Compressibility effects on turbulence structures of axisymmetric mixing layers

A real-time flow visualization system that produces 17 images over a time span of 150 ms is used to visualize the mixing layers of Mach 1.3 (M c50.59) and Mach 2.0 (M c50.87) ideally expanded high Reynolds number axisymmetric jets. These image sequences reveal details about the influence of compressibility on the dynamics of turbulence structures. In general, the behavior observed in axisymmetric jets is similar to that observed in planar shear layers at similar convective Mach numbers. In addition, large streamwise vortices are apparent in cross-stream images of the flow. Large-scale structures become more three-dimensional and less organized with increasing compressibility and more difficult to identify and track. Planar space–time correlations are used to track structures as they convect downstream. The histogram of the convective velocity for the Mach 1.3 jet revealed a broad distribution of convective velocities with an ensemble average of 266 m/s, which is much higher than the theoretical prediction of 206 m/s. The Mach 2.0 jet, however, exhibited a bimodal convective velocity distribution with an ensemble average of 402 m/s for the ‘‘fast’’ and 190 m/s for the ‘‘slow’’ mode. These modes are equally spaced from the theoretical convective velocity of 303 m/s. Approximately 2/3 of the measured velocities were in the slow mode. © 2003 American Institute of Physics. @DOI: 10.1063/1.1570829#