SAM
http://sam.ensam.eu:80
SAM captures, stores, indexes, preserves, and distributes digital research material.Wed, 12 Sep 2018 18:10:22 GMT2018-09-12T18:10:22ZInstabilités bi-et tridimensionnelles dans une couche limite décollée compressible subsonique
http://hdl.handle.net/10985/7396
MERLE, Matthieu; EHRENSTEIN, Uwe; ROBINET, Jean-Christophe
Flow separation is a common feature in wall-bounded flow, where it is generally induced by an adverse pressure gradient. Here we reconsider a bump-type geometry which has been used in previous numerical investigations of the stability of the laminar recirculation bubble for incompressible flow. It has been shown for low Reynolds number that the first bifurcation of the 2D stationnary flow is characterized by a zero-frequency 3D instability mode. For larger Reynolds number a second bifurcation appears (Hopfbifurcation) and separated boundary-layer is then subject to a low frequency phenomenon known as’flapping’. The influence of compressibility for this type of flow is assessed.We first solve the compressible Navier-Stokes equations in order to obtain an equilibrium solution for increasing compressibility effects. Two-dimensional global stability of this solution is then investigatesand we assess the influence of Mach number on the critical Reynolds number for which the separated flow becomes unstable with respect to oscillatory perturbations.Three-dimensional transverse instabilities are addressed as well and in particular the evolution of growth rate and transverse wave length of the most unstable mode for several Mach numbers.
Mon, 26 Aug 2013 00:00:00 GMThttp://hdl.handle.net/10985/73962013-08-26T00:00:00ZMERLE, MatthieuEHRENSTEIN, UweROBINET, Jean-ChristopheFlow separation is a common feature in wall-bounded flow, where it is generally induced by an adverse pressure gradient. Here we reconsider a bump-type geometry which has been used in previous numerical investigations of the stability of the laminar recirculation bubble for incompressible flow. It has been shown for low Reynolds number that the first bifurcation of the 2D stationnary flow is characterized by a zero-frequency 3D instability mode. For larger Reynolds number a second bifurcation appears (Hopfbifurcation) and separated boundary-layer is then subject to a low frequency phenomenon known as’flapping’. The influence of compressibility for this type of flow is assessed.We first solve the compressible Navier-Stokes equations in order to obtain an equilibrium solution for increasing compressibility effects. Two-dimensional global stability of this solution is then investigatesand we assess the influence of Mach number on the critical Reynolds number for which the separated flow becomes unstable with respect to oscillatory perturbations.Three-dimensional transverse instabilities are addressed as well and in particular the evolution of growth rate and transverse wave length of the most unstable mode for several Mach numbers.Toward improved simulation tools for compressible turbomachinery: assessment of RBC schemes for the transonic NASA Rotor 37 benchmark case
http://hdl.handle.net/10985/7658
CINNELLA, Paola; MICHEL, Bruno
Residual-based-compact schemes (RBC) of 2nd and 3rd-order accuracy are applied to a challenging 3D ow through a transonic compressor. The proposed schemes provide almost mesh-converged solutions in good agreement with experimental data on relatively coarse grids, which allows computational cost reductions by a factor between 2 and 4 with respect to standard solvers for a given accuracy level.
Mon, 26 Aug 2013 00:00:00 GMThttp://hdl.handle.net/10985/76582013-08-26T00:00:00ZCINNELLA, PaolaMICHEL, BrunoResidual-based-compact schemes (RBC) of 2nd and 3rd-order accuracy are applied to a challenging 3D ow through a transonic compressor. The proposed schemes provide almost mesh-converged solutions in good agreement with experimental data on relatively coarse grids, which allows computational cost reductions by a factor between 2 and 4 with respect to standard solvers for a given accuracy level.Two-dimensional numerical simulations of nonlinear acoustic streaming in standing waves
http://hdl.handle.net/10985/8594
DARU, Virginie; BALTEAN-CARLES, Diana; WEISMAN, Catherine; DEBESSE, Philippe; GANDIKOTA, Gurunath
Wave Motion
Numerical simulations of compressible Navier–Stokes equations in closed two-dimensional channels are performed. A plane standing wave is excited inside the channel and the associated acoustic streaming is investigated for high intensity waves, in the nonlinear streaming regime. Significant distortion of streaming cells is observed, with the centers of streaming cells pushed toward the end-walls. The mean temperature evolution associated with the streaming motion is also investigated.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/85942013-01-01T00:00:00ZDARU, VirginieBALTEAN-CARLES, DianaWEISMAN, CatherineDEBESSE, PhilippeGANDIKOTA, GurunathNumerical simulations of compressible Navier–Stokes equations in closed two-dimensional channels are performed. A plane standing wave is excited inside the channel and the associated acoustic streaming is investigated for high intensity waves, in the nonlinear streaming regime. Significant distortion of streaming cells is observed, with the centers of streaming cells pushed toward the end-walls. The mean temperature evolution associated with the streaming motion is also investigated.Experimental investigation on ducted counter-rotating axial flow fans
http://hdl.handle.net/10985/8585
NOURI, Hussain; RAVELET, Florent; BAKIR, Farid; SARRAF, Christophe
