Guidance on Boundary layer meshing and Misc.

[Nishanth Surianarayanan]

Hello Developers,

I had some conceptual questions about boundary layer generation. I am working on abdominal aortic aneurysm simulations and I am interested in estimating WSS/OSI. How to define the boundary layer dimensions? Do cardiovascular field researchers use any formula to determine the boundary layer thickness or first cell height? How do you decide the number of layers and the layer decreasing ratio?

Any guidance on these aspect would be really helpful. Thanks for your time and help!

Best,
Nishanth

[Nishanth Surianarayanan]

Hello Developers,

I found my systolic Reynolds number to be 2850. Since SimVascular does not have turbulence models, does this mean I cannot use SimVascular for my simulations?

SimVascular flow solver is not just for laminar flows, right?

Thanks,
Nishanth

[David Parker]

Hi Nishanth,

The svSolver does not model turbulent flow.

Cheers,
Dave

[Nishanth Surianarayanan]

Hi Dave,

Thanks a lot for your reply! I really appreciate it!

I have a question on regional refinement. I want to add regional refinement to the aneurysm region alone and in order to cover the aneurysm volume, I added multiple regional refinements as shown below. It was difficult for me to cover the entire aneurysm volume with a single regional refinement.

regional refinement Picture 3.png (79.14 KiB) Viewed 1430 times

This is the result which I am getting and a closer view of the problem is also shown:

regional refinement Picture 2.png (299.1 KiB) Viewed 1430 times

Even though all the regional refinements are given the same size, some of the elements are becoming squished, which I think is due to the intersection of the regional refinement spheres. If you look at the aneurysm picture (on the left), you will see three lines of elements getting squished.

Is my reasoning for this occurrence correct? Do you have any suggestions on how I could avoid this issue?

It does seem like I would need multiple spheres to cover the aneurysm volume though!

Thanks,
Nishanth

[David Parker]

Hi Nishanth,

I've not much experience with regional refinement. Let me experiment with it a bit, try to verify your hypothesis. And thanks you for those excellent pictures!

Cheers,
Dave

[David Parker]

Hi Nishanth,

I've experimented a bit and have not been able to reproduce what you are seeing. Are you using boundary layer meshing?

If you can put your project (just the Model and Mesh directories) someplace I can download it then I'll have a look.

Cheers,
Dave

[Nishanth Surianarayanan]

Hi Dave,

Thanks for your reply! I have attached the model at the link below. It is possible for me to create a single regional refinement sphere for the entire aneurysm, but it also covers the smaller lumbar vessels. I gave local refinement to the lumbar artery caps, but the regional refinement overrides the local refinement specification. The mesh size for the regional refinement is a bit coarse for the outlet faces of the lumbar arteries. That is the reason for using multiple regional refinements so that it doesn't cover the lumbar arteries.

https://drive.google.com/open?id=16LUuZ ... 99WZb6GQOW

Thanks,
Nishanth

[David Parker]

Hi Nishanth,

Thanks for the project! I've downloaded it and have been experimented with it. It is a bit large for my laptop!

I remeshed the model at a coarser resolution of 0.25 and was able to do the sphere refinement ok so I'm thinking that this is caused by some sort of tolerance problem, not sure though. Your model mesh is perhaps too fine.

I recommend creating separate faces for each vessel in your model. That allows you to set a local edge size for each face. This will reduce the element size in the aorta region. The SimTK SimVascular Coronary example does this (see attached image).

Cheers,
Dave

[Nishanth Surianarayanan]

Hi Dave,

Thank you for your reply! Just a few follow-up questions:
1) I created the geometry using ITK-SNAP. Is it possible to create separate faces for different regions in the model using SimVascular?
2) I am checking my mesh quality based on aspect ratio, minimum dihedral angle and volume as suggested by the SimVascular meshing guide. From your experience, do you have any suggestions on what range (or values that I should avoid) for these three parameters for a good quality mesh?
3) I don't see the latest version of SV for Windows. When will it release?

Thanks,
Nishanth

[David Parker]

Hi Nishanth,

1) I created the geometry using ITK-SNAP. Is it possible to create separate faces for different regions in the model using SimVascular?

I don't think it is possible to do this with ITK-SNAP. Let me think about how to do this, maybe extract centerlines and use that information to assign face IDs.

2) I am checking my mesh quality based on aspect ratio, minimum dihedral angle and volume as suggested by the SimVascular meshing guide. From your experience, do you have any suggestions on what range (or values that I should avoid) for these three parameters for a good quality mesh?

You want to avoid long, skinny tets.

3) I don't see the latest version of SV for Windows. When will it release?

It has just been built and put on SimTK.

Cheers,
Dave