Effect of Lip Shape on Shock Wave-Boundary Layer Interactions in Transonic Intakes at Incidence

Abstract

The flow field around five transonic inlet lips at high incidence is investigated for a variety of flow conditions around a design point representative of a takeoff scenario. Generally, the flow on the surface of the lip is characterised by a supersonic region, terminated by a near-normal shock wave. For the baseline lip profile at the nominal design point, the shock is not strong enough to cause large flow separation, resulting in marginal losses in pressure recovery. Four more parametric shapes were investigated at this design point, obtained by changing the aspect ratio and 'sharpness' of the super ellipse defining the lip contour. Furthermore, off-design conditions are also explored by altering the angle of incidence as well as changing the mass flow rate over the lip, intended to mimic the effect of an increase in engine flow. The parametric investigation revealed a significant effect of lip shape on the position and severity of the shock wave-boundary layer interaction. In particular, the high aspect ratio slim nacelle performed poorly, favouring shock development very close to the lip nose and promoting large scale separation as the incidence increases. From correlation studies based on the parametric investigation, it appears that the extent of shock-induced separation is the main factor affecting the aerodynamic performance. Somewhat surprisingly, this was found to be independent of shock strength but potentially related to the severity of the diffusion downstream of the shock. Alongside delaying flow reattachment, this diffusion is also likely to have a direct detrimental effect on the boundary layer development close to the engine fan.