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  • 151.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Aidanpää, Jan-Olov
    Glavatskikh, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Karlsson, Thommy
    Group dynamics [hydro power equipment]2005In: International Water Power and Dam Construction, ISSN 0306-400X, E-ISSN 1538-6414, Vol. 57, no 12, p. 40-45Article in journal (Other academic)
    Abstract [en]

    An interdisciplinary research group was created at the Lulea University of Technology in Sweden to help develop world leading competencies and knowledge for cutting-edge technologies in hydro power generation. Based on the studies of this Swedish research group, this paper provides a description of the power system from a dynamic point of view to obtain an overall picture, and is completed with a detailed description of the bearings and turbines. The state-of-the-art in each field is presented, together with suggestions for further development

  • 152.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Amiri, Kaveh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Rotor-Stator interaction in Kaplan turbines during steady and transient operations2013Conference paper (Refereed)
  • 153.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Andersson, Uban
    Vattenfall Research & Development.
    Lövgren, Magnus
    Vattenfall Research & Development.
    Turbine-99 unsteady simulations: validation2010In: 25th IAHR Symposium on Hydraulic Machinery and Systems: 20-24 September 2010 'Politehnica' University of Timişoara, Timişoara, Romania, London: IOP Publishing Ltd , 2010, p. 12014-Conference paper (Refereed)
    Abstract [en]

    The Turbine-99 test case, a Kaplan draft tube model, aimed to determine the state of the art within draft tube simulation. Three workshops were organized on the matter in 1999, 2001 and 2005 where the geometry and experimental data were provided as boundary conditions to the participants. Since the last workshop, computational power and flow modelling have been developed and the available data completed with unsteady pressure measurements and phase resolved velocity measurements in the cone. Such new set of data together with the corresponding phase resolved velocity boundary conditions offer new possibilities to validate unsteady numerical simulations in Kaplan draft tube. The present work presents simulation of the Turbine-99 test case with time dependent angular resolved inlet velocity boundary conditions. Different grids and time steps are investigated. The results are compared to experimental time dependent pressure and velocity measurements

  • 154.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Andrée, Gunilla
    Vattenfall.
    Klason, Peter
    Sundström, Joel
    Elforsk.
    Flow measurements in low-head hydro power plants2012Report (Other academic)
    Abstract [en]

    Hydropower stands for a large part of the energy production portfolio in Sweden and provides about 50% of the electricity needs. Most of the turbines were built some decades ago and are in a need of refurbishment. An important refurbishment period started some years ago and will be continuous. Substantial production gains and adaptation to new market demands may be achieved with such refurbishments. Refurbishments are also stimulated by the government through the electricity certificate system. Efficiency step-ups are thus of importance but challenging due to the presence of mainly low head (H<50 m) machines in Sweden. During the last decades, the Winter-Kennedy method has been used to verify improvements of the efficiency by measuring before and after a refurbishment. The results have for a number of cases shown unpredictable results. There is a need of development to measure accurately the efficiency in order to evaluate the outcome of different refurbishment projects. A workgroup within the Swedish Hydropower Centre (Svenskt Vattenkraftcentrum, SVC) has been formed together with representatives from the majority of the hydro turbine industry in Sweden to address the challenge of flow measurements in low head hydraulic turbines. The present report presents the different methods available with their actual development status and potential to meet low head hydraulic machines constraints. The working group suggests several actions for the development of flow measurements in low head machines. They are divided in 2 categories: long term and short term. The long term actions are typical SVC projects for PhD or/and senior researcher while short term actions are projects for consultant or/and senior researcher. The following actions are suggested in a hierarchical order:Long term projects1. Development of the pressure-time method as an absolute and relative method2. Evaluation of scale-up formula and influence of the parameters differing between model and prototype such spiral inlet boundary conditionsShort term projects1. Procedure/road book for implementation, evaluation and reporting of the Winter-Kennedy method. Continue working on the common guideline drafted in SEK-TK4.2. Systematic error analysis of the Winter-Kennedy method3. Testing of the volumetric method on a full-scale unit to investigate capabilities and evaluate necessary development for low head hydro power plants4. Testing of the tracer dilution method on a full-scale unit to investigate capabilities and evaluate necessary development for low head hydro power plants

  • 155.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cupillard, Samuel
    Hydro-Québec Research’s Institute.
    3D thermodynamic analysis of a turbulent textured slider2010Conference paper (Refereed)
    Abstract [en]

    Sliding contacts under laminar regime have been extensively investigated under the last years. The results indicate the possibility to increase load carrying capacity in a slider bearing with more than 10%. The effect of dimples on a slider bearing under a turbulent regime has not yet been investigated. It is the object of the present study. The numerical analysis of a 3D textured slider bearing with fore-region and extended channels at the outlet and on the sides of a pad is considered with a temperature dependent fluid, 2 different types of dimple shape and different operating conditions. The simulations are carried out for a turbulent flow (Re=4.4•10^3- 15•10^3) using Detached Eddy Simulation. The results indicate no gain on the load carrying capacity with the dimple shapes (rectangular and oblique) investigated. A higher operating temperature is found in the presence of dimples.

