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

许多研究致力于估算游泳运动员在爬泳中的推进力。估算通常应用称为准定常分析的定常流理论。然而，游泳者周围的流场是极不稳定的。为了阐明力的产生机理，有必要确定游泳者周围流场的非定常特性。在这项研究中，我们仔细研究了在前爬泳中手周围产生的流场。用于检测非定常流场的最复杂的技术是粒子图像速度场仪（PIV）。测量进行了使用这种技术在几个水平面的受试者在水槽中游泳。该方法可同时得到100个时间序列流场。每个流场计算从两个粒子图像使用互相关方法。对于推进力的估计，不仅涡流的强度;它们的位置也很重要。由于涡在产生推进力，特别是升力中起着重要作用，因此计算了非定常动力学中的涡量分布。使用许多时间序列图，可以明显看出在手附近产生了一个强涡。然后，在从向内扫掠到向外扫掠的过渡阶段脱落。旋涡脱落后，产生一个新的旋涡，与旋转方向相反。这对涡引起的速度分量的方向相反的游泳方向。由这个速度分量引起的动量变化是手的推力的来源。

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.