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http://hdl.handle.net/10985/8534
Evidence for Forcing-Dependent Steady States in a Turbulent Swirling Flow
SAINT-MICHEL, Brice; DUBRULLE, Bérengère; MARIÉ, Louis; RAVELET, Florent; DAVIAUD, François
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, LouisRAVELET, FlorentDAVIAUD, FrançoisWe 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.Experimental and Numerical Analysis of the Flow Inside a Configuration Including an Axial Pump and a Tubular Exchanger
http://hdl.handle.net/10985/8586
Experimental and Numerical Analysis of the Flow Inside a Configuration Including an Axial Pump and a Tubular Exchanger
SOLIS, Moises; RAVELET, Florent; KHELLADI, Sofiane; BAKIR, Farid
In centrifugal and axial pumps, the flow is characterized by a turbulent and complex behavior and also by physical mechanisms such as cavitation and pressure fluctuations that are mainly due to the strong interactions between the fixed and mobile parts and the operating conditions. These fluctuations are more important at the tip clearance and propagate upstream and downstream of the rotor. The control of the fluctuating signal amplitudes can be achieved by incrementing the distance between the components mentioned above. 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 were carried out using a test bench built for this purpose at the DynFluid Laboratory which was accomodated conveniently with a variety of instruments. Firstly, the characteristic curves were drawn for the pump at 1500 rpm and then a set of measurements concerning the use of pressure sensors was done in order to recover for different flow rates the static pressure signals upstream and downstream the pump and the exchanger. The pressure fluctuations and the performance curve were compared to the numerical results. The numerical simulations were carried out by using a Fluent code, the URANS (Unsteady Reynolds Averaged Navier-Stokes) approach and the k-ω SST turbulence model were applied to solve the unsteady, incompressible and turbulent flow. To record the fluctuating pressure signal, virtual sensors were necessary and placed at the same positions as in the experiments.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10985/85862010-01-01T00:00:00ZSOLIS, MoisesRAVELET, FlorentKHELLADI, SofianeBAKIR, FaridIn centrifugal and axial pumps, the flow is characterized by a turbulent and complex behavior and also by physical mechanisms such as cavitation and pressure fluctuations that are mainly due to the strong interactions between the fixed and mobile parts and the operating conditions. These fluctuations are more important at the tip clearance and propagate upstream and downstream of the rotor. The control of the fluctuating signal amplitudes can be achieved by incrementing the distance between the components mentioned above. 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 were carried out using a test bench built for this purpose at the DynFluid Laboratory which was accomodated conveniently with a variety of instruments. Firstly, the characteristic curves were drawn for the pump at 1500 rpm and then a set of measurements concerning the use of pressure sensors was done in order to recover for different flow rates the static pressure signals upstream and downstream the pump and the exchanger. The pressure fluctuations and the performance curve were compared to the numerical results. The numerical simulations were carried out by using a Fluent code, the URANS (Unsteady Reynolds Averaged Navier-Stokes) approach and the k-ω SST turbulence model were applied to solve the unsteady, incompressible and turbulent flow. To record the fluctuating pressure signal, virtual sensors were necessary and placed at the same positions as in the experiments.High Accuracy Volume Flow Rate Measurement Using Vortex Counting
http://hdl.handle.net/10985/8577
High Accuracy Volume Flow Rate Measurement Using Vortex Counting
ZAARAOUI, Abdelkader; RAVELET, Florent; MARGNAT, Florent; KHELLADI, Sofiane
A prototype device for measuring the volumetric flow-rate by counting vortices has been designed and realized. It consists of a square-section pipe in which are placed a two-dimensional bluff body and a strain gauge force sensor. These two elements are separated from each other, unlike the majority of vortex apparatus currently available. The principle is based on the generation of a separated wake behind the bluff body. The volumetric flow-rate measurement is done by counting vortices using a flat plate placed in the wake and attached to the beam sensor. By optimizing the geometrical arrangement, the search for a significant signal has shown that it was possible to get a quasi-periodic signal, within a good range of flow rates so that its performances are well deduced. The repeatability of the value of the volume of fluid passed for every vortex shed is tested for a given flow and then the accuracy of the measuring device is determined. This quantity is the constant of the device and is called the digital volume (V_p). It has the dimension of a volume and varies with the confinement of the flow and with the Reynolds number. Therefore, a dimensionless quantity is introduced, the reduced digital volume (V_r) that takes into account the average speed in the contracted section downstream of the bluff body. The reduced digital volume is found to be independent of the confinement in a significant range of Reynolds numbers, which gives the device a good accuracy.