Numerical and Experimental Investigations on Subsonic Air Intakes with Serpentine Ducts for UAV Configurations

Aerodynamic integration of diverterless air intakes with increasingly compact serpentine shaping and the optimization of their performance as well as engine/intake compatibility are challenging tasks for innovative design of advanced unmanned aerial vehicles (UAVs) featuring superior combat or reconnaissance abilities. Within the Aerodynamics Action Group AG-46 "Highly Integrated Subsonic Air Intakes" of the Group for Aeronautical Research and Technology in Europe (GARTEUR) various technological aspects were investigated in order to advance intake design solutions. By applying modern hybrid Computational Fluid Dynamics (CFD) methods flow simulations were carried out for the EIKON UAV configuration which was previously designed and wind tunnel tested at FOI in Sweden. A major objective was to assess the capability of hybrid methods for the analysis of unsteady phenomena of serpentine air intakes and the accuracy levels of the computations. Numerical results for a variety of wind tunnel conditions were compared with Reynolds-Averaged Navier-Stokes (RANS) and unsteady RANS (URANS) data as well as experimental results. The time evolutions of distortion coefficients (e.g. DC60) at the aerodynamic interface plane (AIP) very well demonstrate the highly turbulent flow in the separated region downstream of the S-duct and allow the comparison of the dynamic intake distortion behavior with steady-state performance as well as experimental data, revealing an improved prediction of the time-averaged DC60 value with hybrid methods. A numerical study on intake lip shaping was conducted allowing an improved assessment of the sources of the aerodynamic forces. The impact of boundary layer ingestion versus boundary layer diversion was investigated in a trade-off study. Eliminating the boundary layer resulted in improved total pressure recoveries at the intake throat by approximately 2%. Internal passive flow control was studied by employing numerical models for the simulation of vortex generators in the intake duct, and active flow control was researched by applying devices in form of micro-jets. Results were compared with experimental data. At DLR in Gottingen experiments with a generic high aspect ratio diverterless intake model were performed in the cryogenic blowdown wind tunnel DNW-KRG with the goal to contribute to a better understanding and correlation of installed performance predictions of highly integrated innovative intake designs. In a parametric study the combined effects of boundary layer ingestion and an S-shaped intake diffuser on total pressure recovery and dynamic distortion at the engine face were investigated as a function of Mach number, Reynolds number, boundary layer thickness and intake mass flow ratio.

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