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The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 06 Dec 2023 08:07:20 GMT2023-12-06T08:07:20ZDense-gas effects on compressible boundary-layer stability
http://hdl.handle.net/10985/18556
Dense-gas effects on compressible boundary-layer stability
GLOERFELT, Xavier; ROBINET, Jean-Christophe; SCIACOVELLI, Luca; CINNELLA, Paola; GRASSO, Francesco
A study of dense-gas effects on the stability of compressible boundary-layer flows is conducted. From the laminar similarity solution, the temperature variations are small due to the high specific heat of dense gases, leading to velocity profiles close to the incompressible ones. Concurrently, the complex thermodynamic properties of dense gases can lead to unconventional compressibility effects. In the subsonic regime, the Tollmien–Schlichting viscous mode is attenuated by compressibility effects and becomes preferentially skewed in line with the results based on the ideal-gas assumption. However, the absence of a generalized inflection point precludes the sustainability of the first mode by inviscid mechanisms. On the contrary, the viscous mode can be completely stable at supersonic speeds. At very high speeds, we have found instances of radiating supersonic instabilities with substantial amplification rates, i.e. waves that travel supersonically relative to the free-stream velocity. This acoustic mode has qualitatively similar features for various thermodynamic conditions and for different working fluids. This shows that the leading parameters governing the boundary-layer behaviour for the dense gas are the constant-pressure specific heat and, to a minor extent, the density-dependent viscosity. A satisfactory scaling of the mode characteristics is found to be proportional to the height of the layer near the wall that acts as a waveguide where acoustic waves may become trapped. This means that the supersonic mode has the same nature as Mack’s modes, even if its frequency for maximal amplification is greater. Direct numerical simulation accurately reproduces the development of the supersonic mode and emphasizes the radiation of the instability waves.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10985/185562020-01-01T00:00:00ZGLOERFELT, XavierROBINET, Jean-ChristopheSCIACOVELLI, LucaCINNELLA, PaolaGRASSO, FrancescoA study of dense-gas effects on the stability of compressible boundary-layer flows is conducted. From the laminar similarity solution, the temperature variations are small due to the high specific heat of dense gases, leading to velocity profiles close to the incompressible ones. Concurrently, the complex thermodynamic properties of dense gases can lead to unconventional compressibility effects. In the subsonic regime, the Tollmien–Schlichting viscous mode is attenuated by compressibility effects and becomes preferentially skewed in line with the results based on the ideal-gas assumption. However, the absence of a generalized inflection point precludes the sustainability of the first mode by inviscid mechanisms. On the contrary, the viscous mode can be completely stable at supersonic speeds. At very high speeds, we have found instances of radiating supersonic instabilities with substantial amplification rates, i.e. waves that travel supersonically relative to the free-stream velocity. This acoustic mode has qualitatively similar features for various thermodynamic conditions and for different working fluids. This shows that the leading parameters governing the boundary-layer behaviour for the dense gas are the constant-pressure specific heat and, to a minor extent, the density-dependent viscosity. A satisfactory scaling of the mode characteristics is found to be proportional to the height of the layer near the wall that acts as a waveguide where acoustic waves may become trapped. This means that the supersonic mode has the same nature as Mack’s modes, even if its frequency for maximal amplification is greater. Direct numerical simulation accurately reproduces the development of the supersonic mode and emphasizes the radiation of the instability waves.Global Stability Analyses Unraveling Roughness-induced Transition Mechanisms
http://hdl.handle.net/10985/17848
Global Stability Analyses Unraveling Roughness-induced Transition Mechanisms
LOISEAU, Jean-Christophe; ROBINET, Jean-Christophe; CHERUBINI, Stefania; LERICHE, Emmanuel
