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.

This study aims to investigate the intricate dynamic characteristics of the high-speed duct during the over-under Turbine-Based Combined Cycle (TBCC) inlet mode transition process while operating in an off-design state under throttled conditions. A typical over-under TBCC inlet, designed for a working Mach number range of 0–6 with a transition Mach number of 3.5, is examined through experimental studies in a supersonic wind tunnel with a freestream Mach number of 2.9. The investigation focuses on the complex oscillatory flow and unique hysteresis observed in the mode transition process of the high-speed duct under the mildly throttled condition, utilizing high-speed schlieren and dynamic pressure acquisition system. The findings reveal that the high-speed duct undergoes four distinct oscillation stages akin to those in a higher throttled state during the mode transition, albeit with smaller dominant frequency and energy. Moreover, an irregular alternating “big/little buzz” mode is observed in the early stage of the large oscillation stage. Notably, the mildly throttled state exhibits three intriguing hysteresis properties compared to the unthrottled and higher throttled states. Firstly, hysteresis is observed in the shock train motion stage in the duct before unstart, along with the corresponding inverse process. Subsequently, hysteresis is noted in the unstart and restart of the high-speed duct, with a smaller hysteresis interval than in the unthrottled state. Finally, the hysteresis characteristics of oscillation mode switching and the corresponding inverse process are explored. Based on the analysis, the first two hysteresis phenomena are associated with the formation and dissipation of the separation bubble. The significant adverse pressure gradient constrains the cross-sectional capacity of the channel, rendering the disappearance of the separation bubble more challenging. The hysteresis in oscillation mode switching is linked to not only the channel cross-sectional capacity but also the state of the incoming boundary layer.

The study presents an experimental exploration into the mode transition of an over-under TBCC (Turbine-Based Combined Cycle) inlet, with a specific emphasis on the flow characteristics at off-design transition Mach number. A systematic investigation was undertaken into the mode transition characteristics in both unthrottled and throttled conditions within a high-speed duct, employing high speed Schlieren and dynamic pressure acquisition systems. The results show that the high-speed duct faced flow oscillations primarily dictated by the separation bubble near the duct entrance during the downward rotation of splitter, leading to the duct’s unstart under the unthrottled condition. During the splitter’s reverse rotation, a notable hysteresis of unstart/restart of the high-speed duct was observed. Conversely, hysteresis vanishes when the initial flowfield nears the critical state owing to downstream throttling. Moreover, the oscillatory diversity, a distinctive characteristic of the high-speed duct, was firstly observed during the mode transition induced by throttling. The flow evolution was divided into four stages: an initial instability stage characterized by low-frequency oscillations below 255 Hz induced by shock train self-excitation oscillation and high-frequency oscillations around 1367 Hz caused by the movement of separation bubble. This stage is succeeded by the “big buzz” phase, comprised of pressure accumulation/release within the overflow-free duct and shock motion outside the duct to retain dynamic flow balance. The dominant frequency escalated with the increase of the internal contraction ratio in the range of 280 Hz to 400 Hz. This was followed by a high-frequency oscillation stage around 453 Hz dominated by a large internal contraction ratio with low pulsating energy, accompanied by a continuous supersonic overflow. Lastly, as the splitter gradually intersected the boundary layer of the first-stage compression surface, the capture area and the turbulence intensity of the incoming flow underwent a sudden shift, leading to a more diverse flow oscillation within the duct, manifested as various forms of mixed buzz.