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http://hdl.handle.net/10985/8538
Influence of Reynolds number and forcing type in a turbulent von Karman flow
SAINT-MICHEL, Brice; DUBRULLE, Bérengère; MARIÉ, Louis; DAVIAUD, François; RAVELET, Florent
We present a detailed study of a global bifurcation occuring in a turbulent von Kármán swirling flow. In this system, the statistically steady states progressively display hysteretic behaviour when the Reynolds number is increased above the transition to turbulence. We examine in detail this hysteresis using asymmetric forcing conditions—rotating the impellers at different speeds. For very high Reynolds numbers, we study the sensitivity of the hysteresis cycle—using complementary particle image velocimetry and global mechanical measurements—to the forcing nature, imposing either the torque or the speed of the impellers. New mean states, displaying multiple quasi-steady states and negative differential responses, are experimentally observed in torque control. A simple analogy with electrical circuits is performed to understand the link between multi-stability and negative responses. The system is compared to other, similar 'bulk' systems, to understand some relevant ingredients of negative differential responses, and studied in the framework of thermodynamics of long-range interacting systems. The experimental results are eventually compared to the related problem of Rayleigh–Bénard turbulence.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/85382014-01-01T00:00:00ZSAINT-MICHEL, BriceDUBRULLE, BérengèreMARIÉ, LouisDAVIAUD, FrançoisRAVELET, FlorentWe present a detailed study of a global bifurcation occuring in a turbulent von Kármán swirling flow. In this system, the statistically steady states progressively display hysteretic behaviour when the Reynolds number is increased above the transition to turbulence. We examine in detail this hysteresis using asymmetric forcing conditions—rotating the impellers at different speeds. For very high Reynolds numbers, we study the sensitivity of the hysteresis cycle—using complementary particle image velocimetry and global mechanical measurements—to the forcing nature, imposing either the torque or the speed of the impellers. New mean states, displaying multiple quasi-steady states and negative differential responses, are experimentally observed in torque control. A simple analogy with electrical circuits is performed to understand the link between multi-stability and negative responses. The system is compared to other, similar 'bulk' systems, to understand some relevant ingredients of negative differential responses, and studied in the framework of thermodynamics of long-range interacting systems. The experimental results are eventually compared to the related problem of Rayleigh–Bénard turbulence.Influence of global rotation and Reynolds number on the large-scale features of a turbulent Taylor–Couette flow
http://hdl.handle.net/10985/6784
Influence of global rotation and Reynolds number on the large-scale features of a turbulent Taylor–Couette flow
DELFOS, René; WESTERWEEL, Jerry; RAVELET, Florent
We experimentally study the turbulent flow between two coaxial and independently rotating cylinders. We determined the scaling of the torque with Reynolds numbers at various angular velocity ratios Rotation numbers and the behavior of the wall shear stress when varying the Rotation number at high Reynolds numbers.We compare the curves with particle image velocimetry analysis of the mean flow and show the peculiar role of perfect counter-rotation for the emergence of organized large scale structures in the mean part of this very turbulent flow that appear in a smooth and continuous way: the transition resembles a supercritical bifurcation of the secondary mean flow.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10985/67842010-01-01T00:00:00ZDELFOS, RenéWESTERWEEL, JerryRAVELET, FlorentWe experimentally study the turbulent flow between two coaxial and independently rotating cylinders. We determined the scaling of the torque with Reynolds numbers at various angular velocity ratios Rotation numbers and the behavior of the wall shear stress when varying the Rotation number at high Reynolds numbers.We compare the curves with particle image velocimetry analysis of the mean flow and show the peculiar role of perfect counter-rotation for the emergence of organized large scale structures in the mean part of this very turbulent flow that appear in a smooth and continuous way: the transition resembles a supercritical bifurcation of the secondary mean flow.Study of passive control study of the cavitation instability on a venturi profile
http://hdl.handle.net/10985/11254
Study of passive control study of the cavitation instability on a venturi profile
DANLOS, Amélie; MEHAL, Jean-Elie; RAVELET, Florent; SARRAF, Christophe
