Predicting the High-frequency Airloads and Acoustics Associated with Blade-Vortex Interaction

As a rotorcraft descends or manoeuvres, the interactions which occur between the rotor blades and vortical structures within the rotor wake produce highly impulsive loads on the blades and with these a highly intrusive external noise. Brown’s Vorticity Transport Model has been used to investigate the influence of the fidelity of the local blade aerodynamic model on the quality of the prediction of the high-frequency airloads associated with blade-vortex interactions and thus on the accuracy with which the acoustic signature of the aircraft can be predicted. Aerodynamic, wake structure and acoustic predictions using the Vorticity Transport Model are compared against the HART II wind tunnel data for an experimental rotor based on the characteristics of the Bo105 rotor. The model can resolve very accurately the structure of the wake, and allows significant flexibility in the way that the blade loading can be represented. The predictions of two models for the local blade aerodynamics are compared for all three of the HART II flight cases. The first model is a simple lifting-line model and the second is a somewhat more sophisticated lifting-chord model based on unsteady thin aerofoil theory. The predicted positions of the vortex cores agree with measured data to within a fraction of the blade chord, and the strength of the vortices is preserved to well downstream of the rotor, essentially independently of the resolution of the calculation or the blade model used. A marked improvement in accuracy of the predicted high-frequency airloads and acoustic signature of the HART II rotor is obtained when the lifting-chord model for the blade aerodynamics is used instead of the lifting-line type approach. Errors in the amplitude and phase of the loading peaks are reduced and the quality of the prediction is affected to a lesser extent by the computational resolution of the wake. Predictions of the acoustic signature of the rotor are