The linear global instability and resulting transition to turbulence induced by a cylindrical roughness element of heighth and diameter d=3h immersed within an incompressible boundary layer flow along a flat plate is investigated using the joint application of direct numerical simulations and three-dimensional stability analyses. The configuration investigated is the same as the one investigated experimentally by Fransson et al. Base flow computations show that the roughness element induces a wake composed of a central low-speed region surrounded by a three-dimensional shear layer and a pair of low- and high-speed streaks on each side. Results from the global stability analyses highlight the unstable nature of the central low-speed region and its crucial importance in the laminar-turbulent transition process. For the set of parameters considered, it is able to sustain a varicose global instability for which the predicted critical Reynolds number is only 6% larger than the one reported in Ref. 10. A kinetic energy budget and wavemaker analysis revealed that this mode finds its root in the reversed flow region right downstream the roughness element and extracts most of its energy from the central low-speed region and streaks further downstream. Direct numerical simulations of the flow past this roughness element puts in the limelight the ability for this linear instability to give birth to hairpin vortices and thus trigger transition to turbulence.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10985/178482015-01-01T00:00:00ZLOISEAU, Jean-ChristopheROBINET, Jean-ChristopheCHERUBINI, StefaniaLERICHE, EmmanuelThe linear global instability and resulting transition to turbulence induced by a cylindrical roughness element of heighth and diameter d=3h immersed within an incompressible boundary layer flow along a flat plate is investigated using the joint application of direct numerical simulations and three-dimensional stability analyses. The configuration investigated is the same as the one investigated experimentally by Fransson et al. Base flow computations show that the roughness element induces a wake composed of a central low-speed region surrounded by a three-dimensional shear layer and a pair of low- and high-speed streaks on each side. Results from the global stability analyses highlight the unstable nature of the central low-speed region and its crucial importance in the laminar-turbulent transition process. For the set of parameters considered, it is able to sustain a varicose global instability for which the predicted critical Reynolds number is only 6% larger than the one reported in Ref. 10. A kinetic energy budget and wavemaker analysis revealed that this mode finds its root in the reversed flow region right downstream the roughness element and extracts most of its energy from the central low-speed region and streaks further downstream. Direct numerical simulations of the flow past this roughness element puts in the limelight the ability for this linear instability to give birth to hairpin vortices and thus trigger transition to turbulence.Influence of the Shape on the Roughness-Induced Transition
http://hdl.handle.net/10985/17803
Influence of the Shape on the Roughness-Induced Transition
LOISEAU, Jean-Christophe; CHERUBINI, Stefania; ROBINET, Jean-Christophe; LERICHE, Emmanuel
lobal instability analysis of the three-dimensional flow past two rough- ness elements of different shape, namely a cylinder and a bump, is presented. In both cases, the eigenspectrum is made of modes characterised by a varicose symmetry and localised mostly in the zones of large base flow shear. The primary instabil- ity exhibited is the same in both cases and consists in an isolated unstable mode closely related to streaks local instability. For the cylinder however, a whole branch of modes is in addition destabilised as the Reynolds number is further increased.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10985/178032015-01-01T00:00:00ZLOISEAU, Jean-ChristopheCHERUBINI, StefaniaROBINET, Jean-ChristopheLERICHE, Emmanuellobal instability analysis of the three-dimensional flow past two rough- ness elements of different shape, namely a cylinder and a bump, is presented. In both cases, the eigenspectrum is made of modes characterised by a varicose symmetry and localised mostly in the zones of large base flow shear. The primary instabil- ity exhibited is the same in both cases and consists in an isolated unstable mode closely related to streaks local instability. For the cylinder however, a whole branch of modes is in addition destabilised as the Reynolds number is further increased.Optimal perturbation for two-dimensional vortex systems: route to non-axisymmetric state
http://hdl.handle.net/10985/14216
Optimal perturbation for two-dimensional vortex systems: route to non-axisymmetric state
NAVROSE; JOHNSON, Author H. G.; BRION, Véronique; JACQUIN, Laurent; ROBINET, Jean-Christophe
