A New Coupled Free Wake-CFD Method for Calculation of Helicopter Rotor Flow-Field in Hover

ABSTRACT: This paper concerns a new coupled free wake-CFD method for proper calculation of aerodynamic loads on a two bladed helicopter rotor in hovering flight. Loading is computed by solving the three dimensional Euler equations in a rotating coordinate system. However, since direct simulation of the tip vortices and wake requires a very fine grid, the rotor's wake effects are modeled by a free wake approach and included into the CFD calculation by a transpiration boundary condition at the rotor surface. An influence coefficient solution method is used to find the rotor's wake shape, being steady in a rotating frame. Euler equations are also considered in the form of absolute flow variables and solved by a multi grid Jameson's finite-volume method. The accuracy of the proposed method is illustrated by comparing numerical results to the available experimental results for the pressure distribution on a blade of a model helicopter rotor at different tip Mach numbers.

摘要：本文针对悬停飞行状态下双叶直升机旋翼的气动载荷精准计算问题，研究了一种新型耦合自由尾迹-计算流体力学（Computational Fluid Dynamics）方法。载荷计算通过在旋转坐标系下求解三维欧拉方程完成。然而，由于直接模拟尖涡与尾迹需要极高的网格分辨率，本文采用自由尾迹模型对旋翼尾迹效应进行建模，并通过旋翼表面的渗流边界条件将其融入计算流体力学求解过程。本文采用影响系数求解方法确定旋转坐标系下稳态的旋翼尾迹形态。欧拉方程亦采用绝对流动变量形式进行构建，并通过多重网格詹姆斯逊有限体积法进行求解。通过将不同尖部马赫数下模型直升机旋翼桨叶压力分布的数值计算结果与已有实验数据进行对比，验证了所提方法的计算精度。