Characteristic features of large structures in compressible mixing layers

The large structures of compressible mixing layers have been investigated using planar laser Mie scattering visualizations from condensed ethanol droplets in the flow. Large ensembles of digital images were collected for 10 cases, corresponding to combinations of 3 distinct conditions of compressibility, 2 different image planes, and 2 seeding methods. The three flow conditions were at relative Mach numbers of 0.63, 0.98, and 1.49. The two-dimensional spatial covariance field was computed for each ensemble to characterize the typical large structure observed within the mixing layer. The spatial covariances of the transverse views indicate that the structures are elongated and compressed toward the streamline flow direction with increasing compressibility. Covariances of the oblique images help quantify the influence of compressibility on the three-dimensional nature of the structures, as evidenced by an increase in size and a reduction in eccentricity of the obliquely imaged structures with increasing compressibility.