Research on Ground Effect of Ducted Fans of VTOL UAVs
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摘要: 近年来, 涵道螺旋桨作为一种性能优异的推进器被广泛应用于各种垂直起降无人机中, 但国内关于其地面效应的研究还较少。针对此, 文中使用了基于雷诺平均Navier-Stokes方程和多重参考坐标系的求解方法, 通过数值仿真分别计算了不同离地高度下, 3种不同桨叶数的涵道螺旋桨各部件所受力和力矩的变化情况。并结合计算流体动力学计算结果分析了气动特性变化机理, 结果表明在近地高度达到一定值时, 涵道螺旋桨的气动特性发生显著变化, 螺旋桨部分升力增大但涵道部分升力减小, 系统整体所受反扭矩增加。最终量化成升力系数、功率系数和品质因子的变化, 发现随着离地高度的降低, 功率系数明显增大, 涵道螺旋桨的升力系数和品质因子的变化与螺旋桨桨叶数有关。由此得到了地面效应对涵道螺旋桨气动特性影响的初步结论。Abstract: In recent years, ducted fans, as a kind of propeller with excellent performance, have been widely used in various vertical take-off and landing(VTOL) unmanned aerial vehicles(UAVs). However, there is less research on its ground effect in China. Therefore, in this paper, a solution method based on Reynolds mean Navier-Stokes (RANS) equations and multiple reference coordinate system(MRF) was used. Through numerical simulation, the force and torque changes of each part of the ducted fans with three different blade numbers at different elevations were calculated. The changing mechanism of aerodynamic characteristics was analyzed by calculation results of computational fluid dynamics. The results show that when the altitude near the ground reaches a certain value, the aerodynamic characteristics of the ducted fans change significantly. The lift force of the propeller increases, but that of the duct decreases, and the reverse torque of the whole system increases, which were finally quantified as the variation of lift coefficient, power coefficient, and quality factor. The power coefficient increases significantly with the decrease in the elevation, and the variation of lift coefficient and quality factor of the ducted fans is related to the number of propeller blades. A preliminary conclusion on the influence of ground effect on the aerodynamic characteristics of ducted fans is thus obtained.
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图 10 不同离地高度下涵道对称面处的流线和压力云图
Figure 10. Streamlines and pressure nephograms at the surface of symmetry of culverts at different elevations
图 12 涵道螺旋桨反扭矩随高度变化曲线
Figure 12. The reverse torque of the ducted fan varies with height
图 13 不同离地高度下涵道螺旋桨升力系数和功率系数变化曲线
Figure 13. Changes of ducted fan lift coefficient and power coefficient at different elevations
图 14 不同离地高度下涵道螺旋桨品质因子变化曲线
Figure 14. Changes of ducted fan quality factor at different elevations
表 1 涵道螺旋桨主要参数
Table 1. Main parameters of ducted fan
项目 数值 桨叶数 2.00 浆叶直径/mm 684.00 桨壁间隙/mm 6.00 涵道高度/mm 455.00 涵道入口直径/mm 812.00 涵道出口直径/mm 702.00 桨盘处涵道内径/mm 696.00 面积比 1.05 展弦比 1.52 
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表 2 不同转速下实验所得升力值与参考值对比
Table 2. Comparsion of the experimental lift value and reference value at different rotational speeds
转速/(r/m) 参考值/N 计算值/N 相对误差/% 1 963 2 561 2 419 5.54 2 405 3 556 3 380 4.95 2 696 4 381 4 249 3.00 2 974 5 240 5 170 1.33 3 401 6 935 6 766 2.43 4 353 10 816 11 090 2.54
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表 3 3种不同网格密度及实验结果对比
Table 3. Comparison of three different mesh densities and experimental results
网格
密度旋转
域静止
域网格总
量升力
/N扭矩
/(N·m)低 4.70×105 3.10×105 7.80×105 178.4 −6.8 中 1.65×106 1.46×106 3.11×106 189.7 −7.4 高 2.81×106 2.28×106 5.09×106 192.4 −7.8
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