Aerodynamic/stealth design of S-duct inlet based on discrete adjoint method

It is a major challenge for the airframe-inlet design of modern combat aircrafts, as the flow and electromagnetic wave propagation in the inlet of stealth aircraft are very complex. In this study, an aerodynamic/stealth optimization design method for an S-duct inlet is proposed. The upwind scheme is introduced to the aerodynamic adjoint equation to resolve the shock wave and flow separation. The multilevel fast multipole algorithm (MLFMA) is utilized for the stealth adjoint equation. A dorsal S-duct inlet of flying wing layout is optimized to improve the aerodynamic and stealth characteristics. Both the aerodynamic and stealth characteristics of the inlet are effectively improved. Finally, the optimization results are analyzed, and it shows that the main contradiction between aerodynamic characteristics and stealth characteristics is the centerline and cross-sectional area. The S-duct is smoothed, and the cross-sectional area is increased to improve the aerodynamic characteristics, while it is completely opposite for the stealth design. The radar cross section (RCS) is reduced by phase cancelation for low frequency conditions. The method is suitable for the aerodynamic/stealth design of the aircraft airframe-inlet system.