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• 301.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. 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)

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• 302.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
Lund University. Umeå University. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. Umeå University. Umeå University. Umeå University. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
SFC – Annual Summary from Bio4Gasification (B4G)2020Report (Other (popular science, discussion, etc.))
• 303.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
Gustavsson, HåkanLuleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.Karlsson, Rolf
Proceedings of Turbine-99: workshop on draft tube flow2000Collection (editor) (Other academic)

The Turbine 99 workshop, held in Porjus, Sweden, 20-23 June 1999, was organized to determine the state-of-the-art of CFD simulations of draft tube flows. A total of 16 groups accepted the invitation to compute the draft tube flow. The following experimental data was available before the workshop: the axial and swirl velocity components at the inlet (with rms-values and one Reynolds' stress component) and the pressure distribution around the outlet cross section. Before the workshop, the groups submitted simulation results (using 12 different CFD codes) which were compiled by the organizers to an extensive set of data available at the workshop. The experimental data for the pressure recovery factor, pressure distributions along the draft tube walls and the detailed velocity field in one downstream cross section was presented during the workshop. In this report a summary of the main results and conclusions of the workshop is given, together with the written reports from the experiments and the simulations. In addition, the report contains all relevant background information for the workshop such as the draft tube geometry, provided data, requested information etc. and is thus the main document from the workshop. As the simulations provided more data than was available for comparison with the experiments, the simulation results will be available in a separate document provided by the organizers. One major conclusion of the workshop is that much attention must (still) be payed to the grid quality and the boundary conditions, factors that need to be strictly specified before a fully relevant comparison can be made between different flow models.

• 304.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique1999In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 20, no 1, p. 146-154Article in journal (Refereed)

This report discusses the advantages and drawbacks of the multi-cavity parallel flow technique for permeability measurements. An experimental series with repeated measurements on material from the same roll shows that the repeatability of the technique is very good considering the manufacturing variability of the fabric. The measured standard deviation in the repeatability study is about 10%. It is, however, shown that the permeability can vary considerably- between reinforcements of similar geometry. Furthermore, computer simulations were used to estimate the errors when highly anisotropic materials are oriented at an angle to the material principal direction in the parallel flow technique. The conclusion based on the simulations is that the length to width ratio of the cavity should be larger than the anisotropy of the reinforcement for an acceptable error.

• 305. Gererkiden, Berhanu Mulu
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
LDA measurements in a Kaplan spiral casing model2010In: 13th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery 2010 (ISROMAC-13): Honolulu, Hawaii, USA, 4 - 7 April 2010, Red Hook, NY: Curran Associates, Inc., 2010, p. 85-92Conference paper (Refereed)

This paper presents an experimental investigation of a Kaplan spiral casing turbine model. A two-component laser Doppler anemometry (LDA) apparatus was used to measure the velocity profiles at different locations in the turbine. To improve the signal quality and measurement accuracy, a refractive index matching optical box was mounted on the circular pipe of the spiral casing inlet. The investigations were carried out with a constant runner- blade angle and at three different loads: the best operating point of the turbine and two off-design operating points (left and right side of the propeller curve) with the presence of a vortex breakdown. The mean velocity profiles and corresponding RMS at the spiral casing before the guide vanes and at inlet of the spiral casing are presented for the different loads investigated.

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• 306.
ABB Corporate Research.
ABB Corporate Research. Department of Engineering Sciences, Uppsala University. Department of Fiber and Polymer Technology, Royal Institute of Technology - KTH, Luleå tekniska universitet, Risø National Laboratory, Roskilde, STFI-Packforsk AB, Department of Engineering Sciences, Division of Applied Mechanics, Uppsala University. ABB Figeholm. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Analysis of the Micromechanical Deformation in Pressboard performed by X-ray Microtomography2015In: IEEE Electrical Insulation Conference (EIC): Seattle, Jun 07-10, 2015, Piscataway, NJ: IEEE Communications Society, 2015, p. 89-92Conference paper (Refereed)

A large number of electrical insulation components are produced in paper-based materials. Paper combines good insulating properties with the necessary mechanical and chemical stability. Paper consists of a system of fibers binding to each other creating a strong network. The presence of large open pores allows for impregnability of the material but also causes mechanical weakness in particular in the out-of-plane direction of the material. This aspect is important for pressboard components, where the resistance to compression stress is relevant for e.g. transformer windings. It is therefore relevant to understand the mechanisms that underlay the out-of-plane deformation of pressboard. In order to get a clear picture of the deformation patterns within the material, X-ray micro-computed tomography was used. Pressboard test pieces were subjected to in-situ out-of-plane compressive loading. 3D images of the sample could be captured before, during and after the loading sequence. Image analysis allowed for the definition of strain fields. The results revealed a strong correlation between the density variation within the sample and the strain calculated from the 3D images.