  • 156.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cupillard, Samuel
    Hydro-Québec Research’s Institute.
    Bance, Kevin
    National Institute for Applied Science.
    Kokkolaras, Michael
    McGill University.
    Shape optimization of a 3D slider with dimples2012In: Journal of Civil Engineering and Architecture, ISSN 1934-7359, E-ISSN 1934-7367, Vol. 6, no 12, p. 1599-1607Article in journal (Refereed)
    Abstract [en]

    Sliding contacts in laminar flow regimes have been investigated extensively in recent years. The results indicate the possibility to increase load carrying capacity in a slider bearing for more than 10% with the addition of dimples. Parametric studies have been performed to determine optimal size and position, with emphasis in the optimal shape and position of the dimple for an operating condition. In this article, the numerical analysis of a 2D textured slider bearing with a dimple is initially considered with an isothermal laminar fluid. Position, depth, width and convergence ratio are optimized, the results demonstrate the importance of the width and convergence ratio to increase load. Then, the numerical analysis of a 3D textured slider bearing with fore-region and extended channels at the outlet and on the sides of a pad is considered. The simulations are also carried out for a laminar isothermal flow. Three dimples are considered and their depth is optimized.

  • 157.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cupillard, Samuel
    Hydro-Québec Research’s Institute.
    Kokkolaras, Michael
    University of Michigan.
    Shape optimisation of a 3D slider with a dimple2011Conference paper (Refereed)
    Abstract [en]

    Sliding contacts under laminar regime have been extensively investigated under the last years. The results indicate the possibility to increase load carrying capacity in a slider bearing with more than 10% with the addition of dimples. Parametric studies have been performed on size and position, while an optimisation to determine the true potential have not yet been examined. Of interest is the optimal shape of the dimple function of the operating condition and position. In the present work, the numerical analysis of a 2D textured slider bearing with fore-region is initially considered with an isothermal laminar fluid. One dimple is considered and the shape optimized for different operating conditions and positions. Then, the numerical analysis of a 3D textured slider bearing with fore-region and extended channels at the outlet and on the sides of a pad is considered with a temper-ature dependent fluid. The simulations are also carried out for a laminar flow. One dimple is considered and the shape optimized for different operating conditions and positions.

  • 158.
    Cervantes, Michel
    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.
    Eddy viscosity turbulence models and steady draft tube simulations2005In: Turbine-99 III: Proceedings of the third IAHR/ERCOFTAC workshop on draft tube flow, Luleå: Luleå tekniska universitet, 2005, p. 37-44Conference paper (Refereed)
    Abstract [en]

    Computations of the Turbine-99 benchmark have been performed for two dimensional steady inlet boundary conditions. Three different turbulence models were used: zero equation model, k-ε and shear stress model (SST). The results from the engineering quantities indicate small differences on the mean pressure recovery and the loss factor, while larger differences appear for the wall pressure recovery.

  • 159.
    Cervantes, Michel
    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. Luleå tekniska universitet.
    Factorial design applied to CFD2004In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 126, no 5, p. 791-798Article in journal (Refereed)
    Abstract [en]

    Factorial design, a statistical method widely used for experiments, and its application to CFD are discussed. The aim is to propose a systematic, objective, and quantitative method for engineers to design a set of simulations in order to evaluate main and joint effects of input parameters on the numerical solution. The input parameters may be experimental uncertainty on boundary conditions, unknown boundary conditions, grid, differencing schemes, and turbulence models. The complex flow of the Turbine-99 test case, a hydropower draft tube flow, is used to illustrate the method, where four factors are chosen to perform a 24 factorial design. The radial velocity at the inlet (not measured) is shown to have an important influence on the pressure recovery (7%) and the energy loss factor (49%).

  • 160.
    Cervantes, Michel
    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. Vattenfall Power Consultant.
    Pulsating turbulent flow in a straight asymmetric diffuser2008In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 46, no Suppl. 1, p. 112-128Article in journal (Refereed)
    Abstract [en]

    Pulsating turbulent flow is studied for four regimes: steady, quasi-steady, relaxation, and quasi-laminar in a rectangular straight asymmetric diffuser, a generic model of the diffuser found at the end of most Kaplan and Francis type hydropower turbines. The flow entering the diffuser is a developing duct flow at Reynolds number 20 000, based on mean streamwise velocity and hydraulic diameter. The time averaged velocity and turbulent quantities are not affected by the forced oscillation. The regimes prevail in the diffuser, but are shifted due to the decreasing friction velocity. The oscillating quantities are affected by the adverse pressure gradient in the same way as the time averaged quantities, but with a decreasing effect for higher forcing frequencies. The amplitude of the oscillating wall shear stress is found to be signicantly lower than the Stokes solution in the quasi-laminar regime. The regime is confirmed by the behaviour of several other quantities. The pressure recovery is found to be 30 % higher in the relaxation regime than in the other regimes. Results are compared with experiments in channels and turbulent boundary layers, with and without an adverse pressure gradient, and with large-eddy simulations.

