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-   -   mass flow rate and size of mesh (http://www.cfd-online.com/Forums/cfx/72670-mass-flow-rate-size-mesh.html)

zandi February 13, 2010 16:33

mass flow rate and size of mesh
 
Hi every body
I'm working on a model of spillway in cfx (free surface flow modeling). when I change the mesh size espacialy near wall bc type (bottom of domain), the mass flow rate passing the domain change. could anyone explain this for me that what and why have happened?!
thanks

ghorrocks February 14, 2010 04:59

Good to see you are starting a mesh sensitivity check - many people do not bother with this (or don't know it is important) and their results are rubbish because of the laziness.

When your results are changing between mesh refinements it means your mesh is not adequately refined. You need to use a finer mesh.

The seminal reference for this analysis is "Computational Fluid Dynamics" by Roache. I strongly recommend you read it.

zandi February 14, 2010 07:16

thank you
in the mesh study hat I have done the water surface, pressure, velocity and turbulent kinetic energy respectively reached to a stable point. But the mass flow rate did not. refining mesh make number of my elements more than 6,000,000 (my study started from 50000).the i7 system that I work on it, does runs with 6000000 elements in about 6 days with trouble. it is difficult to continuing mesh refinment.

ghorrocks February 14, 2010 18:37

CFD was never meant to be easy. Besides simulating nuclear explosions, one of the main reasons people build supercomputers is to run CFD.

feizaghaee February 15, 2010 17:09

i have had this problem i work with free surface flow but i could finally solve it. you could set lower physical time scale for continuity equation in solver control rather than general time scale you should try several times to find a effective cobimation of several tme scales for good result. setting the lower tme scale for volume fraction helps better covergence and it's very effective.

zandi February 16, 2010 03:33

realy thank you mr Feizaghaee
I'm going to test it

ghorrocks February 16, 2010 18:32

Sounds like beating about the bush to me. Either way you need to do a mesh sensitivity check and if that shows your computers are not big enough to solve your problem then you need bigger computers.

zandi February 17, 2010 07:10

thanks a lot friends i'm going to test time scale and other way I can do. still any more suggestion would be greatly appreciated

zandi February 20, 2010 16:36

Quote:

Originally Posted by feizaghaee (Post 246121)
i have had this problem i work with free surface flow but i could finally solve it. you could set lower physical time scale for continuity equation in solver control rather than general time scale you should try several times to find a effective cobimation of several tme scales for good result. setting the lower tme scale for volume fraction helps better covergence and it's very effective.

thank you it works for discharge
but not for convergence
could help me please

feizaghaee February 21, 2010 05:31

if you work with cfx 12 go to the multiphase control in sovler control>advance tab check the box of multiphase control and select coupled mode. in sovler control>advance tab select hydro control and set the number of transient itration to for example 5.
can you tell me what time scales do you use for each equation and what is the total time scale?


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