• 307.
Mechanical Engineering Department, Universidad Michoacana de San Nicolas de Hidalgo.
Mechanical Engineering Department, Universidad Michoacana de San Nicolas de Hidalgo. Mechanical Engineering Department, Universidad Michoacana de San Nicolas de Hidalgo. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Assement study of the RNG turbulence model for steady state swirling flow analysis in a draft tube2012In: Avances en Simulacion Computacional y Modelado Numerico: memorias del XI congreso internacional de métodos numéricos en ingeniería y ciencias aplicadas CIMENICS'2012, Isla de Margarita, Venezuela, 26 al 28 de marzo de 2012 / [ed] E. Dávila; G. Uzcátegui; M. Cerrolaza, Facultad de Ingeniería , Universidad Central de Venezuela , 2012Conference paper (Refereed)

The flow in a hydraulic turbine draft tube is a challenging industrial application for CFD because is characterized by a turbulent flow with different flow phenomena, e.g. unsteadiness, flow separation, swirling flow and strong adverse pressure gradient. Thus, its simulation is complex and time-consuming high computational capacities are required Additionally, adequate turbulence modeling is needed to predict such flows accurately. The objective of the present work is to investigate the accuracy of the Renormalization Group Theory (RNG) k-c turbulence model on the draft tube flow close to the best efficiency of the turbine. The effect of the discerization schemes on these models has been studied As the flow is significantlyaffected by the walls, two different grid concentrations near to the wall (y + I and y + 50) were evaluated The numerical results are compared to detailed experimental results at different section and discussed.

• 308.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
FLOW FIELD IN A HIGH HEAD FRANCIS TURBINE DRAFT TUBE DURING TRANSIENT OPERATIONS2017Doctoral thesis, monograph (Other academic)

Hydroelectricity plays an important role to balance the stability of grid network.  In order to improve the stability of presently high loaded grids, hydropower plants are being operated over a wide range of operations and experiencing frequent start-stop, load rejection, and load acceptance. The turbines need to sustain sudden change in their operating condition to balance the grid frequency. Francis turbines have been widely used because of their wider operating range and higher stability in operation during rapid load variation. This has resulted in severe damage to the turbines as they are not normally designed to operate under such transient conditions.

Several low and high frequency pressure fluctuations prevail during transients operating conditions. Generally, wall pressure measurements are performed which may not provide sufficient information to investigate the flow instabilities related to these fluctuations. Thus, the main objective of the present work is to simplify and perform optical measurements in a turbine during transient operating conditions to investigate the flow field. The measurements have been performed at the Water Power Laboratory using a high head model Francis turbine. The turbine is a 1:5.1 scale down model of a prototype operating at the Tokke Power Plant, Norway. The model runner diameter, net head, and discharge at the best efficiency point (BEP) were 0.349 m, 12 m, and 0.2 m3 s-1, respectively. A total ten pressure sensors were mounted at different locations namely, turbine inlet, vaneless space, and draft tube. The data were acquired at a sampling rate of 5 kHz. The instruments and sensors have been calibrated according to guidelines available in IEC standards. The determined total uncertainty in the measurement of hydraulic efficiency was ±0.15% at BEP condition. The velocity measurements in the draft tube cone were performed using a 2D PIV system and the images were sampled at a rate of 40 Hz.

Steady state measurements were carried out considering the realistic design and off-design operating conditions of the prototype turbine. Therefore, the angular speed of the runner was maintained constant for all steady state conditions during the measurements. The maximum hydraulic efficiency (92.4%) was observed at nED = 0.18, QED = 0.15, and a = 9.8º, which is named BEP. It is observed that the turbine experiences significant pressure fluctuations at the vaneless space, runner, and the draft tube. The fluctuations due to rotor-stator interaction (RSI) were observed to be most dominating at high load condition, however, fluctuations due to the rotating vortex rope (RVR) at part load (PL) condition. Two different modes (synchronous and asynchronous) modes of vortex rope are observed at PL condition of the turbine. An asymmetry in the flow leaving the runner was detected at both design and off-design conditions, with a stronger effect during off-design operating condition. Numerical simulations of the model turbine were carried out at PL operating condition. The simulations were performed using two turbulence models, standard k-ε and SST k-ω, with high-resolution advection scheme. The numerical pressure values obtained with both standard k-ε model and SST k-ω showed a small difference with the experimental values. The amplitudes of numerical pressure values were higher (~2.8%) in the vaneless space and lower (~5.0%) in the draft tube than the experimental values. The frequencies of the RSI and RVR were well captured in the turbine but the amplitudes were overestimated.

During load rejection from BEP to PL, the plunging mode of the vortex rope was observed to appear first in the system than that of the rotating mode. Whereas during the load acceptance from PL to BEP, both the modes were observed to disappear simultaneously from the system. In the velocity data, the axial velocity only contributed to the development of the plunging mode and radial velocity to the rotating mode. The region of low velocity, stagnation point, flow separation, recirculation, oscillating flow and high axial velocity gradients were well captured in the system during the transients. The induced high-velocity gradients during the load acceptance from BEP to HL was observed to develop a vortex core in the draft tube.

During startup and shutdown, the guide vanes angular position was moved from one to another steady state condition to achieve the minimum load condition of the turbine. At this condition, the generator of the turbine was magnetized at the synchronous speed during startup and shutdown, respectively. The frequency of wave propagation was observed to vary with the runner angular speed during startup and complete shutdown of the turbine. Comparatively high-pressure fluctuations in the draft tube were observed during the guide vane movement from the high discharge conditions. Some unsteady phenomena such as the formation of dead velocity zone, backward flow, and flow oscillations were observed during startup and shutdown of the turbine.

