Aerodynamic shape optimization and performance improvement of a slot-shaped boundary-layer-ingesting inlet

Achieving high aerodynamic performance in boundary layer ingesting (BLI) inlets is crucial for consistently enhancing propulsion efficiency in BLI propulsion systems. This paper conducts a multi-objective optimization design on an inlet capable of ingesting a substantial boundary layer, with the goal of improving the total pressure recovery coefficient and reducing the total pressure distortion. The gradient-based shape optimization method, based on a discrete adjoint approach, is employed for the inlet optimization. Three typical solutions selected from the Pareto front are compared with the original BLI inlet to analyze their internal flow characteristics and the mechanisms underlying their performance enhancements. The results indicate that, compared to the baseline geometry, the optimization leads to consistent improvements in all objectives of the composite objective function, with the degree of improvement related to the weight of each objective within the composite function. The Pareto curve shows that the optimization can achieve a maximum reduction in total pressure losses of 1.15% and a maximum reduction in total pressure distortion of 84.36%, albeit with an increase in swirl intensity. It was also found that the optimizer increased the downstream cross-sectional area of the inlet, reducing the mainstream velocity and thereby decreasing flow friction losses, which improved the total pressure recovery coefficient. Furthermore, the optimizer altered the flow separation position and type within the flow passage by adjusting the aerodynamic shape and the resulting local pressure gradient, redistributing the low-energy flow at the outlet toward the center of the outlet cross section, thereby enhancing the total pressure distortion performance.