The Researching Of Numerical Simulation Method Of Aeroacoustics For Hovering Rotors Of Helicopter

Aeroacoustics is a subject which investigates the principles of the noise produced by fluid itself or fluid and solid surface, and the aeroacoustics is a crossing branch of aerodynamics and acoustics. With further understand of people about noise pollution of the environment, reduction of environmental noise especially the noise produced from aircraft gets more important. So investigating the principles of aeroacoustics and the numerical calculation of noise plays important significance for environmental protection. The purpose of this paper is to investigate the principles of aeroacoustics, derive the similarity criterion of aeroacoustics and some formulas of aeroacoustics, and furthermore verify these theories by means of numerical calculation of the noise produced from some simple models. In the end, this paper calculates the noise of hovering rotors that is basic researching work for helicopter noise reduction.Because of supplying flow field information before aeroacoustics calculating, this paper first calculates flow field of hovering rotor using finite volume method of the Euler equation, then calculates noise of hovering rotors using the method of solving FW-H equation in time domain.The main works of this paper are as follows:(1). The flow field of hovering rotors is calculated. For the hovering rotor, the flow field can be treated as steady case in the rotary coordinate system that simplifies the problem, so this paper selects the blade-attached coordinate to calculate flow field of hovering rotors by solving Euler equation. Because of the symmetry of the flow field of hovering rotor, one blade is needed to be calculated. Overlap grid is selected for helping calculate aeroacoustics following and apply symmetrical boundary condition. The numerical method of control equation uses cell- centered finite volume in space and five step Runge-kutta scheme in time. Using local time stepping and implicit residual smoothing to accelerate the convergence. The flow field of hovering rotor is calculated for different state and the numerical results agree very well with the experiment data.(2). The principles of aeroacoustics are analyzed and studied. The FW-H equation is fundamental equation of aeroacoustics, which comes from is rearrangement of N-S equation in the form of nonhomogeneous equation, and the FW-H equation can accurately predicts the noise produced by the reaction of moving body and fluid, so it has very important investigation value and application value. This paper derives the equation with two different ways and gives the meaning of the equation. The FW-H equation is non-dimensionalized by using nondimensional theory, and obtains main similar parameter which must be obeyed in the wind tunnel test of aeroacoustics. FW-H equation is a nonhomogeneous equation, which can be solved with Green function in the unbounded space. This paper derives the solution of aeroacoustic equation by means of integral transformation and derivative transformation for Green function, and obtains the aeroacoustic integral formulation about thickness noise and loading noise. The numerical result of the noise of square cylinder and cylinder successfully verifies the deriving method in the paper and indicates the noise of cylinder is inverse proportional to distance and is proportional to the quintic with Mach number.(3). The noise of hovering rotors is calculated. This paper calculates the noise of hovering rotor by the use of aeroacoustic integral formulation, employing Gauss integral formula and constructing discrete scheme in time. Numerical results agree very well with the experiment data, indicating that numerical calculating method employed by this paper for hovering rotor noise calculation can successfully calculate thickness noise and loading noise, so it is appropriate to predict the noise of the helicopter rotor.