ON THE EFFECTS OF NONEQUILIBRIUM ON THE SUBGRID-SCALE STRESSES

An a priori study of the subgrid-scale (SGS) stresses and dissipation in two nonequilibrium, wall-bounded flows is carried out. The velocity fields were computed by direct simulations of two- and three-dimensio nal boundary layers obtained, respectively, by a sudden change in the Reynolds number and by an impulsive motion in the spanwise direction o f the lower wall of a plane channel in fully developed turbulent flow conditions. Several realizations of the transient period of the flow w ere examined. The SGS stresses react to the imposition of the secondar y sheer more rapidly than the large-scale ones, and return to equilibr ium before the resolved stresses do. In general, the subgrid scales ar e less sensitive than the large ones to the near-wall and nonequilibri um effects. Scale-similar and dynamic models appear well-suited to rep roduce the correlation between resolved Reynolds stress production and events with significant production of SGS energy. (C) 1997 American I nstitute of Physics.