Large scale coherent structures in a ZPG turbulent boundary layer for the Mach number range 0.3 − 3.0

The presence of large scale coherent structures in various wall bounded turbulent flows, including turbulent boundary layers, has been of great interest in recent years. These meandering high- and low-momentum structures can extend up to several boundary layer thicknesses and contain a relatively large potion of the layer’s turbulent kinetic energy. Therefore, studying these features is important for understanding the overall dynamics of turbulent boundary layers and 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 over a range of Reynolds numbers and Mach numbers are considered in order to investigate the effect of compressibility on coherent structures. More specifically, measurements are performed on a flat plate model in the Trisonic Wind Tunnel Munich (TWM) for 0.3 < Ma < 3.0 and a friction Reynolds number of 2700 < Re_τ < 14 800 or 19 800 < Re_δ2 = ρ_eu_eθ/µ_w < 40 800. Velocity fields are recorded using planar particle image velocimetry methods (PIV and stereo-PIV) in three perpendicular planes, i.e. streamwise-wall-normal (xz), spanwise-wall-normal (yz), and wall-parallel (xy). Using multi-point statistical methods it was found that the streamwise spatial extent of coherent structures in the log-law layer slightly increases with increasing Mach number. Furthermore, a distinct increase in the spanwise spacing of these structures was found for the supersonic cases when compared to the subsonic and transonic turbulent boundary layers.