15th European Conference on Turbomachinery Fluid dynamics & Thermodynamics

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Luca Boscagli  - Cranfield University, United Kingdom
David G. MacManus - Cranfield University, United Kingdom
Robert Christie - Cranfield University, United Kingdom


Novel high bypass ratio civil aero-engines have the potential to achieve fuel burn reduction but the increase in nacelle weight and drag due to the typical increase in fan diameter challenges the engine cycle benefits. More compact nacelles, with shorter intakes, may help counteract the aero-engine installation drawbacks. The aerodynamics of a short intake under off-design conditions is characterized by a range of steady and unsteady mechanisms that can adversely affect the fan operability. This paper addresses how the aerodynamic interaction between the fan and the intake affects the characteristics of the intake boundary layer, flow distortion as well as the fan behaviour for a short intake under crosswind conditions. A hierarchical computational fluid dynamics approach was used to determine and quantify primary aerodynamic interactions between the fan and the intake design. This included a lower order fan model as well as full unsteady computations. The influence of fan rotational speed and crosswind direction on the separation onset and characteristics of the boundary layer within the intake was determined and quantified. Although the amplitude of the upstream pulsations of the fan was slightly modulated by a change in fan rotational speed, this had no impact on the onset of the intake boundary layer separation and on the intake critical crosswind speed. The strength and characteristics of the ground vortex dominated the loading on the fan blades and the upstream effect on the unsteady static pressure field within the intake. For a powered intake in crosswind, the direction of the wind determines the direction of rotation of the ground vortex relative to the fan. Thus, two different configurations can be distinguished which were referred as co-rotating and counter-rotating. The steady analyses based on the lower order fan model showed that the counter-rotating configuration was more conservative from an intake design point of view. However, based on a critical crosswind speed requirement, when the coupled unsteady intake-fan interaction was taken into account the separation limit of the intake was adversely affected and the polarity for the relative effect under the two crosswind directions also changed. For the full unsteady analyses, the critical crosswind speed reduced by 12kts and 22kts relative to the steady analysis with the lower order fan model for the counter-rotating and co-rotating configuration respectively. The decay of the blade-passing pulsations and the modulation of the amplitude of the shock wave on the intake lip was notably different under the two crosswind configurations. Compared to the counter-rotating configuration, for the co-rotating configuration the azimuthal extent and amplitude of the fan upstream pulsations was notably greater. This resulted in a stronger unsteady interaction with the intake shock and an earlier onset of intake separation. Overall, this work identified and quantified for the first time the unsteady aerodynamic interactions that limit the design of short intakes in association with fan systems.


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