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-   -   boundary layer in gambit (http://www.cfd-online.com/Forums/ansys-meshing/124234-boundary-layer-gambit.html)

kkpal September 30, 2013 09:40

boundary layer in gambit
 
2 Attachment(s)
dear all,
I want to add a boundary layer to this kind of shape, but as much as I tried, I still couldn't get it right.
These two pics shows a mesh I created lately, obviously the boundary layer is in bad shape at the corners.
How can I get a boundary layer closely resembles the geometry?

-mAx- October 1, 2013 00:43

by clicking right mouse (or midlle ?) you can switch the orientation.
But the question is: why do you have fluid cells on both sides of your wall edge?

kkpal October 1, 2013 01:24

hi max,
no, I dont have fluid cells on both sides, it's just on the outside wall.
I'm simulating flow around this kind of cylinder, later this shape will be extruded into 3D.
It seems right or middle clicking did not change the orientation of the boundary.Can you elaborate more about that?

Quote:

Originally Posted by -mAx- (Post 454369)
by clicking right mouse (or midlle ?) you can switch the orientation.
But the question is: why do you have fluid cells on both sides of your wall edge?


-mAx- October 1, 2013 01:53

2 Attachment(s)
if the bl is pointing in this direction, it means the bl can grow in this direction.
Then you have a surface, in which the bl can be applied.
Enable shaded mode, you will see if you have a surface.
I confirmed the middle mouse click for switching BL direction. But it will switch only if it can (understand if you have a surface on the other side)
Attachment 25717 Attachment 25718

kkpal October 1, 2013 03:03

2 Attachment(s)
hi,max
In my case, I'm meshing the outside boundary of the shape, however, at the corner, the mesh quality seems very bad and I want to modify it.
I think what you mean is changing the boundary layer to the inside of the wall.:confused:. that's not quite exactly what I wanted.
Lately I find out that when the boundary layer is thin, the mesh is in good condition. While the layer grows thicker, the mesh at the corner becomes skewed. Here I post two pics for illustration. The first one is thick bl, and the second one is thin bl.


Quote:

Originally Posted by -mAx- (Post 454378)
if the bl is pointing in this direction, it means the bl can grow in this direction.
Then you have a surface, in which the bl can be applied.
Enable shaded mode, you will see if you have a surface.
I confirmed the middle mouse click for switching BL direction. But it will switch only if it can (understand if you have a surface on the other side)
Attachment 25717 Attachment 25718


-mAx- October 1, 2013 03:13

Quote:

Originally Posted by kkpal (Post 454384)
that's not quite exactly what I wanted..

ok I missunderstood, sorry.
Then the problem comes from the depth of your BL.
If your y+ enables you try to reduce first row height, and increase number of layer

kkpal October 1, 2013 03:53

1 Attachment(s)
Quote:

Originally Posted by -mAx- (Post 454385)
ok I missunderstood, sorry.
Then the problem comes from the depth of your BL.
If your y+ enables youm try to reduce first row height, and increase number of layer

yeah ,that seems the only way to get it right. I was hoping there is a way to create mesh like this.
thanks for your kind help!

-mAx- October 1, 2013 03:58

sure you can, but on the picture the red mesh isn't a BL

kkpal October 2, 2013 00:28

Quote:

Originally Posted by -mAx- (Post 454403)
sure you can, but on the picture the red mesh isn't a BL

Your words really gave me hope!
In another thread(http://www.cfd-online.com/Forums/flu...-cylinder.html) you told me to do some split on the cylinder surface, I will try that.
And if my attempt fails, I will ask you for help.
Thanks!

kkpal October 2, 2013 03:13

need help!!
 
hi, max.
My attempt failed. I don't even have a clue where to do the splits:confused:
Given the geometry of cylinder with helix, can you tell me how to generate that kind of beautiful mesh in gambit?
Here is a similar thread I posted some days ago.
http://www.cfd-online.com/Forums/ans...-strategy.html

ghost82 October 2, 2013 09:42

If the boundary layer failed, you can try to add some extra surface and make "your boundary layer" in these surfaces, by manually create the mesh as the boundary..this is just a suggestion..

Daniele

-mAx- October 3, 2013 01:48

Quote:

Originally Posted by kkpal (Post 454615)
hi, max.
My attempt failed. I don't even have a clue where to do the splits:confused:
Given the geometry of cylinder with helix, can you tell me how to generate that kind of beautiful mesh in gambit?
Here is a similar thread I posted some days ago.
http://www.cfd-online.com/Forums/ans...-strategy.html

For having similar mesh as in your picture you need do split your domain with a straight cylinder which will surroung your helicoid.
Then in this new domain, apply a BL on helicoidal walls and apply also a size fonction)

kkpal October 4, 2013 04:12

3 Attachment(s)
hi max,
Inspired by your suggestion, I successfully created good mesh around the helixcoid. Here is my procedure.
1. mesh the top and bottom edges; 2. mesh the helixcoid face with quad/submap, it automatically generated good quad mesh on the cylinder surface; 3. apply bl on the cylinder surface; 4. mesh the domain.
However, in the second step, it seems difficult to control the axial mesh size. Usually the mesh in the axial direction is too dense for my simulation.
Is there a way to solve this problem? I realize there is a SPACING column, I changed the value in it but result did not change.
And is there a way to view the mesh inside the domain? like the third picture.

-mAx- October 4, 2013 04:21

from your first picture I don't see any split as I adviced you.
Create a cylinder with radius greater than your helicoid, then split your domain with the cylinder volume.
At the end you should have 2 volumes: the one next to your helicoid and the rest
Doing that you will have more control on your mesh

kkpal October 4, 2013 05:02

and one more strange thing.
in the face mesh, sometimes the Proj Interval column appears, but most of the times it does not.:confused::confused::confused:
I believe that option can control the axial mesh size.
How can i make it appear?

-mAx- October 4, 2013 05:17

It appears only with quad
https://www.sharcnet.ca/Software/Gam...proj_intervals

kkpal October 4, 2013 05:36

2 Attachment(s)
thanks max,
I created a circular cylinder surround the helixcoid, and meshed the circular cylinder surface with approiate axial mesh size by specifying Proj interval . Then I mesh the domain with Hex, I suppose by doing this the axial mesh size on the helixcoid would be the same with the outer cylinder, but after two trial it didn't, the mesh on the helixcoid is still not controllable.
what is the way to control that?
and btw is there a button in gambit for view the mesh in side volume?

-mAx- October 4, 2013 06:15

mesh helixcoid surface first, then create a size function with helixcoid surface as source and top (or bottom) cap from cylinder as attachment.
Give the desired parameter mesh expansion.
Mesh the cap surface.
Now you have one cap and also the axial direction which are meshed.
You can go and mesh the volume with cooper. Gambit should select automatically the source for cooper.
for your last question, go to examine mesh, and work with plane

kkpal October 5, 2013 12:04

hi max,
I tried meshing the helixcoid surface first, but for this 10m-high helixcoid Gambit automatically generated 566 nodes in the axial direction. And with 180 nodes along the Circumference, the helixcoid surface alone owns 101880 nodes! And when I use Cooper to perform volume mesh, my machine went break!
Now I'm thinking using the journal file to enforce a Proj interval on my helixcoid surface, since this option did not appear even though I chose Quad to do the meshing.
But is there another way to reduce the axial mesh quantity in the axial direction of the helixcoid?

-mAx- October 9, 2013 00:41

Axial node distribution can be reduced, it will only influence the aspect ratio of your cells (hexa or wedge)
You can dramatically reduce mesh size by playing with size function's parameters.


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