Mach Wave Radiation From a Jet at Mach 1.92

R. Darke and J. B. Freund
University of California, Los Angeles

The visualization shows a two-dimensional slice through the center of a Mach 1.92, Reynolds number 2000 round jet computed by direct numerical simulation. The simulation is discussed in detail by Freund, Lele, and Moin. ^[1] The jet turbulence is visualized with color contours of vorticity between 0 U _c/ r _o and 4.5 U _c/ r _o, where U _c is the local centerline velocity and r _o is the nozzle radius. (The peak vorticity in this plane was 7.2 U _c/ r _o at the instant shown.) Despite the low Reynolds number, we see a range of turbulence scales in the flow. The grayscale levels shown are directly proportional to divergence of velocity between solid white which indicates ? u>0.017 U _j/ r _o and solid black which indicates ? u<0.017 U _j/ r _o where U _j is the jet nozzle velocity. In the sound field, narrow regions of compressions (dark) are typically separated by broader expansions (light) which suggests a nonlinear steepening process. Steepening in the far-field was investigated for this flow using weak shock theory. ^[1] The small visualizations, however, reveal that the dark regions appear to originate as already thin areas within the jet, as speculated by Ffowcs Williams et al. ^[2] who did...