Reynolds Number and Wind Tunnel Wall Effects on the Flow Field Around a Generic UHBR Engine High-Lift Configuration

Ultra High Bypass ratio (UHBR) engines represent one opportunity to get closer to this goal, by maximizing the air mass flow passing through the bypass duct and thus reducing the thrust specific fuel consumption. The large nacelle diameter of such engines presents great challenges for aircraft designers, as compliance with ground clearance regulations require a close wing-nacelle coupling. To avoid collision with the nacelle during take-off and approach a large slat cut-out is inevitable. This geometric modification leads to premature flow separation on the wing and thus to an appreciable degradation of maximum lift.

Established passive flow control measures like nacelle-strakes are not sufficient to compensate the losses associated with the missing slat section in the vicinity of the UHBR engine pylons. Active Flow Control (AFC) has already demonstrated its ability regarding postponement of flow separation and is therefore a potential technology, which can be used to counteract the drawbacks caused by the extended slat cut-out.