The current work has been also used to continue a series of workshops, i.e., Francis-99. The first workshop was held on December 2014 with the cooperation of LTU and NTNU. The measurements performed in this work were used for the second workshop which was held on December 2016. The investigations presented in this thesis will be further explored in the third workshop scheduled for December 2018.

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• 309.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Indian Institute of Technology Roorkee.
Waterpower Laboratory, Norwegian University of Science and Technology. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Indian Institute of Technology Roorkee. Waterpower Laboratory, Norwegian University of Science and Technology.
Experimental investigation on a high head model Francis turbine during load rejection2016In: IOP Conference Series: Earth and Environment, ISSN 1755-1307, E-ISSN 1755-1315, Vol. 49, no 8, article id 082004Article in journal (Refereed)

Francis-99 is a set of workshop aiming to determine the state of the art of high head model Francis turbine simulations (flow and structure) under steady and transient operating conditions as well as to promote their development and knowledge dissemination openly. The first workshop (Trondheim, 2014) was concerned with steady state operation. The second workshop will focus on transient operations such as load variation and start-stop. In the present work, 2-D particle image velocimetry (PIV) with synchronized pressure measurements performed in the draft tube cone of the Francis-99 test case during load rejection is presented. Pressure sensors were mounted in the vaneless space and draft tube cone to estimate the instantaneous pressure fluctuations while operating the turbine from the best efficiency point (9.8°) to part load (6.7°) with the presence of a rotating vortex rope (RVR). The time-resolved velocity and pressure data are presented in this paper showing the transition in the turbine from one state to another

• 310.
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.
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. Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
Vortex Rope Formation in a High Head Model Francis Turbine2017In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 139, no 4, article id 041102Article in journal (Refereed)

Francis turbine working at off-design operating condition experiences high swirling flow at the runner outlet. In the present study, a high head model Francis turbine was experimentally investigated during load rejection, i.e., best efficiency point (BEP) to part load (PL), to detect the physical mechanism that lies in the formation of vortex rope. For that, a complete measurement system of dynamic pressure, head, flow, guide vanes (GVs) angular position, and runner shaft torque was setup with corresponding sensors at selected locations of the turbine. The measurements were synchronized with the twodimensional (2D) particle image velocimetry (PIV) measurements of the draft tube. The study comprised an efficiency measurement and maximum hydraulic efficiency of 92.4±0.15% was observed at BEP condition of turbine. The severe pressure fluctuations corresponding to rotor-stator interaction (RSI), standing waves, and rotating vortex rope (RVR) have been observed in the draft tube and vaneless space of the turbine. Moreover, RVR in the draft tube has been decomposed into two different modes; rotating and plunging modes. The time of occurrence of both modes was investigated in pressure and velocity data and results showed that the plunging mode appears 0.8 s before the rotating mode. In the vaneless space, the plunging mode was captured before it appears in the draft tube. The physical mechanism behind the vortex rope formation was analyzed from the instantaneous PIV velocity vector field. The development of stagnation region at the draft tube center and high axial velocity gradients along the draft tube centerline could possibly cause the formation of vortex rope

• 311.
Department of Mechanical and Industrial Engineering, Indian Institute of Technology.
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. 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)

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.

• 312.
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.
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. 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)

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.

• 313.
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.
Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee. 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)

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.

• 314.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Indian Institute of Technology, Department of Mechanics & Industrial Engineering, Roorkee .
Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee . 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)

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

• 315.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Indian Institute of Technology Roorkee.
Indian Institute of Technology Roorkee. 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)

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.

• 316.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Grenoble-INP/CNRS/UJF-Grenoble 1, Grenoble, France.
Indian institute of Technology, Roorkee, India. 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)

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.

• 317.
Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee.
Norwegian University of Science and Technology, Trondheim, . Indian Institute of Technology Roorkee. 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)

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.

• 318.
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.
Norvegian University of Science and Technology. Indian institute of Technology, Roorkee, India. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Norvegian University of Science and Technology.
Transient pressure measurements at part load operating condition of a high head model Francis turbine2016In: Sadhana (Bangalore), ISSN 0256-2499, E-ISSN 0973-7677, Vol. 41, no 11, p. 1311-1320Article in journal (Refereed)

Hydraulic turbines are operating at part load conditions depending on availability of hydraulic energy or to meet the grid requirements. The turbine experiences more fatigue during the part load operating conditions due to flow phenomena such as vortex breakdown in the draft tube and flow instability in the runner. The present paper focuses on the investigation of a high head model Francis turbine operating at 50% load. Pressure measurements have been carried out experimentally on a model Francis turbine. Total six pressure sensors were mounted inside the turbine and other two pressure sensors were mounted at the turbine inlet pipe. It is observed that the turbine experiences significant pressure fluctuations at the vaneless space and the runner. Moreover, a standing wave is observed between the pressure tank outlet and the turbine inlet. Analysis of the data acquired by the pressure sensors mounted in the draft tube showed the presence of vortex breakdown co-rotating with the runner. The detailed analysis showed the rotating and plunging components of the vortex breakdown. The influence of the rotating component was observed in the entire hydraulic circuit including distributor and turbine inlet but not the plunging one

