Quantifying Swirl
 

Dear All,

I'm trying to quantify swirl on an annulus.

I've searched the forums, WWW and my text books but cannot find a comprehensive reference on swirl number. Plenty of sources refer to Guptas "Swirl Flows" but it's out of print. I have Michael Bo Hansens definition (thanks MBH!) but would like to understand more.

My specific question relates to the extent of applicability of the swirl number - is there a point at which it becomes meaningless?

For example, given one large vortex, or swirl, around a single axis - the angular component is simple - it will be either a positive or negative number depending on direction of rotation. I can see the validity of swirl number for this type of flow.

My case is more complicated - a number of small vortices within the annulus, packed together around its circumference, and these themselves are in rotation down it's length. What happens here? I can imagine a scenario where one half of each vortice gives a positive swirl number, the other half gives negative and overall effect is swirl number of zero despite strongly swirling flow. Maybe swirl number is the wrong way to approach this?

Further, can anyone advise how tangential velocity shold be calculated? I was going to use SQRT(U^2 + V^2) but that makes all tangential velocity positive which I don't think is correct.

Thanks in anticipation!

Ianto.