Do floating objects clump together on curves in rivers?

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zuz

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I recall reading somewhere about when anything, (eg. cars on the road, balls in a flowing stream) tend to clump together on curves. I don't remember where I read it but I seem to think there was some principal involved. Has anyone ever heard of this before, or am I mistaken. Thanks.
 

FactChecker

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I'll take an amateur stab at this (I probably shouldn't, but I have no willpower):
A river often digs into the bank at a curve and leaves a stagnation point where the water just swirls around. Anything washed into it will tend to stay there and get tangled up with other things. And the decreased water velocity lets stuff settle there.

EDIT: The following posts seem to indicate that I got it wrong. On a curve, the outside of the curve has higher velocity water which caries away dirt and debris and the inside of the curve has slower velocity water where dirt and debris are deposited.
 
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256bits

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pinball1970

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I recall reading somewhere about when anything, (eg. cars on the road, balls in a flowing stream) tend to clump together on curves. I don't remember where I read it but I seem to think there was some principal involved. Has anyone ever heard of this before, or am I mistaken. Thanks.
@FactChecker and 256bits have given details and I am not educated in this so @daven could give insight on this.
This is cool and gives examples across the world.
 

Baluncore

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Casimir effect, Coanda effect? Bernoulli effect?
 

Baluncore

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It is clear that if objects cannot pass, then they will tend to bunch up with fast ones at the rear and slow ones in the lead. Car drivers reaching a curve slow down, overcautious drivers slow down more, so tend to obstruct the others, which results in natural bunching.
If cars enter a road section two seconds apart, and remain 2 seconds apart, then they will be closer where the speed is lower, such as at corners. Because cars do not have zero length, the linear separation is reduced quickly and could cause a pileup when it takes a vehicle more than two seconds to travel it's own length.

Where flowing water forms an eddy, that is drained by a central downward movement, any floating objects that enter the eddy will collect there, unless they sink in the whirlpool.

A hydrophobic floating object, such as a waxed wooden block floating on water, will form a local depression in the surface. Two such objects will move together as they slide down the slope of the water surface. Indeed, one hydrophobic object will attract any other floating object.

Ships close together, becalmed at sea are drawn to each other because one will shelter the other from light variable winds. Maybe that is an analogue of the Casimir effect.
If there is a swell then some wave energy will be reflected from the boats. That should also push them together. I believe the presence of the oscillatory swell makes that the Casimir effect.
Where wind blows through the gap between two close vessels, the wind speed is slightly greater than outside the vessels. Faster flow implies lower pressure between the vessels, so they should be attracted together. That is the Bernoulli effect. It requires the wind be sufficient to flow around and between the boats, while not strong enough for the boats to sail apart.
 

zuz

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Actually I mentioned this on another site and someone mentioned density waves and I think that is what I had heard earlier. Thanks everyone.
 

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