S弯收扩喷管过膨胀状态下流动分离特性研究

In order to investigate the flow separation characteristics of S-shaped convergent-divergent nozzle under overexpanded condition，the flow characteristics in S-shaped convergent-divergent nozzle were numerically simulated under different nozzle pressure ratios（NPR）. The results show that at a nozzle pressure ratio of 2.0，asymmetric separation occurs in the nozzle，and an asymmetric double“λ”shock wave structure appears. The jet is deflected upward，with restricted shock separation（RSS）on the upper side and free shock separation （FSS）on the lower side. The three-dimensional separation vortices are generated，and the jet appears to have a significant three-dimensional effect. As the NPR increases from the highly overexpanded condition，the lower side of the nozzle maintains the FSS，and the RSS on the upper side of the nozzle gradually moves backward until it is connected to the outside atmosphere and transforms into FSS. Then as the NPR increases，the upper and lower side separation structure gradually becomes symmetrical. The thrust vector angle increases with the nozzle pressure ratio and then decreases rapidly to 0° in a very small variation range of nozzle pressure ratio，and then remains constant at 0°.