This paper presents experimental and numerical results concerning the operation of a configuration that includes an axial pump and a bundle of tubes that mimics the cool source of a heat exchanger. The pump used in the tests has a low solidity and two blades designed in forced vortex, the tip clearance is approximately 3.87% of tip radius. The experimental measures of the characteristic curves and of the wall static pressure fluctuations signals upstream and downstream the pump and the exchanger were compared to the numerical results. The numerical simulations were carried out by using a Fluent code, with URANS (Unsteady Reynolds Averaged Navier-Stokes) approach and k-w SST turbulence model.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10985/112542010-01-01T00:00:00ZDANLOS, AmélieMEHAL, Jean-ElieRAVELET, FlorentSARRAF, ChristopheThis paper presents experimental and numerical results concerning the operation of a configuration that includes an axial pump and a bundle of tubes that mimics the cool source of a heat exchanger. The pump used in the tests has a low solidity and two blades designed in forced vortex, the tip clearance is approximately 3.87% of tip radius. The experimental measures of the characteristic curves and of the wall static pressure fluctuations signals upstream and downstream the pump and the exchanger were compared to the numerical results. The numerical simulations were carried out by using a Fluent code, with URANS (Unsteady Reynolds Averaged Navier-Stokes) approach and k-w SST turbulence model.Evidence for Forcing-Dependent Steady States in a Turbulent Swirling Flow
http://hdl.handle.net/10985/8534
Evidence for Forcing-Dependent Steady States in a Turbulent Swirling Flow
SAINT-MICHEL, Brice; DUBRULLE, Bérengère; MARIÉ, Louis; DAVIAUD, François; RAVELET, Florent
We study the influence on steady turbulent states of the forcing in a von Karman flow, at constant impeller speed, or at constant torque. We find that the different forcing conditions change the nature of the stability of the steady states and reveal dynamical regimes that bear similarities to low-dimensional systems. We suggest that this forcing dependence may be applicable to other turbulent systems.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/85342013-01-01T00:00:00ZSAINT-MICHEL, BriceDUBRULLE, BérengèreMARIÉ, LouisDAVIAUD, FrançoisRAVELET, FlorentWe study the influence on steady turbulent states of the forcing in a von Karman flow, at constant impeller speed, or at constant torque. We find that the different forcing conditions change the nature of the stability of the steady states and reveal dynamical regimes that bear similarities to low-dimensional systems. We suggest that this forcing dependence may be applicable to other turbulent systems.On the dynamics and breakup of a bubble immersed in a turbulent flow
http://hdl.handle.net/10985/8540
On the dynamics and breakup of a bubble immersed in a turbulent flow
COLIN, Catherine; RISSO, Frédéric; RAVELET, Florent
Experimental investigations of the dynamics of a deformable bubble rising in a uniform turbulent flow are reported. The turbulence is characterized by fast PIV. Time-resolved evolutions of bubble translation, rotation and deformation are determined by three-dimensional shape recognition from three perpendicular camera views. The bubble dynamics involves three mechanisms fairly decoupled: (i) average shape is imposed by the mean motion of the bubble relative to liquid; (ii) wake instability generates almost periodic oscillations of velocity and orientation; (iii) turbulence causes random deformations that sometimes lead to breakup. The deformation dynamics is radically different from that observed in the absence of a significant sliding motion due to buoyancy. Large deformations that lead to breakup are not axisymmetric and correspond to elongations in the horizontal direction. The timescale of decay of shape oscillations is of the same order as their natural frequency f2, so that breakup always results from the interaction with a single turbulent eddy. This overdamping causes the statistics of large deformations and the statistics of breakup identical to the statistics of turbulence. The bubble response time f2 however controls the duration of individual breakup events.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/10985/85402011-01-01T00:00:00ZCOLIN, CatherineRISSO, FrédéricRAVELET, FlorentExperimental investigations of the dynamics of a deformable bubble rising in a uniform turbulent flow are reported. The turbulence is characterized by fast PIV. Time-resolved evolutions of bubble translation, rotation and deformation are determined by three-dimensional shape recognition from three perpendicular camera views. The bubble dynamics involves three mechanisms fairly decoupled: (i) average shape is imposed by the mean motion of the bubble relative to liquid; (ii) wake instability generates almost periodic oscillations of velocity and orientation; (iii) turbulence causes random deformations that sometimes lead to breakup. The deformation dynamics is radically different from that observed in the absence of a significant sliding motion due to buoyancy. Large deformations that lead to breakup are not axisymmetric and correspond to elongations in the horizontal direction. The timescale of decay of shape oscillations is of the same order as their natural frequency f2, so that breakup always results from the interaction with a single turbulent eddy. This overdamping causes the statistics of large deformations and the statistics of breakup identical to the statistics of turbulence. The bubble response time f2 however controls the duration of individual breakup events.Kinematic Alpha Tensors and dynamo mechanisms in a von Karman swirling flow
