Mechanism Analysis of Start and Unstart Flow Characteristic Structures of a Supersonic Inlet

Based on the supersonic inlet model in Wagner's experiment, RANS-SST was used in numerical simulation to analyze the inlet start and unstart flow field characteristics. By broadening the computational domain and adopting the free development method of boundary layer, the possible periodic oscillatory flow phenomena in the process of the inlet unstart, such as the high-pressure generation and reduction inside the inlet, the expansion and contraction of large-scale separation bubbles at the lower wall, and the unstart shock propagation, were accurately predicted. For inlet start at the flap angle of 0, the wave structures, wall pressure, and flow velocity distributions were consistent with the existing experimental results. For inlet unstart at the flap angle of 28, the oscillation period and amplitude of the flow field were in agreement with the experimental data. The dynamic mode decomposition of the unsteady flow of the inlet unstart was carried out. In the flow field mode with the main frequency f₁=69.8 Hz, the pressure oscillation at the outlet of the inlet is the strongest, while the velocity oscillation at the inlet and the upper wall is the strongest. In addition, the flow field modes captured by double frequency f₂=139.7 Hz and triple frequency f₃=209.5 Hz are mainly discrete small-scale high-energy structures. In the oscillation process of the inlet unstart, the flow field outside the inlet produces large velocity and pressure pulsation. Therefore, the accurate description of the interaction between the fields inside and outside the inlet is an important factor to predict the inlet unstart oscillatory flow.