Can a fish swim in incredibly viscous-dominated liquids?

[member 428835]

I was wondering, can a fish swim in incredibly viscous-dominated liquids? I'm thinking it cannot. My first thought is to consider the Navier-Stokes equations and see that the inertial and transient terms are small, so flow is not accelerating, and thus the fish couldn't accelerate either.

Any thoughts?

 

[mfb]

You need a different swimming motion - one that is not time-symmetric. For single-celled organisms, water is a very viscous liquid, and they can swim (some of them). Something similar to the way snakes moved on the ground is an interesting option.

 

[member 428835]

You need a different swimming motion - one that is not time-symmetric.

Could you elaborate why you need something different?

 

[mfb]

Scallop theorem
For low Reynolds numbers, time-symmetric motion in Newtonian fluids won't lead to position changes.

 

[member 428835]

Scallop theorem
For low Reynolds numbers, time-symmetric motion in Newtonian fluids won't lead to position changes.

Could you prove this, or intuitively explain it, or direct me to a source where this is explained? I believe you but would like to understand why.

Alongside what you've said, what if the fish fins press back very fast and then reset forward very slow (not time-symmetric, right?)

 

[mfb]

The article has references, and you can search the literature for "scallop theorem".
If the motion is time-symmetric, and all equations describing the water are time-symmetric as well (low Reynolds number!), how could be there net motion? If you reverse the time direction, the motion looks the same, the reaction of the water is the same, but the swimming direction suddenly has to reverse? Moving faster or slower doesn't break the time symmetry enough. You need asymmetric motion.

 

[member 428835]

Thanks for this! I didn't see the link before, but I'll check it out now.

 

[Andy SV]

 

[mfb]

That was still with large Reynolds numbers. You would need something honey-like to get low Reynolds numbers with humans.

The arm stroke would still work there, it is asymmetric in time. The legs are more problematic.

 

[Andy SV]

Syrup is very honey like it's sort of a defining characteristic of syrup
Honey is in fact a syrup
So I don't understand what you mean by that

 

[mfb]

The syrup they used in the video is certainly not as viscous as every honey I ever saw (at reasonable swimming temperatures).
Look how much it splashes around.