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-   -   Is it normal? (http://www.cfd-online.com/Forums/cfx/19393-normal.html)

David December 25, 2002 10:01

Is it normal?
 
A pipe is 20cm long with a diameter of 2cm. The inlet velocity is 0.4m/s(V direction). Now I found the values of RMS for velocity U and W are nearly the same for each iteration although the solution is converged. Is it an normal phenomena? Thanks.

David

Sascha Becker January 6, 2003 09:57

Re: Is it normal?
 
Maybe your grid is too coarse to get a converged solution --> refine grid and rerun the case.

Neale January 6, 2003 20:54

Re: Is it normal?
 
What exactly do you mean by "values of RMS for velocity U and W"?

David January 6, 2003 21:59

Re: Is it normal?
 
Hi Neale

Thanks for your interests. When I am looking at the out file for each iteration, I found RMS Res for U-mom and W-mom always have the same varying value. I also checked the out file of Tutorial 18, it as well has similar results. By the way, may I ask you another question about scale-up problem. For example, I got a converged solution on a short pipe (20cm and 2cm). Now I want to extend the pipe to ten times but the pipe diameter is kept the same (200cm and 2cm). However I can not get a converged solution in the extended pipe. So what's the reason? Are there any ways to solve the problem. All settings in CFX 5.5.1 for both cases are the exactly same. The max. mesh length is 0.4cm and stretch factor is (1 0.5 1), inflation is also considered with defaults. Thanks

David

Neale January 7, 2003 21:27

Re: Is it normal?
 
David,

1) What timestep are you using?

2) Are you running high resolution advection or upwind?

For the long pipe you should be using something like 200 cm divided by a typical velocity scale. If you are running high resolution or specified blend factor then you need to ensure that you have a fairly decent grid to get a reliable converged result.

Neale



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