The impact of surface roughness geometry on aero-engine intakes at incidence

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Shock Wave-Boundary-Layer Interactions, or SBLI’s, are known to form on engine inlets within a complex transonic flow-field during typical take-off and climb configurations. On the engine inlet, there are a number of potential sources of surface roughness, such as novel de-icing and acoustic systems, or surface contamination. The impact on the flow-field structure, as a result of this roughness, may lead to detrimental side effects, such as losses in engine efficiency or intake flow stability. Previous research into two-dimensional roughness shapes demonstrated flow-field changes, for example a thicker downstream-boundary layer compared to a smooth surface. This paper compares the impact of a two-dimensional ridge roughness to a three-dimensional cubed roughness on the inlet flow-field. The effect of these rough surfaces is examined with schlieren photography and Laser Doppler Velocime-try (LDV) techniques. At an on-design condition, a rough surface promotes a smaller supersonic region, and a thicker boundary-layer downstream of the interaction compared to a smooth surface. At off-design upper surface mass flow rate conditions, modelling a higher mass flow engine demand, the supersonic region grows, leading to a shock location further downstream. Under these conditions, roughness also promotes a thicker downstream boundary-layer. However, comparing the two-dimensional with three-dimensional roughness at an approximate fan-face location, shows that three-dimensional roughness is more benign for all off-design cases. This suggests that the topology of the roughness is influencing the condition of the boundary-layer at this location.