Investigation of the impact of splitter rotation speed on mode transition characteristics of an over-under turbine-based combined cycle inlet

This paper presents the findings of a wind tunnel experiment aimed at investigating the mode transition process of a two-dimensional over-under turbine-based combined cycle inlet at an incoming flow Mach number of 2.9. The study utilized high-speed schlieren and dynamic pressure acquisition systems to examine the evolution process of the shock-dominated flow structure of the high-speed duct during the mode transition process. Additionally, the impact of mode transition speed on the unstart/restart characteristics of the high-speed duct was analyzed. The results indicate that, during the forward mode transition process, the increasing captured airflow of the high-speed duct leads to a higher number of shock reflections and the shock train moves forward in the duct, ultimately resulting in unstart. The unstarting flow field exhibits a small oscillation characteristic dominated by the separation bubble located at the entrance. However, evident hysteresis characteristics were observed in the restart process during the reverse transition. Furthermore, a higher mode transition speed delays the unstart and restart of the high-speed duct, consequently increasing the hysteresis interval. Theoretical analysis suggests that a larger mode transition speed leads to lower mass accumulation efficiency in the high-speed duct, thereby slowing the pressure response and causing the shock train to lag forward, resulting in delayed unstart. The delay in the restart process is attributed to the relative slip motion of the separation bubble with the upper surface of the splitter, in addition to its forced motion.