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Problem creating mesh smaller than 1mm

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Message 1 of 8
cohkka
947 Views, 7 Replies

Problem creating mesh smaller than 1mm

I am having problems trying to generate a 2D mesh to model fluid flow in a gap of 75μm between two plates, each 1090μm long. I mesh a rectangle (with the model scale in μm) with corner points at A (0,0,0); B (0, 0, 37.5); C(0,1090,37.5); D(0,1090,0) so that I can mirror it to model the fluid flow.

 

I set the divisions for AB to 6 and geometric. I wanted the number of divisions for BC to be as large as possible but the largest I can input is 10 without Autodesk Simulation crashing. Even when I set the number of divisions for BC to be 10, when I produce the mesh, the vertical lines do not show up (I am not sure if this is a problem but it does seem strange). [See attachment 1]

 

I carried on with my model, applying an input velocity and an outlet. When I tried to run the analysis, an error popped up saying it had encountered problems with geometry. The log said I had defined no thickness despite it being a 2D model. [See attachment 2]

 

I have done the exact same model but scaling it up by 10^3 and it works fine. Is there some sort of limitation on mesh size that means this won't work??? Any help would be much appreciated!

7 REPLIES 7
Message 2 of 8
Joey.X
in reply to: cohkka

Once you have changed the unit to Micron for this specific model, the working digital is in normal range (not too tiny), so the software should handle it normally.  The real problem goes to the CFD modeling issue since the force from surface tension is more trivial (or probally dominate) in such a micro channel so that simulation result without considering surface tension is not accurate.

 

The second decoder error seems from first meshing problem. If you can attach the model, we can help to find the error root.

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 3 of 8
xli
Alumni
in reply to: cohkka

Hi Cohkka,

 

From the 1st figure you attached we can see that sketched mesh lacks at least one verticle edge for every element. Therefore the program (DCX1.DLL) making elements from connected lines WON't be able to make any 2D element successfully. This is the reason you got error message: DCX_ERROR.

 

Based what you have described the way you sketched the mesh, it seems Fea Editor has some kind of resolution setting preventing smaller size in 2D mesh sketching. As you said that was 10 for BC setting. Even with that size allowed the resulting 2D mesh lacks some of verticle lines after divisions. I am also wondering what is largest size you have in the mesh, usually the ratio of the largest to the smallest size matters. And what unit you used probably matters too. I would image that these are two facts likely dictate the tolerance used to set the resolution there. You may double check them.

 

Yes, if you can send this model that would greatly help us to identify problem and probably seeking some kind of work around if possible. Thanks a lot.

 

-xli 

Message 4 of 8
cohkka
in reply to: cohkka

Thanks very much for such fast replies! I've attached below the fem file I created (in a zip file). xli, I am not sure what you mean by the largest size...hopefully you will be able to see from the file though.

 

Thanks to you both for the help!!!

Message 5 of 8
xli
Alumni
in reply to: cohkka

I briefly looked your model. It seems it is troubling on size. And I found its model unit is inch. I think you have to first change your model unit to mu or mm etc. smaller units, then you will be able to make mesh in that unit as you want. Under a model unit system, the range you can sketch is limited. That limit depends on largest element size and smallest size ratio. 

 

So see if change to correct unit system before modeling can resolve your problem. Good luck.

 

-xli

Message 6 of 8
cohkka
in reply to: xli

Thanks xli, I hadn't realised the difference between model units and display units. I have now changed my model units to mm but when I try to run the analysis, I get an error message saying that the Jacobian of the element is non-positive. I have no idea how to fix this! I tried searching the software help but all that came up was help for solid meshes and I didn't think that applied here. I've added a screenshot of the error message and the simulation file below.

Message 7 of 8
Joey.X
in reply to: cohkka

I got your model from your last post, applied material and run. It works fine in my local PC using V2012, I attahced the compelete archive file.

Note again the result is not accurate since the surface tension is not trivial in such a micro channel since Autodesk simulation Mp does not consider surface tension in fluid flow.

 

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 8 of 8
cohkka
in reply to: Joey.X

Thanks Joey.X! I tried what you said and it worked! I think it's because I was looking at a model example and it told me to change several analysis parameters ('maximum length scale' field and 'maximum expected velocity' field) but without changing these it works. Thanks so much to the both of you!!!

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