Courant number and time step

dsn2491 · March 7, 2012, 03:01

How can we find for a fixed courant number the time step that fluent is using when an explicit density based solver is used?

I have looked in the manual of ansys fluent for the definition if CFL and it is given in some very complicated terms like flux and volume and etc.

SO can please someone tell me the way to know the time step for an explicit scheme when the courant number is fixed?

Thanks in advance!!

ghost82 · March 7, 2012, 03:36

Hi,
as I know Courant number is u*deltat/deltax, where

u is the velocity
deltat is the time step
delta x is cell dimension

Daniele

dsn2491 · March 8, 2012, 18:35

Hi

delta x = 0.025 mm; u = 595.40 m/s; C =0.12;

using the formula i get delta t = 5.0385 nano sec;

Therefore the physical time after 200 iterastions must be: t*200 = 1.00772 micro sec

Fluent simulation:

Since I am using an explicit formulation I couldnt think of any other way of finding out the time-step.

After 200 iterations the physical time shown by fluent is 10.191 micro sec.

I am assuming that as the the solution goes ahead in time the difference will become larger.

Can anyone tell me why this is happening? Am I doing any mistake?

ndabir · September 18, 2012, 13:59

have you activated time step print/plot from monitors panel? That will should unless your method of solution disables it.
Also the rough estimation you made using CFL will not be applicable always during simulation. So time step is not fix and fluent's result is correct since time step is changing every time step based on fixed CFL.

March 7, 2012, 03:01	Courant number and time step	#1
dsn2491 New Member Aditya Join Date: Dec 2011 Posts: 28 Rep Power: 14	How can we find for a fixed courant number the time step that fluent is using when an explicit density based solver is used? I have looked in the manual of ansys fluent for the definition if CFL and it is given in some very complicated terms like flux and volume and etc. SO can please someone tell me the way to know the time step for an explicit scheme when the courant number is fixed? Thanks in advance!!

March 8, 2012, 18:35		#3
dsn2491 New Member Aditya Join Date: Dec 2011 Posts: 28 Rep Power: 14	Hi delta x = 0.025 mm; u = 595.40 m/s; C =0.12; using the formula i get delta t = 5.0385 nano sec; Therefore the physical time after 200 iterastions must be: t200 = 1.00772 micro sec* Fluent simulation: Since I am using an explicit formulation I couldnt think of any other way of finding out the time-step. After 200 iterations the physical time shown by fluent is 10.191 micro sec. I am assuming that as the the solution goes ahead in time the difference will become larger. Can anyone tell me why this is happening? Am I doing any mistake? Last edited by dsn2491; March 8, 2012 at 19:18.

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March 7, 2012, 03:36		#2
ghost82 Senior Member Rick Join Date: Oct 2010 Posts: 1,016 Rep Power: 26	Hi, as I know Courant number is u*deltat/deltax, where u is the velocity deltat is the time step delta x is cell dimension Daniele

September 18, 2012, 13:59		#4
ndabir Senior Member navid Join Date: Jan 2010 Posts: 110 Rep Power: 16	have you activated time step print/plot from monitors panel? That will should unless your method of solution disables it. Also the rough estimation you made using CFL will not be applicable always during simulation. So time step is not fix and fluent's result is correct since time step is changing every time step based on fixed CFL.