Shock Wave Generation and Radiation from a Turbofan Engine Under Flow Distortion

This paper presents aeroacoustic investigations on a full-scale ultrahigh-bypass-ratio engine with inflow distortion at transonic conditions. Computational fluid dynamics (CFD) simulations are first realized to compute the shocks in the vicinity of the fan. These shocks are then radiated in the near field thanks to computational aeroacoustics (CAA) simulations. The chaining between CFD and CAA simulations is done by injecting the shocks in terms of usual conservative variables using a nonreflecting boundary condition. The CAA solver is based on the nonlinearized Euler equations, which allows definition of the CFD/CAA interface close to the fan where the propagation of shocks is highly nonlinear. Both shock generation and propagation mechanisms are investigated, and the effects of inflow distortion are highlighted by comparison with a baseline case without distortion. The flow distortion is shown to be responsible for a modification of shock amplitudes along the circumferential direction. Thus, azimuthal modes appear in addition to the rotor-locked mode present without distortion. A particular feature of the studied configuration is the presence of a supersonic flow region in the bottom part of the nacelle. This supersonic pocket blocks the shocks in the lower half of the nacelle and modifies the angle at which they leave the nacelle in its upper half. Consequently, the far-field radiation (obtained using Kirchhoff's integral method) is mainly oriented skyward and toward the side of descending blades. Acoustic power estimates are provided to quantify these effects.