• 319. Granqvist, S.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Speckle photography applied to detect structure displacement in high-speed laryngoscopic recordings2003Conference paper (Refereed)
• 320.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Modelling the aerodynamics of iron ore pelletizing kilns2012Licentiate thesis, comprehensive summary (Other academic)

In an iron ore pelletizing plant, crude ore is upgraded to pellets to be used as feedstock in steel-making plants. As part of a grate-kiln pelletizing plant, the rotary kiln is an indurating furnace in which the pellets are sintered. The rotary kiln involves complex flow of large amounts of gas and the process is strongly coupled to the fluid dynamics, which is not well understood. The present work focuses on increasing the understanding of the aerodynamics of the rotary kiln.Though the kiln geometry is relatively simple a rather complex flow arises, which is known to occur for turbulent flows in similar geometries. In order to isolate the underlying flow mechanisms, simplified models of the kiln are studied both numerically using Computational Fluid Dynamics (CFD) and experimentally using Particle Image Velocimetry (PIV). The understanding of the flow phenomena that arises for the simplified models is essential for maintaining a solid comprehension of the fluid dynamics when increasing the complexity of the models. Computations are validated against available experimental data to evaluate the capability of the numerical procedure in capturing the underlying physics of the flow. In this way, the reliability of the predictions is improved when increasing the complexity of the model.In Paper A the unsteady non-reacting flow is computed and a preliminary coal combustion model is proposed, which is in need of further development to yield reliable predictions of the reacting flow. Paper B is an experimental investigation of a down-scaled model of the kiln and also an extension to previous experimental work by introducing an inclination of the upper inlet duct to the kiln and carrying out a more thorough analysis of the fluid dynamics. In Paper C, the periodic flow observed in Paper A is investigated further using a more sophisticated turbulence closure and carefully validating the predictions against available experimental data.For the simplified models under investigation, it is concluded that the flow is dominated by the periodic shedding and downstream convection of von-Karman-like vortices originating in the free shear layers enclosing the recirculation zone formed in the inlet end of the kiln. Both numerical and experimental investigations show a strong dependence of momentum flux ratio between the two inlet ducts on the flow field. The large-scale periodic fluctuations, which are resolved in an unsteady computation but completely neglected in a steady computation, are seen to contribute significantly to the turbulent transport in the recirculation zone. This indicates the need for unsteady computations to accurately predict the transport processes. The recirculation zone is important for flame stabilization as it feeds back hot gas to the near-burner region. Hence, a challenging requirement of the numerical model is to accurately capture the physics of the recirculating flow. The use of a second-moment turbulence closure is shown to significantly improve the predictions over the use of an eddyviscosity turbulence model and give promising results for further work on more complex models of the kiln.

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• 321. Granström, Reine
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. LKAB. LKAB.
CFD modelling of the flow through a grate-kiln2009In: Proceedings from Seventh International Conference on Computational Fluid Dynamics in Minerals and Process Industries: CSIRO, Melbourne, Australia, 2009Conference paper (Refereed)

As part of an investigation regarding secondary reduction of NOx emissions in a grate-kiln iron ore pelletizing plant, the aim of this specific research is to develop a CFD model that enhances the understanding of the aerodynamics and mixing of species inside the rotary kiln. At first, a parametric study of the pure airflow through the kiln is carried out and for certain conditions, a transient behaviour similar to that of vortex shedding was observed, with a dependence on momentum flux ratio between the secondary air jets. Further, the development of a preliminary coal combustion model is set out, which is in need of continued work in order to produce reliable predictions of various parameters relevant to the reduction process. However, the effect of the combustion on the flow field is limited, indicating that the pure airflow model can be used to broadly investigate the influence of the secondary air flow.

• 322.
Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest.
Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest. Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest. Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest. 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)

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.

• 323.
Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest, Bucharest, Romania .
Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest, Bucharest, Romania . Department of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest, Bucharest, Romania . Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Wall-Layer Treatment Considering the Pressure Gradient for RANS Simulations of Turbulent Flows2019In: Proceedings of 2019 International Conference on ENERGY and ENVIRONMENT (CIEM), IEEE, 2019, p. 289-293Conference paper (Other academic)

The paper presents two wall-treatments employed with the standard k-ε turbulence model together with their consequences on the estimation of the flow characteristics (separation zone, velocity distribution, adverse/favourable pressure gradient, wall shear stress) and simulation resources (time and computational power). The two wall-treatments were developed by Manhart (2008) and Duprat (2010). The results are compared to those obtained with the standard k-ε turbulence model and the k-ω SST turbulence model, using the numerical computation software Code_Saturne CFD. The test case considered for the numerical simulations is a 3D diffuser geometry with water as fluid which undergoes flow separation. The results show that the two wall-treatments lead to an improvement of the accuracy in the prediction of the separation zone. The required time and computational power are improved, compared to the k-ω SST turbulence model.