We investigate perturbations that maximize the gain of disturbance energy in a two-dimensional isolated vortex and a counter-rotating vortex pair. The optimization is carried out using the method of Lagrange multipliers. For low initial energy of the perturbation ( ), the nonlinear optimal perturbation/gain is found to be the same as the linear optimal perturbation/gain. Beyond a certain threshold , the optimal perturbation/gain obtained from linear and nonlinear computations are different. There exists a range of for which the nonlinear optimal gain is higher than the linear optimal gain. For an isolated vortex, the higher value of nonlinear optimal gain is attributed to interaction among different azimuthal components, which is otherwise absent in a linearized system. Spiral dislocations are found in the nonlinear optimal perturbation at the radial location where the most dominant wavenumber changes. Long-time nonlinear evolution of linear and nonlinear optimal perturbations is studied. The evolution shows that, after the initial increment of perturbation energy, the vortex attains a quasi-steady state where the mean perturbation energy decreases on a slow time scale. The quasi-steady vortex state is non-axisymmetric and its shape depends on the initial perturbation. It is observed that the lifetime of a quasi-steady vortex state obtained using the nonlinear optimal perturbation is longer than that obtained using the linear optimal perturbation. For a counter-rotating vortex pair, the mechanism that maximizes the energy gain is found to be similar to that of the isolated vortex. Within the linear framework, the optimal perturbation for a vortex pair can be either symmetric or antisymmetric, whereas the structure of the nonlinear optimal perturbation, beyond the threshold, is always asymmetric. No quasi-steady state for a counter-rotating vortex pair is observed.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10985/142162018-01-01T00:00:00ZNAVROSEJOHNSON, Author H. G.BRION, VéroniqueJACQUIN, LaurentROBINET, Jean-ChristopheWe investigate perturbations that maximize the gain of disturbance energy in a two-dimensional isolated vortex and a counter-rotating vortex pair. The optimization is carried out using the method of Lagrange multipliers. For low initial energy of the perturbation ( ), the nonlinear optimal perturbation/gain is found to be the same as the linear optimal perturbation/gain. Beyond a certain threshold , the optimal perturbation/gain obtained from linear and nonlinear computations are different. There exists a range of for which the nonlinear optimal gain is higher than the linear optimal gain. For an isolated vortex, the higher value of nonlinear optimal gain is attributed to interaction among different azimuthal components, which is otherwise absent in a linearized system. Spiral dislocations are found in the nonlinear optimal perturbation at the radial location where the most dominant wavenumber changes. Long-time nonlinear evolution of linear and nonlinear optimal perturbations is studied. The evolution shows that, after the initial increment of perturbation energy, the vortex attains a quasi-steady state where the mean perturbation energy decreases on a slow time scale. The quasi-steady vortex state is non-axisymmetric and its shape depends on the initial perturbation. It is observed that the lifetime of a quasi-steady vortex state obtained using the nonlinear optimal perturbation is longer than that obtained using the linear optimal perturbation. For a counter-rotating vortex pair, the mechanism that maximizes the energy gain is found to be similar to that of the isolated vortex. Within the linear framework, the optimal perturbation for a vortex pair can be either symmetric or antisymmetric, whereas the structure of the nonlinear optimal perturbation, beyond the threshold, is always asymmetric. No quasi-steady state for a counter-rotating vortex pair is observed.Optimal perturbation for two-dimensional vortex systems: route to non-axisymmetric state
http://hdl.handle.net/10985/17877
Optimal perturbation for two-dimensional vortex systems: route to non-axisymmetric state
NAVROSE; JOHNSON, Author H. G.; BRION, Véronique; JACQUIN, Laurent; ROBINET, Jean-Christophe
We investigate perturbations that maximize the gain of disturbance energy in a two-dimensional isolated vortex and a counter-rotating vortex pair. The optimization is carried out using the method of Lagrange multipliers. For low initial energy of the perturbation (E.0/), the nonlinear optimal perturbation/gain is found to be the same as the linear optimal perturbation/gain. Beyond a certain threshold E.0/, the optimal perturbation/gain obtained from linear and nonlinear computations are different. There exists a range of E.0/ for which the nonlinear optimal gain is higher than the linear optimal gain. For an isolated vortex, the higher value of nonlinear optimal gain is attributed to interaction among different azimuthal components, which is otherwise absent in a linearized system. Spiral dislocations are found in the nonlinear optimal perturbation at the radial location where the most dominant wavenumber changes. Long-time