Now I know what the spiral is for

[Harry Bloomfield]

The spiral on metal chimneys that is. I used to think it was about having the wind help draw gases out of the chimney. In fact it about preventing wind eddies shaking the chimney to destruction.

[tony sayer]

In article , Harry Bloomfield scribeth thus

Yes quite correct, as can be seen by some spirals here on the top of the Emley mast in Yorkshire;)..

[Graham.]

Yes, I knew about that 30 years ago, but what I couldn't get my head around (if it's true) is that it only works when the spiral is in one particular direction, not the other.

[Tim+]

Yep. Vortex shedding.

Tim

[Bob Eager]

Does it depend which hemisphere you're in? :)

[harryagain]

Not true.

[jrwalliker]

To bring this closer to DIY - wrapping a spiral of cable around the top rung of a stepladder being transported on the roof rack of my car results in a much quieter journey. (If the top rung is at the front.)

John

[Brian-Gaff]

That is what I was thinking, as correolis would suggest it should be the other way darn sarf. Brian

[CB]

Yes as well as industrial chimneys the effect can be seen at the other end of the scale on some car antennae, to stop them resonating at certain speeds

?ref=list

[CB]

But its far to localized for Coriolis to have any effect.

[John Chance]

Must explain why it's seen with plug holes.

[CB]

It isn't

(and an infinite supply of other links if you choose to look)

[Scion]

Harry Bloomfield put finger to keyboard:

Greek bridge prog last night by any chance? I was impressed by the video of the dampers in the earthquake.

[soup]

Where you watching "Impossible Engineering" on Discovery science?

[Tim+]

Whoosh!

Tim

[Tim Streater]

It isn't.

You were obviously never a small boy. This small boy soon discovered that you can force the water going down a plug hole to rotate in either direction by use of a mark 1 finger. Further, that the water continues to rotate that way until there is none left.

[newshound]

Urban myth. It works because it suppresses vortex shedding by disrupting the axial symmetry.

The problem with vortex shedding is resonance when the natural frequency of the chimney, tower, or whatever coincides with the vortex shedding frequency.

[Bob Eager]

Brian probably couldn't see the smiley.

[soup]

Witness the Tacoma narrows bridge.

Usually described as forced resonance the full story is of aeroelastic flutter but for the purposes of a Usenet post it can be thought of as forced resonance.

[The Natural Philosopher]

Yerrs, BUT left to its won devices it nearly always rotated anticlockwise, the same ways as a depression rotates in the Northern hemisphere.