Turbofan Inlet Distortion Noise Prediction with a Hybrid CFD-CAA Approach

Modern turbofan engines have drooped inlets which cause the mean flow into the fan to be circumferentially non-uniform at practically all flight conditions. This effect leads to the appearance of additional fan noise sources which propagate even at subsonic rotor tip speeds. In this paper we demonstrate a hybrid CFD-CAA methodology for predicting fan inlet noise in the presence of such distorted mean flows. A compressible URANS simulation is performed to first capture the unsteady acoustic source terms, which are then propagated using a linear potential-based wave equation solver. Coupling between the two simulations relies on Mohring’s acoustic analogy which is applicable to non-uniform, irrotational flow and uses the stagnation enthalpy as the acoustic variable. The CFD-CAA coupling is successfully verified and shown to accurately reproduce the acoustic field directly simulated by highly resolved CFD. It is shown that the current methodology offers significant improvement in accuracy over the commonly used approach of specifying sources in terms of uniform-flow duct modes. The methodology demonstrated here enables, for the first time, high fidelity simulation of the acoustic field within an inlet using computationally affordable finite-element based methods. The methodology therefore allows rapid parametric studies of inlet liner impedance effects on far-field sound in the presence of realistic acoustic sources.