nonlinear evolution of linear and nonlinear optimal perturbations is studied. The evolution shows that, after the initial increment of perturbation energy, the vortex attains a quasi-steady state where the mean perturbation energy decreases on a slow time scale. The quasi-steady vortex state is non-axisymmetric and its shape depends on the initial perturbation. It is observed that the lifetime of a quasi-steady vortex state obtained using the nonlinear optimal perturbation is longer than that obtained using the linear optimal perturbation. For a counter-rotating vortex pair, the mechanism that maximizes the energy gain is found to be similar to that of the isolated vortex. Within the linear framework, the optimal perturbation for a vortex pair can be either symmetric or antisymmetric, whereas the structure of the nonlinear optimal perturbation, beyond the threshold E.0/, is always asymmetric. No quasi-steady state for a counter-rotating vortex pair is observed.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10985/178772018-01-01T00:00:00ZNAVROSEJOHNSON, Author H. G.BRION, VéroniqueJACQUIN, LaurentROBINET, Jean-ChristopheWe investigate perturbations that maximize the gain of disturbance energy in a two-dimensional isolated vortex and a counter-rotating vortex pair. The optimization is carried out using the method of Lagrange multipliers. For low initial energy of the perturbation (E.0/), the nonlinear optimal perturbation/gain is found to be the same as the linear optimal perturbation/gain. Beyond a certain threshold E.0/, the optimal perturbation/gain obtained from linear and nonlinear computations are different. There exists a range of E.0/ for which the nonlinear optimal gain is higher than the linear optimal gain. For an isolated vortex, the higher value of nonlinear optimal gain is attributed to interaction among different azimuthal components, which is otherwise absent in a linearized system. Spiral dislocations are found in the nonlinear optimal perturbation at the radial location where the most dominant wavenumber changes. Long-time nonlinear evolution of linear and nonlinear optimal perturbations is studied. The evolution shows that, after the initial increment of perturbation energy, the vortex attains a quasi-steady state where the mean perturbation energy decreases on a slow time scale. The quasi-steady vortex state is non-axisymmetric and its shape depends on the initial perturbation. It is observed that the lifetime of a quasi-steady vortex state obtained using the nonlinear optimal perturbation is longer than that obtained using the linear optimal perturbation. For a counter-rotating vortex pair, the mechanism that maximizes the energy gain is found to be similar to that of the isolated vortex. Within the linear framework, the optimal perturbation for a vortex pair can be either symmetric or antisymmetric, whereas the structure of the nonlinear optimal perturbation, beyond the threshold E.0/, is always asymmetric. No quasi-steady state for a counter-rotating vortex pair is observed.Bifurcation analysis and frequency prediction in shear-driven cavity flow
http://hdl.handle.net/10985/18039
Bifurcation analysis and frequency prediction in shear-driven cavity flow
BENGANA, Y.; LOISEAU, Jean-Christophe; ROBINET, Jean-Christophe; TUCKERMAN, L. S.
A comprehensive study of the two-dimensional incompressible shear-driven flow in an open square cavity is carried out. Two successive bifurcations lead to two limit cycles with different frequencies and different numbers of structures which propagate along the top of the cavity and circulate in its interior. A branch of quasi-periodic states produced by secondary Hopf bifurcations transfers the stability from one limit cycle to the other. A full analysis of this scenario is obtained by means of nonlinear simulations, linear stability analysis and Floquet analysis. We characterize the temporal behaviour of the limit cycles and quasi-periodic state via Fourier transforms and their spatial behaviour via the Hilbert transform. We address the relevance of linearization about the mean flow. Although here the nonlinear frequencies are not very far from those obtained by linearization about the base flow, the difference is substantially reduced when eigenvalues are obtained instead from linearization about the mean and in addition, the corresponding growth rate is small, a combination of properties called RZIF (real zero imaginary frequency). Moreover growth rates obtained by linearization about the mean of one limit cycle are correlated with relative stability to the other limit cycle. Finally, we show that the frequencies of the successive modes are separated by a constant increment.