Compressibility effects on large-scale structures and entrainment in turbulent planar mixing layers

In this work, experiments on planar free shear layers are conducted to obtain a set of benchmark computational fluid dynamics-validation data and to examine compressibility effects on shear layer turbulence. Five different dual-stream air mixing layers are studied, with levels of compressibility that range in convective Mach number from 0.19 to 0.88. The chief benchmark experimental data are high resolution, three-component velocity fields on the streamwise-transverse plane that are acquired via stereoscopic particle image velocimetry. Large ensembles (> 3000) of instantaneous measurements are obtained to confirm fully-developed, self-similar conditions of the mean velocity and each component of the Reynolds stress tensor. Transverse-spanwise plane velocity measurements are also acquired, and their mean velocity results confirm spanwise symmetry. Other flow conditions that are documented include the incoming boundary layer integral parameters on four different transverse location walls and sidewall static pressure distributions for the full streamwise extent of each mixing layer to verify that the test-section pressure is constant. A full uncertainty analysis is provided for each measured and calculated quantity, including the individual Reynolds stresses. Mean spanwise velocity magnitudes are shown to be below the maximum uncertainty values (for a 95% confidence interval). All experimental results for each case, as well as the wind tunnel geometries, are made available to the public on the project website: https://wiki.illinois.edu/wiki/display/NCSLF/. Novel experimental fluid dynamic analyses that are performed regarding compressibility effects on mixing layer turbulence include trends of the full Reynolds stress tensor and its anisotropy, turbulence length scales, dominant dynamic eigenmodes, evolution of the large-scale turbulent structures, and differing entrainment mechanisms, among others. As compressibility is increased, the reduction of the transverse normal, spanwise normal, and primary shear stresses, in conjunction with the constant streamwise normal stress, causes the mixing layer turbulence to become more anisotropic and trend toward one-component, streamwise-dominated turbulence from more isotropic turbulence in the incompressible case. This result is likely related to the turbulence length scales increasing for the streamwise velocity fluctuations in the transverse and streamwise directions, and the flow becoming dominated by streamwise pulsing motions, as compressibility is increased. In contrast, the length scales of transverse velocity fluctuations decrease in the transverse direction with increasing compressibility, and the large vortical structures that span the entire transverse height of the mixing layer, which are present in incompressible cases, become smaller in size and elongated in the streamwise direction. The evolution of the large structures in the mixing layer results in differing turbulent interface geometries for different levels of compressibility, ultimately reducing the length scales of entrainment in more compressible mixing layers. The compressibility effects listed here can also be linked to the unanimously agreed upon result in the literature (including here) of reduced normalized mixing layer growth rate for increased compressibility