POLITEHNICA of Bucharest.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Dunca, Georgiana
    University POLITEHNICA of Bucharest.
    Maximum pressure evaluation during expulsion of entrapped air from pressurized pipelines2017In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 10, no 1, p. 11-20Article in journal (Refereed)
    Abstract [en]

    Pressurized pipeline systems may have a wide operating regime. This paper presents the experimental analysis of the transient flow in a horizontal pipe containing an air pocket, which allows the ventilation of the air after the pressurization of the hydraulic system, through an orifice placed at the downstream end. The measurements are made on a laboratory set-up, for different supply pressures and various geometries of water column length, air pocket and expulsion orifice diameter. Dimensional analysis is carried out in order to determine a relation between the parameters influencing the maximum pressure value. A two equations model is obtained and a criterion is established for their use. The equations are validated with experimental data from the present laboratory set-up and with other data available in the literature. The results presented as nondimensional quantities variations show a good agreement with the previous experimental and analytical researches.

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