Ducted rotors are configurations known to outperform their unducted reference baselines when aerodynamic performance is concerned. Aside from aerodynamic benefits in hover, a duct also affects acoustic emissions. One of the most contended design parameters of a duct-rotor assembly is the radial distance between the blade tip and the duct wall, referred to as the “tip gap”. The present study explains how the aerodynamic performance of a ducted-rotor system is affected by the tip-gap distance, taking into account the performance of the rotor and those of the duct's inlet lip and diffuser sections. Separate thrust measurements of the rotor and duct establish that the latter can generate up to half of the total thrust of the assembly. Static wall-pressure measurements along the inner wall of the duct reveal a low pressure suction zone over the duct's inlet lip area. This allows the assembly to generate more thrust than the rotor alone, even though the duct's diffuser section generates a drag component (negative thrust). From the velocity fields it is further shown that the performance-deterioration with an increasing tip gap distance is associated with a contraction of the rotor slipstream in the duct diffuser.

中文翻译：

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旋翼尖端间隙对悬停时管道式旋翼气动和气动声学性能的影响


当涉及空气动力学性能时，管道旋翼是已知的配置，其性能优于其非管道参考基线。除了悬停的空气动力学优势外，管道还会影响声发射。管道转子组件最具争议的设计参数之一是叶尖和管道壁之间的径向距离，称为“尖端间隙”。本研究解释了管道转子系统的空气动力学性能如何受到叶尖间隙距离的影响，同时考虑到转子的性能以及管道的入口唇和扩散器部分的性能。转子和管道的单独推力测量确定，后者可以产生高达组件总推力的一半。沿管道内壁的静态壁压力测量显示，管道的入口唇区域上方存在低压吸入区。这使得组件能够产生比单独使用转子更大的推力，即使管道的扩散器部分产生阻力分量（负推力）。从速度场中进一步表明，随着尖端间隙距离的增加，性能恶化与管道扩散器中转子滑流的收缩有关。