Abstract

Wide-field and high-speed Particle Image Velocimetry measurements were performed in a Mach 2 supersonic turbulent boundary layer to characterise the structure of large-scale coherence. Instantaneous velocity fields in the logarithmic region reveal the presence of elongated uniform low- and high-speed regions. These elongated regions exhibit strong similarities with the large-scale motions found in incompressible boundary layers. Application of Taylor’s hypothesis together with high-speed PIV data indicates that these large-scale structures could extend over 30-40 boundary layer thickness in length. Alternately, these can interpreted as 5-10 boundary layer thickness long structures lasting over several integral time scales. Regardless of interpretation, these elongated (spatio-temporal) structures appear to be play a significant role in the dynamics of shock-induced turbulent boundary layer separation. A possible mechanism that relates the passage of these elongated structures to the frequency of unsteadiness of shock-induced separation will be presented.