VOF converge problem
 

Hi, I used the vof model to simulate the water in the pipe. I want to see the wave profile,but continuity did not converge.

Continuity diverge......

Could everyone help me??

Thanks!!

Re: VOF converge problem
 

Hi,

Can you give me more details regarding the relaxation factors and which VOF scheme you are using.

Change the relaxation factors and schemes, it will help to converge.

Regards, Titiksh

Re: VOF converge problem
 

Hi,

there are some options as I see. you can increase the number of iterations for each time step, that way the flow can stabilize faster. You can use a different scheme in VOF itself. I think there is an implicit scheme, you can used this instead of the default "Geo-construct". You can refer to one of the examples in Fluent online users help. There is an example, where they have done a wave simulation.

hope this helps.

Re: VOF converge problem
 

Hi,

Thanks!

This problem used VOF Geo-construct shceme,Crount number is default 0.25.

under relexation factor:

pressure 0.8

Density 1

Body force 1

momentum 0.6

shceme = PISO

Time step = 0.0005

Max Iteration per Time Step = 15

Re: VOF converge problem
 

Hi

Set all the values as default and use implicit.

Right now dont touch courant number as i dont know how it can be calculated. It may be one of the reasons for divergence.

Titiksh

Re: VOF converge problem
 

Hi,

As Titkish said use the implicit scheme. The courant no. is del X/del T*u where u is the velocity, del T is the time step and del X is the mesh size. At a given time step if the movement of the fluid is more than the mesh size then the solution blows up. Also if you increase the number of time steps per iteration, make it 50 instead of 15, you may be able to get converged solution faster. Also you use PRESTO and PISO schemes. Hope this helps.

Aravind

Re: VOF converge problem
 

Hi Arvind

I am not able to understand what do you mean by: At a given time step if the movement of the fluid is more than the mesh size then the solution blows up.

Regards, Titiksh

Re: VOF converge problem
 

Hi Titiksh,

There is a number called CFL number or Courant number. If this number exceeds 1 then the solution will blow up, if you are not using a deforming mesh, i.e if you mesh is fixed, the fluid should not move by a distance (velocity X del T) which is greater than the mesh size in a give time step. If it exceeds the solver cannot solve such a problem. Please refer to the VOF chapter in the manual, there is an explanation given for this number. If the equation which i wrote before was wrong please correct me. I hope you understand what I am saying here.

Aravind

Re: VOF converge problem
 

Hi Aravind

Got yr point, but tell me are you sure that Courant number can't exceed 1, is there any minimum value as well.

Titiksh

Re: VOF converge problem
 

hi Titiksh,

I am not 100% certain, but as far as I have used ANSYS and Fluent, both have prescribed that the Courant number should not exceed 1, in the case of a mesh that cannot move. Actually one of my Profs. told me this.He told me that most of CFD/FEA codes (and hence many cannot solve moving boundary problmes)would blow up if this number is exceeded, meaning the mesh cannot move and the code will not give you a solution if this happens. You can also refer to the book " Computation Fluid Dynamics " by John Anderson Jr. where the explanation of this number has been dealt with in detail. Hope this helps. Aravind

Re: VOF converge problem
 

Hi Aravind

Could you mail me your details at my e mail. Where are you workinn now..!!

Thanx.

Titiksh