The constructal-law physics of why swimmers must spread their fingers and toes.

Here we show theoretically that swimming animals and athletes gain an advantage in force and speed by spreading their fingers and toes optimally. The larger force means larger body mass lifted and greater speed, in accord with the constructal theory of all animal locomotion. The spacing between fingers must be twice the thickness of the boundary layer around one finger. This theoretical prediction is confirmed by computational fluid dynamics simulations of flow across two and four cylinders of diameter D. The optimal spacing is in the range 0.2D-0.4D, and decreases slightly as the Reynolds number (Re) increases from 20 to 100. For example, the total force exerted by two optimally spaced cylinders exceeds by 53% the total force of two cylinders with no spacing when Re=20. These design features hold for both time-dependent and steady-state flows.