旋翼地面效应的气动建模与特性

地面对旋翼气动特性影响明显，且导致旋翼流场更加复杂。为分析地面效应下的旋翼桨尖涡和流场变化特性，基于涡面和无滑移边界条件，求解第2类Fredholm方程获得地面涡面矢量分布，且将涡面矢量按涡扩散方程扩散到流体中，建立考虑黏性效应的地面气动模型，并耦合非定常面元/黏性涡粒子混合法以体现旋翼桨叶气动特性和旋翼尾迹的非定常效应，构建旋翼地面效应气动分析方法。通过计算Lynx尾桨地面效应下的性能和桨尖涡轨迹，并计算Maryland大学模型旋翼和NASA缩比旋翼地面效应下的垂向、径向速度分布，且与试验和CFD计算结果对比，验证了本文方法能较好捕捉地面效应下的旋翼尾迹变化特性和复杂速度场特性，且结果表明本文方法能较好模拟地面效应下旋翼桨尖涡的收缩、扩散、井喷、地面射流等物理现象。

Aerodynamic model and characteristics of rotor in ground effect

The presence of a ground plane significantly alters the rotor aerodynamics and generates a more complex flow-field. To analyze the characteristics of the rotor tip vortices and flow-field in ground effect, a ground aerodynamic model is then proposed based on the vorticity concept and no-slip boundary condition. In this model, the distribution of vorticity vector is solved through the second kind equation of Fredholm, and is diffused to the flow based on the diffusion equations, accounting for the viscous effects of the ground plane. An aerodynamic method of the rotor in ground effect is then established by coupling the ground aerodynamic model with an unsteady panel/viscous vortex particle hybrid method in which the unsteady aerodynamics of rotor and unsteady behavior of the rotor wake are taken into account. After then, the performance and geometry of tip vortex of Lynx tail rotor, vertical and radial velocities of a Maryland scaled rotor and NASA scaled rotor in ground effect are computed by the present method. Compared with the experiments and CFD results, the computed results show that the present method can capture the unsteady behavior of rotor wake and the complex flow-field of rotors in ground effect with high accuracy. Furthermore, the physical phenomena including the radial contraction and expansion of tip vortex, fountain flow, and wall jet of rotors in ground effect are captured by the present method.