CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT > Fluent Multiphase

VOF : reproduce Variable Time Stepping Algorithm

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 22, 2016, 10:59
Default VOF : reproduce Variable Time Stepping Algorithm
  #1
New Member
 
Lars
Join Date: Jul 2015
Posts: 8
Rep Power: 10
sral is on a distinguished road
Hello,

I am simulating a two-phase flow (water and air) and in order to understand the variable time stepping algorithm better, I try to calculate the next physical time step FLUENT will use following the equation given in the manual (Link)



So I found the tui command
Code:
/report/fluxes/mass-flow
which will give me the mass-flows for my four cell zones (i.e. zone1, zone2, zone3, zone4) in my domain.

And
Code:
/report/volume-integrals/mass
will give me the mass for each of my two phases in the four cell zones. (I didn't find any command to report the volume fluxes, so I think using the mass fluxes divided by the mass will work)

So I set up an excel sheet to calculate myself with the values given by the aforementioned tui commands:

click for larger image

For this arbitraty state in the simulation process I get a physical step size of 0.006746321 seconds for the next iteration, but when I start the iteration in FLUENT, it defines 0.0013777 seconds.

I defined high values for the maximum time step size and the maximum step change factor and low values for the minimum time step size and minimum step change factor respectively. So none of these boundaries will take effect.



Can you please point out, where I am missing something?

When the manual says
Quote:
where the ratio is calculated for each cell.
does it mean for each cell zone or for each of the finite volume elements?


Thank you!
sral is offline   Reply With Quote

Old   February 23, 2016, 07:12
Default
  #2
Senior Member
 
Join Date: Mar 2014
Posts: 375
Rep Power: 13
hwet is on a distinguished road
do you specify a courant number as well? Maybe it is just trying to keep to the maximum courant number value you have specified by keeping the time step small
hwet is offline   Reply With Quote

Old   February 23, 2016, 07:52
Default
  #3
New Member
 
Lars
Join Date: Jul 2015
Posts: 8
Rep Power: 10
sral is on a distinguished road
Thank you for your reply.

Quote:
Originally Posted by hwet View Post
do you specify a courant number as well?
In Fluent as well as in my Excel calculation I specify a courant number (CFLglobal) of 2.


Quote:
Originally Posted by hwet View Post
Maybe it is just trying to keep to the maximum courant number value you have specified by keeping the time step small
Yes, the variable time stepping method of Fluent tries to stay at CFLglobal = 2. In order to do so, Fluent uses the equation .

But when I calculate myself, I get a different value. So the values for "outgoing fluxes" and "volume" I extract from Fluent seem to be different from the values, which Fluent uses to calculate .



Based on the statement
Quote:
Originally Posted by Fluent Documentation
The variable time step is based on the maximum Courant number near the VOF interface. To calculate that Courant number, ANSYS Fluent uses a flux-based definition where, in the region near the fluid interface, ANSYS Fluent divides the volume of each cell by the sum of the outgoing fluxes.
it seems, that Fluent uses the "outgoing fluxes" and "volume" of selected finite volume elements (called "cells" in this case), maybe something like VOFwater = 0.4 ... 0.6, near the water-air-interface and not the whole cell zones.

So when I use the cell zone values, which are averages of all containing finite volume elements, the fluxes are not as critical and therefore leading to a bigger in my manual calculation.


Is there a way in Fluent to report the element based volume fluxes? Preferably of a preselected set of elements (e.g. VOF = 0.4 ... 0.6) ?

.
sral is offline   Reply With Quote

Old   February 24, 2016, 17:37
Default
  #4
Senior Member
 
Join Date: Mar 2014
Posts: 375
Rep Power: 13
hwet is on a distinguished road
in the vof window there is a courant number option as well which is for the phase and different from the global courant number, check that out as well.
hwet is offline   Reply With Quote

Reply

Tags
cfl, courant, step size, vof

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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 Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
courant number in vof reza_gharib1369 FLUENT 49 February 26, 2020 16:55
Help for the small implementation in turbulence model shipman OpenFOAM Programming & Development 25 March 19, 2014 10:08
dynamic Mesh is faster than MRF???? sharonyue OpenFOAM Running, Solving & CFD 14 August 26, 2013 07:47
variable time stepping text commands moe FLUENT 0 September 2, 2009 11:19
IcoFoam parallel woes msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 02:58


All times are GMT -4. The time now is 23:48.