  • 161.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Engström, FredrikLuleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.Gustavsson, Håkan
    Turbine-99 III: Proceedings of the third IAHR/ERCOFTAC workshop on draft tube flow2005Collection (editor) (Other academic)
    Abstract [en]

    The third IAHR/ERCOFTAC workshop on draft tube flows, Turbine-99 III, is based on the experience gained during the first two workshops and the development of the computational capacities. It is expected to be a step towards better understanding of draft tube flow simulation capabilities. Three cases based on the Turbine-99 benchmark were proposed to the participants: steady calculation, unsteady calculation, optimization of the draft tube performance. More than 30 simulations have been performed by the participants with several turbulence models, near wall treatment, grids and boundary conditions. The complexity of the turbulence models ranges from zero equation model to large eddy simulations. The contribution of the different participants, the protocol of their simulations and a comprehensive comparison of experimental data with the simulations are included in the present document.

  • 162.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Engström, T. Fredrik
    Luleå tekniska universitet.
    Influence of boundary conditions using factorial design2001In: Proceedings of the Turbine-99 II: Workshop on Draft Tube Flow, 2001Conference paper (Refereed)
  • 163.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Enström, Christer
    MPC Metal Process Control AB.
    Kelvesjö, Håkan
    MPC Metal Process Control AB.
    Ohlsson, Willy
    MPC Metal Process Control AB.
    Method and apparatus for non-contact measuring of metal bed parametersPatent (Other (popular science, discussion, etc.))
    Abstract [en]

    In a method and device for measuring at least one parameter of a metal bed, measuring is obtained by a magnetic field being generated from one side of the metal bed so that eddy currents are generated in the metal bed as the metal bed moves relative to the magnetic field. The metal bed and the magnetic field are made to move relative to one another. A force related to the eddy currents affects a body and the action of the force on the body is detected by a detecting device, the detected force being a function of a desired parameter of the metal bed.

  • 164.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gustavsson, Håkan
    Development of a reaction probe for flow measurements in continuous casting of steel1998In: Proceedings - The 9th International Conference on Flow Measurement: FLOMEKO, Kista: ITF , 1998Conference paper (Refereed)
  • 165.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gustavsson, Håkan
    Estimation of the Radial Velocity from the Squire-Long Equation and Experimental Data2005In: Waterpower XIV: conference ; advancing technology for sustainable energy ; July 18 - 22, 2005, Austin, Texas, USA ; technical papers, Kansas City: HCI , 2005Conference paper (Refereed)
  • 166.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gustavsson, Håkan
    Natural wave frequencies in continuous casting moulds1998In: Proceedings of the 3rd European Conference on Continuous Casting, 1998Conference paper (Refereed)
  • 167.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gustavsson, Håkan
    On the use of the Squire-Long equation to estimate radial velocities in swirling flows2007In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 129, no 2, p. 209-217Article in journal (Refereed)
    Abstract [en]

    A method to estimate the radial velocity in swirling flows from experimental values of the axial and tangential velocities is presented. The study is motivated by the experimental difficulties to obtain this component in a draft tube model as evidenced in the Turbine-99 IAHR/ERCOFTAC Workshop. The method uses a two-dimensional nonviscous description of the flow. Such a flow is described by the Squire-Long equation for the stream function, which depends on the boundary conditions. Experimental values of the axial velocities at the inlet and outlet of the domain are used to obtain the boundary conditions on the bounded domain. The method consists of obtaining the equation related to the domain with an iterative process. The radial velocity profile is then obtained. The method may be applied to flows with a swirl number up to about Sw=0.25. The critical value of the swirl number depends on the velocity profiles and the geometry of the domain. The applicability of the methodology is first performed on a swirling flow in a diffuser with a half angle of 3 deg at various swirl numbers, where three-dimensional (3D) laser Doppler velocimeter (LDV) velocity measurements are available. The method is then applied to the Turbine-99 test case, which consists in a model draft tube flow where the radial inlet velocity was undetermined. The swirl number is equal to Sw=0.21. The stability and the convergence of the approach is investigated in this case. The results of the pressure recovery are then compared to the experiments for validation.

  • 168. Cervantes, Michel
    et al.
    Gustavsson, Håkan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Unsteadiness and viscous losses in hydraulic turbines2006In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 44, no 2, p. 249-258Article in journal (Refereed)
    Abstract [en]

    The variation of the mechanical energy for the mean, oscillating and turbulent flows are considered. Their mean point out the contribution of the unsteadiness to the viscous losses and the turbulent production. A two-dimensional turbulent pulsating flow is investigated for different unsteady regimes to illustrate the effects of unsteadiness on the overall losses.

  • 169.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gustavsson, Håkan
    Page, M.
    Engström, F.
    Turbine-99 III: a summary2006In: 23rd IAHR Symposium on Hydraulic Machinery and Systems: Yokohama, Japan, October 17 - 21, 2006 ; [conference proceedings], 2006, article id 117Conference paper (Refereed)
  • 170.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jalk, Mats
    MPC Metal Process Control AB.
    Kelvesjö, Håkan
    MPC Metal Process Control AB.
    Ohlsson, Willy
    MPC Metal Process Control AB.
    Metal flow controlPatent (Other (popular science, discussion, etc.))
    Abstract [en]

    A method and a system are for the control of a gas-containing hidden flow of molten metal in a space defined by a tubular device. From measurements in at least one predetermined layer of the metal flow in the space, an indication is obtained of the appearance of the flow which is compared with stored values. The result of the comparison is used for controlling at least one flow-affecting parameter in such a manner that a desired type of flow is provided at least in the layer.

