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-   -   [GAMBIT] need to learn meshing (http://www.cfd-online.com/Forums/ansys-meshing/67992-need-learn-meshing.html)

Ganeshsswamy September 2, 2009 17:28

need to learn meshing
 
Hi all,

I am using Gambit to mesh a part in my project. I am having problems using boundary layers. Can some one teach me howto use Boundary layers for 3D meshing please.

Thank you

Chris D September 2, 2009 23:11

1 Attachment(s)
Quote:

Originally Posted by Ganeshsswamy (Post 228326)
Hi all,

I am using Gambit to mesh a part in my project. I am having problems using boundary layers. Can some one teach me howto use Boundary layers for 3D meshing please.

Thank you

I struggled with the boundary layer mesh tool in Gambit until I realized two things. The first is that, for a 3D case, a boundary layer should be attached to an edge and grow on a face. I attached a diagram to show what I mean.

The diagram shows a boundary layer mesh applied to a 3D flat plate (imagine that the z-direction is out of the page.) Notice that there are two faces perpendicular to the plate, one coincident with the leading edge and one coincident with the trailing edge. These are the faces onto which the boundary layer grows.

The second thing to know is that, in my opinion, for a structured (hex) grid, the boundary layer mesh is just a fancy way of grading edges. By this I mean that, for a structured grid, you can achieve the same results by either applying a boundary layer mesh or grading the edges manually yourself (via edge meshing with first length and interval count.)

If you are using an unstructured (tet) grid, then the boundary layer tool is necessary, as it allows for the generation of a structured (hex) mesh in the boundary layer and an unstructed (tet) mesh outside of the boundary layer.

This is pretty hard to explain without actually having pictures from Gambit to post. Unfortunately, I don't have it in front of me right now. I'll post more tomorrow for you when I can get in front of Gambit and get some pictures. I hope this can at least answer some of your questions until then.

-mAx- September 3, 2009 02:11

Quote:

Originally Posted by Chris D (Post 228352)
I The first is that, for a 3D case, a boundary layer should be attached to an edge and grow on a face.

hmm... for a 3d case, you may choose a surface as source
Indeed Gambit will inflate this source and will create the BL, which sourrounds your source (here your surface... and the BL is a surrounding volume)

Picking an edge as source is ok for 2D case

Chris D September 3, 2009 11:39

4 Attachment(s)
Quote:

Originally Posted by -mAx- (Post 228377)
hmm... for a 3d case, you may choose a surface as source
Indeed Gambit will inflate this source and will create the BL, which sourrounds your source (here your surface... and the BL is a surrounding volume)

Picking an edge as source is ok for 2D case

I agree that you can pick a face as a source and attach the boundary layer to the volume. I’ve had more luck, though, picking an edge as a source and having the BL grow on a face. I’ve attached some screenshots to help explain how I create a BL mesh. (This explanation mostly aimed at the OP, Max, as I’m sure you are familiar enough with Gambit so as to not require such a detailed explanation.)

First, I created a flat plate, and then I built a volume on top of the plate (see pictures 1 and 2.) Then, I created a boundary layer, using an edge as a source and attaching it to (i.e., having it grow on) a face (see picture 3.) (To get the mesh to “grow in a different direction”, shift-middle click the edge, and the mesh will grow from the same edge onto another face.) Next, I meshed the edges of the volume, and you can see in picture 3 that the BL grades the edge mesh such that the interval size increases with increasing distance from the plate.

Finally, I meshed the volume using a Hex/Map scheme. After the volume was successfully meshed, I cut a plane offset from the y-z plane through the mesh (picture 4.) You can see that the manner in which I used the BL mesh tool allowed for the creation of a volume mesh that is fine at the surface of the plate and grows with increasing distance from the plate.

Keep in mind that the BL mesh tool is just a way of grading an edge mesh, and that you could achieve the same results by using a First Length/Interval count edge mesh scheme, where the first length is the height of the first cell of the “boundary layer”, and the mesh size increases with increasing distance from the plate (i.e., the edge mesh arrow points away from the plate.) This is actually the way I have been creating “boundary layer” meshes lately, as it allows for, in my opinion, more control over the mesh, and you don’t end up with a large jump in the mesh size at the top of the boundary layer.

-mAx- September 3, 2009 13:25

I agree with you Chris, because you are working here with Hexa-
But I am pretty sure it won't work with unstructured grid.

Chris D September 3, 2009 15:21

I think you're right, Max.

Ganeshsswamy September 20, 2009 19:50

Thank you Chris.


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