The purpose is to introduce PIV method, which is established in fluid dynamics using a flume, seeded particles, pulsing laser, CCD camera, Software to calculate the velocity vectors of particles. The Outputs of PIV method are a) colourful velocity-vorticity maps and b) graphic representation of momentum changes. Experiments were concentrating on insweep to outsweep action during crawl stroke swimming. The maps showed. in the beginning of this action there is a bound vortex rotating around the band, next this vortex was shed and a new bound vortex of opposite rotation is starting and then shed vortex and bound vortex are creating jet-flow. The momentum graph reveals a strong change in momentum during that phase of the stroke. The practical consequence is that by this action the propulsive forces are increased considerably.
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