Near-field noise reduction of a ducted helicopter tail rotor due to a modified stator vane geometry

A numerical comparison of the aerodynamics and aeroacoustics of a ducted helicopter tail rotor with two different vane geometries under hover flight conditions is presented. The first stator geometry represented the reference configuration, whereby the modified stator vane was designed to be favourable in terms of noise generation. The investigated stage was modelled as single passage, whereby phase-lagged boundary conditions were used in order to take the exact blade and vane count into account. To obtain the unsteady flow field as well as the acoustic near-field the fully turbulent, three-dimensional, Favre averaged Navier- Stokes (FANS) equations were solved on a structured multi-block mesh using the flow solver TRACE. In order to obtain the stage characteristics of the ducted fan, steady state calculations were performed for four distinct rotor blade pitch angles between 20° and 38°. Additionally, detailed unsteady calculations were carried out for a rotor blade pitch angle of 32°. Besides global parameters (e.g. thrust and shaft-power) the study was mainly focused on determining the differences in stage acoustics due to the modification of the vane geometry. The modified stator vane led to noise reductions at the outlet plane of the tail rotor without negative influence on the aerodynamic stage performance. In the inlet plane the predicted noise level was almost unaltered.