An experimental study on counter-rotating axial-flow fans was carried out. The fans of diameter D = 375 mm were designed using an inverse method. The counter-rotating fans operate in a ducted-flow configuration and the overall performances are measured in a normalized test bench. The rotation rate of each fan is independently controlled. The axial spacing between the fans can vary from 10 to 50 mm by steps of 10 mm. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The effects of varying the rotation rates ratio on the overall performances are studied and show that the system is highly efficient on a wide range of flow-rates and pressure rises. However, the change of the axial distance between rotors from 10 to 50 mm does not seem to change the overall performances. This system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space. Further local studies including velocity measurements and wall-pressure fluctuations in the space between the rotors are needed to better understand the interactions between the rotors and to optimize the system.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/10985/85852011-01-01T00:00:00ZNOURI, HussainRAVELET, FlorentBAKIR, FaridSARRAF, ChristopheAn experimental study on counter-rotating axial-flow fans was carried out. The fans of diameter D = 375 mm were designed using an inverse method. The counter-rotating fans operate in a ducted-flow configuration and the overall performances are measured in a normalized test bench. The rotation rate of each fan is independently controlled. The axial spacing between the fans can vary from 10 to 50 mm by steps of 10 mm. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The effects of varying the rotation rates ratio on the overall performances are studied and show that the system is highly efficient on a wide range of flow-rates and pressure rises. However, the change of the axial distance between rotors from 10 to 50 mm does not seem to change the overall performances. This system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space. Further local studies including velocity measurements and wall-pressure fluctuations in the space between the rotors are needed to better understand the interactions between the rotors and to optimize the system.Experimental study of blade rigidity effects on the global and the local performances of a thick blades axial-flow fan
http://hdl.handle.net/10985/8584
NOURI, Hussain; RAVELET, Florent; SARRAF, Christophe; BAKIR, Farid
An experimental investigation on the aerodynamic performances of thick blades axial-flow fans was carried out in this study. Two fans are considered, the first one is rotomoulded (in plastic) and the second one is milled (in aluminium). Both have exactly the same shape, excepting that the rotomoulded fan has hollow blades. They were designed from an existing fan (manufactured by plastic injection process) used in the cooling system of an automotive vehicle power unit. As far as shape is concerned, the only difference between the two first fans and the traditional injected fan is the blade thickness, whereas as far as rigidity is concerned, the only difference between the rotomoulded and the milled fans is the ability of the rotomoulded fan to be deformed easier than the milled fan. The aim of this study is to determine on the one hand the influence of the blade thickness and on the other hand the way the deformation of the hollow blades may affect the global and the local performances. The global performances of the fans were measured in a test bench designed according to the ISO 5801 standards. The curve of the aerodynamics characteristics (pressure head versus flow rate) and of the global efficiency are slightly lower for the rotomoulded fan. The wall pressure fluctuations were also investigated for three flow rates: one corresponding to the maximum efficiencies of both fans and two others corresponding to an under-flow and an over-flow rate. The power spectral density (PSD) levels, estimated by the Welch method, are between six and nine times higher for the rotomoulded fan at nominal flow rate. At partial flow rate, however, the PSD levels are close for both fans.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10985/85842010-01-01T00:00:00ZNOURI, HussainRAVELET, FlorentSARRAF, ChristopheBAKIR, FaridAn experimental investigation on the aerodynamic performances of thick blades axial-flow fans was carried out in this study. Two fans are considered, the first one is rotomoulded (in plastic) and the second one is milled (in aluminium). Both have exactly the same shape, excepting that the rotomoulded fan has hollow blades. They were designed from an existing fan (manufactured by plastic injection process) used in the cooling system of an automotive vehicle power unit. As far as shape is concerned, the only difference between the two first fans and the traditional injected fan is the blade thickness, whereas as far as rigidity is concerned, the only difference between the rotomoulded and the milled fans is the ability of the rotomoulded fan to be deformed easier than the milled fan. The aim of this study is to determine on the one hand the influence of the blade thickness and on the other hand the way the deformation of the hollow blades may affect the global and the local performances. The global performances of the fans were measured in a test bench designed according to the ISO 5801 standards. The curve of the aerodynamics characteristics (pressure head versus flow rate) and of the global efficiency are slightly lower for the rotomoulded fan. The wall pressure fluctuations were also investigated for three flow rates: one corresponding to the maximum efficiencies of both fans and two others corresponding to an under-flow and an over-flow rate. The power spectral density (PSD) levels, estimated by the Welch method, are between six and nine times higher for the rotomoulded fan at nominal flow rate. At partial flow rate, however, the PSD levels are close for both fans.Determination of wind turbine far wake using actuator disk
http://hdl.handle.net/10985/8925
AMER, Rodeyna; DOBREV, Ivan; MASSOUH, Fawaz
The growth in size of wind turbines over the last years is significant. The rotor diameter becomes somehow comparable to atmospheric boundary layer at the land surface. In this case the assumption of uniform velocity of upcoming wind cannot be valid. The aim of this paper is to create a simplified model of wind turbine rotor which can represent the aerodynamic inter-action of atmospheric boundary layer with a horizontal axis wind turbine. Such model will be also useful for the study of optimal placement of wind turbines in a wind farm when a large number of calculations is needed and when the time required for full CFD calculations be-comes prohibitive. In this study we adopt actuator disk model which takes in account with sufficient precision the influence of blade geometry on wind turbine aerodynamic performance. The proposed actuator disk model is tested in the case of horizontal axis wind turbine using wall-modelled large eddy simulation. The obtained results of aerodynamic performance and wake show the rapidity of calculation and the reliability of proposed approach.
