Nacelle Interaction with Natural Wind Before Take-Off

Abstract

To understand how natural wind close to the ground interacts with a jet engine, tests have been completed with a 1/20-scale model fan and intake rig operating within a simulation of the lower atmospheric boundary layer. Unsteady measurements are used to confirm that turbulence in the ambient flow is attenuated as the intake is approached, and analysis of unsteady pressure data reveals that the nacelle surface pressure field is most sensitive to large length-scale gusts. A new statistical approach to analyze intermittent intake separation is used to show that there is hysteresis in the unsteady separation and reattachment of the inlet flow and that there are similarities between the unsteady and steady intake performance. These findings suggest that the intake response to natural wind can be described as quasi steady, and a novel probabilistic model of nacelle behavior is devised based on this hypothesis. The model shows that by combining some statistical characteristics of the wind with the steady intake performance it is possible to synthesize the observed unsteady performance. This confirms that the intake response can be treated in a quasi-steady manner, and this has significant practical implications for engine intake testing and design.