Large-scale coherent structures in compressible turbulent boundary layers

The presence of large-scale coherent structures in various wall bounded turbulent flows, often called superstructures in turbulent boundary layers (TBLs), has been of great interest in recent years. These meandering high- and low-momentum structures can extend up to several boundary layer thicknesses in the streamwise direction and contain a relatively large portion of the layer's turbulent kinetic energy. Therefore, studying these features is important for understanding the overall dynamics of turbulent boundary layers and for the development of flow control strategies or near-wall flow modifications. However, compared to the extensive number of incompressible investigations, much less is known about the structural characteristics for compressible turbulent boundary layer flows. Therefore, in this investigation turbulent boundary layers developing on a flat plate with zero pressure gradient (ZPG) over a range of Reynolds numbers and Mach numbers are considered in order to examine the effect of compressibility on superstructures. More specifically, measurements are performed on a flat plate model in the Trisonic Wind Tunnel Munich (TWM) for the Mach number range 0.3 <= Ma <= 3.0 and a friction Reynolds number range of 4700 <= Re-tau <= 29 700 or 11 730 <= Re-delta 2 = rho(e)u(e)theta*/mu(w) <= 74 800. Velocity fields are recorded using planar particle image velocimetry methods (PIV and stereo-PIV) in three perpendicular planes. Using multi-point correlation and spectral analysis methods it was found that the most energetic frequencies have slightly longer streamwise wavelengths for the supersonic case when compared to the subsonic case. Furthermore, a distinct increase in the spanwise spacing of the superstructures was found for the supersonic cases when compared to the subsonic and transonic turbulent boundary layers.