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Ellen January 28, 2009 18:16

mesh generation problem

I met a problem generating a mesh using Gambit. This is my first time generating a 3D mesh, and the model I created is:

create a big water tank (cylindrical) with its axis in Z direction;

create an inlet pipe (cylindrical) with its axis in y direction;

insert the inlet pipe into the tank, so one end is open to atmosphere and the other end is inside the tank;

generate a hex mesh for the tank, and another mesh for the pipe, then defined the boundary type for the tank wall, pipe wall and pipe inlet (the end that is outside of the tank);

when I read this mesh into Fluent, the pipe end which is inside the tank becomes a "new-wall", and I can't find any suitable type to re-define it, thus the case couldn't run.

In Gambit Tutorials, I saw meshes with intersecting pipes, but didn't find any cases with one pipe inside another. It seems to be simple problem, but I tried several ways to generate the mesh, and just couldn't make it work. Could anyone here please give me a hint? Thank you so much.

regards, Ellen

Nitin January 29, 2009 01:47

Re: mesh generation problem
when u will import mesh in fluent, for a part of a inlet pipe inside the tank fluent will creat new-wall & new-wall-shadow bcz there is fluid on the both sides of wall so it will assign both wall & shadow to two sides of fluid and it will not be a problem for solver point of view. u can start solving problem with the present case file

mange January 29, 2009 06:36

Re: mesh generation problem
make sure that your volumes are connected in gambit. Sometimes when generating geometry it results in two seperate volumes where you only want one. in this case both volumes have a face at the interface but they are located at exactly the same spatial position. This means that fluent will see these faces as outer walls,

What you want to do is have the two volumes share the faces between them. That way you can specify them as "interior" in fluent

good luck

Ellen January 29, 2009 11:10

Re: mesh generation problem
Thank you for your replies, Nitin and mange. I still have some questions:

1. after the mesh is imported into Fluent, I only see a "new-wall" for the pipe end which is supposed to be the pipe outlet, but didn't see a wall shadow for this face. (for the longitudinal part of the pipe I defined it as pipe wall.) Also, if I don't re-define this new-wall, how can the fluid inside the pipe flow into the water tank?

2. at first, I tried to connect these two volumes (tank and pipe). but if I united them, the part of pipe inside the tank would be gone, and I need that part to introduce another fluid into the center of the tank. So it seems I still need two volumes, right? if so, how can I make these two volumes share the small face of the pipe end?

3. I also tried this: subtract the pipe from the tank while retaining the pipe; resulting in two volumes: the tank with a small hollow cylindrical part, and the pipe. but this way the generated mesh contained highly skewed cells.

I appreciate all your suggestions, thank you.


Nitin January 30, 2009 03:30

Re: mesh generation problem
dear friend try to split tank volume by inlet pipe and select bidirectional & connected options in gambit. and initially solve problem with tet mesh as it will not take much time. then u can go for hex mesh and inssyead of making hex for only tank put hex in inlet pipe also by making volumes cooperable

mange January 30, 2009 06:40

Re: mesh generation problem
I think nr.3 is probably what you want to do.

In order to improve your mesh you can split your domain into several smaller domains, where you make sure that all of them are meshable by themselves. That way you can control exactly your mesh structure. this way is time consuming and a great deal of intuition/luck & patience is needed for gambit.

If you want to use tetras, i advice using size functions. it is faster but leaves less control.

have fun! /M

Ellen January 30, 2009 11:57

Re: mesh generation problem
Thanks a lot for these helpful suggestions. I'll try enjoy experimenting with the mesh...


srbbd February 18, 2009 22:20

Re: mesh generation problem: pls advice
In my research , I need to do a CFD model using GAMBIT and FLUENT. My flow is water and this is an open channel flow. I draw geometry in GAMBIT up to face stage. I considered x, y and z value and I have 7 faces. My query is: (1) Do I need to do 2D modelling or (2) 3D modelling. (3) In case of 2D or 3D modelling what I need to do next? how can i show u the geometry for better understanding?

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