Structure of the Blade Pressure Fluctuations Generated by Helicopter Rotor Blade-Wake Interaction

The paper aims at highlighting some characteristics of the turbulence responsible for the broadband noise generated by helicopter rotor blade wake interactions (BWI). To this end, a deepened investigation is performed on the blade pressure fluctuations measured on the BO-105 main rotor model during the Higher harmonic control Aeroacoustics Rotor Test (HART). Classical signal processing, such as spectral and probability density estimations, first show that the blade pressure fluctuations in the frequency range of BWI have similar features to the velocity field in a turbulent wake. In particular, this analysis allows to identify organized pressure patterns with a specific Strouhal number of about 2.5 and suggests that these blade pressure fluctuations are related to coherent eddies present in the incident flow. These results are then complemented by those provided by Time-Frequency Representations, analysis tools which are not usually employed for the study of helicopter rotor noise. The Continuous Wavelet Transform appears particularly well suited to account for the unsteadiness and the impulsiveness of the BWI phenomenon and is thus employed to isolate the organized part of the pressure from the uncoherent pressure fluctuations. It also shows that this organized part contains most of the energy and is a good candidate for BWI acoustic sources. For the noise predictions, modelling the BWI pressure fluctuations as ”coherent” patterns thus might be a good alternative to the current approaches which consider BWI as an homogenous and stationary random phenomenon.