# compressive discretisation scheme

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 July 26, 2004, 15:13 compressive discretisation scheme #1 Jennifer Haque Guest   Posts: n/a 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

 July 27, 2004, 11:21 Re: compressive discretisation scheme #2 Robin Guest   Posts: n/a It's done automatically when you select the free surface model. -Robin

 July 27, 2004, 12:17 Re: compressive discretisation scheme #3 Jennifer Haque Guest   Posts: n/a 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

 July 27, 2004, 13:10 Re: compressive discretisation scheme #4 Robin Guest   Posts: n/a 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

 July 27, 2004, 14:57 Re: compressive discretisation scheme #5 Jennifer Haque Guest   Posts: n/a 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

 July 27, 2004, 15:55 Re: compressive discretisation scheme #6 Robin Guest   Posts: n/a 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

 July 28, 2004, 04:52 Re: compressive discretisation scheme #7 test Guest   Posts: n/a 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

 July 29, 2004, 05:00 Re: compressive discretisation scheme #8 Prash Guest   Posts: n/a 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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