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/180392019-01-01T00:00:00ZBENGANA, Y.LOISEAU, Jean-ChristopheROBINET, Jean-ChristopheTUCKERMAN, L. S.A comprehensive study of the two-dimensional incompressible shear-driven flow in an open square cavity is carried out. Two successive bifurcations lead to two limit cycles with different frequencies and different numbers of structures which propagate along the top of the cavity and circulate in its interior. A branch of quasi-periodic states produced by secondary Hopf bifurcations transfers the stability from one limit cycle to the other. A full analysis of this scenario is obtained by means of nonlinear simulations, linear stability analysis and Floquet analysis. We characterize the temporal behaviour of the limit cycles and quasi-periodic state via Fourier transforms and their spatial behaviour via the Hilbert transform. We address the relevance of linearization about the mean flow. Although here the nonlinear frequencies are not very far from those obtained by linearization about the base flow, the difference is substantially reduced when eigenvalues are obtained instead from linearization about the mean and in addition, the corresponding growth rate is small, a combination of properties called RZIF (real zero imaginary frequency). Moreover growth rates obtained by linearization about the mean of one limit cycle are correlated with relative stability to the other limit cycle. Finally, we show that the frequencies of the successive modes are separated by a constant increment.Large scale dynamics of a high Reynolds number axisymmetric separating/reattaching flow
http://hdl.handle.net/10985/17990
Large scale dynamics of a high Reynolds number axisymmetric separating/reattaching flow
PAIN, R; WEISS, P-E; DECK, S; ROBINET, Jean-Christophe
A numerical study is conducted to unveil the large scale dynamics of a high Reynolds number axisymmetric separating/reattaching flow at M∞ = 0.7. The numerical simulation allows us to acquire a high rate sampled unsteady volumetric dataset. This huge amount of spatial and temporal information is exploited in the Fourier space to visualize for the first time in physical space and at such a high Reynolds number (ReD = 1.2 × 106) the statistical signature of the helical structure related to the antisymmetric mode (m = 1) at StD = 0.18. The main hydrodynamic mechanisms are identified through the spatial distribution of the most energetic frequencies, i.e., StD = 0.18 and StD ≥ 3.0 corresponding to the vortex-shedding and Kelvin-Helmholtz instability phenomena, respectively. In particular, the dynamics related to the dimensionless shedding frequency is shown to become dominant for 0.35 ≤ x/D ≤ 0.75 in the whole radial direction as it passes through the shear layer. The spatial distribution of the coherence function for the most significant modes as well as a three-dimensional Fourier decomposition suggests the global features of the flow mechanisms. More specifically, the novelty of this study lies in the evidence of the flow dynamics through the use of cross-correlation maps plotted with a frequency selection guided by the characteristic Strouhal number formerly identified in a local manner in the flow field or at the wall. Moreover and for the first time, the understanding of the scales at stake is supported both by a Fourier analysis and a dynamic mode decomposition in the complete three-dimensional space surrounding the afterbody zone.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/179902019-01-01T00:00:00ZPAIN, RWEISS, P-EDECK, SROBINET, Jean-ChristopheA numerical study is conducted to unveil the large scale dynamics of a high Reynolds number axisymmetric separating/reattaching flow at M∞ = 0.7. The numerical simulation allows us to acquire a high rate sampled unsteady volumetric dataset. This huge amount of spatial and temporal information is exploited in the Fourier space to visualize for the first time in physical space and at such a high Reynolds number (ReD = 1.2 × 106) the statistical signature of the helical structure related to the antisymmetric mode (m = 1) at StD = 0.18. The main hydrodynamic mechanisms are identified through the spatial distribution of the most energetic frequencies, i.e., StD = 0.18 and StD ≥ 3.0 corresponding to the vortex-shedding and Kelvin-Helmholtz instability phenomena, respectively. In particular, the dynamics related to the dimensionless shedding frequency is shown to become dominant for 0.35 ≤ x/D ≤ 0.75 in the whole radial direction as it passes through the shear layer. The spatial distribution of the coherence function for the most significant modes as well as a three-dimensional Fourier decomposition suggests the global features of the flow mechanisms. More specifically, the novelty of this study lies in the evidence of the flow dynamics through the use of cross-correlation maps plotted with a frequency selection guided by the characteristic Strouhal number formerly identified in a local manner in the flow field or at the wall. Moreover and for the first time, the understanding of the scales at stake is supported both by a Fourier analysis and a dynamic mode decomposition in the complete three-dimensional space surrounding the afterbody zone.Numerical investigation of three-dimensional partial cavitation in a Venturi geometry
