The picture shows a Direct Numerical Simulation of a Mach 1.92 jet. The jet turbulence is visualized with contours of vorticity magnitude and the near acoustic field is visualized with gray levels indicating divergence of velocity, which highlights the weak shocks (dark) and relatively broad expansions (light).

Reference:

J. B. Freund, S. K. Lele, and P. Moin. "Direct numerical simulation of a Mach 1.92 turbulent jet and its sound field," AIAA Journal, 38 (11), 2023--2031, (2000).