# VOF necessary mesh and timestep

 Register Blogs Members List Search Today's Posts Mark Forums Read

 April 9, 2013, 02:02 VOF necessary mesh and timestep #1 New Member   Join Date: Mar 2013 Posts: 13 Rep Power: 4 Hello, dear community of cfd-online! Since VOF model is of much computational cost, there are 2 important issues: How fine domain should be meshed and what timestep should be used. If we have a 2D case of straight channel and to describe cross-section we use say 20 cells in the core and 6 cells per boundary layer to solve homogeneous flow. We want to solve a flow with gas and liquid phases, which volume fraction is 10%. Does it mean that for same accuracy we have to use 10x more cels to describe cross section? If we have a comparatively large timestep which results in Courant number equal to 30 and each timestep converges, is it still a good solution? What timesteps and Courant numbers could be applied to get a relevant solution with not much time spent on calculation? Thank you in advance for your answers!

April 9, 2013, 10:02
#2
New Member

Join Date: Nov 2011
Posts: 27
Rep Power: 5
are you trying to simulate a stratified flow in a channel?
or the gas phase is diluted in the liquid one? if you are trying to simulate small bubbles or droplets, i guess you should use other multiphase model

mesh resolution of VOF calculations should be fine enough to capture the interface between the fluids, when the length scale of the interface is much larger than mesh size (as in stratified flow)

in a 2D domain, i would use a variable time-step with courant number of 2, as fluent manual suggests

with courant number near 30, you may get qualitatively good results as well, but lower courant numbers will provide more meaningful results

Quote:
 Originally Posted by Andrey.M. Hello, dear community of cfd-online! Since VOF model is of much computational cost, there are 2 important issues: How fine domain should be meshed and what timestep should be used. If we have a 2D case of straight channel and to describe cross-section we use say 20 cells in the core and 6 cells per boundary layer to solve homogeneous flow. We want to solve a flow with gas and liquid phases, which volume fraction is 10%. Does it mean that for same accuracy we have to use 10x more cels to describe cross section? If we have a comparatively large timestep which results in Courant number equal to 30 and each timestep converges, is it still a good solution? What timesteps and Courant numbers could be applied to get a relevant solution with not much time spent on calculation? Thank you in advance for your answers!

 April 10, 2013, 00:45 #3 New Member   Join Date: Mar 2013 Posts: 13 Rep Power: 4 Are there any practical advices for mesh? Any criteria. If I get solution but surface of the liquid in gas is not as clear as it should be, are there any approaches to estimate how far my solution from reliable one?

April 10, 2013, 09:41
#4
New Member

Join Date: Nov 2011
Posts: 27
Rep Power: 5
it's hard to tell a specific criteria for VOF calculations
along with the usual ones (skewness and ortogonality), one has to considerer the mesh size which obviously will dictate how thin the interface will be captured

but it will also have an impact on solution time... since you are running 2D calculations, it will be easier to evaluate the influence of mesh size
I would proceed this way: estipulate three mesh sizes and run transient simulations with variable time stepping during some time, keeping courant number equal to 2

then compare results from them: instantaneous hold-up, mean hold-up, position of liquid or gas front

take a look at this presentation for additional information
http://pt.scribd.com/doc/62308472/Be...otive-Industry

regards

Quote:
 Originally Posted by Andrey.M. Are there any practical advices for mesh? Any criteria. If I get solution but surface of the liquid in gas is not as clear as it should be, are there any approaches to estimate how far my solution from reliable one?

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post mbn1454 STAR-CCM+ 1 November 19, 2011 08:49 Ladnam OpenFOAM 0 September 14, 2011 06:30 rbarrett CFX 4 July 21, 2011 19:25 lr103476 OpenFOAM Running, Solving & CFD 30 November 19, 2007 15:09 Liu Main CFD Forum 3 June 6, 2004 04:04

All times are GMT -4. The time now is 16:46.