Numerical study of the rotor thickness noise reduction based on the concept of sound field cancellation

Numerical studies were performed to investigate the mechanism and potential of several active rotors for reducing low-frequency in-plane thickness noise generated by rotating blades.A numerical method coupling the blade element theory,prescribed wake model and Fowcs Williams-Hawkings(FW-H)equation was established for rotor noise prediction.It is indicated that the excitation force on the blade tip can generate anti-noise that to partly cancel the in-plane thickness noise with an appropriate actuation law.Results from the phase,frequency and amplitude sweeps show that the excitation force direction and actuation law are the crucial factors affecting the noise reduction,which determine the noise reduction area in the elevation and azimuth directions,respectively.The active trailing-flap rotor can generate the in-plane excitation force,but because of large lift-drag ratio the anti-noise is mainly from the vertical lift,which is caused by flap deflection similar to a variable camber airfoil.For the harmonic control rotor and active twist rotor,the excitation force is also attributed to the vertical blade lift.The vertical force can reduce the noise near the rotor plane,it will also cause the noise increase in most other areas.Finally,two new active rotors were proposed to generate the in-plane chordwise and spanwise excitation force.With the modified actuation law,the noise in most areas around the rotor was reduced,which improved the acoustic characteristics of rotor significantly.

Simulation on Helicopter Scissors Tail-Rotor Noise Based on N-S Equations and F1A Formulas

Aiming at the noise of helicopter scissors tail-rotor,an advanced numerical method is established by combining computational fluid dynamics(CFD)model with Farassat 1 A(F1 A)formula.In this method,Navier-tokes(N-S)equations are used as governing equations,and the flow field is solved at quasi-steady and unsteady states in hover and forward fight,respectively,based on two different types of embedded grid systems.A simple and effective solution approach is provided for the generation difficulty of donor cells caused by the close gap among scissors tail-rotor blades.Using the CFD calculation results as input,the thickness noise,loading noise and total noise of tail-rotor are calculated by F1 Aformula.By the method,numerical examples on rotor flowfield and noise are performed and the results are compared with available data.Then,aerodynamic and acoustic characteristics of scissors tail-rotor are emphatically calculated in both hover and forward flight.Furthermore,the research on the effects of blade-tip shape parameters on scissors tail-rotor noise is carried out.Also,the scissors tail-rotor is compared with the conventional tail-rotor,and the results show that in hover,the noise of a scissors tail-rotor is not always the smaller one.