Aerodynamics and Power Balance of Boundary Layer Ingesting Aircraft
By ingesting the fuselage Boundary Layer (BL) and re-energising it, Boundary Layer Ingestion (BLI) offers the potential for considerable fuel savings. However, it remains unclear which the most promising aircraft configuration for BLI is. Furthermore, the tight integration between the propulsor and the airframe poses great challenges to design and analysis. The aim of this thesis is to provide the first detailed assessment of the flow field and BLI performance of a novel distributed aft-fuselage BLI aircraft. In particular, the focus is on the link between the flow features to the mechanical power losses within the flow field and their consequent impact on BLI performance.
A fully-coupled computational test case featuring the aft-fuselage installation and propulsor geometries was set up to resolve the external and propulsor internal flow fields. The unsteady simulations were carried out using the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach. Using the computed flow field, a mechanical power balance was drawn up to analyse its performance and identify the loss sources. By varying the boundary conditions of the simulation and/or BLI fan rotational speed, the effects of changes in fuselage BL thickness, aircraft cruise Mach number and BLI propulsor Fan Pressure Ratio (FPR) were also investigated.
The baseline flow field of the BLI aircraft at cruise shows that the exhaust region incurs a significant amount of loss due to the mixing of the exhaust jet and the BL wake. With a thinner incoming BL, a different mode of flow field was observed, with a significant increase in loss caused by supersonic flow and separation over the cowl. This highlights the challenges associated with the robustness of the installation design and operability of the aircraft. As the flight Mach number is reduced, the flow over the cowl is improved, but with the same FPR, this leads to increased loss in the exhaust region due to the higher ratio of jet to flight speeds, which has a strong effect on the mixing loss. At reduced FPRs, the over-speed in the jet relative to the freestream is reduced, but mixing with the wake can leave behind a larger velocity non-uniformity and therefore energy outflow downstream if the FPR is too low. Reducing the fan speed also leads to a redistribution of the losses within the fan stage, as the rotor shock loss is decreased, whilst the changes in incidence onto the rotor and stator see larger variations.
Besides identifying the important flow features specific to this novel BLI concept and showing their links to mechanical power loss, this thesis makes a contribution to the simulations and analysis of highly integrated BLI systems. Through the URANS simulations, the interaction between the fuselage BL turbulence and the BLI propulsor was identified as an area that warrants further research. It is hoped that this research will be applicable to not only BLI aircraft, but also future aircraft concepts that feature tightly integrated propulsion systems.