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fathan January 11, 2009 06:14


anybody know how to make some prepin validation for drag airfoil.

i have create it but the result didn't match @ reference drag coef is 0.6 but my result is 0.998.

i use it for chambered airfoil.

Stefano January 14, 2009 05:04

Re: validation
hello fathan, what is the minimum cell size, respect the chord lenght? And which momentum approximation ("iorder" variable)do you use for this kind of simulations?

Fathan January 14, 2009 12:32

Re: validation
Hello stefano, i use cell size is 8 mm for chord lenght 0.4 m. And iorder is 1 " first order".

Fathan January 14, 2009 13:33

Re: validation
Hello stefano, my chord lenght is 0.4 meter and cell size is 8mm, i use iorder=1.

Stefano January 15, 2009 03:47

Re: validation
hello Fathan, you can try to make a restart from this your simulation when it is finished, and in the restart simulation you can try to switch to iorder=3 and a minimum cell size of 4mm.

But I forgot to ask you the fluid speed and how many mesh blocks you are using... so I'm worried that if the fluid speed is too high or if you are not using multi-blocks you may have a long simulation time.

Fathan January 15, 2009 10:05

Re: validation
For vel i use 1 m/s (thats low speed) and i do for 1 mesh..

Stefano January 15, 2009 10:33

Re: validation
the simulation than should be quite fast. However if you want to speed up your simulation you can think to use 2 or 3 mesh blocks, nested, with the inner one with the cell of 1% of the length, and the outer ones with a bigger cell.

Fathan January 16, 2009 02:42

Re: validation
Thnks stefano, the mesh now good enough, but if i use idrg not equal 2 the result=1, im use idrg=2. Why its diffrnt, i dont understood? Have i to use obstcle porosity? What are variable i have to use?

Stefano January 16, 2009 03:44

Re: validation
Fathan, why are you using the idrg variable? IDRG is made in case you want to model any porous obstacle, is your geometry porous?

fathan January 16, 2009 09:08

Re: validation
my airfoil doesn't have porousity. i use it bcoz at output a dont see drag coef. and i try to using IDRG and the output appear.

how to appear the drag coeff?? i'm confused

Stefano January 16, 2009 09:15

Re: validation
the drag coefficient that you are looking at is not the drag coefficient of the airfoil, but simply the drag due to the porous obstacle. So not the right output. You don't have a direct output of the drag coefficient in FLOW-3D. In FLOW-3D outputs you have the force that acts on the airfoil, then you have manually to adimensionalize to have the Cd. To have the forces that act on your geometry, the fastest way is to define your airfoil as a GMO with "prescribed motion". When the simulation is done, in "Analyze" -> "Probe" -> "general history" you have the forces that act on the GMO.

fathan January 16, 2009 09:29

Re: validation
there is a lot of output, where i should to use? i use f3d V8.2 and thats have no analize button but in result ->probe.

Stefano January 16, 2009 09:43

Re: validation
with 8.2 you don't have the General Moving Obstacle model. You should use a Force Window then (you can find it in the Manual) or think to upgrade. Newer versions will give you a lot of new features and more accurate simulations!

fathan January 16, 2009 09:58

Re: validation
I see, i use f3d for my graduation so, i need this information.. thank you for attention and your regard..

i will try to use force window.. thats right, it should be upgrade.. i think it will be upgrade in 2009-2010 Btw, where are you from??

Stefano January 16, 2009 11:08

Re: validation
Than you can be succesfully by using the "Force Window"! Use it and the hints of this discussion and you'll have good results also with your 8.2! I'm from Italy.

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