摘要

Analyses of the invariants of the velocity gradient tensor were performed on flow fields obtained by DNS of compressible plane mixing layers at convective Mach numbers M_c = 0.15 and 1.1. Joint pdfs of the 2nd and 3rd invariants were examined at turbulent/nonturbulent (T/NT) boundaries−defined as surfaces where the local vorticity first exceeds a threshold fraction of the maximum of the mean vorticity. By increasing the threshold from very small levels, the boundary points were moved closer into the turbulent region, and the effects on the pdfs of the invariants were observed. Generally, T/NT boundaries are in sheet-like regions at both Mach numbers. At the higher Mach number a distinct lobe appears in the joint pdf isolines which has not been observed/reported before. A connection to the delayed entrainment and reduced growth rate of the higher Mach number flow is proposed.