Experimental characterization and analysis of unsteady normal-shock boundary-layer interaction

This study experimentally investigates the interaction of a normal shock wave and the turbulent boundary layer at an upstream Mach number of 1.35 in the transonic wind tunnel S8Ch at ONERA Meudon. Wall-mounted unsteady pressure measurements and high-speed schlieren imaging were combined to measure the shock–boundary layer interaction (SBLI). A small separation region was observed along the centerline. Spectral analysis of the reconstructed wall pressure signals revealed distinct frequency ranges associated with different flow features: shock wave ( St~ 0.01 S t ∼ 0.01 , peaking at St≈ 0.013 S t ≈ 0.013 ), separation point ( St≈ 0.04 - 0.25 S t ≈ 0.04 - 0.25 ), and mixing layer ( $$St > rsim 0.3$$ S t ≳ 0.3 ). It was observed that in the separation region, time scales associated to the shock motion and mixing layer coexist. Spectral Proper Orthogonal Decomposition (SPOD) of schlieren images revealed coherent structures within the shear layer as well as low-frequency unsteadiness linked to the shock motion. Finally, synthetic image analysis showed that sharp discontinuities such as shocks can introduce additional modes at higher harmonics, highlighting the importance of careful interpretation in SBLI studies.