Study of the unsteady mode transition process for an over-under TBCC exhaust system

The present study focuses on the unsteady mode transition process of an over-under TBCC exhaust system. The method of characteristics is applied to design the over-under TBCC exhaust system according to the entrance parameters of the turbine nozzle and ramjet nozzle at the design point. The dynamic mesh is adopted to adjust to the update of the computational domain, and the unsteady numerical method is employed to simulate the dynamic flowfield of the exhaust system during the mode transition process. The results show that the flowfield structure and the performance vary greatly during the mode transition. Owing to the interaction between the turbine exhaust jet and ramjet plume, the flowfiled in the turbine nozzle is affected by the ramjet exhaust jet considerably. The axial thrust of the turbine nozzle decreases, while that of the ramjet nozzle increases gradually during the mode transition, but the total axial thrust of the entire exhaust system varies smoothly. Both the axial thrust coefficient and pitching moment of the exhaust system increase along with the open of the ramjet nozzle, while the result for the lift is contrary. However, the axial thrust coefficient, lift and pitching moment all decrease rapidly with the shutdown of the turbine nozzle, and the decreases in axial thrust coefficient, lift and pitching moment are 1.04%, 67.15% and 80.92%, respectively. Besides, two sudden change of the axial thrust coefficient exist at the beginning and end of the motion of the splitter plate.