  • 171.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jansson, Ida
    Jourak, Amir
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Porjus U9A full-scale hydropower research facility2008In: Hydro technology and the evironment for the new century: 24th IAHR Symposium on Hydraulic Machinery and Systems, October 27 - 31, 2008, Foz do Iguassu, Brazil, Foz do Iguassu, 2008Conference paper (Refereed)
    Abstract [en]

    Hydropower still faces complex scientific and technical challenges in order to secure the availability and reliability of the power plants despite more than a century of development. The main challenge is due to new market constrains such as electrical market deregulation and introduction of renewable sources of energy. The major problem is related to the dynamic of the rotor involving several fields: hydraulics, power engineering and mechanics. On the other side, the large and growing hydropower world market represents an opportunity for technically advanced companies offering better efficiency. The difficulty to scale rigorously any technical advance makes full-scale experiment a necessity. World unique facilities are available at Porjus, Sweden, for this purpose. The Porjus Hydropower Centre is composed of a Francis (U8) and a Kaplan (U9) turbine of 10 MW, each exclusively dedicated to education, research and development. In order to further investigate specific issues related to availability and reliability, a project was initiated in 2006. The main objective is to make U9 a full-scale hydropower laboratory able firstly to furnish the necessary data for the development of rotor-dynamic models but also turbines and bearings. To this purposes more than 200 sensors have been installed to measure displacements, forces, pressure, film thickness, strains... The work presents an overview of the newly upgrade facility as well as some of the problems faced during the instrumentation of the machine.

  • 172.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lövgren, Magnus
    Radial velocity at the inlet of the Turbine-99 draft tube2007In: Proceedings of the 2nd IAHR International Meeting of the Workgroup on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, Timisoara,Romania, October 24-26, 2007 / [ed] R. Susan-Resiga; S. Bernad; S. Muntean, Timişoara, 2007, p. 137-144Conference paper (Refereed)
    Abstract [en]

    Measurements of the axial, radial and tangential velocities at the inlet and downstream the cone of the Turbine-99 draft tube test case with wedge Pitot tubes are presented. The measurements were principally performed to evaluate the radial velocity: an unknown parameter of the Turbine-99 test case. The results show an acceptable agreement withprevious measurements for the yaw angle and axial and tangential velocities. The radial velocity is found to have very small amplitude at the inlet of the draft tube. The measurements downstream the cone indicates a larger tangential velocity while the radial velocity has a larger amplitude that at the inlet.

  • 173.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Parida, Vinit
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Innovation and Design.
    Higher education in hydropower2006In: International Water Power and Dam Construction, ISSN 0306-400X, E-ISSN 1538-6414, Vol. 58, no 7, p. 25-27Article in journal (Other academic)
  • 174.
    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 1: steady operation of Francis turbines2015In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 579, article id 11001Article in journal (Other academic)
  • 175.
    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)
  • 176.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Videhult, Sebastian
    GE Energy AS, NO-7465 Trondheim.
    Adaptive draft tube for increased efficiency2006In: International Water Power and Dam Construction, ISSN 0306-400X, E-ISSN 1538-6414, Vol. 58, no 3, p. 43-45Article in journal (Other academic)
  • 177.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Videhult, Sebastian
    GE Energy AS, NO-7465 Trondheim.
    Increasing efficiency2006In: International Water Power and Dam Construction, ISSN 0306-400X, E-ISSN 1538-6414, Vol. 58, no 4, p. 25-27Article in journal (Refereed)
    Abstract [en]

    The deregulation of electric markets as well as the introduction of renewable sources of energy such as wind and wave power in several countries has lead to a demand for different operating patterns of hydro power plants. Hydro plants are more frequently operated away from best efficiency, at part-load or full-load, and with an increased number of start-ups and shutdowns. These new market conditions involve new constraints for the hydro machines, which can lead to an overall un-optimised utilisation. An example of this is Sweden, where hydro plants mainly operate Francis and Kaplan turbines built between 1950 and 1970. As a result, an extensive period of renovation has started in the country. New machines need to be designed to take into account the new market conditions.

  • 178.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Videhult, Sebastian
    Unsteady pressure measurements at Porjus U92003In: Hydraulic machinery and systems: proceedings of the XXIst IAHR symposium ; September 9 - 12, 2002, Lausanne / [ed] François Avellan, Lausanne: EPFL Laboratoire de Machines Hydrauliques , 2003Conference paper (Refereed)
  • 179.
    Cervantes, Michel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Wallström, Åsa
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Business Administration and Industrial Engineering.
    Forskarskola för kvinnor omgång 4 (2009-2011)2008In: Genus i norrsken, ISSN 1654-7640, Vol. 1, no 3-4Article in journal (Other academic)
  • 180.
    Chitrakar, Sailesh
    et al.
    Kathmandu University, Nepal.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Thapa, Biraj Singh
    Kathmandu University, Nepal.
    Fully coupled FSI analysis of Francis turbines exposed to sediment erosion2014In: International Journal of Fluid Machinery and Systems, ISSN 1882-9554, E-ISSN 1882-9554, Vol. 7, no 3, p. 101-109, article id 9Article in journal (Refereed)
    Abstract [en]

