Effect of Mach Number on Flow Past Microramps
Micro-vortex generators (μVGs) have the ability to alter the near-wall structure of compressible turbulent boundary layers to provide increased mixing of high speed fluid such that the boundary layer remains healthy even with some disturbance imparted to the flow. Due to their small size, μVGs are embedded in the boundary layer and may provide reduced drag when compared to traditional vortex generators. In particular, micro-ramps are of significant interest as they are cost-effective, physically robust, and do not require a power source. To examine their potential, a detailed computational study of micro-ramps in a supersonic boundary layer at M=1.4, 2.2 and 3.0 was undertaken. The moderate Reynolds number of 3,800 allowed the computations to use monotone integrated Large Eddy Simulations (MILES) which generally reproduced the experimentally obtained mean velocity profiles, although some differences were found with respect to grid resolution. The MILES results indicate that micro-ramps have greater impact at lower Mach number near the device where its influence decays faster than that for the higher Mach number cases. This may be due to the reduced aspect ratio of the turbulent structure at the lower Mach number such that their coherency is easily lost yielding the streamwise vorticity and the turbulent kinetic energy to decay quickly. The normal distance between the vortex core and the wall had similar growth indicating weak correlation with the Mach number; however, the spanwise distance between the two counter-rotating cores further increases with lower Mach number.