How does moving your legs propel you forward in swimming?

CuriousBanker

I mean I know you're right. I also know that swimming works. I just wanted to know why, not by using analogies but by using physical laws

But some posters on this thread have done a good job explaining. Unfortunately my ignorance in this field makes me unable to understand most of it. I don't really understand vortices. But I kinda get it now a little bit. So than you all very much

Also another explaining could be this: I think force=v^3 or something like that in water. So maybe you kick really hard on the way down and not so hard on the way up? So the force on the way down is more than the force on the way up?

Ntstanch

The amount of force applied kicking down, generating the vortex, distances itself from you while losing energy in its interaction with the rest of the water as it pushes against you, and you against it. In other words, the longer you take to kick up or push against the remainder of that force, the less force there will be. Now, if you kick down very hard (say in a speed run), you would want kick up, and repeat, to maximize the energy that you're both generating and being pushed by and pushing against

Imagine someone kicking downwards. Only downwards. Then waiting until their momentum essentially stopped. Compare how far and how fast they would end up moving in a 200 meter swim compared to someone who is pacing themselves by kicking up and down in a fluid like manner (no pun intended).

Really, the current math around this (as far as I'm concerned) isn't adequate and (literally) doesn't really exist. If you can figure out three dimensional fluid dynamic mathematics then you win a million dollars (Clay Institute of Mathematics reward for advancement on Navier-Stokes equations). Personally, just thinking about three dimensional flow of a vortex gives me an actual headache. Thought experiments on it feel like they're sucking the energy out of my damn brain .

Best approach, for me at least, is to visually study the behavior of fluids. From there you get an intuitional understanding that advanced from the basics, then upwards. Either way, fluids are a very difficult and confusing/frustrating field of thought.

Ntstanch

The thing with vortices and the human physical form is that we aren't really aquatically dynamic in our shape. An obvious example being a freestyle swim and how quickly the vortices "crumble" due to our various other movements in the water. Like how a smoke ring does fine on its own until the balance of forces disrupts and disbands the motion.

Most aquatic creatures compliment these forces with both their shape and motions. Even snakes utilize the dynamics of the water well when swimming. They seem to "wiggle" their bodies to compliment the flow and to generate the most speed as they snap at each vortex like a whip. Like with the stick in the water and the vortex shedding... except they contour their bodies with the alternating high and low pressure zones in that snap or whip like motion of their tails. Most fish do the same, but far more efficiently given the shape of their tails.

Also, the breast stroke and butterfly would be the best way to implicate (or at least mess around) with this concept. I found it easier to visualize and "feel" when doing underwater dolphin drills.

Edit: Another factor in how we swim rather poorly compared to submerged creatures is that, if submerged and attempting to freestyle, you obviously won't do well on account of having to bring our arms and hands back to the front... what with air resistance being far less slowing than water. You can sort of compare it to two people in a row boat rowing against the motion of the other person.

H2Bro

I see it coming into play during the breast stroke where your hands move towards the center of the body near the end of the stroke, before pushing up against the water to clear your mouth for air.

Would 'pushing' water together like this, before pushing 'against' it, increase the effectiveness of the force transferred to your upward motion? Or would this go against the "incompressible" aspect of water?

Also, by having the stroke "reset" by moving hands towards center, the vortices are led towards the area your body is about to travel through increasing turbidity and decreasing resistance. Your thoughts?

Human body I think is more remarkable for characteristics making it well suited to aquatic activity compared to a lack of such characteristics. You know, hairless bodies, webbed fingers and toes, the 'dive' reflex for infants, so on. Not so sure about the "aquatic ape" hypothesis but we certainly have SOME adaptations that are useful.

Edit: I think two of my 'geusses' at at odds with each other - vortices are unlikely to help you push off as well as glide through at the same time! if you could hint at what you think is going it I'd appreciate.

Rooted

I am curious about the vortices here. I was told once that vortices were formed by 'a flow of energy' (in an argument to explain the existence of life, as it happens). Could it be here that the vortices are the symptom of an energy gradient rather than the cause of propulsion?

