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

Aerodynamic integration of air intakes with increasingly compact shaping and the optimization of their performance are challenging tasks for innovative design of advanced unmanned aerial vehicles (UAVs) featuring superior combat or reconnaissance abilities. In order to meet configurational requirements, diverterless intake designs with optimized entry shaping and sophisticated serpentine duct layout are primary goals in the overall development process. These design challenges, however, can generate intake flow characteristics, which can adversely impact the aerodynamic performance of the intake and the engine/intake compatibility. The extension of Computational Fluid Dynamics (CFD) into application areas such as dynamic intake distortion prediction and thus engine/intake compatibility is made possible by modern hybrid methods and increasing computer resources. Within the Aerodynamics Action Group AG-46 "Highly Integrated Subsonic Air Intakes" of the Group for Aeronautical Research and Technology in Europe (GARTEUR) CFD computations were carried out for the EIKON UAV configuration which was designed and wind tunnel tested at FOI in Sweden. Partners in the international collaboration of AD/AG-46 were AIRBUS Defence and Space (Germany, Chair), ONERA (France, Vice-Chair), FOI (Sweden), AIRBUS Defence and Space (Spain), SAAB (Sweden), DLR (Germany), ALENIA Aermacchi (Italy), and MBDA (France). A major objective of AG-46 was to investigate the capability of Detached Eddy Simulation (DES) methods to analyse unsteady flow phenomena of serpentine air intakes and their accuracy levels. 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 AIP very well demonstrate the highly turbulent flow in the separated region downstream of the S-duct and allow the comparison of the dynamic distortion behaviour with steady-state performance as well as experimental data, revealing an improved prediction of the time-averaged DC60 value with a DES simulation. Prior to CFD computations, investigations of a potential influence of not considering the wind tunnel walls in the CFD calculations on the computational results were performed. The data revealed that the ventilated walls of the T1500 wind tunnel eliminate the blockage of the model within the closed test section and that free stream conditions can be applied for the computational boundary conditions. According to these results a well validated comparison between the CFD results and the experimental data for the UAV configuration could be expected. A numerical study on intake lip shaping, which is a vital design parameter impacting aerodynamic drag and intake performance, was conducted, comprising an alternative round cowl design while maintaining low-observability features of the original W-shaped sharp intake cowl. A comparison of CFD results for the aerodynamic forces produced by the original sharp cowl design and the modified round cowl was performed. The drag and lift breakdown for the individual parts of the wind tunnel model as well as for the intake cowl itself allowed an improved assessment of the sources of the aerodynamic forces. Valid insight into the design of intake lips for innovative UAV configurations could be gained. The impact of boundary layer ingestion versus boundary layer diversion was investigated in a trade-off study. Computations were performed applying Euler boundary conditions at the forebody, thus simulating the total removal or diversion of the boundary layer. The computed inviscid results were compared with the viscous data. Eliminating the boundary layer resulted in decreased total pressure losses and improved total pressure recoveries at the intake throat by approximately 2%, and led to an improvement of the distortion level in the AIP. Internal passive flow control was investigated by employing numerical models for the simulation of vortex generators in the intake duct, and active flow control was studied by applying devices in form of micro-jets. Results were compared with experimental data. At DLR in Göttingen 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.