• 324.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Particle image velocimetry in practice2009Licentiate thesis, comprehensive summary (Other academic)

Experimental fluid mechanics has for a long time been used to visualize flow phenomenon. An early pioneer was Ludwig Prandtl who used aluminum particles in water flumes to describe the flow in a qualitative manner. In line with the rapid development of Computational Fluid Dynamics, CFD, the need for new validation tools has increased. By combining Prandtls attempt to trace particles and contemporary tools in laser and computer technologies a quantitative non intruisiv whole field technique, so called Particle Image Velocity (PIV) has been developed. The PIV technique has been improved and grown in popularity through recent decades with the increase in computer capacity. This thesis describes three rather different areas of application of PIV measurements. In the first case PIV is used as pure measurement technology tool to describe the flow field inside an attraction channel in connection to fish migration. In the second case, PIV is applied as a validation tool for CFD calculations with Large Eddy Simulation (LES) including an extensive analysis of the results. Finally, a description of how PIV technique can be adopted to study the flow of complex fluids in small geometries by means of microscopy is given.The attraction channel is a U-shaped channel designed to facilitate salmonoid like fihes to migrate upstream to their spawning grounds. The attraction channel has a restriction in the downstream outlet that provides an acceleration of the attraction water up to 38% of the sourunding water velocity according to the PIV measurements. With PIV measurements it is also shown that the depth of displacement over the restriction is significant for how far downstream the acceleration is perceptible. CFD technology is constantly evolving and new methods will become the future standard in the industry. In the current situation Reynolds Avereaged Numerical Simulations (RANS) is the most used method in CFD. But development is approaching LES technology. This is, for instance, motivated by energy production units which has many applications with high turbulence and temperature fluctuations. In the current situation it is required to extend the service life of existing power plants. Therefore it is desirable to be able to estimate these fluctuations impact on thermal loads on the materials inside the plant, for example pipe walls. An LES approach is superior to applying to RANS since the large eddies are resolved. However, LES is still not mature enough to be used without validation in critical applications. Therefore, PIV has been used to create a validation database for a generic T-junction.Double Restriction Sealings (DRS) have been used in bearings and other lubricated applications since the 1940's. A DRS is intended to prevent contamination from entering and is therefore used to increase the life span of lubricated parts, i.e. hinder polutants to reach the rolling elements in bearings for example. Although it is widly applied little is known about the actual function and mechanism of the DRS. To learn more about the flow and particle tracks within a DRS, a new method to visualize and quantify grease flow within a DRS has been developed based upon micro PIV. The main result from this study is that it is possible to make quantititative measurement of the flow within a DRS.

FULLTEXT01
• 325.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
PIV in Practice2011Doctoral thesis, comprehensive summary (Other academic)

Experimental fluid mechanics has for a long time been used to visualize flow phenomenonqualitatively. Traditionally, visualization has been done with dye or tracer particle dueto their ability to follow the flow pattern well. One of the early pioneers in experimentalfluid mechanics was Ludwig Prandtl who used mica particles in water flumes to accuratelydescribe the flow around wing profiles. Due to Prandtl’s results in the early 20thcentury, some of the most important theories in aviation were founded. By combiningPrandtl’s attempt to trace particles, and contemporary laser and computer technologiesa quantitative non-intrusive whole field technique, so called Particle Image Velocimetry(PIV), has been developed. The PIV technique has through the advances in computerscience the recent decades, been improved significantly and has also grown in popularityamong the scientific- and technological community.This thesis describes implementation of PIV in several diverse research areas frommacro- to micro scale. First, it is described how PIV is used as a pure measurementtechnique to understand complex flow phenomena. The technique is demonstrated on asmall U-shaped channel designed to facilitate salmonoid like fishes upstream migrationto their spawning grounds. Second, PIV is used as a validation tool for ComputationalFluid Dynamics, CFD. In the current situation, CFD is undergoing a generation shiftfrom Reynolds Averaged Numerical Simulation, RANS, to Large Eddy Simulations, LES.This is for instance motivated by energy production units which has many applicationswith high turbulence and temperature fluctuations. Hence it is desirable to be able toestimate the impact on thermal loads on the materials inside the plant (e.g. the pipewalls). An LES approach is superior to applying to RANS since the large eddies areresolved. However, LES is still not mature enough to be used without validation incritical applications. Therefore, PIV has been used to create a validation database fora generic T-junction. Finally, a description of how PIV technique can be adopted tostudy the flow of complex fluids in small geometries by means of microscopy, is given andapplied on lubrication grease flow in labyrinth seals which have been used in bearingsand other lubricated applications since the 1940’s. The intention with labyrinth seals isto lubricate the bearing and prevent contamination from entering the rolling elements.Although it is widely applied, little is known about the actual function and mechanismof labyrinth seals. To learn more about the flow and particle migration within a sealgeometry, a new method to visualize and quantify grease flow within a labyrinth seal hasbeen developed based upon micro-PIV.

