# How do I account and solve for mesh convergence issues in ANSYS for rectangular channel?

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I gradually increased the node count, and the pressure drop keeps increasing, diverging more and more away from the analytical solution, but suddenly comes back close to the analytical solution. I was wondering how this can be explained and how the general accuracy of this model and ANSYS calculations can be assessed?

## Answers

3,081As ANSYS Staff, we can not download attachments. Please upload images using upload image functionality.

Obviously coarser mesh will have different results than finer mesh. Please check theory guide for the same.

You will first need to do mesh independent study. You need to have a mesh where results do not change further even if mesh is refined. Then check convergence criteria. Make sure that along with residuals, you also monitor key variables to confirm convergence.

Regards,

Keyur

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17My apologies. I will get the pictures uploaded properly as follows. I understand that I need to keep running it at more and more refined meshes until the values doesn't change. Usually the trend I observe with Pressure drop is that it gradually comes closer and closer to the analytical prediction with more refined meshes. However the trend here was that it started with an ANSYS pressure drop close enough to the analytical value at a coarse mesh, but kept rising dramatically until over 12 million nodes where it suddenly rebounded back close to the analytical value. I am puzzled as to what could account for such dramatic fluctuations?

430I am not able to explain the results. I am just wondering if you were using inflation or not? if yes, did you change the parameters?

17All I changed was element size. I did not do anything to change the default inflation parameters.

2,326You are not changing your inflation layers here. But, you are changing the bulk mesh.

Also, is this a turbulent flow? Which model are you using and what y+ values are you getting for each of the meshes?

Thank you.

Karthik

430@B_Rahman Thank you for the reply. Can you share a photo of the velocity profile near the wall for different element size. The coarse one the fine one and 1 in between. I would appreciate if you do so.

17Thanks for the reply @Kremella . This is a laminar flow model. I don't understand what y+ values of meshes are?

2,326If this is a laminar flow, I take back my comment on Y+. It is only applicable for turbulent flow.

Please consider changing the inflation layer settings. As you refine the bulk region of your flow domain, you might have to refine the inflation layer as well. On the mesh where you are not getting a good match between analytical and computational, please share a screenshot of the mesh resolution near the wall. I'm surmising that there is a jump near the wall that the affecting the results.

Also, what is your inlet condition?

Karthik

17I don't believe I would have a quantity for resolution, since I did not use adaptive sizing

I don't know if it will help to have a visual, but here is how the mesh look in general near the corner and throughout the wall area.

6,642Is this not related to your other post? What is the issue now here? Add more information about model settings etc. (and BC's).

2,326Just to add here, how deep is your convergence for each case? Are you monitoring your residual as well as your parameter of interest (overall pressure drop)?

Karthik

17The other post was looking at why the analytical vs. ANSYS calculations are not matching up. This one is more so focused on the strange trend that is happening with the resulting pressure drop, as mesh size is lowered. The model settings and boundary conditions are the same as the previous post: https://forum.ansys.com/discussion/22491/troubleshooting-different-results-for-different-mesh-on-exact-same-geometry-rectangular-channel#latest

, also including the no slip condition for the falls, and zero pressure at the outlet

430Use inflation near the walls, Mesh size are very large near the wall to capture physics.

430There is pinned post, "introducing boundary layer theory". I recommend you have a look at it.