Aeroacoustic analysis of a helicopter configuration with ducted tail rotor

In the framework of the Bavarian research project FORLärm, numerical aeroacoustic investigations on a light weight transport helicopter with ducted tail rotor are carried out. To understand the noise generation mechanisms, a hybrid approach is applied for a forward flight condition. The flow field is calculated by an Unsteady Reynolds Averaged Navier-Stokes (URANS) method using Shear Stress Transport (SST) turbulence model. The helicopter Fenestron ^® configuration without main rotor has been modeled based on a structured mesh. A sliding mesh approach is employed to model the fan rotation. Based on instantaneous flow data provided by the URANS simulations, the computation of the acoustic far-field is performed by means of the Ffowcs Williams-Hawkings (FW-H) surface integral method. A complex flow topology is predicted by the URANS simulation. Flow separation occurs on the aft part of the fuselage thereby creating vortical structures convected downstream to the ducted tail rotor. A separation bubble caused by boundary layer separation at the fan inlet lip geometry is also observed. Based on results of the FW-H caculations, tonal noise components are identified. The sideband frequencies related to the blade passing frequency (BPF) obtained by the FW-H calculation match well with the analytical solution based on the phase modulation technique.