http://hdl.handle.net/10985/8544
Kinematic Alpha Tensors and dynamo mechanisms in a von Karman swirling flow
DUBRULLE, Bérengère; DAVIAUD, François; RATIÉ, Pierre-Arthur; RAVELET, Florent
We provide experimental and numerical evidence of in-blades vortices in the von Karman swirling flow. We estimate the associated kinematic α-effect tensor and show that it is compatible with recent models of the von Karman Sodium (VKS) dynamo. We further show that depending on the relative frequency of the two impellers, the dominant dynamo mechanism may switch from α^2 to α − Ω dynamo. We discuss some implications of these results for VKS experiments.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10985/85442012-01-01T00:00:00ZDUBRULLE, BérengèreDAVIAUD, FrançoisRATIÉ, Pierre-ArthurRAVELET, FlorentWe provide experimental and numerical evidence of in-blades vortices in the von Karman swirling flow. We estimate the associated kinematic α-effect tensor and show that it is compatible with recent models of the von Karman Sodium (VKS) dynamo. We further show that depending on the relative frequency of the two impellers, the dominant dynamo mechanism may switch from α^2 to α − Ω dynamo. We discuss some implications of these results for VKS experiments.Attached cavitation in laminar separations within a transition to unsteadiness
http://hdl.handle.net/10985/15678
Attached cavitation in laminar separations within a transition to unsteadiness
CROCI, Kilian; DANLOS, Amélie; BARAST, Luc; ROBINET, Jean-Christophe; RAVELET, Florent
Attached sheet cavitation is usually observed in turbulent water flows within small laminar separation bubbles which can provide favorable conditions for inception and attachment of cavities. In the present study, viscous silicone oils are used within a small scale Venturi geometry to investigate attached cavitation into laminar separated flows for Reynolds numbers from 346 to 2188. Numerical simulations about single phase flows are performed with steady simulations for a Reynolds number range Re ∈ [50; 1400] and with unsteady simulations for Re ∈ [1000; 2000]. They reveal the emergence of two large laminar boundary layer separations downstream of the Venturi throat in addition to low pressure zones which can possibly induce both degassing or cavitation features. Experiments are performed with high-speed photography, and several multiphase dynamics are observed in these viscous flows, which are considered as quasisteady flows at low Reynolds numbers Re ≤ 1400. Degassing phenomenon with air bubble recirculation has been first observed at pressures far above liquid vapor pressure whereas typical attached cavities have been identified for low pressure conditions as “band” and “tadpole” cavities into the different separations of the laminar flows. For higher Reynolds numbers, a flow regime transition can be noticed in the wake of well-developed gas structures, characterized by wake instabilities, causing vortex cavitation above a critical Reynolds number associated with the bubble width Rebc≃616 . This regime transition can possibly occur either quasicontinuously in the wake of an attached “band” vapor cavity or intermittently behind a recirculating air bubble generated with degassing. This last phenomenon is associated in our study to classical “patch” cavitation.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/156782019-01-01T00:00:00ZCROCI, KilianDANLOS, AmélieBARAST, LucROBINET, Jean-ChristopheRAVELET, FlorentAttached sheet cavitation is usually observed in turbulent water flows within small laminar separation bubbles which can provide favorable conditions for inception and attachment of cavities. In the present study, viscous silicone oils are used within a small scale Venturi geometry to investigate attached cavitation into laminar separated flows for Reynolds numbers from 346 to 2188. Numerical simulations about single phase flows are performed with steady simulations for a Reynolds number range Re ∈ [50; 1400] and with unsteady simulations for Re ∈ [1000; 2000]. They reveal the emergence of two large laminar boundary layer separations downstream of the Venturi throat in addition to low pressure zones which can possibly induce both degassing or cavitation features. Experiments are performed with high-speed photography, and several multiphase dynamics are observed in these viscous flows, which are considered as quasisteady flows at low Reynolds numbers Re ≤ 1400. Degassing phenomenon with air bubble recirculation has been first observed at pressures far above liquid vapor pressure whereas typical attached cavities have been identified for low pressure conditions as “band” and “tadpole” cavities into the different separations of the laminar flows. For higher Reynolds numbers, a flow regime transition can be noticed in the wake of well-developed gas structures, characterized by wake instabilities, causing vortex cavitation above a critical Reynolds number associated with the bubble width Rebc≃616 . This regime transition can possibly occur either quasicontinuously in the wake of an attached “band” vapor cavity or intermittently behind a recirculating air bubble generated with degassing. This last phenomenon is associated in our study to classical “patch” cavitation.Cavitation control using passive flow control techniques
http://hdl.handle.net/10985/21361
