Mach Number and Separation-Scale Effects on the Unsteady Dynamics of Shock/Boundary-Layer Interaction Using Fast-Response PSP and Time-Resolved PIV*

The dynamics of shock/boundary layer interactions are experimentally investigated at two different Mach numbers and with different separated flow length-scales. The primary diagnostics include fast-response pressure-sensitive paint (PSP) and time-resolved particle image velocimetry (PIV). Three different cases of SBLI are studied: (i) Mach 2 with 20 degree compression ramp (weak interaction), (ii) Mach 5 with 26.5 degree ramp (weak interaction), and (iii) Mach 5 with 28 degree ramp (strong interaction). By analyzing surface pressure fields, the spatio-temporal organization of flow structures was examined, revealing streamwise-elongated structures, likely Gӧrtler vortices, which are present in the Mach 2 SBLI but are much more prominent in the Mach 5 case. The pressure data also show the SBLI dynamics are significantly more influenced by fluctuations in the upstream boundary layer at Mach 2 than Mach 5. Both the weak and strong Mach 5 interactions exhibit similar dynamics. Time-resolved PIV (x-y plane) was taken at 50 kHz for Mach 2 and 100 kHz for Mach 5. From the velocity-field movies, we are able to analyze turbulent structures in the incoming boundary layer as well as those in the shear layer and recovering boundary layer. Statistical analyses, including coherence, cross-correlation and conditional analysis, are used to understand and compare the physical mechanisms underlying the unsteady dynamics of ramp-induced SBLI in each case.