Numerical study on hypersonic nozzle-inlet starting characteristics in a shock tunnel

An unsteady viscous numerical simulation is performed to study the starting process of a hypersonic nozzle coupled with a simplified inlet model in a shock tunnel under the condition of inflow Mach number higher than the design value. And the effect of the initial backpressure of test section in shock tunnel on the pulse-starting characteristics of the hypersonic nozzle-inlet is studied. It is found that the operation mode of inlet changes from local unstart to start with the initial backpressure decreasing. At high initial backpressure, two large separation bubbles are induced by the secondary shock at the nozzle walls, the separation induced shock waves repeat reflections between the nozzle walls and a shock train structure appears. The Mach number of the flow into the test section rises and falls repeatedly as a result of the presence of the successive shocks in the shock train structure. During the hypersonic nozzle-inlet starting process, a large separation bubble occurs at the forebody side and the inlet is choked. After the forebody shock moving close to the cowl lip, a bow shock is generated and moves before the cowl lip subsequently. Finally, the quasi-steady flowfield of the inlet is established with a bow shock before the cow lip and a subsonic region at the cowl side, namely the inlet is local unstarted. At low initial backpressure, the separation bubbles at the nozzle walls vanish and no shock train structure appears. Furthermore, an unsteady expansion wave may appear upstream of the secondary shock and the Mach number of the flow following the secondary shock is higher than that of the nozzle quasi-steady flow. The Mach number of the flow into the inlet increases to the maximum value quickly, the inlet starts successfully. The numerical results show that the inlet starts easier and faster with lower initial backpressure.