Why does bathwater in a plughole spin AT ALL

[Grinkle]

That original angular momentum should decay by viscous friction.

It does not happen that way at least for water, or if it does, the decay does not go all the way to zero. If one experiments with fluids of varying viscosity, one might see the effects your intuition tells you should be present, it does seem to me that viscosity should play a big role in the swirling phenomena.

Edit:

Gravity keeps it going, there is no other force acting on the draining fluid to work against any friction that might be present.

You claim the equilibrium point for all forces acting on the fluid is zero angular momentum as though its obvious it cannot reasonably be any other number than zero. To me this is not at all obvious from inspection, and moreover it does not seem to happen that angular velocity decays to zero in practice.

Edit 2:

Think of a marble swirling down a funnel. It swirls faster and faster as it goes down the funnel. There is viscous friction (wind resistance) acting against it, but the dissipative force is very obviously not sufficient to cause the marble to stop swirling and roll straight down the side of the funnel. This is an example of a system that does have dissipative forces but these forces are not sufficient to bring angular momentum to zero. It not sufficient to simply note that dissipative forces exist in a system to establish that the equilibrium point of the system is zero for whatever state one is talking about (angular momentum in this case).

One can blow a fan against the marble and cause it to stop swirling, but that would be a different system.

 

[AdrianMay]

@Grinkle.
1. It doesn't matter how viscous the stuff is. Yes it will eventually go all the way to zero. My point is that the vortex can keep going for a million years if you keep pouring water in the top despite not injecting angular momentum in the process. If we propose that the only reason why the vortex is turning is the amplification (like a ballerina pulling her arms in) of pre-existing currents, then we'd expect the currents to die away through friction, not intensify as they appear to. So something else must be driving the vortex.

2. I agree that gravity is that something else.

3. Actually, angular momentum is a conserved quantity, so *for an isolated system* it'll maintain whatever value it started with. A bucket on a string with *no* hole is isolated. Any currents in the water will decay to zero over time, but if the whole thing had a *net* angular momentum at the start, then the bucket will be turning on the string at the end. But if there *is* a hole and the water forms a vortex, then angular momentum is spewing out of the bottom and I think the bucket will recoil the other way.

But still we're nowhere near an explanation of why the leaking water arranges itself into a fast vortex after an infinitesimal initial current, let alone, why it goes like the clappers when the bucket is nearly empty.

 

[Grinkle]

then we'd expect the currents to die away through friction

I wouldn't necessarily expect that. See my Edit 2 in post 36.

we're nowhere near an explanation of why the leaking water arranges itself into a fast vortex

In my opinion, we are nowhere near any argument against the answer being the initial angular momentum in the fluid causing the vortex.

 

[AdrianMay]

@Grinkle
You're not distinguishing buckets with holes from buckets without. Your funnel has a hole. I said that buckets with holes can turn under recoil. In a bucket without a hole, I expect the currents to die away.

Edit: Your marble will come to rest before long. Edit 2: It's losing velocity, it's losing gravitational potential energy, and believe it or not its even losing angular momentum to the funnel or the air.

 

[berkeman]

I flush my toilet 10 times in a row (we no longer are in a drought in Northern California thank goodness), and it swirls per the right hand rule every time.

Turns out using the toilet for an experimental apparatus is fundamentally flawed (another Mentor pointed this out to me overnight via a PM). Toilets are designed to swirl the water as they flush. Duh!

Sorry for the misinformation. Filling the bathtub now...

 

[Grinkle]

It's losing velocity

You lost me here.

it's losing gravitational potential energy

Yes, so?

its even losing angular momentum to the funnel or the air

I expect its reasonable to conclude that in any real system dissipation of angular momentum is non-zero, but without modelling and quantifying this, I don't draw any conclusion the presence of some dissipative force in the system is a counter argument to intitial angular momentum causing swirling.

 

[AdrianMay]

@Grinkle
It's a counterargument because I can keep the vortex going forever by topping up the bucket.

When a fluid shears (parallel layers sliding against each other), it experiences friction that opposes the shear. That means the vortex must slow down. But it doesn't. Therefore, something must be continuing to drive the vortex.

If the only thing driving the vortex was initial currents, the energy of those currents would soon be exhausted by the friction and the vortex would stop turning.

The missing energy is harvested from the water's potential energy as it falls through the hole, but what's the mechanism that turns it sideways into a vortex when gravity just pushes downwards?

 

[Charles Link]

In the toilets in my house, it is clear that the water enters the bowl thrust in one direction, (somewhat clockwise or counterclockwise, I haven't looked that closely), and thereby Coriolis effects would be secondary.

 

[PeterDonis]

This thread is marked "B", but as was pointed out in an early post, to really answer the question requires an actual mathematical model, which is (a) beyond the "B" level (it's at least "I" and quite possibly "A"), and (b) not being tried, as far as I can tell, by anyone participating in the discussion.

Accordingly, this thread is closed.