Measurement of boundary layer velocity profiles with spatially and temporally varying surface temperature in high-speed flow

This study presents the first time-resolved LDA measurement of streamwise streak generation induced by non-uniform surface temperature distributions in a Mach 2.75 supersonic turbulent boundary layer. To investigate the boundary layer’s response to transient thermal forcing, a flat plate is constructed from alternating streamwise strips of materials with dissimilar thermal diffusivities (copper and MACOR). Preheating the model prior to wind tunnel start-up generates controlled, spanwise periodic surface temperature gradients. To capture the rapid evolution of the flow field, a continuous-motion traverse system is employed for Laser Doppler Anemometry (LDA). This system enables the acquisition of full velocity profiles within 2 s, a timescale commensurate with the transient wall temperature evolution. The fidelity of this time-resolved technique is validated by the close agreement of the reconstructed velocity profiles with theoretical compound boundary layer models (Musker and non-adiabatic wake laws). A physics-informed thermal model is used to reconstruct the wall temperature history. The measurements successfully capture the evolution of the velocity profiles and quantify the formation of coherent velocity streaks. Streak amplitudes reach approximately 5% of the freestream value in the buffer layer and persist at 2% in the outer layer. Furthermore, the results reveal a linear relationship between the spanwise wall temperature difference and the generated streak amplitude, establishing passive thermal forcing as a robust and predictable mechanism for flow control.