Extensions of prescribed wake modelling for helicopter rotor BVI noise investigations

Abstract

The prediction of blade–vortex interaction noise is highly sensitive to the blade–vortex missdistance and thus requires the knowledge of the vortex position relative to the blade interacting with it. The generally accepted solution to the problem is to apply either free-wake codes (vortex lattice methods) or computational fluid dynamics (CFD) codes, both of which are very time consuming, especially CFD. Prescribed wake codes are computationally faster in comparison, since they are based on steady rotor operational conditions only, but they are considered to be not accurate enough due to the prescription of the geometry based on a few general parameters. First, they lack any wake geometry response due to variations in rotor loading distributions at constant thrust caused by, e.g., non-uniformity of the aerodynamic environment in forward flight, elastic blade motion, or by any means of active control. Second, wake deflections due to the presence of the fuselage are ignored. This paper provides an approximate solution to both problems, based on simple momentum theory considerations for the first problem, and based on the displacement flow of the fuselage for the second. This provides extensions for any existing prescribed wake model to account for rotor loading distribution and fuselage effects on the prescribed wake geometry to a first order of accuracy, sufficient for the investigations of the sensitivity analysis of noise radiation due to variations of blade design or due to applications of active control.