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Christian April 8, 2003 01:54

Rotation flow calc. II. Andy
I hope I am not out of line. This tread is a copy from 03-04 Andy, in case you want to comment the mail, but haven't seen it :) Otherwise I apologise.

Hi Andy.

I am sorry for the delay. I needed time to think about your answer, and to check the calc. I do appreciate your time.

There not much swirl in the physical model. I have not measured it, only a visually evaluation. The calc however, the tangential component are as much as 10%. I don't think that is correct.

I cannot see why a certain amount of solid body rotation is wrong. I thought that it was "normal" close to the axis to have a "solid core". However, it does not seem to be the case in my calc. If the calc shows too mush solid core what will be your recommendation?

The velocity vectors are all pointing in the rotational direction, so I guess that is ok. No funny directions of the flow.

I am curious. What do you mean by more expensive procedures? The k-e cannot capture a rotational flow very well, and is inexpensive. RSTM is more expensive (and more unstable) but should be able to capture the rotational characteristics (well that's what I thought until last week :) Do you mean LES or are you thinking of modifications to the RSTM?

Do you have any recommendations to steps making the calculation work.

Ps. Funny thing is that I have made an calc for about 1 year ago that is very similar to this one, and it worked fine. I have looked it over to find any differences in setup, but cannot see any. I guess that I was lucky the last time, and that my knowledge of rotating flow needs an upgrade.

andy April 9, 2003 08:49

Re: Rotation flow calc. II. Andy
Many of the previous comments were based on the assumption you were solving a strongly swirling flow which is difficult for the solution procedure (stiff equations) and difficult for the turbulence model (curvature can greatly stabilize or destabilize the turbulent motion in a shear layer). "As much as 10% tangential velocity" is well below what would trigger swirling flow treatments.

Solid body rotation is correct at the core of a vortex. In a free vortex (but not a forced) this region is very small.

More expensive solution procedures will solve solution variables in blocks, pre-condition matrices, perform inner iterations and the like. Such solution procedures tend to be more robust and can work with smaller amounts of "numerical fixes" like pressure smoothing. However, they require more memory and more computer time per iteration. I was not refering to turbulence modelling.

If you have an old prediction which works and a current one that does not then studying the differences will tell you where the problem lies. I doubt luck has much to do with it but experience helps.

I would recommend either talking to someone local experienced with CFD or talking to your CFD supplier. I think we have just demonstrated the difficulties of getting this type of help from a forum such as this.

Christian April 10, 2003 04:06

Re: Rotation flow calc. II. Andy
Thank you Andy.

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