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• 326.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
A new method to visualize grease flow in a double restriction seal using microparticle image velocimetry2011In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 54, p. 784-792Article in journal (Refereed)

A new method to visualize and quantify grease flow in between two sealing lips or, in general, a double restriction seal is presented. Two setups were designed to mimic different types of seals; that is, a radial and an axial shaft seal. The flow of the grease inside and in between the sealing restrictions was measured using microparticle image velocimetry. The results show that grease flow due to a pressure difference mainly takes place close to the rotating shaft surface with an exponentially decaying velocity profile in the radial direction. Consequently, contaminants may be captured in the stationary grease at the outer radius, which explains the sealing function of the grease.

• 327.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
Visualisering och kvantifiering av fettströmning i lagertätningar med µPIV2011Conference paper (Other academic)
FULLTEXT01
• 328. Green, Torbjörn
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Flow characterization of an attraction channel as entrance to fishways2011In: Rivers Research and Applications: an international journal devoted to river research and management, ISSN 1535-1459, E-ISSN 1535-1467, Vol. 27, no 10, p. 1290-1297Article in journal (Refereed)

The flow field inside and downstream of an open channel placed near the surface of a free flow (such as the tail water of a turbine) is characterized in detail. The channel cross-section is U-shaped and in the downstream end is placed a ramp on the bottom which accelerates the flow passing through the channel. This flow is intended to catch the attention of fish and improve their entrance to fishways, which has also been successfully demonstrated in field tests.

• 329.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Vattenfall Research & Development. Vattenfall Research & Development. Vattenfall Research & Development. Forsmarks kraftgrupp AB.
Termisk utmattning av T-koppling: en jämförelse mellan LES och PIV2009In: Svenska mekanikdagarna: Södertälje 2009, Stockholm: Svenska nationalkommittén för mekanik , 2009, p. 107-Conference paper (Other academic)
• 330.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Helfältsmätning av strömning genom lockvattenanordning2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 117-Conference paper (Other academic)
FULLTEXT01
• 331.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
A pulsed TV holography system for the study of transients in experimental mechanics2000Doctoral thesis, comprehensive summary (Other academic)

An all-electronic system for pulsed holographic interferometry called pulsed TV holography is developed. This is a whole-field non-contacting optical measurement method suitable for studies of transient events like wave propagation in solids and fluids. Chemical wet processing of holographic film and optical reconstruction of holograms are no longer needed. The technique was first developed using a double pulsed ruby laser as light source. The holograms are recorded directly on a CCD-detector. Quantitative data of changes in optical path length, caused either by a deformation of a solid object or a change in refractive index in a fluid, are calculated directly in a computer. The system for pulsed TV holography has recently been further developed by the purchase of a new pulsed laser (twin cavity, injection seeded pulsed Nd:YAG) and a CCD camera (PCO Sensicam) with higher spatial resolution and dynamic range. In the survey of this thesis the increased versatility compared to a ruby laser based system is discussed. During the development of the pulsed TV holography system a number of experiments in mechanics and acoustics have been accomplished. Bending waves in impacted plates propagating at a speed of about 2000 m/s are easily “frozen” due to the short duration laser pulses (<30 ns). These waves act as supersonic travelling acoustic sources and generate sound waves in the surrounding air. For the first time, transient sound fields from impacted plates have been visualised and measured using pulsed holographic interferometry. In another experiment, we have demonstrated that the pulsed TV holography system is feasible in combination with tomography. By recording a three-dimensional acoustic pressure field from a number of viewing directions followed by a tomographic reconstruction, the pressure in any point can be calculated. Finally, a method to restore fringes lost by large bulk motions is proposed. This technique may become very attractive in the study of vibrations (preferable transient) on moving or rotating objects. In conclusion, pulsed TV holography is proved to be a fast and reliable method to quantitatively study transients in mechanics and acoustics. The technique has a great potential in experimental mechanics in the future.

FULLTEXT01
• 332.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Bending wave propagation in rotating objects measured by pulsed TV holography2002In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 41, no 34, p. 7237-7240Article in journal (Refereed)

Transient bending waves in a rotating hard disk is measured by means of pulsed TV holography. The speckle motion in the detector plane caused by the rotation is compensated for in the interference phase evaluation. The technique is all electronic and needs no image derotator

• 333.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Four-pulse interferometric recordings of transient events by pulsed TV holography2003In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 40, no 5-6, p. 517-528Article in journal (Refereed)

With traditional double-pulsed holographic interferometry or pulsed TV holography, the experiment usually has to be repeated to allow the recording of a time sequence of interferograms of the event. With the proposed technique a sequence of four interferograms of a solitary transient event is measured. A twin oscillator, injection-seeded, pulsed Nd:YAG laser is incorporated into a pulsed TV holography set-up. With orthogonal polarisation and double pulsing of each of the two channels of the laser, four pulses are recorded on two separate CCD-frames. Four interferograms of a laser-impacted plate obtained from the same experiment, show how the bending waves develop and propagate in the plate.

