CFD/CAA coupling for the prediction of fan tone noise propagation and radiation through a drooped intake

Nonlinear CFD and linear CAA are combined to model the generation, propagation and radiation of tonal noise in a lined, drooped, turbofan intake at a high power setting. Single/multiple passage or full rotor RANS CFD is used to model the generation and non-linear propagation of "buzz tones" close to the fan in an unlined intake. The effect of the liner and radiation to the far-field is represented by a linear, frequency domain, CAA numerical scheme. By separating the non-linear source generation and propagation, dealt with in the RANS CFD solution, from the prediction of liner attenuation and far-field radiation, dealt with in the less demanding CAA model, solutions can be obtained at relatively modest computational cost for realistic three-dimensional, drooped intakes, taking into account non-linear propagation, attenuation by intake liners, and the effect of steady flow distortion on far field directivity. An adjustment is also made in the current model to approximate the effect of boundary layer refraction over the surface of the liner which is not accounted for in the 'slip' impedance boundary condition commonly imposed at the liner surface in linear CAA models. Matching of the non-linear CFD source to the linear CAA propagation model is achieved by constructing a notional "equivalent" modal CAA source at the fan plane which is compatible with the RANS solution sampled at the throat of an unlined intake. This source is applied to subsequent CAA calculations for lined and drooped intakes. The above approach is approximate in that the source and non-linear propagation effects are treated as being independent of the presence of the liner. This assumption is tested by comparing predictions obtained by using the current approach to a restricted set of non-linear analytic solutions, to high fidelity CFD data, and to measured data from a drooped intake test rig.