Intertwined vorticity and elastodynamics in flapping wing propulsion
Intertwined vorticity and elastodynamics in flapping wing propulsion
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DOI：
10.1017/jfm.2015.659
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发表时间：
2015-12
影响因子：
3.7
通讯作者：
R. C. Mysa;K. Venkatraman
中科院分区：
文献类型：
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作者：
R. C. Mysa;K. Venkatraman
我们进行了数值实验的一维弹性固体振荡的二维粘性不可压缩流体的目的辨别涡量和弹性动力学在扑翼推进的相互作用。也许是第一次，我们已经建立了翼偏转拓扑结构的作用及其对涡量生成的影响，通过空间和时间上不断变化的翼斜率和曲率。虽然翼型的振荡频率有一定的作用，但决定推力或阻力的是翼型斜率和压力之间的相位关系。类似地，拍动速度与压力和惯性力之间的相位差决定了输入到翼片的功率，并且反过来驱动推进效率。在低频振荡时，翼型变形的共振斜率和曲率允许产生不分离的前缘涡流;它们导致前缘和中弦之间的压力显著升高。压力的循环分量主要由前缘涡流决定，因此，在低频时，推力也主要是循环的。在中频和高频范围内，翼片上的推力和阻力在空间上交替，非循环力占主导地位，而循环力和粘性力占主导地位。对于我们模拟的质量比，由于扑动的推力作为Strouhal数或后缘扑动速度的函数二次变化;此外，后缘扑动速度在相同的频率下达到峰值，其中推力也是最大值。另一方面，推进效率大致上是推力变化相对于斯特劳哈尔数的镜像。考虑到自然界中大多数扑翼推进的情况主要是通过分布式肌肉驱动，从而能够精确控制变形形状，从而获得高推力和效率，因此这里提出的结果是理解一些驱动推力和推进效率的机制的指针。
We performed numerical experiments on a one-dimensional elastic solid oscillating in a two-dimensional viscous incompressible fluid with the intent of discerning the interplay of vorticity and elastodynamics in flapping wing propulsion. Perhaps for the first time, we have established the role of foil deflection topology and its influence on vorticity generation, through spatially and temporally evolving foil slope and curvature. Though the frequency of oscillation of the foil has a definite role, it is the phase relation between foil slope and pressure that determines thrust or drag. Similarly, the phase difference between flapping velocity, and pressure and inertial forces, determine the power input to the foil, and in turn drives propulsive efficiency. At low frequencies of oscillation, the sympathetic slope and curvature of deformation of the foil allow generation of leading-edge vortices that do not separate; they cause substantial rise in pressure between the leading edge and mid-chord. The circulatory component of pressure is determined primarily by the leading-edge vortex and therefore thrust too is predominantly circulatory in origin at low frequencies. In the intermediate and high-frequency range, thrust and drag on the foil spatially alternate and non-circulatory forces dominate over circulatory and viscous forces. For the mass ratios we simulated, thrust due to flapping varies quadratically as a function of Strouhal number or trailing-edge flapping velocity; further, the trailing edge flapping velocities peak at the same set of frequencies where the thrust is also a maximum. Propulsive efficiency, on the other hand, is roughly a mirror image of the thrust variation with respect to Strouhal number. Given that most instances of flapping propulsion in nature are primarily through distributed muscular actuation that enables precise control of deformation shape, leading to high thrust and efficiency, the results presented here are pointers towards understanding some of the mechanisms that drive thrust and propulsive efficiency.