Unstart Sensitivity of Hypersonic Streamtraced Inlets During Angle-of-Attack Operation

The propensity of a streamtraced hypersonic Busemann or “sugar scoop” inlet to unstart is experimentally and computationally investigated at positive and negative angles of attack. The backpressure was applied at the isolator exit in the form of an aerodynamic blockage from a high-momentum jet to trigger inlet unstart. High-speed schlieren photography revealed two distinguishable responses during backpressured operation at different angles of attack. At low backpressures, a “weak response” was observed where pseudoshocks were disgorged from the inlet’s internal contraction passage but with a supersonic flow channel present at the cowl closure. When the backpressure was increased, a “strong response” was observed where the entirety of the flow entering the internal contraction segment of the inlet was subsonic, which evidenced the completion of the unstart process. The weak response was observed at a lower backpressure setting with increasing angles of attack. However, a smaller subsequent increase in backpressure was required to achieve the strong response at lower angles of attack compared to higher angles of attack. Computational simulations anchored on the schlieren imagery and two-dimensional pressure fields, measured in the inlet external contraction region, were performed to probe the disparity in the unstart response across different angles of attack. The differences in operational limits and sensitivity margins were attributed to the viscous effects and momentum fluxes through the inlet/isolator.