59079 - Inlet Flow Distortion in an Advanced Civil Transport Boundary Layer Ingesting Engine Installation 

This paper presents first-of-a-kind measurements, coupled with full aircraft computations, of the flow through the propulsion system of a boundary layer ingesting twin-engine advanced civil transport aircraft. The experiments were carried out in the NASA Langley Research Center 14- by 22-foot Subsonic Wind Tunnel, using a 1:11 (4-meter wingspan) scale model with electric ducted fans to provide the propulsive power. The configuration examined was the D8 “double-bubble”, which has two engines at the rear of the upper fuselage surface, resulting in the ingestion of roughly 40% of the fuselage boundary layer, corresponding to 17% of the total boundary layer, at the simulated cruise point. Overall vehicle performance measurements plus flow field surveys were carried out. In particular, five-hole probe measurements were made in situ at the propulsor inlet and exit to determine the inlet stagnation pressure and swirl distortions at a range of conditions representing the different operating points encountered in a civil transport mission. The computations were carried out with the NASA OVERFLOW code, extended to include a description of the flow within the propulsor. The measurements and computations include a range of aircraft angles of attack and propulsor powers, representing operating points over the range of the aircraft mission. The results thus furnish a unique information set to define both the creation of the inlet flow non-uniformity and the interaction between the aircraft and the propulsor that produces the flow field experienced by the propulsor.

Velocity and pressure distributions at the propulsor inlet and exit, and integral inlet distortion metrics, are presented to illustrate the flow distortion at different operating points. The distortions show qualitative and quantitative changes over the mission, from unidirectional stratified stagnation pressure at cruise to a defined streamwise vortex structure at larger angles of attack. The results show there are three mechanisms that determine the BLI propulsor inlet conditions. The first two, reduced momentum in the ingested boundary layer and the generation of streamwise vorticity through the rolling up of the fuselage boundary layer, are external flow features set by the aircraft angle of attack. The third mechanism, fan-distortion interaction, can be conceptually divided into far-field and near-field effects. In the far-field, the mass flow of the propulsor determines the stream tube capture area far upstream and the bulk trajectory of the ingested flow features. In the near-field, the non-uniform response of the fan to the ingested distortion results in axial velocity non-uniformity attenuation and associated swirl generation just upstream of the fan. Comparison of the experimental and numerical results shows a constant pressure-rise actuator disk well captures the far-field effect, but a higher-fidelity model is required to describe the near-field interactions and distortion transfer across the fan. Finally, comparison of the details of the inlet flow field with conventional integral distortion metrics shows that the latter are ill-suited as surrogates for BLI fan efficiency, which depends on circumferential and radial non-uniformities in axial and swirl velocity.