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June 28, 2011, 05:07 
some question

#1 
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hamid
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Can I simulate two phases flow with VOF in steady state ?
in grid independency study for UNSTEADY flow , how should I consider the time steps in every simulation, i mean i have grid1 with X amount of grids and grid2 with 3X and grid3 with 9X and i compare them, but should I consider different time steps as well? 

June 28, 2011, 06:59 

#2 
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Svetlin Philipov
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there is a kind of unwritten rule  time step is of a magnitude of that allows short phenomena modeling.
it depends on the processes you are going to model and bigger mesh not always results in smaller timestep At the beginning try to use one and the same timestep. If not applicable  divide by 2 each time. 

June 28, 2011, 07:54 

#3 
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hamid
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Hi Philipov
Apparently denser mesh needs smaller time step (due to courant number), is it true? As I understood u mean I better to use the same time step for all three meshes , X,3X,9X, ? thank u Hamid 

June 28, 2011, 09:56 

#4 
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Svetlin Philipov
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As a starting point Yes and if you do not obtain convergence reduce timestep by factor of 2 until you get convergence and proper results.


June 30, 2011, 10:06 

#5 
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Adrien Lemoine
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Hi,
if you have a problems due to the courant number (which means you are using an explicit VOF scheme) then you should switch to an Implicit VOF scheme whch gives good results and is easier to use. You should start with an time around 103 s. If you really want to stay with a explicit VOF scheme then you should calculate the time step with this formula : Time step < Size of the smallest cell / Speed of sound in your phase. Adrien
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June 30, 2011, 11:21 

#6 
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Svetlin Philipov
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I will a little disagree with the last comment as far as in air speed of sound is about 342m/s and if you have a cell size of 0.1m in X direction (for example) then you need dt of 3.0E04 .
From last post seems that the task is independent from velocity. Dont you think velocity of 1m/s; 10m/s and 100 m/s does not matter?!?!?! 

June 30, 2011, 16:38 

#7 
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Adrien Lemoine
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Hi Philipov,
I forgot one thing in my last post : This formula will always make your scheme stable since the velocity will be always under the speed of sound. If you are 100% sure that the maxspeed will be for exemple 50 m/s (EVERYWHERE) then you can calculate the time step with : Time step < Size of the smallest cell / 50. The formula "Time step < Size of the smallest cell / Speed of sound in your phase" is always true but not always the quickest. Sincerly Adrien
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July 1, 2011, 03:02 

#8  
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Svetlin Philipov
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Quote:


July 2, 2011, 16:28 

#9 
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Hey,
if you have problems with the Courant number. You could also try to decrease the under relaxation factors a little bit. That would cause a slower simulation but does not effect the speed so much like decreasing the time step. 

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