Numerical and Experimental Investigations on Highly Integrated Subsonic Air Intakes

Abstract

Aerodynamic integration of air intakes and the optimization of their performance are challenging tasks for innovative design of advanced unmanned aerial vehicles (UAVs). 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 AD/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. The major objectives of AD/AG-46 were to investigate the capability of Detached Eddy Simulation (DES) methods for the analysis of unsteady flow 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 with experimental results. The impact of not considering the wind tunnel walls in the CFD calculations on the computational results was investigated, revealing 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. Since intake lip shaping is a vital design parameter impacting the intake internal flow and performance, the original geometry was compared with a modified cowl while maintaining low-observability features of the W-shaped cowl design. A trade-off study between boundary layer diversion versus ingestion was performed numerically by applying Euler boundary conditions to the walls of the numerical model of the UAV configuration, thus simulating the total removal or diversion of the boundary layer. The computed inviscid results were compared with the viscous data, quantifying the losses in total pressure recovery and the increase in distortion for the ingested test cases. Internal flow control in the intake duct of the UAV configuration was studied by numerically applying vortex generators, and the results were compared with experimental data. Numerical models were employed in order to simulate micro-jets as active flow control devices in the serpentine duct. Increasing of jet velocities resulted in smaller areas of flow separation and thus led to beneficial total pressure recoveries and distortion parameters. 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 of contributing 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 distortion at the engine face were investigated as a function of Mach number, Reynolds number, boundary layer thickness, and intake mass flow ratio.