Changes to invariants of the velocity gradient tensor at the turbulent–nonturbulent interface of compressible mixing layers

• Velocity gradient tensor analysis of compressible plane mixing layers at Re = O(10,000).
• New topology at the turbulent/nonturbulent interface at convective Mach number 1.1.
• Entrainment into tube-like regions, as well as usual sheet-like regions.

Analyses of invariants of the velocity gradient tensor (VGT) at the turbulent–nonturbulent interface of temporal plane mixing layers reveal a new type of joint pdf. When compressibility effects are small, fluid packets from the surrounding nonturbulent freestream enter the turbulent region in sheet-like zones; the VGT has one positive (compression) and two negative (expansion) eigenvalues. When compressibility effects are significant, in some parts of the turbulent–nonturbulent interface two of the eigenvalues are positive and one is negative. The implied local flow is that of streamtubes surrounded by vortex arcs (partial rings). The new topology occurs less frequently than the sheet-like regions.