# Free surface modelling under CFX 4

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 April 18, 2002, 22:12 Free surface modelling under CFX 4 #1 Dahong Guest   Posts: n/a Hi, everyone! I am trying to simulate an air-water two phases flow for obtaining the location of the free surface by using CFX 4. I set two inlets for the solution domain, one is air inlet and another is water inlet. I also set an outlet for the solution domain. I have specified the BC's at the inlets as follows: i) for air inlet: air velocity is 1.0m/s, air volume fraction is 1; water velocity is 1.0m/s, water volume fraction is 0 ii) for water inlet: air velocity is 1.0m/s, air volume fraction is 0; water velocity is 1.0m/s, water volume fraction is 1 The default option is used for the outlet. I also set the initial volume fraction VFRAC(INODE,IPHASE) within the solution domain by using the User-Subroutine USRINT, and use the homogeneous model to simulate the free surface between the air and the water. I run the calculation for some iterations for test and can see the free surface under the Post-processor. The problem is that no free surface is found after the running of 2000 iterations. I don't know why? Please help me! Thanks in advance.

 June 20, 2002, 06:52 Re: Free surface modelling under CFX 4 #2 Prash Guest   Posts: n/a hi could you please tell me whether you are running the simulation in transient or a steady state. free surface modelling using surface tension model and the surface sharpening algorithm works only for transient models. there could be many reasons why you are not able to see the free surface.... 1. it could be a convergence issue ... volume fraction in the entire domain is 0.5.. and you dont get any interface... this is probably due to your boundary conditions. 2. it could be that the timesteps you have specified are too large for the convergence to be achieved... hope this helps... cheers prash

 June 22, 2002, 23:07 Re: Free surface modelling under CFX 4 #3 Dahong Guest   Posts: n/a Hi Prash, Thank you for your help. I run the calculation in the transient state. As a first try, even the turbulence model is not included. The time step used is 0.5. Do you think the time step must be smaller than that? I have checked that the setting of initial fraction should be OK, I can still see the free surface in few time steps. It seems to be somewhere wrong during setting the model, but I can not fix it.