Cavitation control using passive flow control techniques
ZARESHARIF, Mahshid; KINAHAN, David J.; DELAURE, Yan M. C.; RAVELET, Florent
Passive flow control techniques, and particularly vortex generators have been used successfully in a broad range of aero- and hydrodynamics applications to alter the characteristics of boundary layer separation. This study aims to review how such techniques can mitigate the extent and impact of cavitation in incompressible flows. This review focuses first on vortex generators to characterize key physical principles. It then considers the complete range of passive flow control technologies, including surface conditioning and roughness, geometry modification, grooves, discharge, injection, obstacles, vortex generators, and bubble generators. The passive flow control techniques reviewed typically delay and suppress boundary layer separation by decreasing the pressure gradient at the separation point. The literature also identifies streamwise vortices that result in the transfer of momentum from the free stream to near-wall low energy flow regions. The area of interest concerns hydraulic machinery, whose performance and life span are particularly susceptible to cavitation. The impact on performance includes a reduction in efficiency and fluctuations in discharge pressure and flow, while cavitation can greatly increase wear of bearings, wearing rings, seals, and impeller surfaces due to excessive vibration and surface erosion. In that context, few studies have also shown the positive effects that passive controls can have on the hydraulic performance of centrifugal pumps, such as total head and efficiency. It is conceivable that a new generation of design in hydraulic systems may be possible if simple design features can be conceived to maximize power transfer and minimize losses and cavitation. There are still, however, significant research gaps in understanding a range of impact factors such as manufacturing processes, lifetime, and durability, and essentially how a static design can be optimized to deliver improved performance over a realistic range of operating conditions.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10985/213612021-01-01T00:00:00ZZARESHARIF, MahshidKINAHAN, David J.DELAURE, Yan M. C.RAVELET, FlorentPassive flow control techniques, and particularly vortex generators have been used successfully in a broad range of aero- and hydrodynamics applications to alter the characteristics of boundary layer separation. This study aims to review how such techniques can mitigate the extent and impact of cavitation in incompressible flows. This review focuses first on vortex generators to characterize key physical principles. It then considers the complete range of passive flow control technologies, including surface conditioning and roughness, geometry modification, grooves, discharge, injection, obstacles, vortex generators, and bubble generators. The passive flow control techniques reviewed typically delay and suppress boundary layer separation by decreasing the pressure gradient at the separation point. The literature also identifies streamwise vortices that result in the transfer of momentum from the free stream to near-wall low energy flow regions. The area of interest concerns hydraulic machinery, whose performance and life span are particularly susceptible to cavitation. The impact on performance includes a reduction in efficiency and fluctuations in discharge pressure and flow, while cavitation can greatly increase wear of bearings, wearing rings, seals, and impeller surfaces due to excessive vibration and surface erosion. In that context, few studies have also shown the positive effects that passive controls can have on the hydraulic performance of centrifugal pumps, such as total head and efficiency. It is conceivable that a new generation of design in hydraulic systems may be possible if simple design features can be conceived to maximize power transfer and minimize losses and cavitation. There are still, however, significant research gaps in understanding a range of impact factors such as manufacturing processes, lifetime, and durability, and essentially how a static design can be optimized to deliver improved performance over a realistic range of operating conditions.Innovative design method and experimental investigation of a small-scale and very low tip-speed ratio wind turbine
http://hdl.handle.net/10985/21360
Innovative design method and experimental investigation of a small-scale and very low tip-speed ratio wind turbine
BOURHIS, Martin; PEREIRA, Michaël; DOBREV, Ivan; RAVELET, Florent
