Causality in the shock-wave/turbulent boundary layer interaction

The present work tackles the study of the unsteady behaviour of an impinging oblique shock and its interaction with a Mach 2 turbulent boundary layer. The investigation is made through the large-eddy simulation presented in Jiang \textit{et al.}, 2017, which has been extensively validated against experimental data. The main objective is to track the causes of the low-frequency fluctuations within the interaction zone, leading to the unsteadiness of the shock foot position. Evaluation of the spectrum in this region indicates that it is dominated by two-dimensional fluctuations which can be isolated via averaging in the spanwise direction. Data-driven approaches such as empirically derived transfer functions and spectral proper orthogonal decomposition (SPOD) are then performed in the averaged flow field. The results indicate the existence of a feedback mechanism between downstream fluctuations and the shock motion. Furthermore the leading SPOD mode comprises upstream travelling waves and enables the reconstruction of a significant portion of the energy of the shock motion from downstream measurements only. The current results indicate that downstream fluctuations are the driving mechanism behind the unsteady shock fluctuations.