1 Copyright © 2015 by Rolls-Royce plc Proceedings of the 14 th International Symposium on Unsteady Aerodynamics, Aeroacoustics & Aeroelasticity of Turbomachines ISUAAAT14 8-11 September 2015, Stockholm, Sweden I14-S12-1 ON LIP STALL SUPPRESSION IN POWERED INTAKE: HIGH AND LOW FIDELITY APPROACH Mauro Carnevale Department of Mechanical Engineering Imperial college of London, SW7 2BX, UK m.carnevale@imperial.ac.uk Jeff S. Green Rolls-Royce plc Derby, DE24 8BJ,UK Luca di Mare Department of Mechanical Engineering Imperial college of London, SW7 2BX, UK ABSTRACT This work describes a computational and analytical study of the flow in a turbofan intake at high incidence. It is well known that lip separation can occur above a certain angle of flight incidence, depending on the flight Mach number, Reynolds number and engine mass flow rate. There has been a steady rise in the lower lip loading in turbofan installations in recent years. This trend is set to continue because of the necessity to reduce the installation weight and drag in view of the introduction of ultra-high bypass ratio engines. This makes designs with shorter and thinner lips more attractive, but at the same time riskier because of the occurrence of lip separation. Once separation occurs, a distortion with pressure, entropy and vortical components is passed into the fan. While these distortions are potentially detrimental to the life and stability of the fan, it is also known that the presence of the fan itself changes the conditions at which separation occurs. It is important therefore, to investigate and understand the mechanism by which the fan modifies the flow in the intake. In this study, detailed simulations have been performed of powered and aspirated intakes at the same reduced mass flow rate and spanning a wide range of incidence angles. Geometries representative of real engine configuration have been used. The computational results are shown to be in agreement with wind tunnel results and demonstrate the beneficial effect of the fan on the intake flow. Furthermore, the distortions at fan face are analysed using wave decomposition. It is shown that the effect of the fan is carried by the rejected waves generated at fan face - consistently with actuator disc theory - but it is amplified by the nature of the separated flow in the intake. INTRODUCTION It is a commonly accepted design practice to provide the Low Pressure Compression system (LPC) with uniform flow under all operating conditions. Under this point of view the intake represents the most critical component. High distortion levels to fan stage and therefore to the OGV and ESS blades, can cause blades instability and have detrimental effects on whole compression system. At the same time, the presence of the LPC modifies the flow in the intake. A beneficial effect of the presence of the fan has been demonstrated in term of improving the stall margin of fan stage. In recent years Computational Fluid Dynamic CFD provides a robust and high fidelity approach to investigate the interaction mechanisms in intake-fan coupled systems. The weak point of these kind of approaches lies on the high computational costs and make them not suitable for preliminary design process. Fast tools are therefore necessary to investigate the distortion propagation for early stage of the design process or whenever a quick solution is desirable, when minimal assumptions are made about the studied geometry and nature of the gas. A large literature review based on low fidelity methods and their application to problem (typically acoustic) involving distortions in boundary conditions is available. Satyanaryan et al [1] compared theoretical results and experimental data highlight the necessity to overcome the single blade analysis models. In 1969, Kaji & Okazaki [2] have described a model, providing the magnitudes of the reflected and transmitted waves as they approach the blade cascade. This approach represents the widest validation