Passive scalar mixing near turbulent/non-turbulent interface in compressible turbulent boundary layers

Abstract

Direct numerical simulations for compressible temporally evolving turbulent boundary layers (TBLs) at Mach numbers of M = 0.8 and 1.6 are preformed up to the Reynolds number based on the momentum thickness Reθ ≈ 2200 to investigate a passive scalar field near the turbulent/non-turbulent interface (TNTI) layer that is formed at the edge of the TBLs. The passive scalar is diffused from the wall in the TBLs developing on the moving wall at constant speed. The outer edge of the TNTI layer detected by an isosurface of vorticity magnitude and passive scalar are compared by visualization, and it is shown that the passive scalar can be used for detecting the TNTI layer in compressible boundary layers. Conditional statistics are calculated as a function of the distance from the outer edge of the TNTI layer. The mean thicknesses of the TNTI layer, viscous superlayer (VSL), and turbulent sublayer (TSL), is about 15ηI, 4ηI, and 11ηI, respectively (ηI: Kolmogorov length scale in the turbulent core region near the TNTI layer). The conditional mean profiles of scalar dissipation rate have a large peak near the boundary between the VSL and TSL, where the fluid with a low scalar value locally entrained from non-turbulent region encounters the turbulent fluid with a higher scalar value. The scalar dissipation rate near the TNTI depends on the TNTI orientation: it is larger near the TNTI facing the downstream direction with respective to the mean flow in the boundary layer (leading edge). This is partially explained by the dependence of the production rate of passive scalar gradient. The conditional mean production rate of the scalar dissipation rate near the leading edge is as large as in the turbulent core region while it is close to the non-turbulent value when the TNTI faces the upstream region (trailing edge).