topic Applying the same mesh to different models in CFD Forum

Applying the same mesh to different models

Anonymous — Sun, 22 Jul 2018 07:11:05 GMT

Hello all,

I have two models, each consisting of a fluid (round volume) passing through a gas column (vertical rectangular volume). The fluid passes through a pipe and a resistance within the gas column, which represent the fin tube of a heat exchanger. The models are identical except that the model on the right is elongated.

My question is this: how can I duplicate the mesh quality of the small model on the elongated model? To use a point of reference I'll focus on the number of notes on the fluid edges and pipe edges.

Here are screenshots of the models in their default state, after Autosizing, showing:

- The differing number of nodes on the pipe ends and the fluid volume where it enters the pipe/gas column

- The differing "Min refinement length"

The different numbers of nodes on the various features means that refinement, such as "Size adjustment" in the mesh sizes box, doesn't get applied the same. I suppose because the length scales are different. So what can I do to make the meshes the same?

Here's some other related questions:

- How does CFD calculate length scales and apply different numbers of nodes to the same edge length?

- How does CFD calculate the default Minimum Refinement length that appears on the slider when the Diagnostics box is opened?

- In the screenshots below you can see the different default Minimum Refinement Lengths for the two models. But I have set the slider to the same position (96.1), below which the red edges disappear on both models. So CFD seems to recognise that the red edges are the same length on both models. This is if course true, but I was expecting the length values in the boxes to be scaled from the overall model geometry. So what is happening here?

Thanks everyone.

Regards,

Chad Lasance

Re: Applying the same mesh to different models

Jon.Wilde — Sun, 22 Jul 2018 10:35:46 GMT

Hi Chad,

A quick way might be to select the part and press Uniform to give it a totally uniform mesh size - this means that if you have (for example) 8 nodes around the circumference, this would apply along the entire length.
You could then match this to the other design.

To totally match the designs, you could use a manual mesh throughout.

I would add though, well, ask... does it matter? If you can demonstrate that both are mesh independent then they do not necessarily need to have identical meshes. Just be careful you are not matching two meshes with results that are still mesh sensitive 😉

Quick note on the top of the column/some of the cuboid shapes. Ideally you need 4-5 elements (without considering the wall layers) from one edge to another across the thickness. A manual mesh might really help you here as everything is so blocky.

Hope that helps!

Jon

Re: Applying the same mesh to different models

Anonymous — Sun, 22 Jul 2018 21:40:58 GMT

Hi Jon,

Thankyou for your reply. I was considering, on reflection, that the mesh independence test is probably the best way to go. I just thought if there was a simple way of duplicating the mesh that would save me the trouble.

Do you have any comments on the other questions in the post?

Thanks and regards,

Chad Lasance