# VOF mass loss

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November 28, 2012, 05:19
#21
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Smaras
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Quote:
 Originally Posted by RodriguezFatz I am confused... you can not decrease to 1e-3 from 1e-5. This is an increase.
ok that's increase and yes that what i did i.e. increase from 1e-5 and change the time step to 1e-3.

Now if i am using directly 1e-3 from the start. The courant no. is increasing surpassing 250. So what to do now?

 November 28, 2012, 05:31 #22 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 25 Alright! Now, in your residual picture it looks like the first 2000 iterations did not converge at all, is that right? How many iterations per timestep did you set as maximum, 25? That means that the first 80 time steps did not converge? Maybe the loss of mass happens right there. Every single timestep of your simulation should converge in a satisfactory manner, otherwise you can not expect mass conservation. I would try to increase the maximum number of iterations and see if you get convergece for the first timesteps. __________________ The skeleton ran out of shampoo in the shower. Last edited by RodriguezFatz; December 3, 2012 at 04:37.

 December 3, 2012, 04:37 #23 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 25 Did it work out Smaras? __________________ The skeleton ran out of shampoo in the shower.

December 3, 2012, 04:53
#24
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Smaras
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Quote:
 Originally Posted by RodriguezFatz Alright! Now, in your residual picture it looks like the first 2000 iterations did not converge at all, is that right? How many iterations per timestep did you set as maximum, 25? That means that the first 80 time steps did not converge? Maybe the loss of mass happens right there. Every single timestep of your simulation should converge in a satisfactory manner, otherwise you can not expect mass conservation. I would try to increase the maximum number of iterations and see if you get convergece for the first time-steps.
Well thanks Rodriguez,

i tried to refine the mesh (adopt) in the regions of inlet and the interaction of two phases and used 10 iterations per step. Using 1e-5 as step size and 10000 steps. Got convergence after 3000 iterations i.e. 300 ts. And then onward every step got convergence.

I don't know why there is mass loss, may be because of viscous heating even though i have not selected that option or either it was something else. But in this run i have got relatively less mass loss and was getting almost similar penetration as in the research papers.

now i want to use K-w SST model what would you suggest???

 December 3, 2012, 04:59 #25 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 25 Hold on. You really need convergence every single timestep. Otherwise you can not take things such as mass conservation into account: What, if during your first 100 non-converged time steps something unphysical happens? Your equations are not converged and it can easily happen, that you lose some mass or whatever. You need to do as many iterations as it takes to get a converged solution right from the start, for time step number one, number two... __________________ The skeleton ran out of shampoo in the shower.

December 3, 2012, 05:03
#26
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Smaras
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Quote:
 Originally Posted by RodriguezFatz Hold on. You really need convergence every single timestep. Otherwise you can not take things such as mass conservation into account: What, if during your first 100 non-converged time steps something unphysical happens? Your equations are not converged and it can easily happen, that you lose some mass or whatever. You need to do as many iterations as it takes to get a converged solution right from the start, for time step number one, number two...
Ok got it....ill try to make more iteration i.e. 20 now an see

December 3, 2012, 05:17
#27
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Smaras
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Quote:
 Originally Posted by RodriguezFatz Hold on. You really need convergence every single timestep. Otherwise you can not take things such as mass conservation into account: What, if during your first 100 non-converged time steps something unphysical happens? Your equations are not converged and it can easily happen, that you lose some mass or whatever. You need to do as many iterations as it takes to get a converged solution right from the start, for time step number one, number two...
Yup getting convergence in K-eps with 20 iteration per step and step size being 1e-6 and for k-w with 25 iteration per step and step size being 1e-5. From the start.

Now running both simulations and will take around a day or so to complete. Then see what is the penetration. And hope now there would be no mass loss.

Thanks once again Rodriguez fro help.

Regards,
Smaras

 December 4, 2012, 03:41 #28 Member   Smaras Join Date: Sep 2012 Location: Germany Posts: 78 Rep Power: 12 Thanks Rodriguez, For the help, the problem lies in the pressure residual monitor. Even though it wasn't not changing too much but it wasn't giving converging.....i have also change it to 0.01 and now getting converging at every iteration. Further no mass loss and the penetration is also kept the step size 1e-5 and iteration per step 20. Getting converge solution. Better results in both models. Thanks for the help, learnt a lot. Regards, Smaras

 December 4, 2012, 10:09 #29 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 25 Just one more comment: By "change it to 0.01" i guess you mean that you increased the residual convergence criterion to 0.01 ? You should better try to increase the max. number of iterations (and lower the residual) than relaxing the threshold of "good" and "bad" convergence. __________________ The skeleton ran out of shampoo in the shower.

December 4, 2012, 10:37
#30
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Smaras
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Quote:
 Originally Posted by RodriguezFatz Just one more comment: By "change it to 0.01" i guess you mean that you increased the residual convergence criterion to 0.01 ? You should better try to increase the max. number of iterations (and lower the residual) than relaxing the threshold of "good" and "bad" convergence.
Ok doki...am working on it.

December 14, 2012, 05:29
#31
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Smaras
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Quote:
 Originally Posted by RodriguezFatz Just one more comment: By "change it to 0.01" i guess you mean that you increased the residual convergence criterion to 0.01 ? You should better try to increase the max. number of iterations (and lower the residual) than relaxing the threshold of "good" and "bad" convergence.
Thanks for the help Rodriguez. Had increase the number of iteration and getting the desired result i mean the result approximately matching the research results.

now working on 3D. Need some help, do you have some idea where to find good resource on meshing techniques in ICEM. And mesh refinement for 3D.

Regards.

 December 14, 2012, 05:44 #32 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 25 Watch the youtube tutorial by Simon (PSYMN in this forum). Search for "ICEM CFD Hexa 2D Airfoil meshing" in youtube. It has three parts and is really helpful, although it's just 2d. Mesh refinement can be done pretty easy in ICEM since you can increase the number of gridpoints on edges quickly. __________________ The skeleton ran out of shampoo in the shower.

 Tags penetration, phase change, vof modeling

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