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Size functions and boundary layer with Gambit 2.2

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Old   July 21, 2006, 04:45
Default Size functions and boundary layer with Gambit 2.2
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Hi all!

I'm fairly new to CFD and I'm trying to model 3D wings in order to compare their induced drag in function of their shape (racked wing tip, twisted wing, tapered, winglets...)

After spending some time reading the forum, I came to the conclusion that using the Boundary layer tool in Gambit will not work in 3D for a newbie like me and that size function could do the trick.

I'm not sure I need a fine boundary layer for my study as the wing is meant to be a hydrofoil working at low Re (working Re between 300k and 600k).

Anyway, I'm wondering how you guys would mesh these wings. Would you only use size functions? Looks like if I want to have my first few cells in the 30-300y+ region I'm gonna need a lot of cells arround my wing.

I have a fairly limited CPU ressource so max number of cells I'm expecting is arround 800k - 1M.

I'm also thinking of only using the laminar solver in Fluent as the Re is low and I don't think I really need a turbulence model for this comparison study. Would this assumption sound right?

any thoughts or tips I could use for my study?

Thanks in advance,

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Old   July 21, 2006, 21:27
Default Re: Size functions and boundary layer with Gambit
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i think the Reynolds number that you mentioned 300K to 600K should require turbulence model.nonetheless, the laminar solver that you mentioned can be used at first and later on the turbulence model can be switched on. to model the wing, why not just try the boundary layer tool first and examine the results?

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Old   July 22, 2006, 05:13
Default Re: Size functions and boundary layer with Gambit
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Thanks for your reply. I've tried the boundary layer tool already but without success. Also, I want to have a mesh as similar as possible between all the wings I'll be testing.

Could I mesh on Gambit without the boundary layer tool and refine the mesh arround the foil in fluent afterwards?


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Old   July 23, 2006, 19:34
Default Re: Size functions and boundary layer with Gambit
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I've managed to mesh my first wing using a curvature size function for the wing faces, a mesh size function for the symetry plane and a fixed size function for the volume mesh with the wing mesh as source. I get a reasonable mesh with about 900k cells before refining it in Fluent.

But I've been trying to do exactly the same thing on my second wing (I've added a raked tip on it) and I can't get Gambit to mesh it... I've checked if the tree size was enough, the worst skewed cell I have is about 0.47 and still, Gambit fails by saying that the problem comes usually from my faces skewness bla bla bla...

I'm not clever enough to understand all the meanings of the default settings in the Gambit help so does anybody knows what should I change?

Maybe the fact that I have a virtual volume doesn't help? But I can't change it to real anyway...


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Old   August 1, 2006, 08:54
Default Re: Size functions and boundary layer with Gambit
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If you are willing to go to a hybrid mesh, I would recommend you use TGRID to generate the boundary layers. Build the surface meshes in Gambit and then add 5-10 prismatic boundary layer cells on top of that.

I have been studying winglets as well and would be interested in seeing some of your results. Please send me an email and we can talk.
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