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/85772013-01-01T00:00:00ZZAARAOUI, AbdelkaderRAVELET, FlorentMARGNAT, FlorentKHELLADI, SofianeA prototype device for measuring the volumetric flow-rate by counting vortices has been designed and realized. It consists of a square-section pipe in which are placed a two-dimensional bluff body and a strain gauge force sensor. These two elements are separated from each other, unlike the majority of vortex apparatus currently available. The principle is based on the generation of a separated wake behind the bluff body. The volumetric flow-rate measurement is done by counting vortices using a flat plate placed in the wake and attached to the beam sensor. By optimizing the geometrical arrangement, the search for a significant signal has shown that it was possible to get a quasi-periodic signal, within a good range of flow rates so that its performances are well deduced. The repeatability of the value of the volume of fluid passed for every vortex shed is tested for a given flow and then the accuracy of the measuring device is determined. This quantity is the constant of the device and is called the digital volume (V_p). It has the dimension of a volume and varies with the confinement of the flow and with the Reynolds number. Therefore, a dimensionless quantity is introduced, the reduced digital volume (V_r) that takes into account the average speed in the contracted section downstream of the bluff body. The reduced digital volume is found to be independent of the confinement in a significant range of Reynolds numbers, which gives the device a good accuracy.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
RAVELET, Florent; DUBRULLE, Bérengère; DAVIAUD, François; RATIÉ, Pierre-Arthur
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/10985/85442012-01-01T00:00:00ZRAVELET, FlorentDUBRULLE, BérengèreDAVIAUD, FrançoisRATIÉ, Pierre-ArthurWe 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.Experimental comparison between a counter-rotating axial-flow fan and a conventional rotor-stator stage
http://hdl.handle.net/10985/8588
Experimental comparison between a counter-rotating axial-flow fan and a conventional rotor-stator stage
WANG, Juan; RAVELET, Florent; BAKIR, Farid
Based on the requirement of energy consumption level and weight and dimension restriction, compact axial machines are highly demanded in many industrial fields. The counter-rotating axial-flow fans could be a promising way to achieve these requirements. Because of the reduction of rotational speed and a better homogenization of the flow downstream of the rear rotor, these machines may have very good aerodynamic performances. However, they are rarely used in subsonic applications, mainly due to poor knowledge of the aerodynamics in the mixing area between the two rotors, where very complex structures are produced by the interaction of highly unsteady flows. The purpose of the present work is to compare the global performances (static pressure rise and static efficiency) and the wall pressure fluctuations downstream of the first rotor for three different stages operating at the same point: a single subsonic axial-flow fan, a conventional rotor-stator stage and a counter-rotating system that have been designed with in-house tools. The counter-rotating system allows large savings of energy with respect to the other two systems, for lower rotation rates and by adjusting the distance between the two rotors, a solution with comparable wall pressure fluctuations levels for the three systems is found.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/85882013-01-01T00:00:00ZWANG, JuanRAVELET, FlorentBAKIR, FaridBased on the requirement of energy consumption level and weight and dimension restriction, compact axial machines are highly demanded in many industrial fields. The counter-rotating axial-flow fans could be a promising way to achieve these requirements. Because of the reduction of rotational speed and a better homogenization of the flow downstream of the rear rotor, these machines may have very good aerodynamic performances. However, they are rarely used in subsonic applications, mainly due to poor knowledge of the aerodynamics in the mixing area between the two rotors, where very complex structures are produced by the interaction of highly unsteady flows. The purpose of the present work is to compare the global performances (static pressure rise and static efficiency) and the wall pressure fluctuations downstream of the first rotor for three different stages operating at the same point: a single subsonic axial-flow fan, a conventional rotor-stator stage and a counter-rotating system that have been designed with in-house tools. The counter-rotating system allows large savings of energy with respect to the other two systems, for lower rotation rates and by adjusting the distance between the two rotors, a solution with comparable wall pressure fluctuations levels for the three systems is found.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
RAVELET, Florent; DELFOS, René; WESTERWEEL, Jerry
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/10985/67842010-01-01T00:00:00ZRAVELET, FlorentDELFOS, RenéWESTERWEEL, JerryWe 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.Experimental study of hydraulic transport of large particles in horizontal pipes
http://hdl.handle.net/10985/8564
Experimental study of hydraulic transport of large particles in horizontal pipes
RAVELET, Florent; BAKIR, Farid; KHELLADI, Sofiane; REY, Robert