    Sediment erosion is one of the key challenges in hydraulic turbines from a design and maintenance perspective in Himalayas. The present study focuses on choosing the best design in terms of blade angle distribution of a Francis turbine runner which has least erosion effect without influencing the efficiency and the structural integrity. A fully coupled Fluid-Structure-Interaction (FSI) analysis was performed through a multi-field solver, which showed that the maximum stress induced in the optimized design for better sediment handling, is less than that induced in the reference design. Some numerical validation techniques have been shown for both CFD and FSI analysis

  • 181.
    Concina, Isabella
    et al.
    SENSOR Lab, Department of Information Engineering, University of Brescia.
    Vomiero, Alberto
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Metal Oxide Semiconductors for Dye- and Quantum-Dot-Sensitized Solar Cells2015In: Small, ISSN 1613-6810, E-ISSN 1613-6829, Vol. 11, no 15, p. 1744-1774Article in journal (Refereed)
    Abstract [en]

    This Review provides a brief summary of the most recent research developments in the synthesis and application of nanostructured metal oxide semiconductors for dye sensitized and quantum dot sensitized solar cells. In these devices, the wide bandgap semiconducting oxide acts as the photoanode, which provides the scaffold for light harvesters (either dye molecules or quantum dots) and electron collection. For this reason, proper tailoring of the optical and electronic properties of the photoanode can significantly boost the functionalities of the operating device. Optimization of the functional properties relies with modulation of the shape and structure of the photoanode, as well as on application of different materials (TiO2, ZnO, SnO2) and/or composite systems, which allow fine tuning of electronic band structure. This aspect is critical because it determines exciton and charge dynamics in the photoelectrochemical system and is strictly connected to the photoconversion efficiency of the solar cell. The different strategies for increasing light harvesting and charge collection, inhibiting charge losses due to recombination phenomena, are reviewed thoroughly, highlighting the benefits of proper photoanode preparation, and its crucial role in the development of high efficiency dye sensitized and quantum dot sensitized solar cells.

  • 182. Cupillard, Samuel
    et al.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Influence of the thrust bearing on the natural frequencies of a72-MW hydropower rotor2016In: IAHR symposium on Hydraulic Machinery and Systems, 2016, article id 236Conference paper (Refereed)
    Abstract [en]

    The thrust bearing is an essential element of a hydropower machine. Not only does itcarry the total axial load but it also introduces stiffness and damping properties in the system.The focus of this study is on the influence of the thrust bearing on the lateral vibrations of theshaft of a 72-MW propeller turbine. The thrust bearing has a non-conventional design with alarge radius and two rows of thrust pads. A numerical model is developed to estimate naturalfrequencies. Numerical results are analyzed and related to experimental measurements of arunaway test.The results show the need to include the thrust bearing in the model. In fact, the vibrationmodes are substantially increased towards higher frequencies with the added properties fromthe thrust bearing. The second mode of vibration has been identified in the experimentalmeasurements. Its frequency and mode shape compare well with numerical results.

  • 183.
    Cupillard, Samuel
    et al.
    Hydro-Québec Research Institute, Varennes, Québec, Canada.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Influence of the thrust bearing on the natural frequencies of a72-MW hydropower rotor2016In: IOP Conference Series: Earth and Environment, ISSN 1755-1307, E-ISSN 1755-1315, Vol. 49, no 8, article id 082024Article in journal (Refereed)
    Abstract [en]

    The thrust bearing is an essential element of a hydropower machine. Not only does it carry the total axial load but it also introduces stiffness and damping properties in the system. The focus of this study is on the influence of the thrust bearing on the lateral vibrations of the shaft of a 72-MW propeller turbine. The thrust bearing has a non-conventional design with a large radius and two rows of thrust pads. A numerical model is developed to estimate natural frequencies. Numerical results are analyzed and related to experimental measurements of a runaway test. The results show the need to include the thrust bearing in the model. In fact, the vibration modes are substantially increased towards higher frequencies with the added properties from the thrust bearing. The second mode of vibration has been identified in the experimental measurements. Its frequency and mode shape compare well with numerical results.

  • 184. Cupillard, Samuel
    et al.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A CFD study of a finite textured journal bearing2008In: Hydro technology and the evironment for the new century: 24th IAHR Symposium on Hydraulic Machinery and Systems, October 27 - 31, 2008, Foz do Iguassu, Brazil, Foz do Iguassu, 2008Conference paper (Refereed)
    Abstract [en]

    An analysis of a lubricated journal bearing is performed with special attention to the influence of textured surfaces which may improve hydrodynamic performance. The bearing is subjected to an external applied load and the force balance is fulfilled with the force of the flow acting on the bearing. The position of the shaft is fixed whereas the bearing, centred at the starting time, moves under the forces until equilibrium is reached. A mesh deformation technique is used with CFD (Computational Fluid Dynamics) in order to perform the simulations. The flow is laminar, isothermal, three-dimensional (3D) and unsteady. Cavitation is taken into account. Results are analysed with smooth and textured surfaces on the bearing. Friction force and eccentricity ratio are compared for different configurations.It is found that shallow grooves under light loading (ε < 0.15) enhance the minimum film thickness while reducing the friction force. Under high loading (ε > 0.5), deep grooves are able to reduce the friction force despite a reduced minimum film thickness. For the second case, the predicted performance is superior to those of a smooth journal bearing with thinner lubricant.

