Wednesday, August 08, 2012

Swimming and Viscosity

I really love the comic XKCD, but his output today is just plain wrong.

In it, two people are trying to catch a swimming Michael Phelps with a net, but can't because he is too fast. So they decide to dump Jello in the pool to slow him down.

Unfortunately, this will do nothing to slow him down. While the viscosity of the water will increase and hence the drag, he will also be able to push off the viscous water more effectively. The net effect will be that nothing changes.

This has in fact been demonstrated back in 2004 by one of my previous chemical engineering professors here at the University of Minnesota, Ed Cussler. Himself an avid swimmer, he was somehow able to convince the maintenance people here to let him gel the pool with guar gum. Additionally, he was able to convince the swim team to swim in the goo. A comparison of times in water vs. goo showed no statistical difference, and in doing so, won an Ignobel Prize in 2005.

Back to the cartoon, the mouseover shows that the two pursuers were unsuccessful in catching Phelps anyway, as he ate all the Jello, a nod to his well known appetite. Right result, but wrong reason.

UPDATE: I have additional comments on this matter here.

14 comments:

Anonymous said...

But the comic says nothing about whether or not the scheme worked. It is plausible that both phelps, and the people trying to capture him didn't know the jello wouldnt work

Anonymous said...

There's a big difference between a syrup and a gel. The point of the comic was for the jello to set, becoming a gel, which is very different from a viscous fluid. Your criticisms are meaningless.

John said...

Sure it does:

From the original post:"...the mouseover shows that the two pursuers were unsuccessful in catching Phelps anyway, as he ate all the Jello, a nod to his well known appetite. Right result, but wrong reason."

Anonymous said...

set jello is not a viscous fluid but a colloid. i know it's tempting to disprove an xkcd comic, but you fell short here. ask your professor about it.

Anonymous said...

Guys, guys... If he ate all the Jello, then there would be nothing to swim through, which could have been their plan all along. Perhaps the point was not to slow him down in Jello, but to make him the architect of his own demise.

Anonymous said...

Actually, you, too, are wrong about the comic because you miss an important distinction:

The characters of the comic are wrong, not the comic nor the author himself.

Furthermore, though it is implied, the comic says nothing about Jello slowing him down . . . and you assume that slowing him down means speed and not strength. Won't the Jello make the swimmer tire more quickly?

Anonymous said...

You need to read (or understand) the article you cite better. It cannot be extrapolated nearly as far as you are extrapolating it.

1. They only increased the viscosity by a factor of 2. Wikipedia has a nice table of some common viscosities if you'd like to look at them. A factor of two is nothing. Things like corn syrup are 1000x as viscous. In this experiment, they are still virtually swimming in water

2. At the end of the article, they make it a point that human swimming is a turbulent process, unlike many of the other commonly studied swimming processes. The Reynolds number they reference is inversely proportional to viscosity. As the viscosity goes up, the Reynolds number goes down, and you move towards laminar flow. Jello swimming, even if it was completely newtonian, which it's not, would be a completely different beast.

3. Jello is non newtonian! I can't even begin to express here how much that changes things. Look it up if you don't know.

4. Even *if* you could robustly say that for all levels of viscosity, the fundamental nature of the flow (turbulent newtonian) doesn't change and that increased drag can always be canceled by increased propulsion, then a swimmer *still* wont be able to go as fast in a highly viscous fluid. The viscosity is directly linked to an energy sink. The body can only push so hard, and has a firm cap on how much energy it can expend for so long. Over long swims in small viscosity increases the swimmer will tire faster, and with large increases in viscosity they wont be able to exert enough to maintain normal speeds regardless of duration.

Anonymous said...

There was a Mythbusters episode where they had a professional swimmer swim in syrup and in water to see exactly what would happen. According to the swimmer, he said it was much different in the syrup, which he believes slowed him down just because his body was not used to anything of the sort. So I think it would be the same case for Michael Phelps. I side with XKCD

Charles Allen said...

Link's broken.

DOI: 10.1002/aic.10389

Curtis Gantt said...

I am going to buy pool supplies soon because my old supplies need to be replaced because those are really old already. I think I should include Jello in my list too because it can add fun to my kids’ swimming experience.

Richard Holmes said...

Maybe if they made the jello mixture the night prior and kept the whole thing airtight overnight under an electric pool cover, there will e a significant difference.

Albert Wood said...

Funny how these pool fences tampa bay could be visible on the story too. Very funny and interesting though.

Kellie Klybertie said...

The organizer of that event should buy balustrade Perth for the safety of the swimmers. Swimming in goo is fun but I don’t like the cold sluggish texture of it.

Mike said...

Nice read! I like the suggestions.