FLUENT VOF FSI Courant number question

Val69290 · November 21, 2017, 05:15

Hello guys,

I come to you today because I'm a bit stuck with a work. I'm trying to modelize the sloshing in a closed water tank which is submitted to a seism. For this, I do the simulation in FLUENT and then I import the CFD loads in Mechanical.

First, I'd like to run a simple analysis to see the behaviour of my fluid domain when the earth gravity is applied, plus an other constant gravitationnal acceleration in the x direction. For this, I use the VOF model and the transient type because I read the solution were more accurate like this for a VOF model, even if the problem is actually steady.

My question is : how do I chose my time-step size ? I have to determine the right size to have the accurracy of my solution but I don't know how because of the facts it's a steady problem.

Could you help me here ?

Thank you very much and sorry for my bad english (I'm French).
I wish you a nice day !

Val

hamed.majeed · March 19, 2018, 13:00

For transient case are you using explicit scheme for vof or implicit?

There are two courant number (I hope I don't confuse you at this point). A global courant number (usually referred to as courant number) for the flow variables of velocity, pressure, etc. Then there is a local courant number (vof courant number) for the volume fraction transport equation.
Note: For explicit you have both local and global courant number. For implicit case you just have the global courant number.

You might know that courant number should be less than 1. Say we want a courant number of C=0.25
By definition,

C=(time_step)/(mesh_size/velocity) C is a number (no units)
Now if you have mesh size of say 0.001m and velocity of 1m/s, and fixing courant number C to 0.25.

Using above equation the time_step =0.0025s. This is the time-step you need for C=0.25.

November 21, 2017, 05:15	FLUENT VOF FSI Courant number question	#1
Val69290 New Member Valentin Join Date: Nov 2017 Posts: 1 Rep Power: 0	Hello guys, I come to you today because I'm a bit stuck with a work. I'm trying to modelize the sloshing in a closed water tank which is submitted to a seism. For this, I do the simulation in FLUENT and then I import the CFD loads in Mechanical. First, I'd like to run a simple analysis to see the behaviour of my fluid domain when the earth gravity is applied, plus an other constant gravitationnal acceleration in the x direction. For this, I use the VOF model and the transient type because I read the solution were more accurate like this for a VOF model, even if the problem is actually steady. My question is : how do I chose my time-step size ? I have to determine the right size to have the accurracy of my solution but I don't know how because of the facts it's a steady problem. Could you help me here ? Thank you very much and sorry for my bad english (I'm French). I wish you a nice day ! Val

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March 19, 2018, 13:00		#2
hamed.majeed Senior Member Hamed Abdul Majeed Join Date: May 2012 Location: New Orleans, LA, US Posts: 147 Rep Power: 14	For transient case are you using explicit scheme for vof or implicit? There are two courant number (I hope I don't confuse you at this point). A global courant number (usually referred to as courant number) for the flow variables of velocity, pressure, etc. Then there is a local courant number (vof courant number) for the volume fraction transport equation. Note: For explicit you have both local and global courant number. For implicit case you just have the global courant number. You might know that courant number should be less than 1. Say we want a courant number of C=0.25 By definition, C=(time_step)/(mesh_size/velocity) C is a number (no units) Now if you have mesh size of say 0.001m and velocity of 1m/s, and fixing courant number C to 0.25. Using above equation the time_step =0.0025s. This is the time-step you need for C=0.25.