  • 185. Cupillard, Samuel
    et al.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pressure buildup mechanism in a textured inlet of a hydrodynamic contact2008In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 130, no 2Article in journal (Refereed)
    Abstract [en]

    A flow analysis is carried out for an inclined slider bearing with the aim of showing the governing mechanism at conditions where an optimum in load carrying capacity is achieved. The effects of surface texture on pressure buildup and load carrying capacity are explained for a textured slider bearing geometry. Numerical simulations are performed for laminar, steady, and isothermal flows. The energy transferred to the fluid from the moving wall is converted into pressure in the initial part of the converging contact and into losses in the second part. The convergence ratio can be increased, in order to get the greatest pressure gradient, until the limiting value where flow recirculation begins to occur. The texture appears to achieve its maximum efficiency when its depth is such that the velocity profile is stretched at its maximum extent without incurring incoming recirculating flow. The wall profile shape controlling the velocity profile can be optimized for many hydrodynamic contacts

  • 186. Cupillard, Samuel
    et al.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pressure build-up mechanism in a textured inlet of a slider bearing2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 33-Conference paper (Other academic)
  • 187.
    Cupillard, Samuel
    et al.
    Hydro-Québec Research’s Institute.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Glavatskih, Sergei
    Kungliga tekniska högskolan, KTH.
    Thermohydrodynamic analysis of a journal bearing with a microgroove on the shaft2011Conference paper (Refereed)
    Abstract [en]

    In this study, thermohydrodynamic performance of a journal bearing with a microgroove created on the shaft is analysed. A plain journal bearing is modelled using a computational fluid dynamics (CFD) software package. Navier-Stokes and energy equations are solved. The rotor-stator interaction is treated by using a computational grid deformation technique. Results are presented in terms of typical bearing parameters as well as flow patterns. Results are also compared to the bearing with a smooth shaft. The effect induced by a microgroove on pressure distribution is explained for different bearing configurations, eccentricities and microgroove depths. It is shown that the microgroove produces a local drop in pressure which, averaged over one revolution, decreases the load carrying capacity. The load carrying capacity is further decreased by using deeper microgrooves. With thermal effects considered, the microgroove carries more cold lubricant into the warmest regions of the bearing. This effect, more pronounced with deeper microgrooves, is due to a global flow recirculation inside the microgroove, which improve mixing

  • 188. Cupillard, Samuel
    et al.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Glavatskih, Sergei
    Thermohydrodynamic analysis of a journal bearing with a microgroove on the shaft2014In: Computational Thermal Sciences, ISSN 1940-2503, E-ISSN 1940-2554, Vol. 6, no 1, p. 47-57Article in journal (Refereed)
    Abstract [en]

    In this study, thermohydrodynamic performance of a journal bearing with a microgroove created on the shaft is analyzed. A plain journal bearing is modeled using a computational fluid dynamics (CFD) software package. Navier-Stokes and energy equations are solved. The rotor-stator interaction is treated by using a computational grid deformation technique. The goal is to examine the pressure/temperature distribution in the bearing film. Results are presented in terms of typical bearing parameters as well as flow patterns. Results are also compared to the bearing with a smooth shaft. The effect induced by a microgroove on pressure distribution is explained for different bearing configurations, eccentricities, and microgroove depths. It is shown that the microgroove produces a local drop in pressure which, averaged over one revolution, decreases the load carrying capacity. The load carrying capacity is further decreased by using deeper microgrooves. With thermal effects considered, the microgroove carries more cold lubricant into the warmest regions of the bearing. This effect, more pronounced with deeper microgrooves, is due to a global flow recirculation inside the microgroove, which improves mixing

  • 189. Cupillard, Samuel
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    3D thermodynamic analysis of a textured slider2008In: Proceedings of NORDTRIB 2008, 13th Nordic Symposium on Tribology: Scandic Rosendahl Hotel, Tampere, Finland, June 10 - 13, 2008 / [ed] Jaakko Kleemola; Arto Lehtovaara, Tampere: Tampere University of Technology, 2008Conference paper (Refereed)
    Abstract [en]

    Analysis of a 3D inlet textured slider bearing with a temperature dependent fluid is performed. Numerical simulations are carried out for a laminar and steady flow. Hot and cold lubricant mixing in the groove is modelled and examined for different operating conditions. Thermohydrodynamic performance of the bearing is analysed for different texture lengths. Results show that texture has a stronger and positive influence on load carrying capacity when thermal effects are considered. This beneficial effect is at a maximum for the longest dimples with a length shorter than the pad length. Texture is also beneficial for the load carrying capacity when the sliding speed and inlet flow rate are varied: with an appropriate supply mass flow, load carrying capacity can be increased by 6-7% at low sliding speeds.