Small horizontal axis wind turbines operating at low wind speeds face the issue of low performance compared to large wind turbines. A high amount of torque is required to start producing power at low wind speed to overtake friction of mechanical parts. A low design tip-speed ratio (λ) is suitable for low power applications. The relevance of the classical blade-element/ momentum theory, traditionally used for the design of large wind turbines operating at high tip-speed ratio, is controversial at low tip-speed ratio. This paper presents a new design methodology for a 300 mm horizontal axis wind turbine operating at very low tip-speed ratio. Chord and blade angle distributions were computed by applying the Euler’s turbomachinery theorem. The new wind turbine has multiple fan-type blades and a high solidity. The rotor was tested in wind tunnel. The power and torque coefficients have been measured, and the velocities in the wake have been explored by stereoscopic particle image velocimetry. The results are compared to a conventional 3-bladed horizontal axis wind turbine operating at higher tip-speed ratio λ = 3. The new wind turbine achieves a maximum power coefficient of 0.31 for λ = 1. The conventional wind turbine achieves similar performance. At low tip-speed ratio, the torque coefficient (Cτ) is higher for the new wind turbine than for the conventional one and decreases linearly with the tip-speed ratio. The high magnitude of torque at low tip-speed ratio allows it to have lower instantaneous cut-in wind speed (2.4 m.s−1) than the conventional wind turbine (7.9 m.s−1). The order of magnitude of the axial and tangential velocities in the near wake are closed to the design requirements. The current method could still be improved in order to better predict the profiles. The analysis of the wake shows that the new wind turbine induces a highly stable and rotating wake, with lower wake expansion and deceleration than the conventional one. This could be useful to drive a contra-rotating rotor.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10985/213602021-01-01T00:00:00ZBOURHIS, MartinPEREIRA, MichaëlDOBREV, IvanRAVELET, FlorentSmall horizontal axis wind turbines operating at low wind speeds face the issue of low performance compared to large wind turbines. A high amount of torque is required to start producing power at low wind speed to overtake friction of mechanical parts. A low design tip-speed ratio (λ) is suitable for low power applications. The relevance of the classical blade-element/ momentum theory, traditionally used for the design of large wind turbines operating at high tip-speed ratio, is controversial at low tip-speed ratio. This paper presents a new design methodology for a 300 mm horizontal axis wind turbine operating at very low tip-speed ratio. Chord and blade angle distributions were computed by applying the Euler’s turbomachinery theorem. The new wind turbine has multiple fan-type blades and a high solidity. The rotor was tested in wind tunnel. The power and torque coefficients have been measured, and the velocities in the wake have been explored by stereoscopic particle image velocimetry. The results are compared to a conventional 3-bladed horizontal axis wind turbine operating at higher tip-speed ratio λ = 3. The new wind turbine achieves a maximum power coefficient of 0.31 for λ = 1. The conventional wind turbine achieves similar performance. At low tip-speed ratio, the torque coefficient (Cτ) is higher for the new wind turbine than for the conventional one and decreases linearly with the tip-speed ratio. The high magnitude of torque at low tip-speed ratio allows it to have lower instantaneous cut-in wind speed (2.4 m.s−1) than the conventional wind turbine (7.9 m.s−1). The order of magnitude of the axial and tangential velocities in the near wake are closed to the design requirements. The current method could still be improved in order to better predict the profiles. The analysis of the wake shows that the new wind turbine induces a highly stable and rotating wake, with lower wake expansion and deceleration than the conventional one. This could be useful to drive a contra-rotating rotor.Numerical Assesment of a Small-Scale and Very Low Tip Speed Ratio Wind Turbine
http://hdl.handle.net/10985/21379
Numerical Assesment of a Small-Scale and Very Low Tip Speed Ratio Wind Turbine
BOURHIS, Martin; DOBREV, Ivan; RAVELET, Florent; PEREIRA, Michaël
The aim of this paper is to study by CFD the performance and to characterize the velocity fields in the wake of an horizontal axis wind turbine. The design of this wind turbine is far from classical as it has been designed to work at very low angular velocity to promote torque. The 8 blades are not isolated but form a high solidity blade cascade. The numerical simulation compares well to experimental data regarding the power coefficients. The analysis of the wake does show that high tangential velocities, close to the order of magnitude that was used as a design requirement, are generated and form a stable rotating wake. This rotating kinetic energy in the wake may be used to rotate a second rotor in a counter-rotating arrangment.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10985/213792021-01-01T00:00:00ZBOURHIS, MartinDOBREV, IvanRAVELET, FlorentPEREIRA, MichaëlThe aim of this paper is to study by CFD the performance and to characterize the velocity fields in the wake of an horizontal axis wind turbine. The design of this wind turbine is far from classical as it has been designed to work at very low angular velocity to promote torque. The 8 blades are not isolated but form a high solidity blade cascade. The numerical simulation compares well to experimental data regarding the power coefficients. The analysis of the wake does show that high tangential velocities, close to the order of magnitude that was used as a design requirement, are generated and form a stable rotating wake. This rotating kinetic energy in the wake may be used to rotate a second rotor in a counter-rotating arrangment.