Gehrke Alexander, Mulleners Karen
École polytechnique fédérale de Lausanne, School of Engineering, Institute of Mechanical Engineering, Unsteady Flow Diagnostics Laboratory, Lausanne 1015, Switzerland.
Proc Natl Acad Sci U S A. 2025 Feb 11;122(6):e2410833121. doi: 10.1073/pnas.2410833121. Epub 2025 Jan 28.
Airborne insects generate a leading edge vortex when they flap their wings. This coherent vortex is a low-pressure region that enhances the lift of flapping wings compared to fixed wings. Insect wings are thin membranes strengthened by a system of veins that does not allow large wing deformations. Bat wings are thin compliant skin membranes stretched between their limbs, hand, and body that show larger deformations during flapping wing flight. This study examines the role of the leading edge vortex on highly deformable membrane wings that passively change shape under fluid dynamic loading maintaining a positive camber throughout the hover cycle. Our experiments reveal that unsteady wing deformations suppress the formation of a coherent leading edge vortex as flexibility increases. At lift and energy optimal aeroelastic conditions, there is no more leading edge vortex. Instead, vorticity accumulates in a bound shear layer covering the wing's upper surface from the leading to the trailing edge. Despite the absence of a leading edge vortex, the optimal deformable membrane wings demonstrate enhanced lift and energy efficiency compared to their rigid counterparts. It is possible that small bats rely on this mechanism for efficient hovering. We relate the force production on the wings with their deformation through scaling analyses. Additionally, we identify the geometric angles at the leading and trailing edges as observable indicators of the flow state and use them to map out the transitions of the flow topology and their aerodynamic performance for a wide range of aeroelastic conditions.
空中飞行的昆虫在扇动翅膀时会产生前缘涡。这个连贯的涡是一个低压区域，与固定翼相比，它能增强扑翼的升力。昆虫的翅膀是由脉系加强的薄膜，不允许翅膀有大的变形。蝙蝠的翅膀是在其四肢、手部和身体之间伸展的薄而柔顺的皮肤膜，在扑翼飞行过程中会表现出更大的变形。本研究考察了前缘涡在高度可变形膜翼上的作用，这种膜翼在流体动力载荷下会被动改变形状，在整个悬停周期中保持正弯度。我们的实验表明，随着柔韧性增加，不稳定的机翼变形会抑制连贯前缘涡的形成。在升力和能量最优的气动弹性条件下，不再有前缘涡。相反，涡量积聚在一个附着剪切层中，该剪切层覆盖机翼上表面从前缘到后缘。尽管没有前缘涡，但与刚性机翼相比，最优的可变形膜翼仍表现出增强的升力和能量效率。小型蝙蝠有可能依靠这种机制进行高效悬停。我们通过尺度分析将机翼上的力产生与其变形联系起来。此外，我们将前缘和后缘的几何角度确定为流动状态的可观测指标，并利用它们来描绘广泛的气动弹性条件下流动拓扑结构的转变及其气动性能。
