Here are a few movies from a simulation of a turbulent boundary layer at supersonic Mach number (M = 2.25) and low Reynolds number (Reθi = 2000, δ+ = 560). The data correspond to case 3VW from Poggie, Bisek, and Gosse (Computers and Fluids, v. 120, 2015, DOI: 10.1016/j.compfluid.2015.07.015). Each case is a planar slice of the domain, and flow is from left to right. For the side-view movie, the field of view is 8δ x 2δ, and for all the plan-view movies, the field of view is 8δ x 8δ.
The dark regions in the density field correspond to hot, low-density, turbulent fluid near the wall. The lighter regions represents cold, high-density, irrotational freestream fluid. The dark δ-scale structures in the turbulence are traditionally called bulges, and the incursions of light, freestream fluid are called valleys. The interface between the light and dark regions is called the viscous superlayer; here turbulent and irrotational fluid meet. The turbulent boundary layer grows by capturing freestream fluid in the valleys, stretching and folding that fluid into thin sheets, and making it turbulent through viscous diffusion.