Unstart/Restart Boundary Broadening Method for a Two-Dimensional Supersonic Variable Inlet Based on a Distributed Bleed System

Compared to the fixed-geometry inlet, the supersonic variable inlet improves its wide Mach number operating capability and efficient cruise property, mainly by regulating the throat area. However, the magnitude of its performance improvement is constrained by the unstart and restart boundaries. To broaden the operating boundaries of the inlet, a distributed bleed system capable of suppressing the shock-dominated internal flow was developed. Its control effect and influence mechanism were investigated by an unsteady numerical simulation with a dynamic grid technique at freestream Mach numbers of 2.5, 3.0, 3.5, and 4.0. The results indicate that the proposed distributed bleed system can adaptively tune the bleed ratio to maintain the dynamic stability of the inlet's flowfield according to the varying wave system in the internal duct. Additionally, the inlet's unstart and restart boundaries were significantly improved. The unstart internal contraction ratio (defined as the area ratio of the duct entrance and throat, denoted as ICR) increased from 1.96 to 2.18, and the restart ICR increased from 1.37 to 1.47 when the freestream Mach number was 3.0. The mechanism of improving the unstart boundary is that the distributed bleed system alters the boundary-layer characteristics of the bleed region, making it thinner and fuller, with stronger resistance to reverse pressure gradients. The mechanism of improving the restart boundary is that the distributed bleed system reduces the spillage by increasing the inlet's throat flow capacity, making the separation shock at the duct entrance impinge earlier on the cowl lip than that of the uncontrolled inlet. Moreover, the control effects of the distributed bleed system at different bleed ratios and under a wide Mach number range were investigated, which will provide guidance for practical engineering design and application.