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The viscosity of water creates drag on swimmer's body so its effect is to slow down the swimmer. However the viscosity seems to be essential for pushing the water backwards by the swimmer's arms and legs. Would a human be able to swim in water with much lower viscosity? What standard stroke (front crawl, breaststroke, butterfly) would work best/worst? How would a lower viscosity affect fish motion in water?

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  • $\begingroup$ Drag coefficient depends rather weakly on Reynolds number and hence on viscosity. So even if viscosity decreased several times (if the density remained unchanged) the drag coefficient change would be much smaller so the dynamic of swimming probably wouldn't change much. $\endgroup$
    – user23660
    Commented Jun 11, 2013 at 15:04
  • $\begingroup$ Ref $\endgroup$
    – Trimok
    Commented Jun 11, 2013 at 18:39
  • $\begingroup$ Somehow related: will a propeller work in a superfluid? physics.stackexchange.com/q/311069/226902 $\endgroup$
    – Quillo
    Commented Oct 10, 2022 at 12:16

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There are two forces you need to consider when calculating fluid flow, viscous and inertial. The former comes from the viscosity of the fluid and the latter from the momentum of the fluid - momentum is mass (of a fluid element) times velocity just as for a particle. In general at very low shear rates viscous forces dominate while at high shear rates inertial forces dominate.

I would guess that when swimming (at least high speed swimming) the thrust you can generate is dominated by inertial forces. When you swim you are in effect throwing water backwards so conservation of momentum propels you forwards. Therefore you would be able to swim even in fluids with very low viscosity. The drag is probably dominated by viscosity, and this would be reduced in a thin liquid, so you might even be able to swim faster as the viscosity is reduced.

Calculating exactly how the efficiency of the various strokes is affected seems hard to me and I'm not sure it would be wise to speculate. It would be easy to do some basic experiments with different fluids and a swimming toy.

Later: it appears Myth Busters have had a play with this and there is an article here. They don't seem to have done any experiments though.

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    $\begingroup$ As a demonstration of the fact that our swimming is mostly inertial, here is the classic article "Life at Low Reynolds Number" which talks about how weird it looks in the other limit. jila.colorado.edu/perkinsgroup/… $\endgroup$ Commented Jun 11, 2013 at 15:41
  • $\begingroup$ Why should we assume that drag is dominated by viscosity while the stroke is dominated by inertia forces? The Reynolds number R is the ratio of the inertial to viscous forces, and R goes as $v l$, so a smaller object (arm) which is moving faster would have about same R as the whole body - correct? $\endgroup$ Commented Jun 11, 2013 at 19:33
  • $\begingroup$ I would guess the Reynolds number for the flow around the arm is a lot higher than for the flow around the torso. I've never done the calculation, but it certainly feels that way when I swim. Someone is bound to have done the calculations. It would probably be worth some concentrated Googling if you want to pursue the question. $\endgroup$ Commented Jun 12, 2013 at 6:33

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