DIRECT SIMULATION OF A SELF-SIMILAR TURBULENT MIXING LAYER

Three direct numerical simulations of incompressible turbulent plane m ixing layers have been performed. All the simulations were initialized with the same two velocity fields obtained from a direct numerical si mulation of a turbulent boundary layer with a momentum thickness Reyno lds number of 300 computed by Spalart [J. Fluid Mech. 187, 61 (1988)]. In addition to a baseline case with no additional disturbances, two s imulations were begun with two-dimensional disturbances of varying str ength in addition to the boundary layer turbulence. After a developmen t stage, the baseline case and the case with weaker additional two-dim ensional disturbances evolve self-similarly, reaching visual thickness Reynolds numbers of up to 20 000. This self-similar period is charact erized by a lack of large-scale organized pairings, a lack of streamwi se vortices in the ''braid'' regions, and scalar mixing that is charac terized by ''marching'' probability density functions (PDFs). The case begun with strong additional two-dimensional disturbances only become s approximately self-similar, but exhibits sustained organized large-s cale pairings, clearly defined braid regions with streamwise vortices that span them, and scalar PDFs that are ''nonmarching.'' It is also c haracterized by much more intense vertical velocity fluctuations than the other two cases. The statistics and structures in several experime nts involving turbulent mixing layers are in better agreement with tho se of the simulations that do not exhibit organized pairings.