CNRS Fluid Mechanics and Acoustics Laboratory

Lyon, France

CNRS Fluid Mechanics and Acoustics Laboratory

Lyon, France
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Venaille A.,University of Lyon | Gostiaux L.,CNRS Fluid Mechanics and Acoustics Laboratory | Sommeria J.,CNRS Laboratory of Geophysical and Industrial Flows
Journal of Fluid Mechanics | Year: 2017

Predicting how much mixing occurs when a given amount of energy is injected into a Boussinesq fluid is a long-standing problem in stratified turbulence. The huge number of degrees of freedom involved in these processes renders extremely difficult a deterministic approach to the problem. Here we present a statistical mechanics approach yielding a prediction for a cumulative, global mixing efficiency as a function of a global Richardson number and the background buoyancy profile. Assuming random evolution through turbulent stirring, the theory predicts that the inviscid, adiabatic dynamics is attracted irreversibly towards an equilibrium state characterised by a smooth, stable buoyancy profile at a coarse-grained level, upon which are fine-scale fluctuations of velocity and buoyancy. The convergence towards a coarse-grained buoyancy profile different from the initial one corresponds to an irreversible increase of potential energy, and the efficiency of mixing is quantified as the ratio of this potential energy increase to the total energy injected into the system. The remaining part of the energy is lost into small-scale fluctuations. We show that for sufficiently large Richardson number, there is equipartition between potential and kinetic energy, provided that the background buoyancy profile is strictly monotonic. This yields a mixing efficiency of 0.25, which provides statistical mechanics support for previous predictions based on phenomenological kinematics arguments. In the general case, the cumulative, global mixing efficiency predicted by the equilibrium theory can be computed using an algorithm based on a maximum entropy production principle. It is shown in particular that the variation of mixing efficiency with the Richardson number strongly depends on the background buoyancy profile. This approach could be useful to the understanding of mixing in stratified turbulence in the limit of large Reynolds and Péclet numbers. © 2016 Cambridge University Press?.

Godeferd F.S.,CNRS Fluid Mechanics and Acoustics Laboratory | Moisy F.,University Paris - Sud
Applied Mechanics Reviews | Year: 2015

Rotating turbulence is a fundamental phenomenon appearing in several geophysical and industrial applications. Its study benefited from major advances in the recent years, but also raised new questions. We review recent results for rotating turbulence, from several numerical and experimental researches, and in relation with theory and models, mostly for homogeneous flows. We observe a convergence in the statistical description of rotating turbulence from the advent of modern experimental techniques and computational power that allows to investigate the structure and dynamics of rotating flows at similar parameters and with similar description levels. The improved picture about the anisotropization mechanisms, however, reveals subtle differences in the flow conditions, including its generation and boundary conditions, which lead to separate points of view about the role of linear mechanisms-the Coriolis force and inertial waves-compared with more complex nonlinear triadic interactions. This is discussed in relation with the most recent diagnostic of dynamical equations in physical and spectral space. © 2015 by ASME.

Kadoch B.,French National Center for Scientific Research | Bos W.J.T.,CNRS Fluid Mechanics and Acoustics Laboratory | Schneider K.,French National Center for Scientific Research
Physical Review Letters | Year: 2010

The Lagrangian velocity statistics of dissipative drift-wave turbulence are investigated. For large values of the adiabaticity (or small collisionality), the probability density function of the Lagrangian acceleration shows exponential tails, as opposed to the stretched exponential or algebraic tails, generally observed for the highly intermittent acceleration of Navier-Stokes turbulence. This exponential distribution is shown to be a robust feature independent of the Reynolds number. For small adiabaticity, algebraic tails are observed, suggesting the strong influence of point-vortex-like dynamics on the acceleration. A causal connection is found between the shape of the probability density function and the autocorrelation of the norm of the acceleration. ©2010 The American Physical Society.

Rozenberg Y.,ONERA | Roger M.,École Centrale Lyon | Roger M.,CNRS Fluid Mechanics and Acoustics Laboratory | Moreau S.,Université de Sherbrooke
AIAA Journal | Year: 2010

This paper deals with the experimental validation of an analytical trailing-edge noise model dedicated to low-speed fans operating in free field. The model is intrinsically related to the aerodynamics of the blades and should lead to a useful fast-running tool to be included in a blade-design process in an industrial context. The investigations are made on a two-bladed low-speed axial fan without shroud, installed inside an anechoic room. The blades are instrumented with two sets of embedded small-size microphones (2.5 mm diam), and the wall-pressure signals are acquired via a slip ring mounted on the fan axis. The chord-based Reynolds number is about 200,000, and the tip Mach number about 0.07. The data base is completed by far-field measurements made with a single microphone on a moving support. The analytical model is based on a previously published extension of Amiet's trailing-edge noise theory. A blade is split into several strips in the spanwise direction, and the model is applied to each strip. For this the input data are interpolated from the measurements performed with the aforementioned sets of microphones. The trailing-edge noise model is more reliable for observer positions within ±30° from the fan-rotation plane. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

Bogey C.,École Centrale Lyon | Marsden O.,CNRS Fluid Mechanics and Acoustics Laboratory
AIAA Journal | Year: 2016