• 334.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Pulsed TV holography combined with digital speckle photography restores lost interference phase2001In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 40, no 14, p. 2304-2309Article in journal (Refereed)

The measuring of situations with optical measuring methods is difficult when a deformation field must be determined while it is superposed to comparatively large rotating or translating object motion. Interferometric methods such as pulsed TV holography might be suitable to measure the small transient deformation, but the often-large bulk motion makes the phase information disappear. However, by a combination of digital speckle photography (DSP) (also called digital image correlation) with pulsed TV holography, such measuring problems can be mastered. A method to calculate the bulk in-plane motion by DSP from the usual pulsed TV holography recordings and then to use this information to restore the interference phase is proposed. This technique may be attractive in the study of transient vibrations overlaid on rotating or translating motions.

• 335.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Structured surfaces and turbulence1987Licentiate thesis, comprehensive summary (Other academic)
• 336.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
Feasibility of using digital speckle correlation in the study of seal contacts2009In: Lubrication Science, ISSN 0954-0075, E-ISSN 1557-6833, Vol. 21, no 4, p. 123-134Article in journal (Refereed)

This paper presents studies of the contact between a soft rubber specimen and glass counterface using the Digital Speckle Correlation method, which provides information of displacements and structural similarities between recorded images. The setup is designed with a real contact and changes in the contact can be varied. Microscopic images using laser light illumination for different displacements are recorded and correlated. The results show that the contact area can be identified both for dry and lubricated contacts. The method can be applied on different geometries, surface roughness and lubricants. Influences of scars and contaminations, e.g. wear particles, may also be analysed.

• 337.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Aeronautical Research Institute of Sweden.
Deformation during impact on an orthotropic composite plate1990In: Hologram interferometry and speckle metrology : proceedings ; 25. anniversary edition, Society for Experimental Mechanics , 1990Conference paper (Refereed)
• 338.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå tekniska universitet.
Transient wave fields in mechanics and acuostics recorded by pulsed TV holography1998In: Experimental mechanics: advances in design, testing and analysis, Balkema Publishers, A.A. / Taylor & Francis The Netherlands , 1998Conference paper (Refereed)
• 339.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå tekniska universitet. Department of Modern Mechanics and Engineering, University of Science and Technology of China.
Tomographic reconstruction of transient acoustic fields recorded by pulsed TV holography1998In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 37, no 5, p. 834-840Article in journal (Refereed)

Pulsed TV holography together with CT reconstruction were used to measure the three-dimensional distribution of transient acoustic fields in air. Holograms from several directions were directly recorded onto a CCD detector. From the recorded holograms, phase maps were quantitatively evaluated

• 340.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Department of Speech, Music and Hearing, Royal Institute of Technology (KTH).
Laser vibrometry measurements of vibration and sound fields of a bowed violin2006In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 17, no 4, p. 635-644Article in journal (Refereed)

Laser vibrometry measurements on a bowed violin are performed. A rotating disc apparatus, acting as a violin bow, is developed. It produces a continuous, long, repeatable, multi-frequency sound from the instrument that imitates the real bow-string interaction for a 'very long bow'. What mainly differs is that the back and forward motion of the real bow is replaced by the rotating motion with constant velocity of the disc and constant bowing force (bowing pressure). This procedure is repeatable. It is long lasting and allows laser vibrometry techniques to be used, which measure forced vibrations by bowing at all excited frequencies simultaneously. A chain of interacting parts of the played violin is studied: the string, the bridge and the plates as well as the emitted sound field. A description of the mechanics and the sound production of the bowed violin is given, i.e. the production chain from the bowed string to the produced tone

• 341.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Direct resonance of nonaxisymmetric disturbances in pipe flow1989In: Studies in applied mathematics (Cambridge), ISSN 0022-2526, E-ISSN 1467-9590, Vol. 80, no 2, p. 95-108Article in journal (Refereed)

On considère un mécanisme qui peut conduire à la croissance algébrique, suivie d'une décroissance exponentielle de petites perturbations non axisymétriques dans un écoulement en conduite. Le mécanisme est interprété comme une résonance directe entre les perturbations de pression et de vitesse d'écoulement. Résolution numérique des problèmes de valeur propre pour les modes de pression et de vitesse. Influence du nombre de Reynolds

• 342.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Energy growth of three-dimensional disturbances in plane Poiseuille flow1991In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 224, p. 241-60Article in journal (Refereed)

The development of a small three-dimensional disturbance in plane Poiseuille flow is considered. Its kinetic energy is expressed in terms of the velocity and vorticity components normal to the wall. The normal vorticity develops according to the mechanism of vortex stretching and is described by an inhomogeneous equation, where the spanwise variation of the normal velocity acts as forcing. To study specifically the effect of the forcing, the initial normal vorticity is set to zero and the energy density in the wavenumber plane, induced by the normal velocity, is determined. In particular, the response from individual (and damped) Orr-Sommerfeld modes is calculated, on the basis of a formal solution to the initial-value problem. The relevant timescale for the development of the perturbation is identified as a viscous one. Even so, the induced energy density can greatly exceed that associated with the initial normal velocity, before decay sets in.