Thu, 19 Jun 2014 00:00:00 GMThttp://hdl.handle.net/10985/89252014-06-19T00:00:00ZAMER, RodeynaDOBREV, IvanMASSOUH, FawazThe growth in size of wind turbines over the last years is significant. The rotor diameter becomes somehow comparable to atmospheric boundary layer at the land surface. In this case the assumption of uniform velocity of upcoming wind cannot be valid. The aim of this paper is to create a simplified model of wind turbine rotor which can represent the aerodynamic inter-action of atmospheric boundary layer with a horizontal axis wind turbine. Such model will be also useful for the study of optimal placement of wind turbines in a wind farm when a large number of calculations is needed and when the time required for full CFD calculations be-comes prohibitive. In this study we adopt actuator disk model which takes in account with sufficient precision the influence of blade geometry on wind turbine aerodynamic performance. The proposed actuator disk model is tested in the case of horizontal axis wind turbine using wall-modelled large eddy simulation. The obtained results of aerodynamic performance and wake show the rapidity of calculation and the reliability of proposed approach.Numerical Study of the Effect of Freestream Turbulence on by-pass Transition in a Boundary Layer
http://hdl.handle.net/10985/9011
CHERUBINI, Stefania; ROBINET, Jean-Christophe; DE PALMA, Pietro
Energy Procedia
We use direct numerical simulations in the presence of free-stream turbulence having different values of intensity, T u, and integral length scale, L, in order to determine which kind of structures are involved in the path to transition of a boundary-layer flow. The main aim is to determine under which conditions the path to transition involves structures similar to the linear or non-linear optimal perturbations. For high values of T u and L, we observe a large-amplitude path to transition characterized by localized vortical structures and patches of high- and low-momentum fluctuations. Such a scenario is found to correlate well with the L and hairpin structures resulting from the time evolution of non-linear optimal perturbations, whereas, for lower T u and L, a larger correlation is found with respect to linear optimal disturbances. This indicates that a large-amplitude path to transition exists, different from the one characterized by elongated streaks undergoing secondary instability. To distinguish between the two transition scenarios, a simple parameter linked to the streamwise localisation of high- and low-momentum zones is introduced. Finally, an accurate law to predict the transition location is provided, taking into account both T u and L, valid for both the transition scenarios.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/90112014-01-01T00:00:00ZCHERUBINI, StefaniaROBINET, Jean-ChristopheDE PALMA, PietroWe use direct numerical simulations in the presence of free-stream turbulence having different values of intensity, T u, and integral length scale, L, in order to determine which kind of structures are involved in the path to transition of a boundary-layer flow. The main aim is to determine under which conditions the path to transition involves structures similar to the linear or non-linear optimal perturbations. For high values of T u and L, we observe a large-amplitude path to transition characterized by localized vortical structures and patches of high- and low-momentum fluctuations. Such a scenario is found to correlate well with the L and hairpin structures resulting from the time evolution of non-linear optimal perturbations, whereas, for lower T u and L, a larger correlation is found with respect to linear optimal disturbances. This indicates that a large-amplitude path to transition exists, different from the one characterized by elongated streaks undergoing secondary instability. To distinguish between the two transition scenarios, a simple parameter linked to the streamwise localisation of high- and low-momentum zones is introduced. Finally, an accurate law to predict the transition location is provided, taking into account both T u and L, valid for both the transition scenarios.Minimal perturbations approaching the edge of chaos in a Couette flow
http://hdl.handle.net/10985/8971
CHERUBINI, Stefania; DE PALMA, Pietro
Fluid Dynamics Research
