How does moving your legs propel you forward in swimming?

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Moving the legs in swimming contributes to propulsion primarily through a downward kick that displaces water, creating thrust. The upward movement of the legs encounters less resistance, allowing for a more efficient return to the starting position without significant backward motion. The effectiveness of the kick is enhanced by the synchronization of hip movement, which increases the arc of the kick and improves thrust. Additionally, kicking helps reduce the vacuum created behind the swimmer, minimizing drag and aiding forward motion. Overall, while the legs do propel the swimmer, their primary role is to mitigate suction and enhance overall efficiency in the water.
  • #31
marcusl said:
I'm speaking of vortices in the fluid, not rotational motion of a flagellum. At the low Reynolds numbers that Purcell considers, inertia effects are negligible compared to viscous effects (he points out that inertial forces die away in distance of order 0.1 Angstrom!). How are vortex rings sustained in that regime?

As I mentioned, Purcells' paper is a *starting* point, kind of like how frictionless surfaces and massless pulleys are used in introductory mechanics. Since you (apparently) don't have access to Lighthill:

http://maeresearch.ucsd.edu/~elauga/research/references/LaugaPowers09_RPP.pdf

and
http://www.pnas.org/content/early/2011/07/28/1106904108.full.pdf

Figure 8 of #2 is instructive, as vortices are clearly present. In the fluid. More than 0.1 Angstrom away.
 
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  • #32
Thank you, I will read these.
 

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