It is currently held that propulsion in human competitive swimming is achieved by a mixture of lift and drag forces predominantly generated by the hands. Calculation of these propulsive forces relies on the quasi-steady assumption that the fluid dynamic behaviour of a hand model in a flow channel (constant velocity and orientation) is similar to that of a hand of a real swimmer swimming the front crawl. Recently, it has been shown that quasi-steady calculations cannot account for the observed propulsive forces and propelling efficiency during front crawl swimming , suggesting that significant unsteady mechanisms must be present. To gain insight into the nature of such unsteady mechanisms, we used woollen tufts to study the flow pattern around the arm and hand during the front crawl stroke, which consists of a glide -, an insweep - and a powerful outsweep phase. It was shown that: 1) the velocity and movement direction of the fingertip and elbow varied strongly throughout the insweep and outsweep, 2) the arm movements were largely rotational, 3) the flow direction varied strongly throughout the insweep and outsweep, 4) a strong, accelerating axial flow component, not in the direction of the arm movement, was observed during the late insweep and the outsweep. These observations discredit the quasi-steady analysis of front crawl swimming. Furthermore, during most of the stroke the flow direction was inconsistent with circulation around the hand, and associated lift force production may be assumed to be of secondary importance. Therefore, the notion that the main propulsive forces are generated by conventional hydrofoil action of the hand was abandoned. Instead, we show how rotation of the arm could lead to a proximo-distal pressure gradient, which could account for the observed axial flow. We further demonstrate that such axial flow along the trailing side of the arm could greatly enhance the pressure difference over the hand, thus assisting propulsion by paddling. Axial flow was also observed during hawkmoth hovering. Therefore, we suggest that axial flow could play an important role wherever a propelling element is rotated (e.g. fins, paddles, wings, legs). (siehe auch unter http://www.ifkb.nl/B4/PUMPED.html)
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