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cfdiscool February 12, 2010 09:59

finding the size of smallest cell

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 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

jack1980 February 12, 2010 10:26

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.

cfdiscool February 13, 2010 11:12

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 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
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


jack1980 February 14, 2010 05:52

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!

hamed1985 May 24, 2013 23:50

size define
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

sangthuongmai May 25, 2013 05:21

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khan_faizan01 June 18, 2013 05:20

how u calculated the delX ie the minimum dimension of your cell

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