THE EFFECTS OF WING ROTATION ON UNSTEADY AERODYNAMIC PERFORMANCE AT LOW REYNOLDS NUMBERS
THE EFFECTS OF WING ROTATION ON UNSTEADY AERODYNAMIC PERFORMANCE AT LOW REYNOLDS NUMBERS
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发表时间：
1994-07
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通讯作者：
Michael H. Dickinson
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作者：
Michael H. Dickinson
许多昆虫从下击到上击的转变是以快速的翅膀旋转为特征的。这些攻角的快速变化的空气动力学后果已经使用一个力学模型进行了研究，该模型动态缩放到适合于果蝇等小型昆虫飞行的雷诺数。几个运动学参数的翼翻转进行了检查，包括速度和旋转轴，以及持续时间和角度的攻击在翼中风旋转之前。这些运动学参数的改变以各种方式改变了随后冲程期间的力产生。1.当旋转轴靠近机翼后缘时，模型机翼可以捕获旋转过程中产生的涡量，大大提高了气动性能。这种涡流捕获最明显地表现在迎角为0°时产生的升力上。在绕前缘旋转后，也会产生0°迎角的升力，但前提是下冲角足够大，足以产生冯卡门街。升力可能是由于下冲程尾流中的涡间流引起的有效迎角的改变。2.如果机翼向后平移通过前一次冲程产生的尾流，则获得的最大升力（在所有迎角下）大大提高。当机翼在76.5°迎角下通过冯卡门道向后平移时，获得的瞬态升力系数值接近4。这种效应也可以通过涡间流的影响来解释，涡间流在平移方向上对流体速度贡献了一个小的分量。3.在76.5°迎角下，7.5弦冲程后，升力随迎角的增长显著增加，尽管在所有其他运动学条件下，升力随迎角的增长相对恒定。4.结果还表明，瞬态和时间平均性能的措施时出现的不稳定的机制是负责力的产生之间的差异。虽然机翼旋转的影响在前几个弦的平移是强烈的，平均性能在短短的6.5弦的运动大大削弱了旋转的影响。5.总之，这些模拟结果表明，由简单的翅膀翻转产生的非定常机制可以为昆虫飞行中产生的气动力提供重要来源。此外，对几乎所有运动参数的微小变化的极端敏感性可以为理解主动飞行控制的空气动力机制提供基础。
The downstroke-to-upstroke transition of many insects is characterized by rapid wing rotation. The aerodynamic consequences of these rapid changes in angle of attack have been investigated using a mechanical model dynamically scaled to the Reynolds number appropriate for the flight of small insects such as Drosophila. Several kinematic parameters of the wing flip were examined, including the speed and axis of rotation, as well as the duration and angle of attack during the wing stroke preceding rotation. Alteration of these kinematic parameters altered force generation during the subsequent stroke in a variety of ways. 1. When the rotational axis was close to the trailing edge, the model wing could capture vorticity generated during rotation and greatly increase aerodynamic performance. This vortex capture was most clearly manifested by the generation of lift at an angle of attack of 0°. Lift at a 0° angle of attack was also generated following rotation about the leading edge, but only if the downstroke angle was large enough to generate a von Karman street. The lift may be due to an alteration in the effective angle of attack caused by the inter-vortex stream in the downstroke wake. 2. The maximum lift attained (over all angles of attack) was substantially elevated if the wing translated backwards through a wake generated by the previous stroke. Transient lift coefficient values of nearly 4 were obtained when the wing translated back through a von Karman street generated at a 76.5° angle of attack. This effect might also be explained by the influence of the inter-vortex stream, which contributes a small component to fluid velocity in the direction of translation. 3. The growth of lift with angle of attack was significantly elevated following a 7.5 chord stroke with a 76.5° angle of attack, although it was relatively constant under all other kinematic conditions. 4. The results also indicate the discrepancies between transient and time-averaged measures of performance that arise when unsteady mechanisms are responsible for force generation. Although the influence of wing rotation was strong during the first few chords of translation, averaging the performance over as little as 6.5 chords of motion greatly attenuated the effects of rotation. 5. Together, these modeling results suggest that the unsteady mechanisms generated by simple wing flips could provide an important source for the production of aerodynamic forces in insect flight. Furthermore, the extreme sensitivity to small variations in almost all kinematic parameters could provide a foundation for understanding the aerodynamic mechanisms underlying active flight control.