Predicting the High-frequency Airloads and Acoustics Associated with Blade-vortex Interaction PDF Download

Author: Mary E. Kelly
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Languages : en
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Book Description
As a rotorcraft descends or manoeuvres, the interactions which occur between the rotorblades and vortical structures within the rotor wake produce highly impulsive loads onthe blades and with these a highly intrusive external noise. Brown?s Vorticity TransportModel has been used to investigate the influence of the fidelity of the local blade aerodynamicmodel on the quality of the prediction of the high-frequency airloads associated withblade-vortex interactions and thus on the accuracy with which the acoustic signature ofthe aircraft can be predicted. Aerodynamic, wake structure and acoustic predictions usingthe Vorticity Transport Model are compared against the HART II wind tunnel data for anexperimental rotor based on the characteristics of the Bo105 rotor. The model can resolvevery accurately the structure of the wake, and allows significant flexibility in the way thatthe blade loading can be represented. The predictions of two models for the local bladeaerodynamics are compared for all three of the HART II flight cases. The first model isa simple lifting-line model and the second is a somewhat more sophisticated lifting-chordmodel based on unsteady thin aerofoil theory. The predicted positions of the vortex coresagree with measured data to within a fraction of the blade chord, and the strength ofthe vortices is preserved to well downstream of the rotor, essentially independently of theresolution of the calculation or the blade model used. A marked improvement in accuracyof the predicted high-frequency airloads and acoustic signature of the HART II rotoris obtained when the lifting-chord model for the blade aerodynamics is used instead ofthe 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 similarly affected, with the lifting-chord model at the highest resolution producing the best representation of the distribution of sound pressure on the ground plane below therotor.