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Bubble column simulation with Lift coefficient UDF

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Hii ..

Iam doing two phase cfd simulations of bubble column in FLUENT.I want to study the effect of lift force,so i wrote UDF.when i running with UDF in FLUENT its getting divergence.While compiling UDF,its not showing any errors.How to overcome this problem??

 m20 June 29, 2013 04:22

Quote:
 Originally Posted by raju.vadlakonda (Post 436017) Hii .. Iam doing two phase cfd simulations of bubble column in FLUENT.I want to study the effect of lift force,so i wrote UDF.when i running with UDF in FLUENT its getting divergence.While compiling UDF,its not showing any errors.How to overcome this problem?? Please suggest me.
I think there is nothing wrong with your UDF, just one coefficient in line 43:
"f1=0.0105*pow(mod_etvos,3)-0.0159*pow(mod_etvos,2)-0.0204*mod_etvos+0.474;"
This equation is Tomiyama' equation and the first coefficient is 0.00105 instead of .0105.
Getting convergence by considering lift force is not simple in bubble column and you should have a look on solution controls and AMG solver.

 hossein65 June 30, 2013 00:41

the problem of writing a UDF aside, did you obtain reasonable velocity distribution? because when I simulate a bubble column, every thing looks fine, except velocity. the inlet velocity is 0.1m/s (air) but after some time steps, the maximum velocity in the system goes to 1.1 m/s. it seems like a air jet in the inlet, so it messes every thing up

Quote:
 Originally Posted by m20 (Post 436699) I think there is nothing wrong with your UDF, just one coefficient in line 43: "f1=0.0105*pow(mod_etvos,3)-0.0159*pow(mod_etvos,2)-0.0204*mod_etvos+0.474;" This equation is Tomiyama' equation and the first coefficient is 0.00105 instead of .0105. Getting convergence by considering lift force is not simple in bubble column and you should have a look on solution controls and AMG solver.
Hiii,

Yes,the first coefficient is 0.00105.
In simulations iam getting usually "Divergence detected in AMG solver". How to handle this problem??

 m20 June 30, 2013 06:21

Quote:
 Originally Posted by raju.vadlakonda (Post 436860) Hiii, Yes,the first coefficient is 0.00105. In simulations iam getting usually "Divergence detected in AMG solver". How to handle this problem??

http://www.cfd-online.com/Forums/flu...ying-lift.html

Quote:
 Originally Posted by m20 (Post 436864) Please have a look on following thread: http://www.cfd-online.com/Forums/flu...ying-lift.html

I have started simulations with that idea,but after some iterations simulation getting divergence.

The error is coming like this "turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 120589 cells ".

Regards,
Raju

 m20 July 6, 2013 08:47

You can consider one of the following things:
1. Change AMG solver coefficients
2. Run the simulation in laminar condition at first then switch to turbulent
3. Do initialization with best initial guess (this Is very important)