  • 190. Cupillard, Samuel
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    3D thermohydrodynamic analysis of a textured slider2009In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 42, no 10, p. 1487-1495Article in journal (Refereed)
    Abstract [en]

    Analysis of a 3D inlet textured slider bearing with a temperature dependent fluid is performed. Numerical simulations are carried out for a laminar and steady flow. Hot and cold lubricant mixing in the groove is modelled and examined for different operating conditions. Thermohydrodynamic performance of the bearing is analysed for different texture lengths. Results show that texture has a stronger and positive influence on load carrying capacity when thermal effects are considered. This beneficial effect is at a maximum for the longest dimples with a length shorter than the pad length. Texture is also beneficial for the load carrying capacity when the sliding speed and inlet flow rate are varied. The load carrying capacity of the slider can be increased by up to 16% in severe operating conditions (high sliding speed).

  • 191. Cupillard, Samuel
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Computational fluid dynamics analysis of a journal bearing with surface texturing2008In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 222, no 2, p. 97-107Article in journal (Refereed)
    Abstract [en]

    An analysis of a lubricated conformal contact is carried out to study the effect of surface texture on bearing friction and load carrying capacity using computational fluid dynamics. The work focuses on a journal bearing with several dimples. Two- and three-dimensional bearing geometries are considered. The full Navier-Stokes equations are solved under steady-state conditions with a multi-phase flow cavitation model.The coefficient of friction can be reduced if a texture of suitable geometry is introduced. This can be achieved either in the region of maximum hydrodynamic pressure for a bearing with high eccentricity ratio or just downstream of the maximum film for a bearing with low eccentricity ratio. An additional pressure build-up produced as a result of the surface texture has been shown at low eccentricity ratios.

  • 192. Cupillard, Samuel
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Effect of surface texturing on journal bearing performance2006In: ASIATRIB 2006: proceedings of the third Asia International Conference on Tribology, October 16-19, Kanazawa, Japan, Minato-ku: Kikai Shinkou Kaikan , 2006Conference paper (Refereed)
  • 193. Cupillard, Samuel
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Inertia effects in textured hydrodynamic contacts2010In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 224, no 8, p. 751-756Article in journal (Refereed)
    Abstract [en]

    A flow analysis is carried out for a parallel textured slider to investigate the role of fluid inertia. Numerical simulations are performed for a laminar, two-dimensional, steady and isothermal flow. Stokes solutions are compared with Navier-Stokes solutions at the same Reynolds number. A range of texture depths is analysed. It is shown that there is an optimum value of texture depth that provides maximum load-carrying capacity. It is also shown that there is a critical depth value. Inertia has a negative effect on the load-carrying capacity for depths higher than the critical value, whereas it has a positive effect for lower depths. For a given texture depth, these effects are amplified as the Reynolds number increases. The global effect of inertia is positive when a realistic configuration of a parallel textured slider with a fore-region is considered.

  • 194. Cupillard, Samuel
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Thermal analysis of lubricant flow in a textured inlet contact2008In: Proceedings of the 19th International Symposium on Transport Phenomena (ISTP-19): Reykjavik, Iceland, August 17-21, 2008 / [ed] Sigurdur Brynjolfsson; Olafur Petur Palsson; Jong H. Kim, University of Iceland, Faculty of Industrial Engineering, Mechanical Engineering and Computer Science , 2008Conference paper (Refereed)
    Abstract [en]

    Performance of an inlet textured contact is analysed for a temperature dependent lubricant flow. Thermal effects are analysed for different shear rates. Shearing of lubricant and subsequent heating reduce load carrying capacity and frictional losses compared with isothermal conditions. Load carrying capacity of a parallel surface contact can be improved by using texturing at the inlet when thermal effects are considered. It is also shown that for the different convergence ratios considered, the texture enables the sustaining of a load until a certain critical shear rate is reached. This critical shear rate depends on a number of factors such as the convergence ratio and lubricant parameters including the viscosity-temperature coefficient and the dynamic viscosity at reference temperature.

  • 195.
    Dagli, Deniz
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Zeinali, Amin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Laue, Jan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Image analyses of frost heave mechanisms based on freezing tests with free access to water2018In: Cold Regions Science and Technology, ISSN 0165-232X, E-ISSN 1872-7441, Vol. 146, p. 187-198Article in journal (Refereed)
    Abstract [en]

    A freezing test apparatus was supplemented with a camera to allow for recording and monitoring one-dimensional freezing tests to analyze the development of ice lenses via particle image velocimetry (PIV) in the laboratory. Two tests on disturbed, partially saturated samples of silt loam were conducted. Image recording and correlation analyses provided detailed information about frost front penetration and ice lens formation(s) under varying temperature boundary conditions. Thawing has also been regarded in further studies.

    Results of the image analyses were compared to readings from conventional displacement measurements during the same test. Significant agreement between the results of image analyses and displacement measurements has been found. Test results were also used to establish a qualitative relationship between heat extraction and heave rates. Advantages and disadvantages of utilizing image analysis methods were discussed. Potential remedies for overcoming the drawbacks of using image analysis are suggested.

    Image analysis is shown to be a viable method in further understanding of frost heave mechanisms.

