Wind tunnel testing of a nacelle bypass concept for a quiet supersonic aircraft

Sonic boom attenuation is a considerable design challenge to enable civilian aircraft to operate at supersonic flight conditions. One technology proposed by Gulfstream Aerospace for the production of low-noise supersonic aircraft is the high-flow nacelle bypass concept in which an outer nacelle surface is used to encircle the asymmetric external engine protuberances of a traditional turbine engine. Although this bypass flow may reduce the overall sonic boom signature of the vehicle, the engine mechanics and mounting create a highly complex 3-D flow in the annular bypass region. To better understand the 3-D flow features, an approximately 1/6^th engine model has been tested complete with fairings to transition the flow smoothly around the protuberances. The model was installed in a newly constructed 11.1 inch diameter axisymmetric tunnel at the University of Illinois to simulate the bypass flow path. Pressure measurements were taken over a range of circumferential angles and radial positions and are used to create planar maps of total and dynamic pressure upstream and downstream of the bypass model. Wind tunnel testing was performed on the empty axisymmetric wind tunnel followed by model configurations of increasing complexity until a full test configuration of the engine model with all the gearbox fairings and crane beam mounts was achieved. Results show significant pressure losses behind the gearbox fairing with a large flow recirculation zone, which was also characterized by surface flow visualizations. Due to the blockage created by the gearbox fairing mounted at the underside of the model, results also show increased flow velocity in the upper section of the bypass duct. These results are currently being utilized to confirm computational results of the flow field and to aid in the design of the next generation of fairings and crane beam supports.