Hybrid Prediction of Fan Tonal Noise

A hybrid method is used to simulate the downstream rotor-vane interaction tonal noise associated with the NASA Source Diagnostic Test (SDT) 22-in fan rig. A 3-D, unsteady, Reynolds-averaged Navier-Stokes (URANS) CFD simulation is used to predict the unsteady surface pressure on the exit guide vanes (EGVs). The resulting downstream duct acoustics are then computed using two methods. The first is an analytical Green's function method approximation for an infinite, fixed duct geometry. The second is a finite element model for a realistic duct geometry. One case from the SDT matrix is the main focus of this paper: approach condition for 22 rotor blades and 54 vanes. The configuration was designed for cut-off at the first blade-passing frequency. Comparison of the exhaust power level results for the 22x54 case reveal that the nonuniformity of the duct does not significantly influence the overall power level but it does cut-off the fifth radial mode and redistributes the energy between the other modes. The current acoustic simulation which models only the vane-wake interaction predicts an exhaust power approximately 13 dB lower than the experimental exhaust power.