THE HAND-FOREARM SHOULD BE VERTICAL IN PROPULSION
Keys, M., Lyttle, A., Cheng, L., & Blanksby, B. A. (2010). Wave formation as a possible mechanism of propulsion in the freestyle stroke. A paper presented at the XIth International Symposium for Biomechanics and Medicine in Swimming, Oslo, June 16-19, 2010.
"Computational Fluid Dynamics (CFD) allows simulation of complex fluid flow regimes and geometry to answer problems that physical testing techniques cannot provide. One such area arising from CFD analyses is that wave formation effects might aid some propulsion phases of freestyle."
In a case study approach, one world class swimmer underwent a 3D body scan. A Finite Volume Method of CFD modeling was used incorporating a realizable K-epsilon turbulence model with a multi-phase fluid domain. The fully submerged streamlined simulation was compared with a static streamlined position just under the water surface and the wave created around the body was compared to properties obtained via Linear Wave Theory. Manual video digitizing then provided the 3D kinematics of the swimmer’s freestyle stroke to animate the model; and resultant forces throughout the freestyle stroke were compared to the wave properties and sub-surface water pressure along the swimmer’s length.
Critical points along the wave described in Linear (Airey) Wave Theory correlated well with resultant force changes via the CFD simulations of the static position when near the surface compared to at depth. A 122% drag increase was found around the upper body at the surface, with the lower body recording a 104% reduction in drag, leading to a propulsive force on these components. The freestyle wave location found by CFD calculations of sub-surface pressure seems to move with changes in swimmer length when moving arms from the front to the back of the body. This changing wave appears to create a short wave having a high relative acceleration component as when two waves join; and, in turn, create a short surge in the swimming direction at the same time as the peak net force occurs in the stroke.
In the wave that surrounds a swimmer, acceleration and velocity of the water varies greatly and can influence the forces of the body components in those regions. The transient pressure wave at 0.3 m underwater occurs at the same location and time as when the forearm and hand pass through during the upsweep and may have contributed to the peak force occurring later in the stroke. Clarification of this situation is needed to determine the exact cause of this scenario and how it may benefit swimmers. A high velocity with forearm and hand perpendicular to the direction of flow to ensure maximum volume and added mass capacity at this point, may improve freestyle stroke efficiency.
Implication. The best position for the hand-forearm combination is vertical, that is, at right angles to the intended direction of propulsion.
[This editor would argue the simplicity of the wave modeling in this study. A good case could be made for the wave factors that exist in a "barge-in-a-canal" model being more appropriate for swimming. In that model, there is more than one form of wave developed and also the situations of submersion and surface swimming are very different. The major waves on the surface are bow wave, stern wave, and lateral waves (which actually move sideways and downward). Those waves interfere with progression in that they consume energy and create resistance of a high magnitude.]
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