Onset of unsteadiness in aero-engine intakes at incidence

Abstract

The flow field around a transonic engine inlet lip at high incidence is investigated for a variety of flow conditions around the design point. Generally, the flow on the upper surface of the lip is characterised by a supersonic region, terminated by a near-normal shock wave. At the nominal design point, the shock is not strong enough to cause significant flow separation for each of the shapes investigated. Off-design conditions were explored by altering the angle of attack as well as changing the mass flow rate over the upper lip, intended to mimic a greater mass flow demand by a turbofan engine. The results suggest that angle of attack is the dominant parameter, where an even relatively small increase of 2 • can lead to large and highly unsteady flow separation with an associated shock oscillation. This is a consequence of the significantly stronger shock compared to the on design case. Both qualitative and quantitative measurements suggest a noticeably reduced aerodynamic performance resulting from higher incidence operation. In contrast, an increase of up to 5.2% in mass flow did not result in large separated regions or flowfield unsteadiness. However, a trend of increasing separation with greater mass flow was observed.

Normal Shock Wave-Turbulent Boundary Layer Interactions in Transonic Intakes at Incidence

“…The first stage of this project investigated the flow field occurring during typical take-off or climb conditions and found the shock-boundary interaction to be relatively benign. 4,5 Separated flow was found to be confined to a small region of space. However, aero-engine intakes are designed to operate over a wide range, defined by inflow speed, incidence and engine demand.…”