Analyse physique des effets de rotation de paroi en écoulements transitionnels et modélisation d'écoulements turbulents autour de structures portantes

Cette thèse a étudié les effets de rotation pariétale sur la transition laminaire-turbulente en nombre de Reynolds modéré et la modélisation de la turbulence d'écoulements instationnaires fortements détachés. Les étapes successives de la transition de l'écoulement autour d'un cylindre en rotation sont analysées par simulations numériques 2D et 3D. Les effets de rotation peuvent amplifier, maintenir ou atténuer les modes d'instabilité qui apparaissent d'une façon naturelle dans l'écoulement. L'amplification de l'instabilité 3D est étudiée à partir de la DNS et du modèle d'oscillateur global de Landau pour évaluer le nombre de Reynolds critique d'apparition de l'instabilité secondaire. L'analyse des structures organisées est réalisée par la POD. Les approches de macrosimulation statistique OES, Organised Eddy Simulation et hybride DES, Detached Eddy Simulation sont étudiées quant à leur capacité prédictive d'écoulements turbulents autour d'obstacles à nombre de Reynolds élevé. ABSTACT : This thesis studied the effect of wall rotation on the laminar-turbulent transition at moderate Reynolds number, and the turbulence modelling in highly unsteady detached flows. The successive stages of the transition in the flow around a rotating cylinder are analysed by 2D and 3D numerical simulations. The rotation effects can amplify, maintain or attenuate the instability modes that appear inherently in the flow. The amplification of the 3D instability is studied by means of the DNS and the Landau global oscillator model in order to quantify the critical Reynolds number for the appearance of the secondary instability. The analysis of the coherent pattern is carried out by the Proper Orthogonal Decomposition. Statistical and Hybrid macrosimulation approaches, OES, Organised Eddy Simulation and DES, Detached Eddy Simulation are studied at high Reynolds number, according to their ability to predict the strongly detached turbulent flows around obstacle

Anisotropic Organised Eddy Simulation for the prediction of non-equilibrium turbulent flows around bodies

The unsteady turbulent flow around bodies at high Reynolds number is predicted by an anisotropic eddy-viscosity model in the context of the Organised Eddy Simulation (OES). A tensorial eddy-viscosity concept is developed to reinforce turbulent stress anisotropy, that is a crucial characteristic of non-equilibrium turbulence in the near-region. The theoretical aspects of the modelling are investigated by means of a phase-averaged PIV in the flow around a circular cylinder at Reynolds number 1.4×10^5. A pronounced stress–strain misalignment is quantified in the near-wake region of the detached flow, that is well captured by a tensorial eddy-viscosity concept. This is achieved by modelling the turbulence stress anisotropy tensor by its projection onto the principal matrices of the strain-rate tensor. Additional transport equations for the projection coefficients are derived from a second-order moment closure scheme. The modification of the turbulence length scale yielded by OES is used in the Detached Eddy Simulation hybrid approach. The detached turbulent flows around a NACA0012 airfoil (2-D) and a circular cylinder (3-D) are studied at Reynolds numbers 105 and 1.4×10^5, respectively. The results compared to experimental ones emphasise the predictive capabilities of the OES approach concerning the flow physics capture for turbulent unsteady flows around bodies at high Reynolds numbers