Comparison of two low-order models for the prediction of fan broadband noise

Abstract

Two low-order methods for predicting the broadband noise downstream of a fan exit guide vane (FEGV) due to interaction with rotor wake turbulence are compared. The noise is predicted with a two-step approach in each method. The methods share the exact same first step. The unsteady vane surface pressure is computed using a strip approach. At each strip, the 2D flat-plate cascade response to a gust in the wave-number, frequency domain is calculated. An integral approach is used for this calculation. The two methods then differ in the second step. One method integrates the unsteady surface pressure together with the Green's function for a cylindrical annulus to obtain the downstream duct acoustic pressure modes. The other method computes rectilinear acoustic modes relevant to the cascade at each strip. In the second method, the total power is obtained by adding the acoustic power downstream of each strip. Both methods neglect mean flow swirl effects on acoustic propagation. The fan broadband noise workshop RC1 and FC2 cases are used as a platform to compare the two methods. When the FEGV inflow turbulent parameters are taken from different sources (e.g. hot-wire, CFD) the predictions from both methods respond similarly. The trends predicted with both methods based on turbulence level or rotor speed are similar. The trends with vane count predicted by the two methods do not match and it is unclear which prediction is correct. The actual spectrum predicted by the Green's function method for all of the cases is shown to be slightly closer to experimental results.