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-   -   difference between true vof and two-fluid vof (http://www.cfd-online.com/Forums/flow-3d/110519-difference-between-true-vof-two-fluid-vof.html)

 jacky831 December 13, 2012 03:36

difference between true vof and two-fluid vof

Hi, there. i want to simulate the flow on the spillway, what i concern is the high-speed flow's aeration problem. i don't know whether FLOW3D's two-fluid vof can sovle this problem. Looking forward to your advice

 JBurnham December 13, 2012 11:54

FLOW-3D can solve most spillway problems, including high-speed aeration. It is easier (and usually much more accurate) to use a one-fluid VOF method. If it is desirable to have a clearly-defined free surface, then 1-fluid VOF (IFVOF=4, 5, or 6) is the best choice. Usually IFVOF=4 is the place to start unless the free surface is highly curved. In these cases, once the hydraulic flow model is complete, then do a restart and activate 'air entrainment' physics. Specify the air density, surface tension coefficient (do not use full surface tension physics, just air entrainment), and a calibration coefficient for the rate of entrainment. You will probably have to calibrate the coefficient for each mesh resolution you try, using a similar known case with experimental data to calibrate the coefficient correctly. If deaeration is important in the stilling basin, then after calibrating the coefficient, activate 'drift flux' physics, and specify the air density and a drag coefficient of 0.95 (from Karamenev and Nikolov's data on bouyant spheres). Use the viscosity of water for all viscosities, to reflect the fact that the air bubbles are always separated by at least a thin film of fluid, and turn on 'escape at free surface' to allow the air to exit the flow when it rises to the surface. Good luck!

 jacky831 December 13, 2012 12:16

Quote:
 Originally Posted by JBurnham (Post 397368) FLOW-3D can solve most spillway problems, including high-speed aeration. It is easier (and usually much more accurate) to use a one-fluid VOF method. If it is desirable to have a clearly-defined free surface, then 1-fluid VOF (IFVOF=4, 5, or 6) is the best choice. Usually IFVOF=4 is the place to start unless the free surface is highly curved. In these cases, once the hydraulic flow model is complete, then do a restart and activate 'air entrainment' physics. Specify the air density, surface tension coefficient (do not use full surface tension physics, just air entrainment), and a calibration coefficient for the rate of entrainment. You will probably have to calibrate the coefficient for each mesh resolution you try, using a similar known case with experimental data to calibrate the coefficient correctly. If deaeration is important in the stilling basin, then after calibrating the coefficient, activate 'drift flux' physics, and specify the air density and a drag coefficient of 0.95 (from Karamenev and Nikolov's data on bouyant spheres). Use the viscosity of water for all viscosities, to reflect the fact that the air bubbles are always separated by at least a thin film of fluid, and turn on 'escape at free surface' to allow the air to exit the flow when it rises to the surface. Good luck!
thank you very much.i lerarn a lot from your reply! you must be a professional.

 JBurnham December 13, 2012 12:21

Thanks. There's a tech note here that gives more details of the air entrainment model: http://www.flow3d.com/pdfs/tn/FloSci-TN61.pdf. Good luck; I probably won't have time to respond to further questions on this topic. - Jeff

 Jing_min December 20, 2012 05:44

Quote:
 Originally Posted by JBurnham (Post 397368) FLOW-3D can solve most spillway problems, including high-speed aeration. It is easier (and usually much more accurate) to use a one-fluid VOF method. If it is desirable to have a clearly-defined free surface, then 1-fluid VOF (IFVOF=4, 5, or 6) is the best choice. Usually IFVOF=4 is the place to start unless the free surface is highly curved. In these cases, once the hydraulic flow model is complete, then do a restart and activate 'air entrainment' physics. Specify the air density, surface tension coefficient (do not use full surface tension physics, just air entrainment), and a calibration coefficient for the rate of entrainment. You will probably have to calibrate the coefficient for each mesh resolution you try, using a similar known case with experimental data to calibrate the coefficient correctly. If deaeration is important in the stilling basin, then after calibrating the coefficient, activate 'drift flux' physics, and specify the air density and a drag coefficient of 0.95 (from Karamenev and Nikolov's data on bouyant spheres). Use the viscosity of water for all viscosities, to reflect the fact that the air bubbles are always separated by at least a thin film of fluid, and turn on 'escape at free surface' to allow the air to exit the flow when it rises to the surface. Good luck!

Hi
First off all, TOO MUCH Thanks for JBurnha.
I have some questions:
1- Another way to simulate these problem, is using 2pahse model. Is it better than air entrainment?
2- What and Why do you mean by "do not use full surface tension physics"?
3- Are the coefficients in the air ent. model functions of mesh size? And what are their approximation? Who do they change with the mesh size?
4-What are the differences and applications between IFVOF=4, 5, or 6 ?