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Jack Martinez July 1, 2007 11:05

Gambit help: Cube inside cube

We intend to create a geometry (cube inside cube) in Gambit. The inner cube has to cut off because we dont want any cell in that region. The outer cube is a 3*3*3 mm^3 and the inner cube is 1*1*1 mm^3 (both have same center).

We have written our own grid file, because we need only 26 cells in the entire volume, and imported it in Gambit. The gambit is able to display the geometry properly but couldn't identify different selections assigned before.

Our requirement is, when we import this 3d mesh-grid from Gambit in Fluent, there should be following two zones in the boundary conditions selection panel:- 1. The inner cube. 2. The volume between the inner cube and outer cube.

Our grid file is correct, I have checked on different platform like Fire and radtherm but when I imported it in Fluent it couldn't identify different zones. This is a problem I am facing since long with Fluent.

So I am trying to make selections in gambit and then export it in Fluent. However the gambit is giving warning like "A default unspecified boundary entity was created" and again Fluent couldn't identify the internal cube selection. In Gambit, I tried to group all the faces of inner cube to have a selection before exporting in Fluent.

An urgent help is required, as we have a deadline very close.

If required I will send my grid file for your reference. Let me know about ur mail id..

With regards Jack Martinez.

AJ July 1, 2007 17:13

Re: Gambit help: Cube inside cube
Why don't you model this and control your grid size in Gambit itself? If the model is simple enough you could create in Gambit and control the mesh very well...line level, face level to volume level.... Are you trying to mesh it 26 cells on outer volume or others??? Why?

Any face not selected under any boundary condition it will create default unspecified boundary entity... which you could see under Fluent as either wall or enterior defaults...

What are you trying to do with this model? Thermal analysis etc...?

Too many questions from me... but it could help...

Good luck... AJ

amir July 2, 2007 03:53

Re: Gambit help: Cube inside cube
Dear Jack as AJ said i bilive that you can create your mesh by gambit,but also as i founn out , it is not your problem and you have a problem about BC. gambit diplay this warn "A default unspecified boundary entity was created" when you dont specify any BC for one or more face. Best Regards Amir

Jack Martinez July 2, 2007 05:46

Re: Gambit help: Cube inside cube
Dear AJ,

Thanks for your reply, I really appreciate time and effort you spend in replying...

As mentioned in your reply, I would be really thankful if you can please elaborate on how to control the mesh. I am required to mesh the grid with 26 equivalent cells. I have mentioned the cube size in my previous post.

Actually the grid contained 27 equivalent cells fron which the center most cell was deleted to finally have 26 cells in the volume.

Yes, we are trying to perform thermal analysis. Even I am facing problems with my own model because we have referenced intotal 64 nodes in our model and for further processing when I created a 2 d rectangle on left boundary face of outer cube in Gambit and tried to import it in Fluent. The Fluent automatically deleted the face selection with a warning that extra node from referenced value has been deleted...

I really would have preferred modelling it in Gambit but I am unable to export required selection using Gambit. I hope I can get a way out to mesh it in Gambit.


Jack Martinez.

AJ July 2, 2007 16:30

Re: Gambit help: Cube inside cube
In Gambit you could mesh the line or face first to comtrol the mesh of the volume. This method works well and you could divide the line or face exactly to the size you want...

1) Model your first cube by selecting create Real Brick ( leave the default centered ) use 3 in X, Y and Z

2) Model your second cube by selecting again the same as above, but now using 1 in X, Y and Z

3) Split the two volumes to make sure that these two volumes have connection...

4) Start meshing --> DIvide the edges to equal intervals using mesh --> Edge --> select the edges... and use spacing as interval count use 26 or so what ever needed...

Complete the meshing of volumes...after completing lower level mesh...

5) Define the boundary conditions --> Select all the faces which have same BC and apply it... do it till you complete all faces... Also specify continum type to the volumes...

Hope this helps...

Make sure you select all the boundaries before you export the mesh from Gambit else it will export few walls/interior unknown as default.... Even if it happend (you see default BC) you could visualize them under Fluent and change the boundary condition!!!

Good luck... AJ

AJ July 2, 2007 16:34

Re: Gambit help: Cube inside cube
If you really want HEX mnesh then you have to divide the volumes and mesh them using mapped mesh under Gambit..


Jack Martinez July 3, 2007 03:07

Re: Gambit help: Cube inside cube
Hello AJ,

thanks for your reply. I followed the procedure suggetsed by you. I need only hex meshes.

1) Created outer cube of edge .003m.

