Journal of the Brazilian Society of Mechanical Sciences
Print version ISSN 0100-7386
SILVESTRINI, Jorge H.. Dynamics of coherent vortices in mixing layers using direct numerical and large-eddy simulations. J. Braz. Soc. Mech. Sci. [online]. 2000, vol.22, n.1, pp. 53-67. ISSN 0100-7386. http://dx.doi.org/10.1590/S0100-73862000000100005.
Coherent vortices in turbulent mixing layers are investigated by means of Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES). Subgrid-scale models defined in spectral and physical spaces are reviewed. The new "spectral-dynamic viscosity model", that allows to account for non-developed turbulence in the subgrid-scales, is discussed. Pseudo-spectral methods, combined with sixth-order compact finite differences schemes (when periodic boundary conditions cannot be established), are used to solve the Navier- Stokes equations. Simulations in temporal and spatial mixing layers show two types of pairing of primary Kelvin-Helmholtz (KH) vortices depending on initial conditions (or upstream conditions): quasi-2D and helical pairings. In both cases, secondary streamwise vortices are stretched in between the KH vortices at an angle of 45° with the horizontal plane. These streamwise vortices are not only identified in the early transitional stage of the mixing layer but also in self-similar turbulence conditions. The Re dependence of the "diameter" of these vortices is analyzed. Results obtained in spatial growing mixing layers show some evidences of pairing of secondary vortices; after a pairing of the primary Kelvin-Helmholtz (KH) vortices, the streamwise vortices are less numerous and their diameter has increased than before the pairing of KH vortices.
Keywords : Coherent Vortices; Mixing Layer; Direct Numerical Simulation; Large-Eddy Simulation; Subgrid Scales Models.