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-   -   [GAMBIT] Few newbie questions (http://www.cfd-online.com/Forums/ansys-meshing/85459-few-newbie-questions.html)

ajkratos February 25, 2011 21:18

Few newbie questions
 
Hi all
I just have few basic mesh related qeustion and hoping someone will answer them. If i have 3 different meshes to simulate a jet flow and 1 of the meshes gives me the best result for a single phase jet, will the same mesh give me best result also if simulating two-phase jet flow? I am getting some strange results if i make the mesh finer. For the most coarse mesh (40,000 mesh faces) i get a nicely shaped jet, spreading downstream but as i make the mesh finer (640,000 mesh faces at most), the shape changes significantly and starts looking very uneven. I do not have any data to compare my results with at the moment but for a rough guide what is the relationship between the mesh and results? is finer mesh always better?

PSYMN February 26, 2011 07:46

No finer mesh is not always better. Sometimes the quality drops, depending on the mesh topology. For instance, imagine a case where you have mapped quads to a circle. At the corners, you will have elements where there are 3 out of the 4 nodes of a quad on the perimeter of the circle. If your mesh only had 4 quads, the elements would be decent quality (the angle between the quads would be minimal, but the finer your mesh gets, the more that corner angle opens up... Ultimately as the number of nodes approaches infinity, the corner element angle approaches 180 degrees.

However, if your quad elements against the wall only had 2 nodes each on the circle and were orthogonal (topologically), they would not get worse with refinement...

Refinement usually allows for more smoother flexibility because a smaller percentage of the nodes are constrained by the geometry, so the quality is usually better.

ajkratos February 27, 2011 12:09

4 Attachment(s)
Hi
thanks for your reply. I get your point although geometery of my problem is very simple and the mesh is a structured mesh but the result changes significantly with increasing the mesh faces. I have attached the pics of the results produced by 3 diffrerent meshes (40,000, 160,000 and 640,000 mesh faces). Also attached the pic of 1 of meshes with 40,000 mesh faces, other 2 meshes look the same except that they are denser. If i dont have anythng to compare the results with how do i determine which mesh provides the best reults. As you can see in the pictures the geometry is very simple and therefore resulting mesh is simple also, so in this case will it be right to say finer mesh=better results ?

PSYMN February 27, 2011 15:33

If the mesh is just mapped quads, then finer should be better... But looking at your post images, the first looks best...

How is the mesh quality in the finer examples?

ajkratos February 28, 2011 15:00

2 Attachment(s)
Hi
All three meshes are quad mapped meshes. Everytime i made the mesh finer I just increased the number of grid points propotionately in both directions. I have attached 2 pics of zoomed view of the most fine mesh which has 640,000 mesh faces. The result from the simulation using this mesh is in my last post. Thanks

PSYMN February 28, 2011 23:26

Perfect...
 
So the mesh quality in both cases is pretty much perfect... Perfect angles, perfect (lack of ) skewness, perfect uniform (lack of) transition, aspect ratio of 1, etc. Perfect, maybe pretty inefficient (more elements than you need in some directions), but perfect quality.

The mesh doesn't get better when it is refined because it was already perfect, but taking smaller chunks at a time should let you resolve the flow features better...

You have nothing wrong with your mesh.

Perhaps it is just capturing solver problems better?

Now you need to post this interesting question on the solver boards ( if you have not already)... Please let us know how it turns out.


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