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Old 6th June 2015, 01:17 PM
STOKER STOKER is offline
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Default Vortex Physics

Can someone explain to me the physics/fluid dynamics behind a vortex? You know like water spiralling down a plughole. Although the specific example I'm thinking of is the funnel vortex created by using a stirrer bar in a beaker of water. I believe it has something to do with Bernoulli's Principle, and the conservation of energy by water travelling in a curved path around a central axis. I'm guessing the funnel is narrow at the bottom of the beaker because of the greater pressure at that depth by the water column, and the rest is a balance between different speeds and pressures.
Thanks in advance
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Old 6th June 2015, 04:38 PM
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Default Re: Vortex Physics

I'd love to help - unfortunately, the only thing I know about fluid dynamics is that it is very difficult! Fortunately, there are many here more clever than I.
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Old 6th June 2015, 05:04 PM
stevebrooks stevebrooks is offline
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Default Re: Vortex Physics

Quote:
odd said View Post
I'd love to help - unfortunately, the only thing I know about fluid dynamics is that it is very difficult! Fortunately, there are many here more clever than I.
I always heard it was a bit chaotic but since that's exactly how my head works we get along fine, specially if the fluid is in frosty cold bottle with a small neck, hmmm beeeeer!
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Old 6th June 2015, 05:08 PM
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Default Re: Vortex Physics

Quote:
STOKER said View Post
Can someone explain to me the physics/fluid dynamics behind a vortex? You know like water spiralling down a plughole. Although the specific example I'm thinking of is the funnel vortex created by using a stirrer bar in a beaker of water. I believe it has something to do with Bernoulli's Principle, and the conservation of energy by water travelling in a curved path around a central axis. I'm guessing the funnel is narrow at the bottom of the beaker because of the greater pressure at that depth by the water column, and the rest is a balance between different speeds and pressures.
Thanks in advance
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Last edited by Darwinsbulldog; 6th June 2015 at 05:14 PM.
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Old 6th June 2015, 05:08 PM
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Default Re: Vortex Physics

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Old 6th June 2015, 05:35 PM
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Default Re: Vortex Physics

It isn't something I've come across yet in my studies, without digging further ( I don't really have much time for it, sorry.) your original post sounds about right. I do know vortexes have vorticity, that is a vector that describes the rotary motion at a point within some fluid. (gas, dust, plasma etc.) and that vorticity is important in regards to vortexes.
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Old 18th July 2015, 06:13 PM
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DanDare DanDare is offline
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Default Re: Vortex Physics

Given the paucity of responses this might help:

https://en.wikipedia.org/wiki/Vortex

Quote:
Irrotational vortices




Pathlines of fluid particles around the axis (dashed line) of an ideal irrotational vortex. (See animation)


In the absence of external forces, a vortex usually evolves fairly quickly toward the irrotational flow pattern, where the flow velocity u is inversely proportional to the distance r. For that reason, irrotational vortices are also called free vortices.
For an irrotational vortex, the circulation is zero along any closed contour that does not enclose the vortex axis and has a fixed value, , for any contour that does enclose the axis once.[6] The tangential component of the particle velocity is then . The angular momentum per unit mass relative to the vortex axis is therefore constant, .
However, the ideal irrotational vortex flow is not physically realizable, since it would imply that the particle speed (and hence the force needed to keep particles in their circular paths) would grow without bound as one approaches the vortex axis. Indeed, in real vortices there is always a core region surrounding the axis where the particle velocity stops increasing and then decreases to zero as r goes to zero. Within that region, the flow is no longer irrotational: the vorticity becomes non-zero, with direction roughly parallel to the vortex axis. The Rankine vortex is a model that assumes a rigid-body rotational flow where r is less than a fixed distance r0, and irrotational flow outside that core regions. The Lamb-Oseen vortex model is an exact solution of the Navier–Stokes equations governing fluid flows and assumes cylindrical symmetry, for which
In an irrotational vortex, fluid moves at different speed in adjacent streamlines, so there is friction and therefore energy loss throughout the vortex, especially near the core.
Rotational vortices




The cloud vortex Saturn's hexagon is at the north pole of the planet Saturn.


A rotational vortex – one which has non-zero vorticity away from the core – can be maintained indefinitely in that state only through the application of some extra force, that is not generated by the fluid motion itself.
For example, if a water bucket is spun at constant angular speed w about its vertical axis, the water will eventually rotate in rigid-body fashion. The particles will then move along circles, with velocity u equal to wr.[6] In that case, the free surface of the water will assume a parabolic shape.
In this situation, the rigid rotating enclosure provides an extra force, namely an extra pressure gradient in the water, directed inwards, that prevents evolution of the rigid-body flow to the irrotational state.
There is a fair bit of dense maths on the page and lots of links to relevant stuff. I didn't know about the two types of vortex before so it was fun to find.
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Old 18th July 2015, 06:15 PM
STOKER STOKER is offline
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Default Re: Vortex Physics

Thanks DanDare,
much appreciated
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