DIRECT NUMERICAL-SIMULATION OF A 3-DIMENSIONAL TEMPORAL MIXING LAYER WITH PARTICLE DISPERSION

The three-dimensional mixing layer is characterized by both two-dimens ional and streamwise large-scale structures. Understanding the effects of those large-scale structures on the dispersion of particles is ver y important. Using a pseudospectral method, the large-scale structures of a three-dimensional temporally developing mixing layer and the ass ociated dispersion patterns of particles were simulated, The Fourier e xpansion was used for-spatial derivatives due to the periodic boundary conditions in the streamwise and the spanwise directions and the free -slip boundary condition in the transverse direction. A second-order A dam-Bashforth scheme was used in the time integration. Both a two-dime nsional perturbation, which was based on the unstable wavenumbers of t he streamwise direction, and a three-dimensional perturbation, derived from an isotropic energy spectrum, were imposed initially, Particles with different Stokes numbers were traced by the Lagrangian approach b ased on one-way coupling between the continuous and the dispersed phas es. The time scale and length scale for the pairing were found to be t wice those for the rollup. The streamwise large-scale structures devel op from the initial perturbation and the most unstable wavelength in t he spanwise direction was found to be about two thirds of that in the streamwise direction. The pairing of the spanwise vortices was also fo und to have a suppressing effect on the development of the three-dimen sionality. Particles with Stokes number of the order of unity were fou nd to have the largest concentration on the circumference of the two-d imensional large-scale structures, The presence of the streamwise larg e-scale structures causes the variation of the particle concentrations along the spanwise and the transverse directions, The extent of varia tion also increases with the development of the three-dimensionality, which results in the 'mushroom' shape of the particle distribution.