Two isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number of 2 × 105 have been computed by compressible large-eddy simulation using high-order finite differences on a grid of 3.1 billion points. At the exit of a straight pipe nozzle in which a trip forcing is applied, the jet flow velocity parameters, including the momentum thickness and the shape factor of the boundary layer, the momentum-thickness-based Reynolds number, and the peak turbulence intensity, roughly match those found in experiments using two nozzles referred to as the ASME and the conical nozzles. The boundary layer is in a highly disturbed laminar state in the first case and in a turbulent state in the second. The exit flow conditions, the shear-layer and jet flowfields, and the far-field noise provided by the large-eddy simulation are described. The jet with the ASME-like initial conditions develops a little more rapidly, with slightly higher turbulence levels than the other. Overall, however, the results obtained for the two jets are very similar, and they are in good agreement with measurements available forMach0.9 jets. In particular, this similarity holds for the far-field spectra. Because the ASMEnozzle has been reported to yield higher noise levels than the conical nozzle, this suggests that the nozzle-exit conditions in the large-eddy simulation do not adequately reflect those in the experiments and/or that the link between the noise differences and the jet initial conditions using the two nozzles is not as simple as was first thought, and that other parameters, associated for instance with the nozzle geometry such as the presence of pressure gradients, may also play an important role. © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Bos W.J.T.,CNRS Fluid Mechanics and Acoustics Laboratory | Kadoch B.,Aix - Marseille University | Schneider K.,Aix - Marseille University
Physical Review Letters | Year: 2015

The angle between subsequent particle displacement increments is evaluated as a function of the time lag in isotropic turbulence. It is shown that the evolution of this angle contains two well-defined power laws, reflecting the multiscale dynamics of high-Reynolds number turbulence. The probability density function of the directional change is shown to be self-similar and well approximated by an analytically derived model assuming Gaussianity and independence of the velocity and the Lagrangian acceleration. © 2015 American Physical Society.

Riviere N.,CNRS Fluid Mechanics and Acoustics Laboratory | Travin G.,CNRS Fluid Mechanics and Acoustics Laboratory | Perkins R.J.,University of Lyon
Water Resources Research | Year: 2011

Subcritical flow in an intersection composed of four similar orthogonal channels has been studied experimentally in a configuration with two inflows and two outflows for a wide range of experimental conditions. The results have been used to develop a relationship between the incoming flow rates and the flow distribution in the two outlet channels, based on the conservation of discharge and momentum in the intersection, and suitable stage-discharge relationships for the downstream controls in the outflow channels. A final equation is provided by an empirical correlation for the outflow in one of the channels, based on the experimental data obtained from these experiments; this correlation agrees with all the available data to within ±5%. It is shown how the resulting set of equations can be used to compute the discharge distribution in any similar intersection, given the incoming flow rates and some form of stage-discharge relationship for the outlet conditions. © 2011 by the American Geophysical Union.

Koussa F.,French Scientific and Technical Center for Building | Defrance J.,French Scientific and Technical Center for Building | Jean P.,French Scientific and Technical Center for Building | Blanc-Benon P.,CNRS Fluid Mechanics and Acoustics Laboratory
Applied Acoustics | Year: 2013

This research work aims at evaluating the acoustic performance of conventional and low height gabions noise barriers. On one hand, in situ as well as scale model measurements at a scale of 1:10 have been carried out to assess the intrinsic acoustic properties of a 3 m high gabions barrier. Single number ratings of transmission and reflection indices reached 20 dB and 5 dB, respectively. On the other hand, numerical simulations using a 2D boundary element method (BEM) and scale model measurements are carried out to study the effectiveness of low height gabions noise barriers when they are inserted in dense urban areas. The agreement between numerical and scale model measurements results is satisfactory. The effectiveness of low height gabions noise barriers is significant for receivers of limited height and the insertion loss values can reach 8 dB(A) behind the barrier. This confirms that gabions noise barriers are possible candidates as useful devices for environmental noise reduction. © 2012 Elsevier Ltd. All rights reserved.

Sebastien C.,Geoservices | Michel L.,CNRS Fluid Mechanics and Acoustics Laboratory
International Journal of Multiphase Flow | Year: 2011

New technology combined to the rise of the barrel price make wet gas flow metering of primary importance. A Venturi and a multienergy gamma ray hold-up meter provide capital information to estimate gas and liquid flow rates with the required metering accuracy. Starting from Navier-Stokes' equations, the two-phase flow is modeled with a three 1D equations system for gas, liquid droplets and liquid film. Hypotheses are added to reduce the dynamical system to a simplified and operational two scalar equation model. Based on this physical description of the film-core flow, this new model is tested over experimental data. Finally, three remaining coefficients are calibrated and modeled thanks to empirical correlation with non-dimensional numbers. © 2010 Elsevier Ltd.

Dragna D.,CNRS Fluid Mechanics and Acoustics Laboratory
Proceedings of Meetings on Acoustics | Year: 2013

This paper deals with modeling of sources in motion in time-domain solvers. In the context of transportation noise, acoustic sources are complex. Indeed, they are in motion, and they are generally not compact. Equivalent point sources are often used to simplify the problem. Heuristic methods are then applied to handle acoustic propagation over complex sites. Besides, time-domain solutions of the linearized Euler equations have proved to be an attractive approach to study outdoor sound propagation, and can then be used to validate these models. However point sources in arbitrary motion are difficult to account for in these approaches. Distributed volume sources can be used instead. First, influence of the spatial support of the source on the acoustic field is investigated. The case of a harmonic source moving at a constant speed is studied. Then, simulations of a broadband moving source above a rigid ground surface in a three-dimensional geometry are presented, and ground effect is highlighted. © 2013 Acoustical Society of America.

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