http://hdl.handle.net/10985/20502
Numerical investigation of three-dimensional partial cavitation in a Venturi geometry
GOUIN, Camille; JUNQUEIRA-JUNIOR, Carlos; GONCALVES DA SILVA, Eric; ROBINET, Jean-Christophe
Sheet cavitation appears in many hydraulic applications and can lead to technical issues. Some fundamental outcomes, such as, the complex topology of 3-dimensional cavitation pockets and their associated dynamics need to be carefully visited. In the paper, the dynamics of partial cavitation developing in a 3D Venturi geometry and the interaction with sidewalls are numerically investigated. The simulations are performed using a one-fluid compressible Reynolds-averaged Navier–Stokes solver associated with a nonlinear turbulence model and a void ratio transport equation model. A detailed analysis of this cavitating flow is carried out using innovative tools, such as, spectral proper orthogonal decompositions. Particular attention is paid in the study of 3D effects by comparing the numerical results obtained with sidewalls and periodic conditions. A three-dimensional dynamics of the sheet cavitation, unrelated to the presence of sidewalls, is identified and discussed.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10985/205022021-01-01T00:00:00ZGOUIN, CamilleJUNQUEIRA-JUNIOR, CarlosGONCALVES DA SILVA, EricROBINET, Jean-ChristopheSheet cavitation appears in many hydraulic applications and can lead to technical issues. Some fundamental outcomes, such as, the complex topology of 3-dimensional cavitation pockets and their associated dynamics need to be carefully visited. In the paper, the dynamics of partial cavitation developing in a 3D Venturi geometry and the interaction with sidewalls are numerically investigated. The simulations are performed using a one-fluid compressible Reynolds-averaged Navier–Stokes solver associated with a nonlinear turbulence model and a void ratio transport equation model. A detailed analysis of this cavitating flow is carried out using innovative tools, such as, spectral proper orthogonal decompositions. Particular attention is paid in the study of 3D effects by comparing the numerical results obtained with sidewalls and periodic conditions. A three-dimensional dynamics of the sheet cavitation, unrelated to the presence of sidewalls, is identified and discussed.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; RAVELET, Florent; DANLOS, Amélie; ROBINET, Jean-Christophe; BARAST, Luc
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, KilianRAVELET, FlorentDANLOS, AmélieROBINET, Jean-ChristopheBARAST, LucAttached 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.Automatic Hybrid RANS/LES Strategy for Industrial CFD
http://hdl.handle.net/10985/15320
Automatic Hybrid RANS/LES Strategy for Industrial CFD
PONT, Grégoire; CINNELLA, Paola; ROBINET, Jean-Christophe; BRENNER, Pierre
An automatic HRL (Hybrid RANS/LES) strategy is investigated in FLUSEPA, a finite-volume solver developed by Airbus Defense and Space. A HRL turbulence model is coupled to a high-order hybrid numerical approximation method. Concerning the turbulence model, the well-known k − ε two equations RANS turbulence model is sensitized to the grid as suggested by Perot and Gadebusch (Phy Fluids 19:1–11, 2007). Concerning the numerical strategy, a third-order accurate upwind approximation method is locally re-centered in vortex dominated regions to achieve non-dissipative fourth-order accuracy. Results are presented for a 2D backward facing step and an an axisymmetry backward facing step, which represent good prototypes of after body flows.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10985/153202015-01-01T00:00:00ZPONT, GrégoireCINNELLA, PaolaROBINET, Jean-ChristopheBRENNER, PierreAn automatic HRL (Hybrid RANS/LES) strategy is investigated in FLUSEPA, a finite-volume solver developed by Airbus Defense and Space. A HRL turbulence model is coupled to a high-order hybrid numerical approximation method. Concerning the turbulence model, the well-known k − ε two equations RANS turbulence model is sensitized to the grid as suggested by Perot and Gadebusch (Phy Fluids 19:1–11, 2007). Concerning the numerical strategy, a third-order accurate upwind approximation method is locally re-centered in vortex dominated regions to achieve non-dissipative fourth-order accuracy. Results are presented for a 2D backward facing step and an an axisymmetry backward facing step, which represent good prototypes of after body flows.