In this paper, the effects of rotor-rotor interaction on the wake structure and thrust generation of a quadrotor unmanned aerial vehicle (UAV) are experimentally investigated in the rotor tip Reynolds number range of 34000 - 54000. The interaction strength is manipulated by varying the number of rotating rotors and the normalized rotor separation distance. A stronger rotor-rotor interaction places the inner tip vortices between rotors closer to each other, forming an upflow region through vortex pairing and intensifying the turbulence intensity between rotors. To comprehensively evaluate the effect of interaction on the wake structure, we propose a modified Landgrebe's model that accurately describes the wake boundary of UAV, given the number of rotating rotors and the normalized rotor separation distance. The wake analysis based on the model shows that the stronger the rotor-rotor interaction, the less the wake contracts and the closer the vena contracta moves to the rotor-tip path plane. The momentum theory combined with the modified Landgrebe's model shows that the loss of axial momentum transfer due to the wake inclination is insufficient to account for the thrust loss caused by the rotor-rotor interaction. This paper shows that the shift of the inner tip vortex away from the rotational axis and the corresponding increase of induced axial velocity followed by a decrease in the local effective angle of attack is another important mechanism for the thrust loss.

中文翻译：

---

旋翼相互作用对四旋翼无人机尾流结构和推力产生的影响


本文在旋翼尖端雷诺数范围为 34000 - 54000 的情况下，通过实验研究了旋翼-旋翼相互作用对四旋翼无人机尾流结构和推力产生的影响。通过改变旋转转子的数量和标准化转子间隔距离。更强的转子-转子相互作用使转子之间的内叶尖涡彼此更接近，通过涡旋配对形成上流区域，并增强转子之间的湍流强度。为了全面评估相互作用对尾流结构的影响，我们提出了一种改进的Landgrebe模型，在给定旋转旋翼数量和归一化旋翼间距的情况下，该模型可以准确描述无人机的尾流边界。基于该模型的尾流分析表明，转子-转子相互作用越强，尾流收缩越小，收缩腔越接近转子-叶尖路径平面。动量理论与修正的Landgrebe模型相结合表明，尾流倾斜引起的轴向动量传递损失不足以解释转子-转子相互作用引起的推力损失。本文表明，内尖端涡流远离旋转轴的移动以及引起的轴向速度的相应增加以及随后局部有效攻角的减小是推力损失的另一个重要机制。