The Experts below are selected from a list of 2181 Experts worldwide ranked by ideXlab platform
Thierry Poinsot - One of the best experts on this subject based on the ideXlab platform.
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large eddy simulation of mean and oscillating flow in a side dump ramjet combustor
Combustion and Flame, 2008Co-Authors: A Roux, Laurent Gicquel, Y Sommerer, Thierry PoinsotAbstract:Ramjet flows are very sensitive to combustion instabilities that are difficult to predict using numerical simulations. This paper describes compressible large eddy simulation on unstructured grids used to investigate nonreacting and reacting flows in a simplified twin-inlet ramjet combustor. The reacting flow is compared to experimental results published by ONERA in terms of mean fields. Simulations show a specific flow topology controlled by the impingement of the two air jets issuing from the twin air inlets and by multiple complex recirculation zones. In a second part, all unsteady modes appearing in the reacting LES are analyzed using spectral maps and POD (proper orthogonal decomposition) tools. A Helmholtz solver also computes the frequencies and structures of all acoustic modes in the ramjet. Pure longitudinal, transverse and combined modes are identified by all three diagnostics. In addition, a mode-by-mode analysis of the Rayleigh Criterion is presented thanks to POD. This method shows that the most intense structure (at 3750 Hz) is the first transverse acoustic mode of the combustor chamber and the Rayleigh Criterion obtained with POD illustrates how this transverse mode couples with unsteady combustion.
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Large-eddy simulation and acoustic analysis of a swirled staged turbulent combustor
AIAA Journal, 2006Co-Authors: Charles Martin, Franck Nicoud, Laurent Benoit, Yannick Sommerer, Thierry PoinsotAbstract:The analysis of self-excited combustion instabilies encountered in a laboratory-scale, swirl-stabilized combustion system is presented. The instability is successfully captured by reactive large-eddy simulation (LES) and analyzed by using a global acoustic energy equation. This energy equation shows how the source term due to combustion (equivalent to the Rayleigh Criterion) is balanced by the acoustic fluxes at the boundaries when reaching the limit cycle. Additionally, an Helmholtz-equation solver including flame-acoustics interaction modeling is used to predict the stability characteristics of the system. Feeding the flame-transfer function from the LES into this solver allows to predict an amplification rate for each mode. The unstable mode encountered in the LES compares well with the mode of the highest amplification factor in the Helmholtz-equation solver, in terms of mode shape as well as in frequency.
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Thermoacoustic instabilities : Should the Rayleigh Criterion be extended to include entropy changes?
Combustion and Flame, 2005Co-Authors: Franck Nicoud, Thierry PoinsotAbstract:The Rayleigh Criterion (which measures the correlation between pressure and heat release) is the standard tool used to investigate and predict combustion instabilities in both experimental and numerical studies. However, the Rayleigh term is just one of the terms appearing in the acoustic energy equation. The recent development of large eddy simulations for combustion chambers allows complete closure of the budget and analysis of all terms in this equation. This task leads to unexpected difficulties and requires some basic work, as multiple definitions of the energy of fluctuations in a reacting compressible flow can be derived. The objective of this article is to revisit the theoretical derivations of the fluctuation energy equations. Two forms of energy are defined: The first is the classic acoustic energy (AE) introduced by various authors. The second is the fluctuation energy (FE) presented by B.T. Chu [Acta Mech. (1965) 215–234]. Both equations are rederived in a compact manner starting from full nonlinear forms. It is shown that the classic Rayleigh Criterion naturally appears as the source term of the AE equation, while the FE form leads to a different Criterion stating that temperature and heat release must be in phase for the instability to be fed by the flame/acoustics coupling. The FE form also integrates the fluctuations of three variables (pressure, velocity, entropy), while the AE form uses only pressure and velocity perturbations. It is shown that only the FE form should be used in flames, in contradiction to many current studies performed for combustion instabilities.
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Thermoacoustic instabilities: Should the Rayleigh Criterion be extended to include entropy changes?
Combustion and Flame, 2005Co-Authors: Franck Nicoud, Thierry PoinsotAbstract:International audienceThe Rayleigh Criterion (which measures the correlation between pressure and heat release) is the standard tool used to investigate and predict combustion instabilities in both experimental and numerical studies. However, the Rayleigh term is just one of the terms appearing in the acoustic energy equation. The recent development of large eddy simulations for combustion chambers allows complete closure of the budget and analysis of all terms in this equation. This task leads to unexpected difficulties and requires some basic work, as multiple definitions of the energy of fluctuations in a reacting compressible flow can be derived. The objective of this article is to revisit the theoretical derivations of the fluctuation energy equations. Two forms of energy are defined: The first is the classic acoustic energy (AE) introduced by various authors. The second is the fluctuation energy (FE) presented by B.T. Chu [Acta Mech. (1965) 215-234]. Both equations are rederived in a compact manner starting from full nonlinear forms. It is shown that the classic Rayleigh Criterion naturally appears as the source term of the AE equation, while the FE form leads to a different Criterion stating that temperature and heat release must be in phase for the instability to be fed by the flame/acoustics coupling. The FE form also integrates the fluctuations of three variables (pressure, velocity, entropy), while the AE form uses only pressure and velocity perturbations. It is shown that only the FE form should be used in flames, in contradiction to many current studies performed for combustion instabilities
Carlos Negreira - One of the best experts on this subject based on the ideXlab platform.
