Olivier Métais - Academia.edu (original) (raw)

Papers by Olivier Métais

ERCOFTAC series, 2001

Aims and Scope of the Series ERCOFTAC (European Research Community on Flow, Turbulence and Combus... more Aims and Scope of the Series ERCOFTAC (European Research Community on Flow, Turbulence and Combustion) was founded as an international association with scientific objectives in 1988. ERCOFTAC strongly promotes joint efforts of European research institutes and industries that are active in the field of flow, turbulence and combustion, in order to enhance the exchange of technical and scientific information on fundamental and applied research and design. Each year, ERCOFTAC organizes several meetings in the form of workshops, conferences and summerschools, where ERCOFTAC members and other researchers meet and exchange information. The ERCOFTAC Series will publish the proceedings of ERCOFTAC meetings, which cover all aspects of fluid mechanics. The series will comprise proceedings of conferences and workshops, and of textbooks presenting the material taught at summerschools. The series covers the entire domain of fluid mechanics, which includes physical modelling, computational fluid dynamics including grid generation and turbulence modelling, measuring-techniques, flow visualization as applied to industrial flows, aerodynamics, combustion, geophysical and environmental flows, hydraulics, multiphase flows, non-Newtonian flows, astrophysical flows, laminar, turbulent and transitional flows. The titles published in this series are listed at the end of this volume.

Direct and Large-Eddy Simulation VI

Flow evolution in a rotating mixing layer is investigated using direct and large-eddy simulation.... more Flow evolution in a rotating mixing layer is investigated using direct and large-eddy simulation. The mixing layer rotates about a fixed vertical axis perpendicular to the plane of its mean initial shear. The rotating mixing layer forms oscillatory large-scale columnar structures and rapid horizontal flow-reversals. The frequency of these oscillations varies approximately inversely with the Rossby number, Ro. At low Ro vertical mixing of a passive scalar is strongly reduced. This is quantified by investigating the evolution of level-sets of the scalar field. The surfacearea of the level-sets remains virtually constant even at modest rotation rates. More localized motions are less affected by rotation and yield comparatively high levels of surface-wrinkling. Rotation effects are accurately predicted in large-eddy simulations that involve the dynamic eddy-viscosity model or the LANS-α or Leray regularization models. The small-scale variability is best preserved when using the LANS-α formulation.

Journal of Fluid Mechanics, 1992

Large Eddy Simulations of three-dimensional turbulent compressible flows in curved duct of rectan... more Large Eddy Simulations of three-dimensional turbulent compressible flows in curved duct of rectangular cross-section are presented. Strong secondary flows and turbulent structures are the typical features of this type of flow: these are responsible for most of the heat and mass transfer within the duct. Our goal is here to investigate the influence of the duct aspect ratio on the flow development. The results show that this parameter modifies the flow structures in terms of intensity and localisation. Thus it significantly influences the heat transfers when a higher temperature is imposed on the duct convex wall

The objective of the present work is to improve numerical predictions of unsteady turbulent swirl... more The objective of the present work is to improve numerical predictions of unsteady turbulent swirling flows in the draft tubes of hydraulic power plants. We present Large Eddy Simulation (LES) results on a simplified draft tube consisting of a straight conical diffuser. The basis of LES is to solve the large scales of motion, which contain most of the energy, while the small scales are modeled. LES strategy is here preferred to the average equations strategies (RANS models) because it resolves directly the most energetic part of the turbulent flow. LES is now recognized as a powerful tool to simulate real applications in several engineering fields which are more and more frequently found. However, the cost of large-eddy simulations of wall bounded flows is still expensive. Bypass methods are investigated to perform high-Reynolds-number LES at a reasonable cost. In this study, computations at a Reynolds number about 2 105 are presented. This study presents the result of a new near-wal...

Lecture 1 Recent approaches in large-eddy simulations of turbulence.- Lecture 2 Large eddy simula... more Lecture 1 Recent approaches in large-eddy simulations of turbulence.- Lecture 2 Large eddy simulation: an introduction and perspective.- Lecture 3 Large-eddy simulation and statistical turbulence models: complementary approaches.- Lecture 4 Modeling compressibility effects on turbulence.- Lecture 5 Reynolds averaged and large eddy simulation modeling for turbulent combustion.- Lecture 6 Modeling tools for flow noise and sound propagation through turbulence.- Lecture 7 Vortices in compressible LES and non-trivial geometries.- Lecture 8 LES-DNS: The aeronautical and defense point of view.- lecture 9 Turbulence modeling in aeronautical flows.- Lecture 10 Fluid instabilities in inertial confinement fusion.- Lecture 11 Large eddy simulations in nuclear reactors thermal-hydraulics.- Lecture 12 Coherent vortices in rotating flows.- Lecture 13 Large-eddy simulation of air pollution dispersion: a review.- Lecture 14 Numerical simulations of compressible convection.- Lecture 15 Dynamical evol...

