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Jennifer Haque July 26, 2004 15:13

compressive discretisation scheme
 
Dear All,

I am using CFX5.7, how do I select the compressive discretisation scheme for volume fraction equations when using the homogeneous free surface model?

Regards Jennifer

Robin July 27, 2004 11:21

Re: compressive discretisation scheme
 
It's done automatically when you select the free surface model.

-Robin

Jennifer Haque July 27, 2004 12:17

Re: compressive discretisation scheme
 
Dear Robin,

Many thanks for that. Do you have any experience in free surface modelling as I am experiencing a few issues with flow in the system on adding the free surface model. If you have any experience can you let me know, i can explain what i am trying to do and maybe you can give me some suggestions.

Kind Regards

Jennifer

Robin July 27, 2004 13:10

Re: compressive discretisation scheme
 
Hi Jennifer,

I have some experience, but there may be others on the forum who can help you. You can also try posting it on the CFX Community Forum.

Regards, Robin

Jennifer Haque July 27, 2004 14:57

Re: compressive discretisation scheme
 
Dear Robin,

I am modelling flow in a stirred tank reactor. On application of the free surface model, the flow in the reactor is not as expected. The residuals seem to converge but the flow does not reach a steady state. I am using unstructured meshing. I am using high resolution differencing scheme, and i have used inflation and refined meshing. Have you solved any similar problems or can you give me some general advise on free surface modelling.

Regards Jennifer

Robin July 27, 2004 15:55

Re: compressive discretisation scheme
 
Hi Jennifer,

I haven't solved anything similar. One comment, however, is that the residuals will only converge if you have a steady state.

To what level have your MAX and RMS residuals been reduced?

Regards, Robin

test July 28, 2004 04:52

Re: compressive discretisation scheme
 
Hi, Well Free surface problems are tough to converge and require very fine mesh near the interphase. I suppose you have not used Mesh adaption!! This will be very helpful. CFx uses a step function to determine the free surface.. and uses a surface sharpening algorithm to sharpen the interphase. If the mesh is coarse near the free surface the inetrface will be smeared off. When you create the mesh you may!! have refined the mesh at the free surface, but as the solution goes along, the free surface changes, Thus you will need to use mesh adaption to track the free surface

Cheers, Test

Prash July 29, 2004 05:00

Re: compressive discretisation scheme
 
u must use mesh adaption for accurate interface tracking and overall convergence. another thing, free surface problems don't necessarily have a steady state. They are mostly dynamic and having a dynamic solution is not wrong! what is important is getting convergence at every step of this transient problem.

cheers, prash


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