Role of dynamic lift and its application to the propulsive force in swimming

人们一直致力于估计游泳者在前爬泳中的推进力。估计通常应用一种称为准稳定分析的稳定流动理论。然而，游泳者周围的流场是非常不稳定的。为了阐明力的产生机制，有必要确定游泳者周围的流动的非定常特性。在这项研究中，我们仔细研究了前爬行过程中手周围产生的流场。检测非定常流场最复杂的技术是粒子图像测速仪(PIV)。使用这项技术在几个水平面上对在水槽中游泳的受试者进行了测量。用这种方法可以一次得到100个时间序列的流场。采用互相关法从两幅颗粒图像中计算出每个流场。对于推进力的估计，不仅是旋涡的强度，它们的位置也很重要。由于旋涡在推力，特别是升力的产生中起着至关重要的作用，因此根据非定常动力学计算了旋涡的分布，利用许多时间序列图，可以明显地看到在手部附近产生了一个强烈的旋涡。然后，它在从内扫到外扫的过渡阶段脱落。在旋涡脱落后，产生了一个反向旋转的新旋涡。这对旋涡在与游泳方向相反的方向上诱导出速度分量。可归因于此速度分量的动量变化是手动推力的来源。

Much effort has been devoted to estimating the propulsive force of swimmers in a front crawl. Estimations have typically applied a steady flow theory called quasisteady analysis. Flow fields around a swimmer, however, are extremely unsteady. To elucidate the generation mechanism of force, it is necessary to determine the unsteady properties of flow around a swimmer. In this study, we carefully investigate the flow field that is created around a hand in the front crawl.The most sophisticated technique for detecting an unsteady flow field is Particle Image Velocimetry (PIV). Measurements were carried out using this technique in several horizontal planes for subjects swimming in a flume. Using this method, one hundred time-sequential flow fields can be obtained at once. Each flow field was calculated from two particle images using cross-correlation method. For estimation for the propulsive force, not only the intensity of vortices ; their location is also important. Vorticity distributions were calculated with respect to the unsteady dynamics because vortices play an essential role in generation of propulsive forces, especially lift force.Using many time-sequential figures, it was apparent that a strong vortex was generated near the hand. It then shed in the transition phase from in-sweep to out-sweep. After vortex shedding, a new vortex, rotating with opposite direction, was created. The pair of vortices induced the velocity component in the direction opposite to the swimming direction. The momentum change that is attributable to this velocity component is the origin of thrust force by hand.