NUMERICAL INVESTIGATION OF THE FORMATION OF 3-DIMENSIONAL STRUCTURES IN STABLY-STRATIFIED MIXING LAYERS

A temporally-growing mixing layer has been directly simulated with a p seudospectral technique, for initial bulk Richardson numbers from 0.0 to 0.2 and for Prandtl numbers from 0.00535 to 2.2. Several different initial conditions for the velocity fluctuations were imposed. For the two-dimensional (2-D) case only purely-deterministic conditions were used, whereas purely-deterministic combined deterministic-random, or p urely-random conditions were imposed in the three-dimensional (3-D) ca ses. The numerical procedure allowed fields with very different charac teristic lengths to be resolved, with spectral accuracy maintained. Th e evolution of the velocity, active (temperature), and passive scalar fields were followed independently by adaptively redistributing colloc ation points in the regions of high shear and rapid scalar variations. The vertical boundary conditions were imposed at infinity to eliminat e any boundary-layer effects and an exponential mapping was used to tr anslate infinite physical space into finite computational space. The b irth and time evolution of the longitudinal structures have been inves tigated. Variations in the initial modal forcing are reflected in diff erent outcomes from the competition between core- and braid-centered i nstabilities in unstratified flow. For relatively strong fundamental f orcing (compared to the 3-D forcing) the most unstable mode is braid-c entered, whereas if the fundamental forcing is weak the most-unstable, core-centered mode determines the overall three-dimensionalization of the flow by generating large deformations of the main vortex cores. I n subcritically-stratified air and water flow, instead, the braid-cent ered instability supersedes the core-centered instability, whatever th e initial forcing. In stratified liquid sodium the shear-aligned conve ctive instabilities observed in air and water are not excited. The con version of potential into kinetic energy by convective overturning in the braid region does not occur because the high thermal conduction pr ecludes the existence of the unstably-stratified regions necessary to drive the instabilities. The initial conditions, the Richardson, and t he Prandtl numbers accordingly play a significant role in the free-she ar-layer evolution and need to be explicitly considered for modeling p urposes. (C) 1998 American Institute of Physics.