CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

finding the size of smallest cell

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

Like Tree1Likes
  • 1 Post By cfdiscool

Reply
 
LinkBack Thread Tools Display Modes
Old   February 12, 2010, 09:59
Default finding the size of smallest cell
  #1
New Member
 
sed
Join Date: Feb 2010
Posts: 19
Rep Power: 7
cfdiscool is on a distinguished road
Hallo

I am having 4.2 million cells in my domain and I should try to set up a unsteady case now:

My velocity is 5 m/s for this case


I am stuck with the calculation of time step.....as referred in the forums I tried to follow the rule :

delt< delx/v

1. now How to find this "delx" (The smallest mesh size (should it be length of that cell or volume of that cell?) through ICEM CFD or Grid-->Check?

2. What is Global Courant number? Where Can i Change it in FLUENT? Is there an exact formula to derive this Courant number?


Thank you all
rgd likes this.
__________________
Life is not always converging ... but u can relax
cfdiscool is offline   Reply With Quote

Old   February 12, 2010, 10:26
Default
  #2
Senior Member
 
Jouke de Baar
Join Date: Oct 2009
Posts: 122
Rep Power: 7
jack1980 is on a distinguished road
Try using variable time stepping. Here you can set your desired Global Courant Number. For the first time step I usually just guess a couple of times untill it seems to work. Not that sophisticated but it does the job, in my case.

The Global Courant Number is 'the number of cells the fluid travels during one iteration'. The default is 2.0. Basically, when my case is running well I keep it like that, when my case is causing trouble I lower to 1.0 or even 0.2.

An exact definition of the GCN is in the Fluent User Guide. If I remember correctly, it takes into account if cells are near a boundary etc.
jack1980 is offline   Reply With Quote

Old   February 13, 2010, 11:12
Default
  #3
New Member
 
sed
Join Date: Feb 2010
Posts: 19
Rep Power: 7
cfdiscool is on a distinguished road
Hi Jouke

Thank u
i found that my minimum cell length is in the order of 0.001 mm , indeed very very small.

I tried things as u said ..........by opting for variable time stepping

My time step turns out to be in the order of 10^-8 and i even decreased the minimum step size to 10^-10

in one run FLUENT gave the out put as

courant number is 0 and in other simulation attempt

it gave the following report and quit to iterate:

Updating solution at time level N...
Global Courant Number : 0.28
done.
physical-dt 1.0000e-06
iter continuity x-velocity y-velocity z-velocity k epsilon time/iter
1 1.0000e+00 1.5793e+02 2.2789e-01 2.2312e-02 2.8591e+00 5.6203e+07 0:17:21 19

Internal error at line 8910 in file 'sg_press.c' on Node 0.
couldn't allocate velocity and coefficient array



Did i set the time step too low than FLUENT defaults ...???

my flow velocity is around 220 m/s

regards
__________________
Life is not always converging ... but u can relax
cfdiscool is offline   Reply With Quote

Old   February 14, 2010, 05:52
Default
  #4
Senior Member
 
Jouke de Baar
Join Date: Oct 2009
Posts: 122
Rep Power: 7
jack1980 is on a distinguished road
Ok I can see your challenge here, you have a high velocity and a very fine grid. I'm afraid I don't know what your error message indicates, starting out with GCN=0.28 would seem ok ...

Some approaches I use:

- Start out on a coarser grid. Here you can have larger time steps and (hopefully) converge a bit faster. Then interpolate your data to your fine grid.

- Have longer rectangular cells in your grid that are aligned with the flow. Especially near walls. This will result in a lower GCN, or larger time steps.

- Make both your velocity and your viscosity, say, 100x smaller. You will have the same Re, therefore you should/could find the same effects in your flow. The good thing is the lower velocity means a lower GCN, or again larger time steps. This option is a bit dodgy, I agree, but in some cases it will give you good results. In other cases it might at least give you a much quicker idea of what is going on.

- Another thing that you might want to check is this: if there are sudden jumps in cell size, this might induce high turbulunce, and after some while very high velocities. So maybe, avoid having these jumps and have a smoother reduction of cell size.

Just some thoughts, hope it helps, good luck!
jack1980 is offline   Reply With Quote

Old   May 24, 2013, 23:50
Smile size define
  #5
New Member
 
Hamed
Join Date: May 2013
Posts: 1
Rep Power: 0
hamed1985 is on a distinguished road
Hi
I have a problem with size of my sphere !
I am to finding Cd (drag coefficient) in a steady state air flow current around a sphere. I set "plot" to plot Cd convergence, but the results are very very different from actual data. Where can I define my figure size? (for instance sphere diameter = 1 meter)
Where can I see Drag (not drag coefficient)?

velocity = 0.5 and 3 mach
boundry condition = pressure far field & 101325 pa
matterial = air (Ideal gas)
Cd (fluent result for mach 3) = 1581080.4
Cd (experimental data) = 0.95

thank you
hamed1985 is offline   Reply With Quote

Old   May 25, 2013, 05:21
Default
  #6
New Member
 
no
Join Date: May 2013
Posts: 2
Rep Power: 0
sangthuongmai is on a distinguished road
BĂ*i vi?t hay l?m upppppppppppppppp ............... cho b?n nhĂ©
sangthuongmai is offline   Reply With Quote

Old   June 18, 2013, 05:20
Default
  #7
New Member
 
Faizan
Join Date: Jun 2013
Posts: 1
Rep Power: 0
khan_faizan01 is on a distinguished road
how u calculated the delX ie the minimum dimension of your cell
khan_faizan01 is offline   Reply With Quote

Reply

Tags
cell, courant number, smallest

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
OF 1.6 | Ubuntu 9.10 (64bit) | GLIBCXX_3.4.11 not found piprus OpenFOAM Installation 22 February 25, 2010 14:43
Phase locked average in run time panara OpenFOAM 2 February 20, 2008 15:37
fluent add additional zones for the mesh file SSL FLUENT 2 January 26, 2008 12:55
Trimmed cell and embedded refinement mesh conversion issues michele OpenFOAM Other Meshers: ICEM, Star, Ansys, Pointwise, GridPro, Ansa, ... 2 July 15, 2005 04:15
Warning 097- AB CD-adapco 6 November 15, 2004 05:41


All times are GMT -4. The time now is 12:06.