Research On Calculation And Debasement Method Of Rotor Aerodynamic Noise For Helicopters

The calculation and debasement for rotor noise is a challenging subject in the field of helicopter aerodynamics. In this thesis, on the one hand, the CFD method for rotor flow field is modeled, and the method for the calculation of rotor aerodynamic noise is developed on the basis of FWH and Kirchhoff equations. By the method, the characteristics of the aerodynamic noise for rotors are computed and analyzed. On the other hand, the effects of the atmosphere and ground on rotor noise are analyzed, and a method for separating the overall helicopter noise is developed. Also, the new methods of helicopter rotor noise reduction are studied. The major contributions of the author's research work are as follows:As the background of present work, the objectives of the research are firstly described. Then the researches and developments in the domain of numerical simulation of rotor flow field, the calculation of rotor aerodynamic noise, the effects of atmosphere and ground on rotor noise as well as the rotor noise reduction methods are briefly reviewed. The deficiencies in the current research are also pointed out, and the methods used in the present research are briefly introduced.In order to get the detailed information of the flow field for the rotor noise computation, a CFD method which is suitable for the calculation of rotor noise is presented based on the unstructured overset grid and N-S equation, and the comparisons with the available experimental data for different rotors are made to show the capability of the method. Then, a new model for the calculation of helicopter rotor aerodynamic noise is established based on the FWH and Kirchhoff equations, which is suitable for the calculations of subsonic and transonic rotor aerodynamic noise. The properties for rotor noise in different flight conditions are deeply studied by applying the method and model, and the effects of viscous force on the calculations for rotor aerodynamic noise are also analyzed, then some results of practical significance are obtained.On the basis of last chapter, a new model is established for the analysis of atmospheric absorption influence and ground acoustic effects on the rotor noise. The traditional algorithms used by NASA are improved in this model. By the developed model, the effects of atmospheric absorption as well as the ground reflection and absorption on rotor aerodynamic noise are calculated and analyzed, and some new conclusions are obtained. Afterwards, based on the 2-D frequency analysis, a method which can separate main and tail rotor noise from the helicopter overall noise is developed. The method is applied to separate the noise of the helicopter with scissors tail rotor, and the noise debasement mechanism of the scissor rotor is investigated.Then, the noise reduction methods are emphatically studied. The research on rotor noise reduction method, which has control sources of the monopole and dipole on the blade tips, is made. On the basis of method above, a new analytical expression for rotor noise is derived with the reference source location set on the rotor hub. From the new analytical expression, a new noise reduction method is proposed by the use of the cancellation of two noise waves of main and tail rotors. In addition, a new simplified calculation model for BVI noise is established. Applying this model, the BVI noise is analyzed, and the flight parameters causing strong BVI noise are given, then the methods for avoiding strong BVI noise by the slight adjustment of flight parameters are studied and some new conclutions are obtained.