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Ahmed body 2D -Questions about mesh independence, Y plus value and others

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Old   July 6, 2017, 08:52
Default Ahmed body 2D -Questions about mesh independence, Y plus value and others
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Welcome everyone,

I'm a beginner in CFD and I would like to compute fluid flow around Ahmed body. I was following mostly this tutorial - https://www.youtube.com/watch?v=HyxR5sJk-gU but I wanted Steady simulation
I am working on Ahmed body model in 1/3 scale. I attach model measurements, computational domain (where L is 348mm) and example for the mesh.

I made some unstructured meshes with triangles and wanted to make mesh independence tests by tracking Drag Coefficient but while I was making meshes finer the drag coefficient didn't tend to drop down so I decided to start over with some help.
I used velocity 60m/s, Standard k-epsilon model, Non-equilibrium wall function, SIMPLE scheme and Second Order on Pressure, Momentum, Tubulent Kinetic Energy and Dissipration Rate.

I wanted to make a refinement zone where mesh will be finer around the model for better results.
(To be honest I don't remember why the height of the refinement zone is 243,6mm, anyway can be changed)

1st question - Is it good idea to do this kind of refinement zone? Should I use structural or unstructural mesh?

2nd question - Should I use inflation layer around Ahmed body model to get proper Y+ value?
I read that if I want to use k-epsilon model Y+ should be around 30-200. I checked Y+ calculator but I have no idea how I am supposed to know Boundary Layer Length.

3rd question - How to properly make mesh independence test? Is it basically make a mesh, going to Fluent, performing simulation while tracking certain value, saving files, going back to Meshing, making mesh finer and doing the same thing again?

4th question - What to do if the simulation doesn't converge while making mesh independence test? Is it OK to change different things like turbulence model or changing Solution Method or is it making mesh independence test wrong?

I know that it is really chaotic writing but I hope you will understand and help me solve this problem.
Thanks for help in advance.
Attached Images
File Type: png ahmed.png (25.7 KB, 5 views)
File Type: png Mesh.png (40.9 KB, 9 views)
File Type: png Model.png (12.2 KB, 6 views)
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