ON ONE ASPECT OF WAVE RESISTANCE IN SWIMMING
Prepared for the New Scientist
August 15, 2008
As a swimmer moves through water, energy is transferred to the water causing two waves to form. The sudden outward and downward movement at the front (the "bow" wave) and the sudden stopping inward and upward movement at the back (the "stern" wave) have an effect on the swimmer. The bow wave also contains a forward component as it radiates away from the swimmer. Those waves are responsible for a considerable amount of "drag" resistance that impedes progress. Any time a wave is generated, a pattern of repeat waves follows at a distance depending on the swimmer's velocity. The important feature of these waves is that if the crests of the bow and stern waves coincide, the drag resistance on the swimmer increases.
In a shallow swimming pool, the downward wave portion of the bow wave hits the bottom and reflects back toward the surface. If it coincides with the stern wave or clashes with the swimmer, resistance is increased and the swimmer is slowed. That is why shallow pools are "slow". If the pool is deep, by the time the wave has travelled down and reflected upward, the swimmer has passed and no resistance increase occurs. It seems that a three meter or more depth in a competitive swimming pool is desirable over shallower pools. Most pools deemed to be "fast" by coaches are quite deep and even more than three meters. In a pool that is deep at one end and shallow at the other, elite competitive swimmers can feel the wave turbulence in the shallow water but not in the "quieter" deeper water. The three-meter depth of the Beijing Water Cube pool is helpful to competitive performances.
Depth is not the only structural factor that is important. The phenomenon of the bow wave bouncing off a surface in a confined swimming pool also exists at the pool sides. Swimmers in outside lanes who are bordered by a wall can have the lateral portion of the bow wave hit the wall and reflect back onto the swimmer and the stern wave. In that situation, outside-lane swimmers would be at a disadvantage compared to swimmers not bounded by walls. International swimming is aware of this problem and for top competitions requires 10-lane pools so that with eight finalists swimmers in the outside lanes do not have to contend with their own waves bouncing off a wall. The unused lane provides added distance for the bow wave to travel away sufficiently. The Beijing pool has 10 lanes and so this problem does not occur. One further consideration for outside lanes is that with the still water in the unused lane, swimmers there might incur a minor advantage by only being buffeted by waves from a swimmer on one side as opposed to the remaining swimmers who contend with "interference" from swimmers on both sides.
The design of the Beijing pool ensures that swimmers' progress is not hindered by reflected pressure waves off the bottom or sides. However, there still is the problem of the forward wave component that bounces off the walls at the end of the pool. Perforated wall surfaces are one attempt to dissipate the forward wave but some reflection does occur.
Turning in swimming races also confronts the problem of the pattern of waves that follow swimmers. The faster a swimmer travels, the greater will be the bow and stern waves. Consequently, when big fast swimmers move en masse into a turn in a 100 m race, when they explode back off the wall as part of the turn, they are buffeted by their own stern waves still traveling forward, particularly if they are close to the surface. At these Games, Michael Phelps pushes off the wall deeper than most and largely avoids the retarding effect of his own stern waves and the collision of his own trailing bow wave remnants and the stern wave. That is a desirable feature of his turning technique.
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