Investigation on three-dimensional unsteady flow characteristics during TBCC inlet mode transition using fast-responding PSP

This study investigates the three-dimensional unsteady flow characteristics during the mode transition process of a Turbine-Based Combined Cycle (TBCC) inlet under side-wall constraints. The research focuses on an over-under TBCC inlet with a design transition Mach number of 3.5 and a transition speed of 1.65 (°)/s, in a wind tunnel with incoming flow Mach number 2.9. A novel approach combining fast-responding Pressure-Sensitive Paint (PSP) measurement technique with high-speed schlieren imaging and dynamic pressure transducers was employed to achieve the three-dimensional dynamic measurement of shock-dominated flow evolution during mode transition. The Proper Orthogonal Decomposition (POD) method was utilized to reveal the three-dimensional dynamic mechanisms of two typical unsteady flows following the high-speed duct unstart. When the duct is fully open, the flow separation caused by throat congestion exhibits significant spanwise asymmetry under side-wall constraints. The phase difference between separation vortices in the central and corner regions induces a low-frequency oscillation of the separation shock, predominantly characterized by spanwise fluctuations. Conversely, in the hysteresis state, the corner effect is substantially diminished, leading to increased spanwise flow uniformity. The separation shock oscillation converts to a streamwise motion mode, primarily governed by the self-excited oscillation of the downstream separation bubble and the shoulder shock train. This study provides comprehensive clarification of the multi-scale flow coupling mechanism during the TBCC inlet mode transition process from a three-dimensional spatiotemporal evolution perspective. The findings offer crucial theoretical support for the dynamic stability control of hypersonic combined cycle inlets.