Vortices And Propulsion

VORTICES AND PROPULSION

RaÐ"Ñ"l Arellano

Universidad de Granada (Spain)

A review of the general theory of swimming propulsion is presented relating this

with knowledge about vortices in steady and unsteady flow conditions. Three

methods of flow visualisation have been used in the experiments: a) injecting air

bubbles close to big toe during undulatory underwater swimming and breaststroke

kick; b) putting reflective particles in water to see hand short movements and; c)

injection of air bubbles in the swimming pool creating a "bubble wall", making it

possible for the swimmer to cross and to swim along it. The results of the

experiments showed that: a) vortices are generated during different phases of the

stroke and during the downward kick in undulatory swimming, flutter kick and

breaststroke kick; b) when the hand suddenly changes the direction of its

movement the starting vortex is detached from the hand and; c) the size and

movement characteristics of the vortex seem related to propulsion obtained by the

hand and foot movements.

KEY WORDS: swimming propulsion, vortex, flow visualisation, bubble wall.

INTRODUCTION: THE BASICS OF SWIMMING PROPULSION: The total mechanical

power (Po) produced by the swimmer (assuming a constant velocity) equals the power to

overcome active drag (Pd) plus power expended in giving masses of water pushed away a

kinetic energy change (Pk) (Toussaint, 1992):

Po = Pd + Pk (1)

(Counsilman, 1971) stated that efficient propulsion is obtained by pushing a large mass of

water a short distance without much acceleration. Greater efficiency in water is achieved by

moving a large amount of water a short distance than by moving small amounts of water a

great distance. These statement were developed after observing how good swimmers pull

in the water with complex 3D trajectories that show continuous changes of the direction of

the hand's pulling path. Later (Martin, 1989) speaking about the fundamental principles in

swimming asserted that swimming by propelling water, one may achieve a given amount of

thrust either by accelerating a large mass of water to a small velocity