Jun 9, 2009
The figure illustrate the water flow passing through the hand at different finger spacing (left panel: closed fingers; middle panel: passive ‘optimal’ spacing; right panel: wide open fingers). Flow passing between fingers has the advantage to disrupt a counter current vortex that would decrease the pressure differential between the two hand surfaces. Also, it is apparent that at the optimum finger spacing the wake behind the hand is the largest one.
Q: why elite swimmers use spaced fingers during the power stroke? Since the overall area of the hand surface is independent on finger spacing, is that strategy pointing towards a more comfortable stroke or is it meant to increase the hand drag (thus the thrust)?
A: we used a 3D mesh manipulation software (Poser) to model hands with differently spaced fingers. Successively, the obtained meshes were ‘virtually’ immersed in a water flow at different speed, to assess the drag caused by the different spacings. A CFD (Computational Fluid Dynamics) program (Fluent and CFX) simulated all the eddies and vortices for each water speed and spacing, as by using a real flum. Results focussed on a given spacing, very close to the passive finger posture, which maximizes the hand drag, thus the thrust.
The fluid-dynamics explaining such results are discussed below.
REF Minetti A.E., G. Machtziras & J. Masters, J. Biomech. 42: 2188-2190, 2009.