Flow, Turbulence and Combustion, 2016

In this paper, Unsteady RANS (URANS) simulations and Large Eddy Simulations (LES) in the draft tu... more In this paper, Unsteady RANS (URANS) simulations and Large Eddy Simulations (LES) in the draft tube of a bulb turbine are presented with the objective to understand and locate the head losses in this turbine component. Three operating points of the turbine are considered. Numerical results are compared with experimental velocity measurements for validation. Thanks to a detailed analysis of the energy balance in the draft tube, the physical and hydrodynamic phenomena responsible for head losses in the draft tube are identified. Head losses are due to transfer of mean kinetic energy to the turbulent flow and viscous dissipation of kinetic energy. This occurs mainly in the central vortex structure and next to the walls in the draft tube. Head losses prediction is found to be dependent on the turbulence model used in the simulations, especially in URANS simulations. Using this analysis, the evolution of head losses between the three operating points is understood.

Fluid Mechanics and Its Applications, 2000

This paper studies aspects of particle dispersion and vortex formation in stably stratified turbu... more This paper studies aspects of particle dispersion and vortex formation in stably stratified turbulence with rotation using the direct numerical simulation of the (non-dimensionalized) Navier-Stokes equations within the Boussinesq approximation.

Mécanique & Industries, 2005

-Nous présentons des Simulations des GrandesÉchelles d'écoulements turbulents compressibles tridi... more -Nous présentons des Simulations des GrandesÉchelles d'écoulements turbulents compressibles tridimensionnels se développant dans des conduits courbes de section rectangulaire. Cesécoulements sont caractérisés par la présence de flux secondaires intenses et de structures turbulentes responsables des transferts de masse et thermique. Le but est ici d'évaluer l'influence du rapport d'aspect de la section sur ce type d'écoulements. Les résultats montrent que ce paramètre modifie l'intensité et la localisation des tourbillons et donc indirectement le transfert de chaleur lorsqu'un chauffage est appliqué sur la paroi convexe.

Physics of Fluids A: Fluid Dynamics, 1991

The effect of a solid-body rotation, characterized by an angular velocity Ω, on a two-dimensional... more The effect of a solid-body rotation, characterized by an angular velocity Ω, on a two-dimensional mixing layer (in a plane perpendicular to Ω) of relative vorticity ω2D, upon which is superposed a small three-dimensional turbulent perturbation, is considered. Using the Kelvin theorem in the frame rotating with Ω, and with the aid of arguments based on the straining of absolute vortex filaments by the basic velocity, it is shown that the rotation is always stabilizing (with respect to the nonrotating case) in the cyclonic case. In the anticyclonic case, a slight rotation is destabilizing. At a local Rossby number R0=‖ω2D‖/2‖Ω‖ of the order of 1, the anticyclonic rotation disrupts catastrophically the coherent structures of the mixing layer. Anticyclonic rotation becomes stabilizing again for R0<0.5. Also presented are three-dimensional numerical simulations which support the theory, and agree qualitatively with experimental results. The consequences for oceanic and atmospheric vor...

Ocean Modeling and Parameterization, 1998

Direct-numerical simulation of turbulence (DNS) consists of solving explicitly all the scales of ... more Direct-numerical simulation of turbulence (DNS) consists of solving explicitly all the scales of motion, from the largest l I to the Kolmogorov dissipative scale l D . It is well known from the statistical theory of turbulence that l I /l D scales like \( R_l^{3/4}\), where R l is the large-scale Reynolds number u’l I / v based upon the rms velocity fluctuation u’. Therefore, the total number of degrees of freedom necessary to represent the whole span of scales of a three-dimensional turbulent flow is of the order of \( R_l^{9/4}\) in three dimensions. With the presently available computers, the DNS is then limited to Reynolds numbers which are several orders of magnitude smaller than those encountered in the ocean, the atmosphere, or most of the industrial facilities. In order to increase the Reynolds number in the simulations, it is necessary to introduce a subgrid-scale model representing the action of scales smaller than Δx, the computational mesh, upon the explicitly resolved scales. This is the basis of the Large-Eddy Simulation (LES) techniques.

Frans Nieuwstadt has been a professor of fluid mechanics since 1986. He died under rather tragic ... more Frans Nieuwstadt has been a professor of fluid mechanics since 1986. He died under rather tragic circumstances on May 18, 2005. He made several profound contributions to research in turbulent shear flows, first in meteorology and later in engineering. Frans was born in 1946. After secondary school he studied aerospace engineering at the Technische Hogeschool in Delft (now known as TU-Delft). After his university education he spent two years at the California Institute of Technology, studying computational fluid mechanics. He very much enjoyed this period, especially because of the informal relation between students and professors. At that time the student-professor relation was still very formal in the Netherlands. After leaving Caltech, he accepted a position at the Royal Dutch Institute for Meteorology (KNMI). Here he worked on atmospheric boundary layers. In 1981 he got his PhD, with Henk Tennekes as PhD adviser, on the behavior of the nocturnal boundary layer. This work initiated his international reputation. Best known is his model of the stable boundary layer that (with a few modifications) has since been regarded as the standard model. In 1986 he was appointed Professor of fluid mechanics at TU-Delft. Among his predecessors were renowned persons such as J.M. Burgers and J.O. Hinze. Due to various circumstances, the fluid mechanics Chair at Delft had been vacant for more than two years and a whole new group needed to be formed. Frans proved to be a worthy successor and successfully took up this challenge.

