In this image, Rayleigh scattering is used to visualize a wall-normal streamwise cross-section of the density field of a turbulent flat plate boundary layer at Mach 2.5. The Reynolds number based on momentum thickness is about 25,000. The flow is from left to right. The Rayleigh scattering is enhanced by the presence of nano-scale water droplets that form in the nozzle as the flow expands to the freestream Mach number. The image has been false-colored to emphasize the diffuse edges of the low-density bulges (the "clouds") and the high-density of the edge fluid (the blue "sky"). A plan view is also available.

Smith, M. W. and Smits, A. J., Visualization of the Structure of Supersonic Turbulent Boundary Layers. Experiments in Fluids 18, 288-302, 1995.