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Time reversal of elastic waves in soft solids.
Physical review letters, 2008Co-Authors: Stefan Catheline, Nicolas Benech, Javier Brum, Carlos NegreiraAbstract:When a scalar far-field wave is time reversed, it starts to converge toward its initial point source location, then collapses and finally diverges. Without evanescent waves, the symmetric focus spot is limited by the Rayleigh Criterion. We present an experimental observation of a time-reversal elastic wave in a soft solid cavity using the transient elastography technique. It is observed that the time-reversed far field wave collapses and gives birth to near fieldlike effects. Elastodynamic Green's functions computation confirms the experimental conclusions: the diffraction limit implies a direction dependant Rayleigh Criterion.
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Time Reversed Elastic Waves within Soft Solids
The Journal of the Acoustical Society of America, 2008Co-Authors: Stefan Catheline, Nicolas Benech, Carlos Negreira, Javier BrumAbstract:When a scalar far‐field wave is time reversed from the surface of a cavity, it starts to converge toward its initial point source location, then collapses and finally diverges. Without near field evanescent waves, the symmetric focus spot is limited by the Rayleigh Criterion. We present an experimental observation of a time‐reversal vectorial wave in the volume of a soft solid. The elastic field is measured using the transient elastography technique. It is observed that the time reversed far field wave collapses and gives birth to near field effects. Numerical computation based on elastodynamic Green's functions in a time‐reversal cavity confirms and completes the experimental conclusions: the time symmetry with respect to collapse time is broken and the Rayleigh Criterion is direction dependant; the spatial collapse is larger in the direction of the point body force than in the perpendicular direction. The authors anticipate that the method can be used for shear wave beam forming in soft tissues as well ...
Stefan Catheline - One of the best experts on this subject based on the ideXlab platform.
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Time reversal of elastic waves in soft solids.
Physical review letters, 2008Co-Authors: Stefan Catheline, Nicolas Benech, Javier Brum, Carlos NegreiraAbstract:When a scalar far-field wave is time reversed, it starts to converge toward its initial point source location, then collapses and finally diverges. Without evanescent waves, the symmetric focus spot is limited by the Rayleigh Criterion. We present an experimental observation of a time-reversal elastic wave in a soft solid cavity using the transient elastography technique. It is observed that the time-reversed far field wave collapses and gives birth to near fieldlike effects. Elastodynamic Green's functions computation confirms the experimental conclusions: the diffraction limit implies a direction dependant Rayleigh Criterion.
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Time Reversed Elastic Waves within Soft Solids
The Journal of the Acoustical Society of America, 2008Co-Authors: Stefan Catheline, Nicolas Benech, Carlos Negreira, Javier BrumAbstract:When a scalar far‐field wave is time reversed from the surface of a cavity, it starts to converge toward its initial point source location, then collapses and finally diverges. Without near field evanescent waves, the symmetric focus spot is limited by the Rayleigh Criterion. We present an experimental observation of a time‐reversal vectorial wave in the volume of a soft solid. The elastic field is measured using the transient elastography technique. It is observed that the time reversed far field wave collapses and gives birth to near field effects. Numerical computation based on elastodynamic Green's functions in a time‐reversal cavity confirms and completes the experimental conclusions: the time symmetry with respect to collapse time is broken and the Rayleigh Criterion is direction dependant; the spatial collapse is larger in the direction of the point body force than in the perpendicular direction. The authors anticipate that the method can be used for shear wave beam forming in soft tissues as well ...
Francois Gallaire - One of the best experts on this subject based on the ideXlab platform.
