Impact of Fan Aerodynamics on Inlet Distortion at Crosswind

Abstract This study offers a holistic perspective on fan and intake distortion interaction in crosswind conditions, addressing the factors influencing fan stability during ground operations. Although prior research has extensively examined individual aspects such as intake length, fan operating points, and crosswind speed, a comprehensive analysis encompassing all these parameters remains unexplored. Using two intakes with different length-to-diameter (L/D) ratios (0.5 and 0.25) and a 3D unsteady computational fluid dynamics (CFD) model, this paper investigates how the fan operating point (in terms of mass flow, pressure ratio, and proximity to the stall line) influences intake separation and how the resulting distortion affects the fan operation. The results show that for both intakes, the primary parameter of fan operation on intake flow is the mean mass flow. The intake flow remains largely unaffected by the fan pressure ratio or its proximity to the stall line, as long as the mass flow is kept constant. As the fan speed (mass flow) increases to a critical value, a shock forms at the intake lip, triggering flow separation within the intake. The shorter intake moves the onset of separation to a higher mass flow. However, the increase in critical mass flow is not driven by fan suction, as often suggested in the literature, but is caused by the repositioning of the spinner closer to the intake highlight, which redistributes flow radially near the intake lip. This observation was validated using an aspirated intake, which showed a similar conclusion to the powered configuration provided the hub line remained unchanged. These findings support the use of aspirated intakes for wind tunnel tests of intake designs under crosswind conditions. However, the fan’s response to the same level of distortion depends on the fan design and operating point. The data from aspirated intake measurements can be used as inlet conditions for fan (numerical and experimental) testing, retaining the effect of the operating point.