2)Created inner cube of edge .001m

3)Split the volume (using panel apear in the geometry->Volume->Split panel). Selected the outer cube in first table window and inner cube in second table window. I splited with connected setting.

4)Meshed the edge of outer cube using Iterval count-> 3

5)Meshed the edge of inner cube using Iterval count-> 1

6)Meshed the face of outer cube using count 9.

7)Meshed the face of inner cube using count 1.

8)Mesh the outer volume using interval count 26. There should be 27 cell volumes but due to split the remaianingvolume enclosed between VOl-1 and vol-2 can only accomodate 26 cells.

9)I am unable to obtain Hexagonal mesh.

10) Please tell me, by dividing the volume, do u mean dividing the outer cube volume into 27 cell volume and then mesh them each. Then also suggest how to divide this outer cube volume into cell volume.

Thanks and Regards

Jack Martinez

Jack Martinez July 3, 2007 07:06

Re: Gambit help: Cube inside cube
Hi AJ!!

I have solved the problem of having invalid entity inside inner_cube walls. I imported my own model and created a cube that fits the cavity completely and assigned all the required boundary types and continuum zones. Its working well upto this level !!!

But in Gambit, I am still not able to pursue it. To have Hex mesh, its neccesary to divide the volume after splitting the cube into 26 equi volume cubical cells (one at the cnter will be absent).. and after meshing individual volumes and then export the grid in Fluent but I yet couldn't find a way to divide volume in Gambit, I am looking forward for a suggestion from your side.

However, the current problem is I want to have an element selection on the left face of the cube. Please let me know if it is possible to select a particular face element on meshed face of a 3D geometry either in Gambit or Fluent.

Thanks and Regards

Jack Martinez.

AJ July 3, 2007 16:05

Re: Gambit help: Cube inside cube
To mesh hex in Gambit you have to make sure that you could map mesh or submap mesh the faces....So best thing is split the volumes (outer)such that you could mesh individual volumes by mapped HEX or copper HEX...

You could divide the volume using volume to volume split, or face to volume split... make sure that the vertices/edges are not duplicate that means features are connected....

Alternaltely if you make all 27 volumes then the most center one is your inner cube. Try meshing these volumes you should get HEX mesh... There could be number of ways to split and make this work...

Hope this helps... AJ

AJ July 3, 2007 16:13

Re: Gambit help: Cube inside cube
Other alternative is to create these two cubes and delete the volumes without deleting the lower geometry. Now create 6 volumes by connecting the six faces of inside cube to outer cube.... All volumes can be meshed using HEX mesh... although it mesh would be tilted for outer volumes...

Just a thought... hope it helps... AJ

AJ July 3, 2007 17:01

Re: Gambit help: Cube inside cube
Another option which I could do is following ( little tedious)... Create the outer block (3x3x3 mm^3) Now select a face in X direction and copy it and generate one at 1mm away and another at 2mm away... similarly copy faces from Y and Z sides of the big cube...

Now keep splitting the outer volumes with these faces ( this is tedious process) because you will generate 27 volumes at the end of this....

Now you could mesh the volumes as desired...


mayur July 4, 2007 07:11

Re: Gambit help: Cube inside cube
first create cubes of lenghth 3 and 1... scaling u can do in fluent.. split them...delete the inner cube now.. now mesh the edges with an elemnt lenght of 1 ... go to vol. mesh and select submap or map and mesh it... u will ge the desired result... .

Jack Martinez July 4, 2007 07:31

Re: Gambit help: Cube inside cube
Hello AJ!!

I tried the last which is tedious but it worked out I am able to select the face element as well as define boundary conditions to my requirement... :)

I am having immense pressure as I have to submit the results in one week.. I will definitely try the alternative options and inform you about their compatibilty.

Many thanks for your time and valuable suggestions


Jack Martinez

subha_meter August 11, 2010 06:29

wall formation
Hello All,

I've a problem which I think, could be resolved with suggestions from you all in this active forum.

I have two co-axial cylinders, where part of the inner cylinder is projected into the outer cylinder (Don't wish to UNITE the volumes. The inner cylinder's projected part should act as a WALL).

I want to simulate flow through this assembly.

Desired BC:

inner cylinder top face : velocity inlet
outer cylinder bottom face: pr outlet
All side surface : wall

Now the bottom face of the inner cylinder which is projected into the outer cylinder volume invariably creates a "WALL" face and in spite of providing a
"Interior" BC, I'm not in a position to get rid of this unwanted wall.

After simulation,all flow pathlines stuck at this wall and never reach the outlet.

Kindly help.



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