LARGE-EDDY SIMULATIONS OF STABLE-STRATIFICATION EFFECTS UPON A BACKSTEP FLOW

Large-eddy simulations (LES) of Navier-Stokes equations within the Bou ssinesq approximation are carried out. in order to study the effects o f an upstream heating upon the coherent-vortex dynamics in a backward- facing step Bow. The subgrid-scale models used are the structure-funct ion model (SF) in its original form (Metais and Lesieur, 1992), and a selective version of it (SSF). We study first the non-stratified case, already looked al by Silveria et ni. (1993). In a transient regime, a nd for an expansion ratio of 5 (high step, regular grid, Reynolds numb er R-e = 48000), it is shown that quasi two-dimensional Kelvin-Helmhol tz (KH) vortices are shed behind the step, and stretch intense longitu dinal alternate vortices. When a statistically stationary regime is at tained, KH vortices are subject to dislocations, and transform into ii -vortices. The latter topology is recovered at an expansion ratio of 1 .2 (low-step, grid refinement, R-e = 5000), in the same geometry as th e DNS of Le and Moin (1997). Here, one shows how big Lambda-vortices m ay impinge the lower wall and be carried away downstream. Stably-strat ified simulations are performed in the high-step case, with 80% of the upper inlet flow heated. Four LES using the SF model have been carrie d out at R-e = 48000, and with an upstream Richardson number Ri (based upon the step height) of respectively 0.25 (a), 0.5 (b), 0.7 (c) and 1.0 (d). Case (a) shows no change with respect to the non-stratified s ituation, as far as the vortex dyamics is concerned. Tn case (b), the mixing layer behind the step becomes quasi two-dimensional, but the pa iring is still highly three-dimensional. In (c), the vortex pairing is inhibited, as well as the shear-layer growth. Finally, case (d) shows a very stable density interface. The LES has been redone using the SS F model for Ri = 0.7. Two-dimensional secondary vortices form within t he braids reconnecting the primary Kelvin-Helmholtz vortices, due to b aroclinic effects. This is in agreement with the predictions of Staque t and Riley (1989). In this case, vortex pairing is still observed. Th e same predictions are obtained in a purely two-dimensional simulation using the same code. Finally, it is shown that regions of baroclinic vorticity production to correspond to a local Richardson number lower than 1/4.