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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. [ABSTRACT FROM AUTHOR]

As a practical rounded trailing edge airfoil for coaxial rotors, DBLN-526 is a fore and aft symmetrical airfoil with two steps on its lower side. This airfoil has been used at the inboard section of the coaxial rotor system. As there are always two eddies behind the airfoil because of its rounded trailing edge, the interaction between the separation and transition should be considered. Thus, the unsteady Reynolds-averaged Navier-Stokes- (RANS-) based γ − Re ¯ θ t model was used to analyze the unsteady transition and separation features. Three different rounded trailing-edge airfoils were compared with DBLN-526. Power spectrum density analysis and Δ Cl calculations demonstrated that the lift coefficient fluctuation of DBLN-526 was smaller than that of other rounded airfoils with different angles. Further investigation indicated that the locations of transition for DBLN-526 can be fixed at a wide range of angles by the unique design on its lower side. Because of this settled transition location, the size of separation is decreased, and the position of separation is settled as well, which leads to a lower lift coefficient fluctuation. The turbulent kinetic energy after the transition was higher, which injected a lot of energy into the boundary layer, and the separation zone near the transition position was relatively smaller. This study provides an indication for controlling separation and reducing unsteady fluctuations for rounded trailing edge airfoils. [ABSTRACT FROM AUTHOR]