Compressibility Effects on the Growth and Development of Large-Scale Structures in an Axisymmetric Jet

A temporally resolved flow visualization system that produces 17 flow images over a time span of 128 microseconds is used to visualize the mixing layers of a Mach 1.28 (M c =0.59) and a Mach 2.06 (M c =0.87) ideally expanded high Reynolds number axisymmetric jets. These image sequences revealed details about the effects of compressibility on the dynamics of turbulence structures. In general, the observations made in the axisymmetric jets are similar to observations made in planar shear layers at similar convective Mach numbers. Large-scale structures progressively become more three-dimensional and less organized with increasing compressibility and are more difficult to identify and track in the Mach 2.06 jet. Furthermore, while specific examples of 'tilting', 'stretching', 'tearing' and 'pairing' are demonstrated in a single image sequence obtained in the Mach 1.28 jet, these processes are less pronounced in Mach 2.06 case. Spatial correlation functions are used to extract quantitative information about the size, shape and convective velocity of large-scale structures in these flows. The ensemble average convective velocity of the Mach 1.28 jet was 270 m/sec, which is higher than the theoretical prediction of 206 m/sec. The histogram of the convective velocity, however, revealed a broad distribution of convective velocities. The Mach 2.06 jet, on the other hand exhibited a bimodal ensemble average convective velocity distribution with a 'fast' mode at ~420 m/sec and a 'slow' mode at ~180 m/sec. The histogram showed almost Gaussian distributions centered around both the fast mode and the slow mode. These modes are equally spaced from the theoretical convective velocity of 303 m/sec. Approximately 2/3 of the measured velocities were in the slow mode.