• 343.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Excitation of direct resonances in plane Poiseuille flow1986In: Studies in applied mathematics (Cambridge), ISSN 0022-2526, E-ISSN 1467-9590, Vol. 75, no 3, p. 227-248Article in journal (Refereed)

The direct resonance mechanism between vertical vorticity and vertical velocity is studied for plane Poiseuille flow. The resonance term, obtained from the initial-value problem, has been determined numerically for various initial disturbances. The amplitude, in wave-number space, is found to decay with time for all resonances. Because of its appearance in the Laplace-transform plane as a double pole, we show that a resonance term will have an initial amplitude which grows with Reynolds number, thus indicating that resonances will dominate the initial development of a disturbance at large Reynolds number.

• 344.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Non-linear wave interactions from transient growth in plane-parallel shear flows2009In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 28, no 3, p. 420-429Article in journal (Refereed)

Based on the normal velocity-normal vorticity (v - η) formulation for the development of 3D disturbances in plane-parallel shear flows, the non-linear terms in the governing equations are derived as convolution integrals of the Fourier-transformed variables. They are grouped in three categories: v - v, v - η and η - η terms, and are expressed in a simple geometric form using the modulus of the two wave-vectors (k′ and k″) appearing in the convolution integrals, and their intervening angle (χ). The non-linear terms in the v-equation involving η are all weighted by sin χ (or sin2 χ). This confirms the known result that non-linear regeneration of normal velocity, necessary for a sustained driving of 3D disturbances, is not possible for stream-wise elongated structures (α = 0), only. It is therefore suggested how transiently amplified η can interact with decaying 2D waves to activate (oblique) waves which may be less damped than the 2D wave. This is shown to be possible for Blasius flow. In the η-equation, non-linear effects are possible for elongated structures resulting in shorter spanwise scales appearing at a shorter time-scale than the (linear) transient growth. A numerical example shows the details of this process in plane Poiseuille flow. From an inspection of the y-dependency (wall-normal direction) of the non-linear terms it is suggested that higher y-derivatives may give rise to non-linear effects in the inviscid development of perturbations. Also, a result for the y-symmetry of the non-linear terms is derived, applicable to plane Poiseuille flow.

• 345.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Parameter study of direct resonance in water table flow1984Report (Other academic)
• 346.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Resonant growth of three-dimensional disturbances in plane Poiseuille flow1981In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 112, p. 253-264Article in journal (Refereed)

A linear mechanism for growth of three-dimensional perturbations on plane Poiseuille flow is investigated. The mechanism, resonant forcing of vertical vorticity waves by Tollmien-Schlichting waves, leads to an algebraic growth for small times. Eventually, viscous damping becomes dominant and the disturbance decays. The resonance occurs only at discrete points in the wave-number space. Nine resonances have been investigated. For these, the phase velocities range from 0.67 to 0.81 of the center-line velocity. The lowest Reynolds number for which the resonance can occur is 25. The strongest resonance appears only above a Reynolds number of 341. Also, two cases of degeneracy in the Orr-Sommerfeld dispersion relationship have been found.$• 347. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå tekniska universitet. Degeneracies and direct resonances in water-table flow1990In: Studies in applied mathematics (Cambridge), ISSN 0022-2526, E-ISSN 1467-9590, Vol. 83, no 1, p. 61-89Article in journal (Refereed) Degeneracies of the Orr-Sommerfeld eigenmodes and direct resonances between the Orr-Sommerfeld eigenmodes and vorticity eigenmodes are studied in water-table flow. The sensitivity of the characteristics of these algebraic mechanisms to flow parameters, such as the Reynolds number (R), the slope of the table$(\theta)$, and the material parameter$(\gamma)\$, are investigated. It is found that the mechanisms become operative at subtransitional R, and their damping rates decrease with increasing R. When the mean flow profile is slightly distorted from the ultimate parabolic profile, the characteristics of the direct resonances show remarkable variations. Also, some of the algebraic mechanisms in water-table flow are shown to have the same characteristics and modal structures as some of those in plane Poiseuille flow.

• 348.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå tekniska universitet.
Effect of three-dimensional surface elements on boundary layer flow1990In: Structure of turbulence and drag reduction: Proceedings of the IUTAM Symposium / [ed] Albert Gyr, Berlin: Encyclopedia of Global Archaeology/Springer Verlag, 1990, p. 399-406Conference paper (Refereed)

The perturbation velocity field induced by a three-dimensional surface distortion in a boundary layer flow is considered. For small amplitudes, the kinetic energy is shown to be composed of two factors: one associated with the surface structure and the other with the velocity profile. Level curves of the profile factor, in the (alpha, beta) wavenumber plane, are ridge-like and approach the beta-axis as the Reynolds number increases. Thus, in the inviscid limit, the kinetic energy is confined to structures infinitely extended in the streamwise direction. For a certain class of surface structures, also the level curves for the kinetic energy have been determined. It is shown how a spanwise modulation and an aspect ratio of the surface distortion change the position of the level curves and the amplitudes.

• 349.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering. Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering. Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
Norrländsk vattenkraft och gruvnäring: två svenska välståndsbyggare2009In: Thule: Kungl. Skytteanska samfundets årsbok 2009, Umeå: Kungl. Skytteanska samfundet , 2009, p. 331-345Chapter in book (Other (popular science, discussion, etc.))
• 350.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå tekniska universitet.
Observations of turbulent spots on a water table1982Report (Other academic)
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