Yves Fouillet

Generation of coherent structures in free shear layers

Direct numerical simulations are performed for the turbulent mixing layer and the turbulent plane wake. Coherent structures are qualitatively analyzed by means of graphic visualizations. In all cases, transition to turbulence is triggered by random noise superimposed onto a unidirectional basic flow. In the case of three-dimensional incompressible mixing layers, two regimes are found according to whether this noise is more two-dimensional or more three-dimensional. In the case of three-dimensional incompressible plane wakes, we first observe the formation of two staggered mixing-layers of opposite signs. Each of them features spanwise primary vortices connected together by streamwise secondary vortices. The interaction between the two layers then intensifies, until a Karman street forms. At this moment, closed vortex loops bridging primary vortices of unlike signs are observed. Two-dimensional compressible mixing-layer simulations verify that compressibility effects inhibit the spreading of mixing layers. Eddy-shocklets are found when the convective Mach number reaches 0.8. A two-dimensional simulation of a spatially-growing plane wake is also presented.

Large-Eddy Simulation of a Three-Dimensional Mixing Layer

With the aid of an appropriate parameterization, we simulate numerically a three-dimensional turbulent mixing layer at high Reynolds number. Results are compared with corresponding direct simulations performed with the same pseudo-spectral numerical code. Periodicity is assumed in streamwise and spanwise directions. The evolution of a passive temperature is calculated simultaneously.

Three-Dimensional Numerical Simulations of Coherent Structures in Free-Shear Flows

We investigate, with the aid of three-dimensional direct-numerical simulations, the origin and dynamics of coherent structures in the following mixing-layers: temporal case, flow behind a backwards-facing step, and spatially-developing case.