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CFXPostuser June 19, 2006 03:31

How to calculate inlet swirl number in Post?
Hi.... I read some papers comparing effects of inlet swirl on a diffuser. They used inlet swirl number in the graphs.

Can anyone here please tell me how to construct a formula using the function given in CFX Post 10 to calculate the inlet swirl number?

Thank you very much for your help.

Joe June 19, 2006 07:13

Re: How to calculate inlet swirl number in Post?
Exactly what analytical formula are the papers using for swirl number?

Michael Bo Hansen June 19, 2006 07:20

Re: How to calculate inlet swirl number in Post?
Here's how I do it on the "out" plane. - Michael Bo

Ram June 19, 2006 09:08

Re: How to calculate inlet swirl number in Post?
Hi Mr.Michael Bo Hansen,

I have to calculate swirling flow in a reactor. First i used k-e model and it converged well. As it is not accurate for swirling flows i tried to calculate with SSG turbulence model. I am getting convergence problems. I tried with very fine mesh, initial guess & interploation of k-e model solution, and different physical time scales, auto time scale etc but in vain.

I used same conditions as for K-e model. Do I need to specify any additional conditions for SSG model to get convergence?

Thank you.

Michael Bo Hansen June 19, 2006 09:21

Re: How to calculate inlet swirl number in Post?
The SSG model will give you a better prediction of the swirl as you said, and thereby also a more complex flow, which can be harder to convergence.

Try reducing the Advection Scheme from Second Order to Specified Blend Factor = 0.7 in the Solver Control. Of Course it is a compromise.

CFXPostUser June 21, 2006 05:02

Thank you
Thank you so much, Michael. That's exactly what I'm looking for.

Joe June 26, 2006 02:09

Re: How to calculate inlet swirl number in Post? *NM*

Joe June 26, 2006 02:11

Re: How to calculate inlet swirl number in Post?
Ups, sorry.

That it is not converging may be an indicator, that your flow is instationary. The k-eps-modell is very diffusive and can give an unphysical stabilisation for that reason.

Ianto April 29, 2009 13:50

Dear All,
Thanks for the excellent definition Michael!

Could anyone please clarify further for me? My non-comprehension is re. tangential velocity definition, whether or not we can have negative tangential velocity thus swirl number and, if we can have the scenario where two counter-rotating vortices give swirl number of zero?

I'd like to use swirl number to quantify the flow at an annular outlet in order to select the best design (least swirl).

Instead of the flow pattern shown in Michaels image, all rotating around a single central axis, the annulus is filled with a ring of small vortices each rotating around it's own axis.

My first thought was to use SQRT(U^2+V^2) for tangential velocity. Obviously this removes negative tangential velocity. This would make sense to me, as it sidesteps the problem of counter rotating vortices (or even opposite side of the same vortice) cancelling each other out in terms of swirl number. However I read this:

........ which states negative tangential velocity does occur. Thus my Pythagoras based tangential velocity definition is wrong, the prospect of vortices cancelling each other out is possible and I'm confused.

I'd be VERY grateful for some clarification!

Thanks in anticipation,


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