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Generalized Rayleigh Criterion for non-axisymmetric centrifugal instabilities
Journal of Fluid Mechanics, 2005Co-Authors: Paul Billant, Francois GallaireAbstract:The well-known Rayleigh Criterion is a necessary and sufficient condition for inviscid centrifugal instability of axisymmetric perturbations. We have generalized this Criterion to disturbances of any azimuthal wavenumber m by means of large-axial-wavenumber WKB asymptotics. A sufficient condition for a free axisymmetric vortex with angular velocity $\Omega(r)$ to be unstable to a three-dimensional perturbation of azimuthal wavenumber m is that the real part of the growth rate \[\sigma (r) =-{\rm i}m\Omega(r)+\sqrt{-\phi(r)}\] is positive at the complex radius $r{=}r_0$ where $\partial \sigma (r)/\partial r{=}0$ , i.e. \[\phi'(r_0) =-2{\rm i}m\Omega'(r_0)\sqrt{-\phi(r_0)},\] where $\phi{=}(1/r^3)\partial{r^4\Omega^2}/\partial {r}$ is the Rayleigh discriminant, provided that some a posteriori checks are satisfied. The application of this new Criterion to various classes of vortex profiles shows that the growth rate of non-axisymmetric disturbances decreases as m increases until a cutoff is reached. The Criterion is in excellent agreement with numerical stability analyses of the Carton & McWilliams (1989) vortices and allows one to analyse the competition between the centrifugal instability and the shear instability. The generalized Criterion is also valid for a vertical vortex in a stably stratified and rotating fluid, except that ϕ becomes $\phi{=}(1/r^3)\partial{r^4(\Omega+\Omega_b)^2/\partial r$ , where $\Omega_b$ is the background rotation about the vertical axis. The stratification is found to have no effect. For the Taylor–Couette flow between two coaxial cylinders, the same Criterion applies except that $r_0$ is real and equal to the inner cylinder radius. In sharp contrast, the maximum growth rate of non-axisymmetric disturbances is then independent of m .
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Generalized Rayleigh Criterion for non-axisymmetric centrifugal instabilities
Journal of Fluid Mechanics, 2005Co-Authors: Paul Billant, Francois GallaireAbstract:The well-known Rayleigh Criterion is a necessary and sufficient condition for inviscid centrifugal instability of axisymmetric perturbations. We have generalized this Criterion to disturbances of any azimuthal wavenumber m by means of large-axial-wavenumber WKB asymptotics. A sufficient condition for a free axisymmetric vortex with angular velocity Ω(r) to be unstable to a three-dimensional perturbation of azimuthal wavenumber m is that the real part of the growth rate...is positive at the complex radius r=r0 where ∂σ(r)/∂r=0, i.e. where ϕ=(1/r3)∂r4Ω2/∂r is the Rayleigh discriminant, provided that some a posteriori checks are satisfied. The application of this new Criterion to various classes of vortex profiles shows that the growth rate of non-axisymmetric disturbances decreases as m increases until a cutoff is reached. The Criterion is in excellent agreement with numerical stability analyses of the Carton & McWilliams (1989) vortices and allows one to analyse the competition between the centrifugal instability and the shear instability. The generalized Criterion is also valid for a vertical vortex in a stably stratified and rotating fluid, except that φ becomes \phi{=}(1/r^3)\partial{r^4(\Omega+\Omega_b)^2/\partial r, where Ωb is the background rotation about the vertical axis. The stratification is found to have no effect. For the Taylor-Couette flow between two coaxial cylinders, the same Criterion applies except that r0 is real and equal to the inner cylinder radius. In sharp contrast, the maximum growth rate of non-axisymmetric disturbances is then independent of m.
Javier Brum - One of the best experts on this subject based on the ideXlab platform.
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Time reversal of elastic waves in soft solids.
Physical review letters, 2008Co-Authors: Stefan Catheline, Nicolas Benech, Javier Brum, Carlos NegreiraAbstract:When a scalar far-field wave is time reversed, it starts to converge toward its initial point source location, then collapses and finally diverges. Without evanescent waves, the symmetric focus spot is limited by the Rayleigh Criterion. We present an experimental observation of a time-reversal elastic wave in a soft solid cavity using the transient elastography technique. It is observed that the time-reversed far field wave collapses and gives birth to near fieldlike effects. Elastodynamic Green's functions computation confirms the experimental conclusions: the diffraction limit implies a direction dependant Rayleigh Criterion.
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Time Reversed Elastic Waves within Soft Solids
The Journal of the Acoustical Society of America, 2008Co-Authors: Stefan Catheline, Nicolas Benech, Carlos Negreira, Javier BrumAbstract:When a scalar far‐field wave is time reversed from the surface of a cavity, it starts to converge toward its initial point source location, then collapses and finally diverges. Without near field evanescent waves, the symmetric focus spot is limited by the Rayleigh Criterion. We present an experimental observation of a time‐reversal vectorial wave in the volume of a soft solid. The elastic field is measured using the transient elastography technique. It is observed that the time reversed far field wave collapses and gives birth to near field effects. Numerical computation based on elastodynamic Green's functions in a time‐reversal cavity confirms and completes the experimental conclusions: the time symmetry with respect to collapse time is broken and the Rayleigh Criterion is direction dependant; the spatial collapse is larger in the direction of the point body force than in the perpendicular direction. The authors anticipate that the method can be used for shear wave beam forming in soft tissues as well ...