This paper provides an investigation of the structure of the stable manifold of the lower branch steady state for the plane Couette flow. Minimal energy perturbations to the laminar state are computed, which approach within a prescribed tolerance the lower branch steady state in a finite time. For small times, such minimal-energy perturbations maintain at least one of the symmetries characterizing the lower branch state. For a sufficiently large time horizon, such symmetries are broken and the minimal-energy perturbations on the stable manifold are formed by localized asymmetrical vortical structures. These minimal-energy perturbations could be employed to develop a control procedure aiming at stabilizing the low-dissipation lower branch state.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/89712014-01-01T00:00:00ZCHERUBINI, StefaniaDE PALMA, PietroThis paper provides an investigation of the structure of the stable manifold of the lower branch steady state for the plane Couette flow. Minimal energy perturbations to the laminar state are computed, which approach within a prescribed tolerance the lower branch steady state in a finite time. For small times, such minimal-energy perturbations maintain at least one of the symmetries characterizing the lower branch state. For a sufficiently large time horizon, such symmetries are broken and the minimal-energy perturbations on the stable manifold are formed by localized asymmetrical vortical structures. These minimal-energy perturbations could be employed to develop a control procedure aiming at stabilizing the low-dissipation lower branch state.Airfoil Shape Optimization for Transonic Flows of Bethe–Zel’dovich–Thompson Fluids
http://hdl.handle.net/10985/6776
CONGEDO, Pietro; CORRE, Christophe; CINNELLA, Paola
AIAA Journal
High-performance airfoils for transonic flows of Bethe–Zel’dovich–Thompson fluids are constructed using a robust and efficient Euler flow solver coupled with a multi-objective genetic algorithm. Bethe–Zel’dovich– Thompson fluids are characterized by negative values of the fundamental derivative of gasdynamics for a range of temperatures and pressures in the vapor phase, which leads to nonclassical gasdynamic behaviors such as the disintegration of compression shocks. Using Bethe–Zel’dovich–Thompson gases as working fluids may result in low drag exerted on airfoils operating at high transonic speeds, due to a substantial increase in the airfoil critical Mach
number. This advantage can be further improved by a proper design of the airfoil shape, also leading to the enlargement of the airfoil operation range within which Bethe–Zel’dovich–Thompson effects are significant. Such a result is of particular interest in view of the exploitation of Bethe–Zel’dovich–Thompson fluids for the development of
high-efficiency turbomachinery.
Publication précédent le recrutement de l'auteur à l'ENSAM
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10985/67762007-01-01T00:00:00ZCONGEDO, PietroCORRE, ChristopheCINNELLA, PaolaHigh-performance airfoils for transonic flows of Bethe–Zel’dovich–Thompson fluids are constructed using a robust and efficient Euler flow solver coupled with a multi-objective genetic algorithm. Bethe–Zel’dovich– Thompson fluids are characterized by negative values of the fundamental derivative of gasdynamics for a range of temperatures and pressures in the vapor phase, which leads to nonclassical gasdynamic behaviors such as the disintegration of compression shocks. Using Bethe–Zel’dovich–Thompson gases as working fluids may result in low drag exerted on airfoils operating at high transonic speeds, due to a substantial increase in the airfoil critical Mach
number. This advantage can be further improved by a proper design of the airfoil shape, also leading to the enlargement of the airfoil operation range within which Bethe–Zel’dovich–Thompson effects are significant. Such a result is of particular interest in view of the exploitation of Bethe–Zel’dovich–Thompson fluids for the development of
high-efficiency turbomachinery.Numerical investigation of dense gas flows through transcritical multistage axial Organic Rankine Cycle turbines
http://hdl.handle.net/10985/7655
SCIACOVELLI, Lucas; CINNELLA, Paola
Many recent studies suggest that supercritical Organic Rankine Cycles have a great potential for lowtemperature heat recovery applications, since they allow better recovery efficiency for a simplified cycle architecture. In this work we investigate flows of dense gases through axial, multi-stage, supercritical ORC turbines, using a numerical code including advanced equations of state and a high-order discretization scheme. Several working fluids are considered, and performances of supercritical turbines are compared to those of subcritical ones using the same fluids.
Mon, 26 Aug 2013 00:00:00 GMThttp://hdl.handle.net/10985/76552013-08-26T00:00:00ZSCIACOVELLI, LucasCINNELLA, PaolaMany recent studies suggest that supercritical Organic Rankine Cycles have a great potential for lowtemperature heat recovery applications, since they allow better recovery efficiency for a simplified cycle architecture. In this work we investigate flows of dense gases through axial, multi-stage, supercritical ORC turbines, using a numerical code including advanced equations of state and a high-order discretization scheme. Several working fluids are considered, and performances of supercritical turbines are compared to those of subcritical ones using the same fluids.