In this image, Rayleigh scattering is used to visualize a streamwise cross-section of the density field of a turbulent flat plate boundary layer at Mach 2.5 in a plane parallel to the wall at y/delta = 0.72. 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 side 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.