关键词: 旋翼, 地面效应, 非定常气动特性, 地面涡, 配平分析, Navier-Stokes方程

Abstract:

A computational fluid dynamics (CFD) method based on the unstructured grid technique and momentum source method is developed aimming at simulating the unsteady aerodynamic characteristics of a rotor in ground effect (IGE). In this method, the three-dimensional Navier-Stokes equations have been taken as governing equations, the discretization of convective fluxes and the time marching are completed by Jameson scheme and the five-step Runge-Kutta iteration method respectively, and one equation Spalart-Allmaras turbulence model has been employed. The rotor is modeled as a distribution of momentum source to simulate the ground effect more realistically, the boundary of "moving ground" is used instead of the conventional "stationary ground", and the grids near the rotor plane and between the rotor and the ground are refined to capture the ground vortex more accurate. Considering the motion and control of the rotor in the actual flight environment, the rotor trim is taken into account in the simulation of the rotor flowfield. The aerodynamic forces of trim equations are given by coupling of CFD method and momentum-blade element theory model. In order to improve the robustness and efficiency of the trim method, the genetic algorithm/quasi Newton hybrid iterative algorithm is proposed and established. Firstly, the method is used to calculate the rotor thrust increment and rotor power required in ground effect. The calculated results are compared with the experimental results aimming at verifying the validity of the method, the problem of ramp increment of rotor power required at a low speed flight has been solved which is difficult for wake analysis methods. Then, the flowfields of UH-60A helicopter rotor in different flight heights different advance ratios have been calculated, meanwhile the difference of the rotor power required, induced velocity, ground vortex and rotor control are investigated. The calculated results show that the ground vortex appears at small advance ratio; with increase of the advance ratio, it moves along inflow direction in the longitudinal plane and moves close to the ground in axial direction, then, it becomes weak continuously and finally disappears.

Key words: rotor, in ground effect, unsteady aerodynamic characteristics, ground vortex, trim analyses, Navier-Stokes equations