Research On The Flowfield And Noise Of Helicopter Rotors Based On Unstructured Mesh

In this paper, an unstructured adaptive overset grid technique and corresponding numerical method which is suitable for solving the flowfield and noise of helicopter rotors has been developed. Based on the developed technique and method, the flowfield and acoustic characteristics of the helicopter rotor were simulated and analysed. The major contributions of the author's research work are as follows:As the background of the work, the development and application of the CFD method in the rotor flowfield simulation and rotor noise analysis were described firstly, and then the importance of the mentioned research and the significant influence of the grid quality on calculated results were pointed out. Also, the methods which would be used in this paper were briefly introduced.In Chapter 2, after analysing the characteristics of the structured grid, the unstructured grid and the Cartesian grid (can also be regarded as a kind of unstructured grid), a new unstructured adaptive overset grid method which suits for the helicopter flowfield calculation was developed, and a corresponding"donor searching"strategy was presented. As numerical examples, the 2-dimension and 3-dimension illustrations of the grid system were given, respectively.Based on the developed unstructured grid system, a new numerical method and solver were established to solve the flowfield of helicopter rotors in hover and forward flight. In the solution of the N-S equations, the flux was calculated by combining the upwind scheme (MUSCL) with the flux-difference splitting scheme, and the gradients of the primitive variables were achieved by reconstruction technique so as to maintain the precision of computations. Meanwhile, the dual-time stepping method was used for simulating the unsteady phenomenon in helicopter flowfield and the Matrix-free LU-SGS method was used in every pseudo-time step. By the solver, the pressure on blade surface, the blade lift and the tip-vortex trajectory were calculated and the capability of the presented method was demonstrated.Chapter 4 is the application of the method and solver developed in above chapters to the helicopter flowfield simulation. The flowfield characteristics of the rotor which works in ground effects, the aerodynamic force and vortex trajectory of the co-axial twin rotors were studied, the flowfield of the rotor/fuselage combination was also simulated and the influence of the blade flapping motion and the location change among the blades and fuselage on the flowfield were discussed. On the basis of the above simulations, some meaningful conclusions were drawn. In Chapter 5, a solver which is used for simulating the helicopter rotor acoustic characteristic was established by combining the Kirchhoff method and the developed N-S equation solving method on unstructured embedded grid, and the different generation approaches of the integrated surface which were used for sound pressure calculations were given. By this noise solver, the acoustic pressures of the rotor in hover and forward flight were predicted, and the comparisons between the calculation results and the available experimental data were made in order to validate the solver. Taking the rectangular blade tip as the baseline tip, the high-speed impulse (HSI) noise of the rotors with different new blade-tips were calculated and the effects of the new tips on the acoustic characteristics of a rotor were discussed emphatically.Chapter 6 is the application of the noise solver to the helicopter rotor noise computations. The presented method was used for analysing the noise characteristics of a special-configuration rotor or tail rotor, i.e., the scissor rotor. The aerodynamic force was also calculated for the scissor rotor. The effects of the different scissor angles, configurations and operation conditions on the noise of the scissor rotor, and some new results are obtained.