This article presents an experimental study of the hydraulic transport of very large solid particles (above 5 mm) in an horizontal pipe. Two specific masses are used for the solids. The solids are spheres that are large with respect to the diameter of the pipe (5, 10 and 15%) or real stones of arbitrary shapes but constant specific mass and a size distribution similar to the tested spherical beads. Finally, mixtures of size and / or specific mass are studied. The regimes are characterized with differential pressure measurements and visualizations. The results are compared to empirical models based on dimensionless numbers, together with 1D models that are based on mass and momentum balance. A model for the transport of large particles in vertical pipes is also proposed and tested on data available in the Literature, in order to compare the trends that are observed in the present experiments in a horizontal pipe to the trends predicted for a vertical pipe. The results show that the grain size and specific mass have a strong effect on the transition point between regimes with a stationary bed and dispersed flows. The pressure drops are moreover smaller for large particles in the horizontal part contrary to what occurs for vertical pipes, and to the predictions of the empirical correlations.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/85642013-01-01T00:00:00ZRAVELET, FlorentBAKIR, FaridKHELLADI, SofianeREY, RobertThis article presents an experimental study of the hydraulic transport of very large solid particles (above 5 mm) in an horizontal pipe. Two specific masses are used for the solids. The solids are spheres that are large with respect to the diameter of the pipe (5, 10 and 15%) or real stones of arbitrary shapes but constant specific mass and a size distribution similar to the tested spherical beads. Finally, mixtures of size and / or specific mass are studied. The regimes are characterized with differential pressure measurements and visualizations. The results are compared to empirical models based on dimensionless numbers, together with 1D models that are based on mass and momentum balance. A model for the transport of large particles in vertical pipes is also proposed and tested on data available in the Literature, in order to compare the trends that are observed in the present experiments in a horizontal pipe to the trends predicted for a vertical pipe. The results show that the grain size and specific mass have a strong effect on the transition point between regimes with a stationary bed and dispersed flows. The pressure drops are moreover smaller for large particles in the horizontal part contrary to what occurs for vertical pipes, and to the predictions of the empirical correlations.Experimental study of the instationary flow between two ducted Counter-rotating rotors
http://hdl.handle.net/10985/8536
Experimental study of the instationary flow between two ducted Counter-rotating rotors
NOURI, Hussain; DANLOS, Amélie; RAVELET, Florent; BAKIR, Farid; SARRAF, Christophe
The purpose of this work is to study experimentally the aerodynamic characteristics of a subsonic counter-rotating axial-flow fans system operating in a ducted configuration. The fans of diameter D = 375 mm were designed to match the specification point using an original iterative method: the front rotor blade cascade is designed with a conventional inverse method, setting the radial distribution of the Euler work. The through-flow is then computed using an axisymmetric and radial equilibrium asumption, with empirical models of losses. The rear rotor is not conventional but is designed to straighten the radial profile of the tangential velocity. The design of the front rotor is then modified until the stage meets the requirements. The experimental setup is arranged such that the rotation rate of each fan is independently controlled and that the axial distance between the rotors can be varied from 17% to 310% of the mid-span chord length. Systematic measurements of the global performances and local measurements of the velocity field and of the wall pressure fluctuations are performed, in order to first validate the design method, and to explore the effects of the two specific free parameters of the system: the axial spacing and the ratio of rotation rates. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The developed design method slightly over-predicts the pressure rise and slightly under-predicts the best ratio of rotation rates. Flow angle measurements downstream of the stage show that the outflow is not completely straightened at the design point. Finally, the system is highly efficient on a wide range of flow-rates and pressure rises: this system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/85362013-01-01T00:00:00ZNOURI, HussainDANLOS, AmélieRAVELET, FlorentBAKIR, FaridSARRAF, ChristopheThe purpose of this work is to study experimentally the aerodynamic characteristics of a subsonic counter-rotating axial-flow fans system operating in a ducted configuration. The fans of diameter D = 375 mm were designed to match the specification point using an original iterative method: the front rotor blade cascade is designed with a conventional inverse method, setting the radial distribution of the Euler work. The through-flow is then computed using an axisymmetric and radial equilibrium asumption, with empirical models of losses. The rear rotor is not conventional but is designed to straighten the radial profile of the tangential velocity. The design of the front rotor is then modified until the stage meets the requirements. The experimental setup is arranged such that the rotation rate of each fan is independently controlled and that the axial distance between the rotors can be varied from 17% to 310% of the mid-span chord length. Systematic measurements of the global performances and local measurements of the velocity field and of the wall pressure fluctuations are performed, in order to first validate the design method, and to explore the effects of the two specific free parameters of the system: the axial spacing and the ratio of rotation rates. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The developed design method slightly over-predicts the pressure rise and slightly under-predicts the best ratio of rotation rates. Flow angle measurements downstream of the stage show that the outflow is not completely straightened at the design point. Finally, the system is highly efficient on a wide range of flow-rates and pressure rises: this system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space.Study of the cavitating instability on a grooved Venturi profile
http://hdl.handle.net/10985/8535
Study of the cavitating instability on a grooved Venturi profile
