Inlet Distortion Characterization of the Boundary Layer Ingesting D8 Aircraft

Abstract

This thesis experimentally assesses the inflow towards the propulsors and the pressure distribution at the propulsor fan-face for the boundary layer ingesting D8 aircraft, and examines the dependence of the model, the propulsor and the flight condition on the inlet distortion. Use is made of mini-tuft flow visualization and five hole probe pressure surveys. The results are compared with CFD simulations. The experiments were performed at the most important mission points of the D8: cruise, descent, start of climb, and top of climb. CFD was only performed for cruise and top of climb. From the pressure distributions the distortion coefficient, DC(60), was calculated, the maximum variation in pressure over a specified circumferential segment (60°). At cruise the DC(60) equaled ~0.3, compared to DC(60)~0.1-0.2 for conventional aircraft. The D8 model caused cross-flow to the propulsors, the flow is directed towards the sides of the model. Both fans rotate in the same direction, such that one propulsor has the flow in the direction of rotation, and the other has the flow opposite to the direction of rotation, causing an asymmetry between the left and right propulsor. The flight phase is characterized by ?, the angle of attack, ?, the ratio of tip velocity over tunnel speed, and ?, the yaw angle. It is found that at a high value of ? the pressure differences at the fan-face are reduced by engine suction, lowering the distortion and counter-acting the cross-flow. A low value of ? means a relative lower influence of the propulsor on the flow, such that the propulsor is not able to (fully) counter-act the cross-flow, resulting in a higher difference in DC(60) and power required between the left and right propulsor. Changing ? mainly changes the location of the pressure distributions. The results from experiments agree well with CFD, there is a 1% deviation in DC(60) at top of climb condition, and 6% at cruise. The pressure distributions look similar and the pressure coefficient values scale equally, from -0.8 to 0. Further research should focus on the exact fan response on the distortion. The D8 used conventional engines, optimized for uniform inflow. Developing a BLI optimized engine could further increase the BLI benefit. The D8 model induced cross-flow, resulting in an asymmetry between the left and right engine. Eliminating this cross-flow by a change in model design could also decrease the distortion.