Physics of Fluids A: Fluid Dynamics, 1993

Using linear stability analysis, the instability characteristics are examined of both planar wake... more Using linear stability analysis, the instability characteristics are examined of both planar wakes and mixing layers subjected to rigid-body rotation with axis of rotation perpendicular to the plane of the ambient flow. In particular, the tendency of rotation to stabilize or destabilize three-dimensional motions is addressed. In the inviscid limit the results are consistent with the criterion established by Pedley [J. Fluid Mech. 35, 97 (1969)] and Bradshaw [J. Fluid Mech. 36, 177 (1969)]. Cyclonic rotation and strong anticyclonic rotation tend to stabilize three-dimensional motions, whereas weaker anticyclonic rotation (Ro≳1) acts to destabilize these motions. This latter instability is in the form of streamwise rolls, similar to previous results obtained for boundary layer and channel flows. It is found that this instability is stronger than the coexisting Kelvin–Helmholtz instability for roughly the range 1.5<Ro<8, and its effect is maximum for Ro≂2. For the case of constan...

Physics of Fluids, 2002

Direct and large-eddy simulations (DNS/LES) are performed to analyze the vortex dynamics and the ... more Direct and large-eddy simulations (DNS/LES) are performed to analyze the vortex dynamics and the statistics of bifurcating jets. The Reynolds number ranges from ReD=1.5×103 to ReD=5.0×104. An active control of the inlet conditions of a spatially evolving round jet is performed with the aim of favoring the jet spreading in one particular spatial direction, thus creating a bifurcating jet. Three different types of forcing, based on the information provided by a LES of a natural (unforced) jet, are superimposed to the jet inlet in order to cause its bifurcation. The different forcing types mimic the forcing methods used in experimental bifurcating jets (Lee and Reynolds, Parekh et al., Suzuki et al.), but using excitations with relatively low amplitudes, which could be used in real industrial applications. The three-dimensional coherent structures resulting from each specific forcing are analyzed in detail and their impact on the statistical behavior of bifurcating jets is explained. I...

Physics of Fluids, 2007

Direct numerical simulations are performed to analyze the flow dynamics and the mixing properties... more Direct numerical simulations are performed to analyze the flow dynamics and the mixing properties of natural (unforced) and excited coaxial jets at moderate Reynolds number. First, the study of the natural coaxial jet, with species injected in the outer jet alone, allows us to understand the role of the coherent vortices on the mixing process during the transition stage. It is observed that the global flow behavior is controlled by the dynamics of the outer shear layer during the transition. The streamwise vortices are shown to play a significant role in the mixing process since they initiate intense ejections from the seeding regions. Spots of pure (unmixed) species from the outer jet are seen to persist far downstream. Two different types of inflow forcing are then considered based on the information provided by the natural coaxial jet: first, a purely axisymmetric excitation and second, combined axisymmetric and azimuthal excitations all of moderate amplitude. These excitations a...

Physics of Fluids, 2011

In this work, modeling of the near-wall region in turbulent flows is addressed. A new wall-layer ... more In this work, modeling of the near-wall region in turbulent flows is addressed. A new wall-layer model is proposed with the goal to perform high-Reynolds number large-eddy simulations of wall bounded flows in the presence of a streamwise pressure gradient. The model applies both in the viscous sublayer and in the inertial region, without any parameter to switch from one region to the other. An analytical expression for the velocity field as a function of the distance from the wall is derived from the simplified thin-boundary equations and by using a turbulent eddy coefficient with a damping function. This damping function relies on a modified van Driest formula to define the mixing-length taking into account the presence of a streamwise pressure gradient. The model is first validated by a priori comparisons with direct numerical simulation data of various flows with and without streamwise pressure gradient and with eventual flow separation. Large-eddy simulations are then performed ...

Journal of Turbulence, 2005

Large eddy simulations (LES) of a turbulent field within a heated duct are presented. As opposed ... more Large eddy simulations (LES) of a turbulent field within a heated duct are presented. As opposed to a previously performed LES which assumed periodicity in the flow direction, here we consider spatially growing situations for which a thermal boundary layer grows in the longitudinal direction. Characteristic wave boundary conditions previously proposed for subsonic flows are implemented and shown to yield good results. Two different thermal boundary conditions are considered in the spatial configuration: the first case corresponds to an imposed temperature at the wall and the second to an imposed heat flux. As we move downstream in the duct, we observed the progressive formation of a unique ejection of hot fluid localized in the middle plane of the heated wall. This induces significant variations in thermal characteristics along the transverse direction of the heated wall.