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The impact of a drop on a solid surface generates a rapidly expanding thin jet traveling along the surface. We study the evolution of the fingering pattern at the edge of this jet during the impact of a water drop on a glass plate. Multiple-flash photography shows that systematic changes in frontal shapes take place during the expansion. The initial fingers widen and split in two. This splitting is in many cases limited to the development of a double peak on each finger. The subsequent interaction of two such adjacent undulations often results in merging which produces three pronounced fingers. Despite the significant changes in the frontal shapes, the number of fundamental undulations remains approximately constant during the expansion. The progenitors of these azimuthal disturbances are observed right at first contact. Some heuristic arguments based on capillary waves are put forth to explain the splitting and merging. The main focus of this study is on impacts having Reynolds numbers of about 15 000, based on the drop diameter. The corresponding Weber numbers are about 1000.

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Much effort has been undertaken for the estimation of propulsive force of swimmers in the front crawl. Estimation is typically based on steady flow theory: so-called the quasi-steady analysis. Flow fields around a swimmer, however, are extremely unsteady because the change direction of hand produces unsteady vortex motions. To evaluate the force correctly, it is necessary to know the unsteady properties determined from the vortex dynamics because that unsteadiness is known to make the force greater. Unsteady flow measurements were made for this study using a sophisticated technique called particle image velocimetry (PIV) in several horizontal planes for subjects swimming in a flume. Using that method, a hundred time-sequential flow fields are obtainable simultaneously. Each flow field was calculated from two particle images using the cross-correlation method. The intensity of vortices and their locations were identified. A strong vortex was generated near the hand and then shed by directional change of the hand in the transition phase from in-sweep to out-sweep. When the vortex was shed, a new vortex rotating in the opposite direction around the hand was created. The pair of vortices induced the velocity component in the direction opposite to the swimming. Results of this study show that the momentum change attributable to the increase of this velocity component is the origin of thrust force by the hand.