Also I was under the impression that vortices increase drag. Does this mean they are an undesirable thing for a swimmer? This video suggests natural swimmers have laminar flow http://www.youtube.com/watch?v=ojlL03Z5yVQ

Ntstanch

H2bro, Those are both "Tall orders" on my part, as I'm still trying to convince myself and understand their dynamics. Also been busy lately. I'll try and think of a way to explain my thinking on your questions, but can't promise anything.

Except for the pushing off the wall. In this case you're pushing off a solid, then going into a dolphin like kick where the importance of a vortex in general comes into play. When "pushing off" a vortex you're guiding your body and using unique motions to utilize the force of the spin in the vortex to continue propelling yourself on what otherwise would get you no where... or at least veer away from you and disperse its energy as it travels away.

(Edit: Pushing off the wall, and pushing off the vortices, isn't so different in my mind. One is a solid and not going to give way or waste much of that energy. The vortex will however waste energy while being pushed against as all that energy doesn't concentrate in key areas as if it were helping us swim faster. Fins, flippers, big feet - etc... help you to not waste that energy though. Oh, and chimps are terrible swimmers, their bone density and muscle mass usually results in them sinking. So score one for homo-sapiens.... However, that muscle and bone density is also why they're terrifyingly powerful and can dent/bend the frame of a car tire by bashing it with their fists. :P )

Until I really get a solid foundation in the whole concept the theory becomes mostly private as not to confuse or waste anyone time (aside from my own).

Ntstanch

I'm not really interested in this sort of topic. Sorry.

A vortex, or vortices, can increase or decrease drag. It's a spinning streamline flow... depends where you're at and how you're utilizing the flow. Though I think this is more a case for gases than fluids. Gases being much quicker and much more difficult to imagine in general. As for the video, streamline flow is important, but have you ever seen anything in the water lay still and swim quickly? The motions are, from what I can tell, the key to efficient swimming. The idea is that vortices are an important part of this efficiency.

Andy Resnick

Swimming (or any propulsive locomotion based on a repeating cycle of movement) is surprisingly tricky to explain- after all, why can't you simply reverse your motion and swim backwards?

Pucell's excellent paper is a great place to start:

http://jila.colorado.edu/perkinsgrou...lds_number.pdf

and Lighthill's book "Mathematical Biofluiddynamics' has a considerable amount of information and current (as of 1975) theoretical approaches to solving the problem of aquatic locomotion, but I can't claim to understand very much of it.

marcusl

Purcell's paper is not directly relevant here because bacteria swim at *very* low Reynolds numbers where our experience fails to apply. Purcell states that a swimming bacterium is equivalent to a human sitting in a swimming pool of molasses and moving no part of his/her body more rapidly than 1 cm/minute! As just one difference, micro-organisms do not generate the vortices that are an important part of how we swim. Having said that, it is a remarkable paper.

Andy Resnick

Wrongedy-wrong. See flagellar locomotion and nematode swimming. Gray and Hancock's 1955 paper has some nice detail. And again, Lighthill's book is comprehensive.

Unless you mean that having arms and legs makes human swimming radically distinct from other forms of aquatic locomotion.

marcusl

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?

Ntstanch

I'm kind of lost, but wouldn't a vortex waste more energy while it is spreading out... while the ring contains itself far more efficiently?

marcusl

The issue I raised is only that Purcell's paper does not describe human swimming but rather that of bacteria for whom inertial forces in the fluid are negligible. I believe in that case that the fluid cannot support vortices of the sort you described for human swimming and that have been observed shedding off of the tails of fish.

Andy Resnick

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/...wers09_RPP.pdf

and
http://www.pnas.org/content/early/20...04108.full.pdf

Figure 8 of #2 is instructive, as vortices are clearly present. In the fluid. More than 0.1 Angstrom away.

marcusl

Thank you, I will read these.