CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   ANSYS Meshing & Geometry (http://www.cfd-online.com/Forums/ansys-meshing/)
-   -   [ICEM] turbine mesh refinement (http://www.cfd-online.com/Forums/ansys-meshing/89373-turbine-mesh-refinement.html)

Jonathan June 10, 2011 14:20

turbine mesh refinement
 
4 Attachment(s)
Hi,

I have been trying to mesh a fairly highly twisted turbine blade using a standard o-grid configuration and have managed to get a fairly good coarse mesh. However, i need to simulate using a low y+ mesh and so have been trying to refine the pre-mesh, especially in the o-grid (the hub and casing should be relatively straightforward ...).

however, in setting the edge pre-mesh values to sizes which i know are close to what i need for y+ = 1, i start to get inverted cells especially near the trailing edge. I wondered if my blocking and associations are not conducive to this (see o-grid) at TE, where the o-grid starts quite far to the rear of the physcial TE.

was wondering if you guys have any advice, or comment on the mesh / blocking in general.

PS have previously been using gambit and so am still coming up to speed with icem, so any advice would be appreciated.

cheers
jonathan

PSYMN June 12, 2011 18:34

"corners"
 
The Ogrid works best when at the "corners" of the rounded portions. For the leading edge, you have this correct. But for the trailing edge, your Ogrid connects along the airfoil long before the corners. This leads to distorsion at the actual corners...

My suggestion would be to slide those verts back...

Jonathan June 13, 2011 03:08

2 Attachment(s)
Hi Psymn

thanks - originally i had an issue with the blocking / premesh not recognising the trailing edge if i unassociated the block vertices from the points they are associated with on the blade geometry. I could not figure out why the mesh 'collapsed' there when i did, and did not follow the curvature of the trailing edge surface. So, to work around this - i have splined out the blocking and it seems to stick to the blade TE surface ok now.

in refining the premesh now to get BL elements (1st cell thickness 0.01mm), i am getting inverted cells in the tip gap region. see attached - dont really understand why, if both the upper and lower surfaces of the tip gap block are associated to the top of the blade, and the projection of the blade onto the casing?

again, near the inlet where i have specified a first element thickness of around the same (0.02mm) the premesh appears to collapse the first cell in a isolated region for some reason. Again, the upper and lower edges of the block are associated with the curves wich are defining the tip gap region and i have specified the edge mesh refinement to copy to all parallel edges, so it seems a little odd ...

otherwise, this is a great tool - esp the hexa tool for complex geometries!

cheers
jonathan

Jonathan June 13, 2011 05:49

4 Attachment(s)
hi Psymn,

had a bit of free time so just thought I would have a go at moving the o-grid vertices back towards the TE as you suggested - but still get a degenerate TE mesh for some reason - have attached a few images - so not sure why this happens ...

i am fairly certain this must be an association issue, but i dont understand why this should happen? The associations are that the pressure / suction / TE curves are associated as a compound entity to the three corresponding hexa edges. this is different to the LE where the geometry curve corresponds better to the size of the hexa block, and i can associate the LE curve directly with the front edges of the hexa block. having moved the verts near the TE backward, i cannot do this at the TE - perhaps this is the issue??

any advice is much appreciated! thanks v much,
cheers
jon

PSYMN June 17, 2011 11:17

Associations...
 
Yup, it is all about the associations...

They are surface projected edges, which usually means they project to the nearest surface. But in your case, you had probably had issues with mesh projection jumping across your blade (from the convex to concave side), so you went in and applied a "Face to Surface" association. The problem is that surface didn't go all the way back to the rear of the airfoil.

To check these things, right click on edges to display "Projected Mesh Shape". You will probably see that those corner edges suddenly jump back to where you ended up placing the vertex... sliding the verts worked as a psudo fix for you because you happened to move those edges to where they were projecting... You can confirm this by turning on "Faces" and right clicking to display "Face Projection". At 13.0, it just shows a big colored face, but in 14.0, it will actually be color coded with more meaning.

Anyway, lets fix it. There are many ways... You could split that face along the edge of the surface so things line up... You could remove the association and fix the origial prjection problem with edge splits... You could break up the two sides of the airfoil by part and use associate to PART (each part can have multiple surfaces)...

Since you have already split the edges to fit the airfoil, changing the surface associations back to "Closest" seemed to be best solution.

1) Reduce your Index control (make it easier to select faces). Right click on Blocking (or use hotkey i) to turn on the index control. Increase O3 to 1-1 (assuming you created the OGrid in that direction).

2) To reduce clutter, turn off everything else in the tree. You don't need to see surface or curves or mesh or even faces to select a face, they just get in the way. All you should be looking at is edges.

3) Blocking => Associate => Face to Surface, select Closest and pic the two faces that were associated to surface before.

4) Turn on the premesh to make sure it is working properly now...

Post a pic if it worked ;^)

Jonathan June 17, 2011 14:57

2 Attachment(s)
Hi Psymn,

great - thanks - reassociating the faces to the surfaces using the 'closest' condition worked well.

attached are a couple pictures of the remeshed TE ...

cheers
jonathan

Far November 8, 2011 12:09

is this a J-type grid?

Jonathan November 8, 2011 12:51

no, o-grid

Far November 8, 2011 13:06

O-grid in the vicinity of blade and what about the rest of the domain? Is it a H-type, J-type, L-Type or mix?

Jonathan November 8, 2011 15:06

yes, as you say, o-grid around the blades and h-grid in the rest of the domain. there are a couple pictures of the coarse mesh at the start of the thread as far as i recall.

regards
jonathan

Far November 8, 2011 15:09

did you try to reach the mesh quality produced by TurboGrid? at the moment I am trying to do this, since there are many geometries where automatic templates may not work.

Jonathan November 8, 2011 15:13

hi - not sure exactly what you mean - but if i understand correctly you are asking whether i was able to reproduce equivalent mesh quality with ICEM as one would have by using TurboGrid ... ?

No, i meshed everything in ICEM for this project, and in actual fact, for the bulk of the work, i used Gambit ...

rgds
jon

Far November 9, 2011 10:08

J Type grid
 
1 Attachment(s)
@PSYMN

Can we create the J-type for this geometry, and what should be the blocking strategy


All times are GMT -4. The time now is 12:02.