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Bob Gillett September 15, 1999 00:04

Fish Kick
 
I am a swimming coach, with top level world rank athletes. I have been using a technique on starts and turns in the butterfly event, that involves the swimmers kicking on their sides (fish kick), rather than on their front (dolphin kick). The thesis is that more power can be generated with the fish kick as a result of a more advantageous interaction between the counter-rotational vorticies of the forward and backward foil movements. During a dolphin kick, the interaction between the vorticies is disrupted because of the distortion of the vorticies with the bottom of the pool and surface of the water. Can anyone assist with evaluating this thesis and estimations of magnitude using CFD. Reference: "Robo Tuna", Scientific American, Triantafllou, March, 1995.

Md. Ziaul Islam September 15, 1999 19:21

Re: Fish Kick
 
V-SHAPED GROOVES called riblets have been found to inhibit the motion of eddies, thereby lessening the drag they can exert on an aircraft's wing. To be effective, the riblets must be very closely spaced, like phonograph grooves. The skin of a shark has tiny, toothlike denticles that seem to serve the same function as the riblets, lessening the drag on the creatures as they move through the water.

If a swimmer wears a swim suit that has small V-shapes Grooves on all around the whole surface of the suit that comes in contact with water while smimming then due to less drag, the smimmer will be able to swim faster than his counterparts.

Jonas Larsson September 16, 1999 05:36

Re: Fish Kick
 
Interesting problem! It is also a very difficult problem which involves interaction of 3-D vortices, walls and a free-water surface. I haven't seen the reference you mentioned, but looking at it quickly one explanation could be that in the case of when you kick normally, like a dolphin, the counter rotating vortices will generate a low-pressure region between the two vortices, this region might create a surface wave and thus increase the generated wave-drag. These vortices will also be drawn towards eachother due to the low-pressure and this could further increase this effect.

If you swim like a fish the low-pressure region between the two vortices will be down under the upper vortex, and if you swim a bit below the surface, the low-pressure region will in this case pull down the upper vortex (and pull upp the lower, if you are not close to the bottom). This will reduce the generated surface wave.

I'm not at all sure if this theory is correct, anyone have any opinions about it? You could check if this is reasonably by observing the surfave wave generated when kicking in the two different ways. If my theory is correct the surface wave should be smaller in the case when you use a fish-kick.

Someone could do a CFD simulation of this and analyse it in detail, but it would probably take more than 100 hours and thus might be expensive.

Jonas Larsson September 16, 1999 06:25

Re: Fish Kick
 
Thinking about this a second time, my explanation above concerns the streamwise oriented vortices generated along the sides if the fin/feets, like the tip-vortices on an airplaine wing.

It is perhaps more likely that the cross-streamwise vortices generated in each kick are more important. The explanation with the surface interaction and less wave-drag for the fisk-kick also holds well for this case though. A vortex oriented in the direction normal to the bottom/surface will produce a smaller wave than a vortex aligned with the surface.

Does the fish-kick produce less surface waves?

John C. Chien September 17, 1999 02:07

Re: Fish Kick
 
(1). I know nothing about the swimming. So, I can only look at the problem from my point of view. (2). When a fish is swimming in the water under the surface, the surface effect is small. I mean the free surface motion in the vertical direction will be small in this case. (3). As the fish is swimming just under the surface, the added degree of freedom of the surface motion will make the fish motion less effective. The surface layer will have the tendency to move in the kick direction, thus reduce the relative velocity and the effectiveness of the kick. (4). On the other hand, if one turns the fish 90 degree from the normally vertical position, the downward kick will have small effect if the fish is (deep)under the surface. (something like a stingray motion) (5). On the other hands, Near the surface, the downward motion will be more effective, because the surface layer can move(follow) easily with the downward movement. Thus the drag will be lower on the back side of the kick surface. On the other hand, below the surface, the suction pressure will be lower and thus higher drag.) (6). So, it looks like the near the surface, the downward kick (a fish rotated 90 degree along the axial direction.) will be more effective than the normal side kick of a fish. Under the surface, I would say that the same conclusion can be made because the shallow bottom will reduce the vertical motion of the water, thus increase the effectiveness of the downward kick. (7). These conclusions are obtained based on the fact that if the water right in front of the kick can be moved easily when a kick is performed, then the reaction derived will also be small. And if the water on the back side of the kick can also be move easily, then the suction will be less severe with less drag. By adding these two effects on both sides of the kick, one can derive the total effectiveness of a fish motion. (in other words, the fish under water will always symmetric because the force developed from the fish swimming is symmetric. On the other hand, a stingray swimming just under the surface will develop into a bird like wing, non-symmetric )

Jonas Larsson September 17, 1999 02:52

Re: Fish Kick
 
I'm not sure that I agree with points (5) and (6). At the surface when you kick downwards you will generate a surfave wave. This wave represents energy that is not used to drive the swimmer forward and hence it represents a loss - the kick will be less effective in driving the swimmer forward. The kick might well require less force to perform, but that doesn't mean that it is more effective since the propulsive efficiency will also be less.

John C. Chien September 17, 1999 11:49

Re: Fish Kick
 
(1). I think, near the surface, the water will have more places to go. So, it is hard to guess at the result. You could be right. (2). The answer should come from the CFD calculation, or the actual testing I think. My model is probably over-simplified.

clifford bradford September 17, 1999 14:32

Re: Fish Kick
 
hello jonas, i'm a swimmer as well as a fluid dynamicists and i think your explanation of lower wave drag is probably correct. from a swimming point of view the same increase in propulsive force to be gained from turning on the side can be achieved by going deeper. i've not used this fish kick technique myself (i haven't swam competitively for a while) although i think i'll try it when i go to the pool next. i always thought when i was swimming that a lot of these little things don't add up to much in a race. the swimmers who do better turning on their sides to kick when they push off probably do better than conventional swimmers due more to the fact that they have stronger kicks than due to the lower drag.

clifford bradford September 17, 1999 14:36

Re: Fish Kick
 
in any case i think the effect for swimmers is probably more psychological than fluid dynamic. they see a world championship swimmer on TV doing this new style and so they go to the pool and work on something they might not have normally and so they improve whether the wave drag is less or not. i speak from experience as i used to swim competitvely


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