DANLOS, Amélie; MÉHAL, Jean-Elie; RAVELET, Florent; COUTIER-DELGOSHA, Olivier; BAKIR, Farid
Instabilities of a partial cavity developed on a hydrofoil, a converging-diverging step, or in an interblade channel have already been investigated in many previous works. The aim of this study is to evaluate a passive control method of the sheet cavity. According to operating conditions, cavitation can be described by two different regimes: an unstable regime with a cloud cavitation shedding and a stable regime with only a pulsating sheet cavity. Avoiding cloud cavitation can limit structure damage since this regime is less aggressive. The surface condition of a converging-diverging step is here studied as a solution to control the cavitation regime. This study discusses the effect of longitudinal grooves on the developed sheet cavity. Analyzes conducted with laser Doppler velocimetry, visualizations, and pressure measurements show that the grooves geometry, and especially the groove depth act on the sheet cavity dynamics and can even suppress the cloud cavitation shedding.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/85352014-01-01T00:00:00ZDANLOS, AmélieMÉHAL, Jean-ElieRAVELET, FlorentCOUTIER-DELGOSHA, OlivierBAKIR, FaridInstabilities of a partial cavity developed on a hydrofoil, a converging-diverging step, or in an interblade channel have already been investigated in many previous works. The aim of this study is to evaluate a passive control method of the sheet cavity. According to operating conditions, cavitation can be described by two different regimes: an unstable regime with a cloud cavitation shedding and a stable regime with only a pulsating sheet cavity. Avoiding cloud cavitation can limit structure damage since this regime is less aggressive. The surface condition of a converging-diverging step is here studied as a solution to control the cavitation regime. This study discusses the effect of longitudinal grooves on the developed sheet cavity. Analyzes conducted with laser Doppler velocimetry, visualizations, and pressure measurements show that the grooves geometry, and especially the groove depth act on the sheet cavity dynamics and can even suppress the cloud cavitation shedding.Influence of design parameters on the global performances of low-speed counter-rotating axial-flow fans
http://hdl.handle.net/10985/8579
Influence of design parameters on the global performances of low-speed counter-rotating axial-flow fans
WANG, Juan; RAVELET, Florent; BAKIR, Farid
The present work aims at experimentally investigating the effects of some parameters on the performances of a counter-rotating stage, and on the instationary flow between the rotors. Three counter-rotating fans, which have the same design point, have been designed. These systems differ by the distribution of the loading and of the ratio of angular velocity between the front rotor and the rear rotor. All the configurations have been tested in a normalized test rig, where the ratio of angular velocities and the axial distance between the two rotors can be varied. The influence of these parameters are then addressed by analysing the experimental results of the static pressure rise and static efficiency, as well as of the wall pressure fluctuations registered by a microphone at the wall. The three systems achieve the design point with a high efficiency. The counter-rotating systems lead to at least a 10 percentage points gain in static efficiency at the design flow rate, compared to the typical peak efficiency of a traditional rotor-stator stage. Meanwhile, counter-rotating systems display good working stabilities at very low volume flow rates. In addition, at the design speed ratio, the overall performance decreases almost monotonically with the axial distance. Nevertheless, an optimum in axial distance can be found for higher speed ratios. Finally, the investigations of the wall pressure fluctuations show that the amplitudes of power spectral density corresponding to the blade passing frequency of the rear rotor are significantly higher than that of the front rotor. The interaction peaks are also stronger for an equal distribution of the work on the two rotors.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/85792014-01-01T00:00:00ZWANG, JuanRAVELET, FlorentBAKIR, FaridThe present work aims at experimentally investigating the effects of some parameters on the performances of a counter-rotating stage, and on the instationary flow between the rotors. Three counter-rotating fans, which have the same design point, have been designed. These systems differ by the distribution of the loading and of the ratio of angular velocity between the front rotor and the rear rotor. All the configurations have been tested in a normalized test rig, where the ratio of angular velocities and the axial distance between the two rotors can be varied. The influence of these parameters are then addressed by analysing the experimental results of the static pressure rise and static efficiency, as well as of the wall pressure fluctuations registered by a microphone at the wall. The three systems achieve the design point with a high efficiency. The counter-rotating systems lead to at least a 10 percentage points gain in static efficiency at the design flow rate, compared to the typical peak efficiency of a traditional rotor-stator stage. Meanwhile, counter-rotating systems display good working stabilities at very low volume flow rates. In addition, at the design speed ratio, the overall performance decreases almost monotonically with the axial distance. Nevertheless, an optimum in axial distance can be found for higher speed ratios. Finally, the investigations of the wall pressure fluctuations show that the amplitudes of power spectral density corresponding to the blade passing frequency of the rear rotor are significantly higher than that of the front rotor. The interaction peaks are also stronger for an equal distribution of the work on the two rotors.