  • 196.
    Dahl, Jan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Hermansson, Roger
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Benckert, Lars
    Temperatures and velocities near the gradient zone in a short term water heat storage1991In: Proceedings: Thermastock '91, International Conference on Thermal Energy Storage, Scheveningen, 13 - 16 May 1991, Utrecht: Novem , 1991Conference paper (Refereed)
  • 197.
    Das, Samir
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ganesh, S. Jai
    Centre for Energy, Indian Institute of Technology Jodhpur, Ratanada.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Modelling of a groundwater mound in two-dimensional heterogeneous unconfined aquifer in response to precipitation recharge2015In: Journal of hydrologic engineering, ISSN 1084-0699, E-ISSN 1943-5584, Vol. 20, no 7, article id 4014081Article in journal (Refereed)
    Abstract [en]

    The paper deals with the transient semi-analytical solution of linearized Boussinesq equations characterizing the development of groundwater mound in an unconfined two-dimensional heterogeneous aquifer under vertical recharge conditions. The finite aquifer consists of two rectangular basins surrounded by open water bodies and shares a common impermeable or permeable boundary at the mid plane. The governing equations are solved by applying the Laplace transform and finite Fourier sine transform techniques. Accordingly, analytical expressions for water heads for two rectangular basins are obtained for various scenarios. The applicability of the solutions has been illustrated with the help of a case study and numerical examples, considering various cases. The region wise development of the groundwater mound indicates that the effect of heterogeneity becomes significant for small time duration whereas for long time it becomes insignificant. This result can have application in land reclamation problems in the presence of localized recharge where the reclamation displaces the ground water divide and changes the groundwater conditions in the entire region

  • 198.
    Davidsson, Niklas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Stability and transition in the suction boundary layer and other shear flows2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Bypass transition has been studied by theoretical and numerical procedures, with the asymptotic suction boundary layer (ASBL) in focus. As reference cases the Blasius boundary layer (BBL) and a free shear flow have been studied. In order to reduce energy losses associated with flow systems, it is a wish to avoid turbulence in these flows. It is thus necessary to have a fundamental understanding of the mechanisms behind bypass transition, which typically starts with the formation and growth of structures extended in the streamwise direction, so called streaks. One way of delaying the transition to turbulence is to apply wall suction, which is also known to stabilize streaks. In this work, it is shown that the stabilizing mechanism of wall suction is not unambiguous. A theoretical study on the linear evolution of streaks triggered by a localized disturbance is performed. Releasing the disturbance in the free-stream, it will migrate towards the wall and quickly be subject to shear. Consequently, this disturbance is amplified when applying wall suction, provided that for the suction-free case the growth of the BBL may be considered small. When initiating the disturbance inside the boundary layer, on the other hand, it is found that suction stabilizes the growth of such a streak. Also, the non-linear properties of suction are studied using a model with prescribed wall-normal disturbance velocity identical for the ASBL and the BBL. Despite the similarity, suction is shown to dampen the non-linear forcing of the perturbation. Moreover, the non-linear response is shown to favor the forcing of streamwise longitudinal (3D) structures and 2D waves. For the ASBL, also energy thresholds for transition of periodical disturbances have been determined by direct numerical simulations. The least energy required to reach transition is obtained when the initial flow field consists of two oblique waves, for which the threshold value is found to scale as Re^{-2.6}. For transition starting with streamwise vortices or random noise the threshold scales like Re^{-2.1} (Re being the Reynolds number), and the routes to transition are similar to that of other flows. A theoretical framework for evaluating the non-linear interaction terms of the normal- velocity/vorticity equations is also developed. This formulation allows for study of wave interaction throughout the whole wavenumber plane, i.e. for any given wave number of a disturbance. The framework has been applied to a free shear flow, which shows that primarily streamwise elongated structures and Tollmien-Schlichting waves are forced by the non-linear interactions. Besides that, the geometrical interpretation shows that the non-linear interaction involving normal vorticity is most potent for structures inter-angled by 90 degrees in the wavenumber plane.

  • 199.
    Davidsson, Niklas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Streak growth and breakdown in a boundary layer with steady uniform suction2006In: Abstracts: EFMC6 KTH, Kungl. Tekniska Högskolan , 2006, Vol. Vol. 2, p. 203-Conference paper (Other academic)
  • 200. Davidsson, Niklas
    et al.
    Gustavsson, Håkan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Elementary solutions for streaky structures in boundary layers with and without suction2008In: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 40, no 3, p. 212-231Article in journal (Refereed)
    Abstract [en]

    The temporal evolutions of small, streamwise elongated disturbances in the asymptotic suction boundary layer (ASBL) and the Blasius boundary layer (BBL) are compared. In particular, initial perturbations localized (δ-functions) in the wall-normal direction are studied, corresponding to an axi-symmetric jet coming out of a plane parallel to the flat plate. Analytical solutions are presented for the wall-normal and streamwise velocities in the ASBL case whereas both analytical and numerical methods are used for the BBL case. The initial position of the perturbation and its spanwise wave number are varied in a parameter study. We present results of maximum amplitudes obtained, the time to reach them, their position and optimal spanwise scales. Free-stream disturbances are shown to migrate towards the wall and reach their (negative) optimum inside the boundary layer. The migration is faster for the ASBL case and a larger amplitude is reached than for the BBL. For perturbations originating inside the boundary layer the amplitudes are overall larger and show the phenomenon of overshoot, i.e. positive amplitudes moving out of the boundary layer. The overall largest amplitudes are obtained for the BBL case, as in other studies, but it is shown that for free-stream disturbances initiated somewhere downstream the leading edge streak growth may be amplified due to suction since in the BBL the disturbance mainly advects above the boundary layer.

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