共轴刚性旋翼悬停状态地面效应气动特性

为了研究地面效应下共轴刚性旋翼的气动特性，建立了一套基于非定常雷诺平均Navier-Stokes方程的气动干扰数值方法，采用运动嵌套网格模拟双旋翼的反转运动。地面采用无滑移边界条件，并对旋翼和地面附近的网格进行加密，以更好地捕捉旋翼的流场细节和尾迹特征。计算结果与Lynx尾桨试验结果进行对比，验证了所建立方法的有效性。对地面效应下共轴刚性旋翼的气动性能和流场进行分析，结果发现：相对于单独的上下旋翼而言，共轴旋翼地面效应下的拉力增益更大，这是由于上下旋翼桨叶表面的压强干扰受地面高压的影响而减弱；地面的干扰主要影响双旋翼尾迹的径向位置，对其轴向位置影响不大，上下旋翼尾迹在地面附近相互融合、分裂，形成复杂的桨尖涡尾迹；双旋翼在地效下的尾迹径向扩张半径比单旋翼大，这是由于双旋翼的径向射流速度更大；随着旋翼距地面高度的增加，双旋翼间的气动干扰强度逐渐恢复，因此下旋翼拉力增益的下降速度比上旋翼更大；共轴旋翼桨尖涡相对卷起高度和扩张半径均随离地高度增加而减小。

Aerodynamic characteristics of hovering coaxial rigid rotor in ground effect

In order to analyze the aerodynamic characteristics of hovering coaxial rigid rotors in ground effect, a numerical method based on unsteady Reynolds averaged Navier-Stokes equations is established. Moving overset grid is applied to simulate the reversal motion of the coaxial rotors. The non-slip boundary condition is adopted on the ground, and the grid around the rotors and the ground are refined to better capture the details of the flow field and the wake of the rotors. The comparison between the calculations and the experimental results of Lynx tail rotor verifies the effectiveness of the established method. The analyses of aerodynamic characteristics and the flow field of coaxial rigid rotors in ground effect show that the thrust increment of coaxial rotors in ground effect is larger than that of single upper and lower rotors. The increment is caused by the weakening of the interference between the surface of upper and lower blades due to high ground pressure. The ground interference mainly affects the radial position of the wake of the coaxial rotors, but it has little effect on its axial position. Upper and lower rotor wakes fuse and split with each other near the ground, forming complex tip vortex. The radius of the coaxial rotors wake in ground effect is larger than that of single rotor, because the radial jet velocity of the coaxial rotors is larger. With the increase of the height of the rotor from the ground, the aerodynamic interference intensity between the coaxial rotors gradually recovers, making the descending speed of the thrust increment of the lower rotor larger than that of the upper rotor. The relative roll height and the expansion radius of the coaxial rotors tip vortex decrease with the increase of off-ground height.