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C-grid or O-grid for airfoil?

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Old   July 6, 2005, 06:20
Default C-grid or O-grid for airfoil?
  #1
Quarkz
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Hi,

I've coded a NS solver based on cartesian grids and I'm now extending it to structured grids. My main objective is to simulate airfoils, possibly extending to 3D. Hence, is there a clear choice to use c or o-grid for airfoils?

Please explain why.

Also I've download a grid generator for airfoil known as UGRID which can generates 2d structured o-grid for airfoils.

Is there similar type of program which can generate c-grid for airfoil as well?

Thanks alot!
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Old   July 6, 2005, 10:59
Default Re: C-grid or O-grid for airfoil?
  #2
ganesh
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Dear Quarkz,

As far as airfoil simulations as concerned, I find no distictive reason why an O-grid should be preferred over a C-grid or viceversa for all kinds of problems. However, a C-grid CAN be preferred as the grid will be aligned to the wake or slipstream at the trailing edge of the airfoil. Particularly, in context of adaptation, wherin you have a viscous flow past airfoil, a boundary layer and the wake are common features. The BL and the wake are to be adapted in the streamwise direction ( to resolve it better). The use of a C-grid will help you achieve this easily, whereas it is not the case for an O-grid. However on a relatively fine grid, results on both O- and C- grids are found satisfactory. If you are intersted in handling more complicated geometries you can try UNSTRUCTURED grids as well.

I am not sure of any program for C-grid generation available, but if you are able to use an O-grid generator with a cartesian meshing and match them, a C-grid can be generated, though some effort is needed in that direction.

Hope this helps

Regards,

Ganesh
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Old   July 7, 2005, 13:07
Default Re: C-grid or O-grid for airfoil?
  #3
Mani
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For viscous flow a C-grid is preferred because it allows you to resolve the wake. To really resolve the wake, you would actually need a different C-grid for each angle of attack. Often enough this is not done, which somewhat weakens this argument for using a C-grid. More important than resolving the complete far wake is the nice resolution of the region right behind the trailing edge.

O-grids are good enough for inviscid flow, where they have the advantage of reduced cell count as compared to C-grids. The resolution in normal direction is high only where you really need it: right on the airfoil surface. The region behind the trailing edge is not well resolved, which makes the use of O-grids questionable in case of viscous flow. You may still get good results even for viscous flow, depending on the particular case.

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Old   July 8, 2005, 21:57
Default Re: C-grid or O-grid for airfoil?
  #4
Quarkz
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Hi,

thank everyone! I'm writing an NS solver hence it seems that I will need to use C-grid.

However, I only 've got a free 2D O-grid generator, Ugrid which is simple to use and user-friendly. Does anyone know of a c-grid generator (free/commercial) for airfoil. I prefer one where i just enter the x/y coord and a reasonable mesh can be generated.

Thanks again
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Old   July 8, 2005, 22:17
Default Re: C-grid or O-grid for airfoil?
  #5
Quarkz
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Hi,

just to add that my main interest is in finding lift and drag of airfoil. in that case, is c-grid still more suitable than o-grid?

thanks again
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Old   July 12, 2005, 01:35
Default Re: C-grid or O-grid for airfoil?
  #6
Kyung-Seok, Kim
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There is C-grid Generator for free.

The name of the program is "MeshPilot"

And you can download this program through the following website.

http://www.shore-cfd.com/html/shore_...eshpilot.shtml

and maybe this program support naca series airfoil.

And I have questions. Now I trying to make 2-D airfoil grid for N-S(Spalart-Allamas).

Airfoil is RAE 2822.

But it is difficult to make this grid.

Do you know how to determine

1. Boundary layer thickness 2. Grid spacing 3. Etc. ^^

I'm poor at CFD.

Is there reference or technical guide book for grid making?

Thanks, in advance~
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Old   July 12, 2005, 05:15
Default Re: C-grid or O-grid for airfoil?
  #7
na
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You can see again the "Fletcher book" T2, there is a source programm (in Fortran) for mech generation (C-grid) around naca airfoil utilising the algebraica methods (multi-surfaces elements)
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