Crosswind aerodynamics of low pressure ratio fans

Ultra-high bypass ratio (UHBPR) turbofans offer significant reductions in fuel burn and pollutant emissions due to their higher propulsive efficiency. To minimise drag and weight penalties with larger fan diameters, shorter and slimmer intakes are used. Compact intakes pose a risk of delivering highly distorted flow into the fan when operating at high power in strong crosswind. The fan-distortion interaction can degrade the thrust delivered by the engine, jeopardise the stable operability and threaten the mechanical integrity of the fan. This paper aims to understand the impact of crosswind direction on the aerodynamics of a low pressure ratio fan with both a conventional and a short intake. A fully coupled numerical model of the fan, intake and external flow-field is used to quantify how crosswind direction and intake length impact the loss generation and work through the fan. For crosswind from the left (looking into the engine) the ground vortex is co-rotating relative to the fan direction. This causes co-swirl at the fan tip at entry to the windward lip separation leading to reduced incidence and fan work. In contrast, crosswind from the right causes counter-swirl towards the fan tip as it leaves the separated region, leading to higher work and large corner separations extending down the span. A shorter intake length has a more closely coupled fan and intake lip, which tends to suppress the extent of the windward lip separation, but increases the swirl variations at the casing. This leads to reduced rotor losses for crosswind from the right but increased loss for crosswind from the left. Overall, for a short intake, the fan work is found to be around 6% higher for crosswind from the right relative to crosswind from the left. Despite the large levels of distortion in extreme crosswind, the prototype fan demonstrates in all cases it can operate stably without a significant drop in performance.