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Grid Independence Study, how to start?

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Old   August 28, 2021, 06:53
Default Grid Independence Study, how to start?
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Hey guys,

for my steadyState multiRegion case I would like to perform a grid convergence study. The case is a flow around an object.

I have never done that before, so I am a bit confused on how to start. I was thinking about first doing a coarse grid and then two finer grids. The question is: what about adding layers near the wall?
Am I first doing a study without any layers, find the best mesh and then do another study to find the right amount of layers??
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Old   August 30, 2021, 06:49
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You are right to be cautious here - essentially you have two different grid effects that you are discussing here:
1. Wall resolution of the boundary layer(s); this is dependent primarily on the cell y+ values, but also potentially their shape (are you running RANS, LES, ... ?).
2. Grid resolution of the flow (and scalar) gradients in the main body of the flow.

The nature of your test case (aerofoil flow? bluff body separation? mixing vessel? etc etc) will determine the relative importance of the above two effects. Have a think about what is dominant in your flow, and make sure you test these independently (eg keep wall y+ fixed while you vary the mesh size in the interior of the flow, or vice versa), since otherwise it will get very confusing for you. Good luck!
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Old   May 3, 2024, 12:34
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Hi Tobermory,

I'm trying to perform a grid independence test on turbulence modeling for my problem, which involves the mixing process in a micromixer through LES simulation using OpenFOAM.
In a laminar case, I could easily refine the background mesh by choosing smaller values for dx, dy, and dz to obtain a finer mesh and conduct the grid test analysis straightforwardly. However, since my simulation involves turbulence modeling through LES, I am unsure how to proceed. My geometry is quite complex and not symmetrical, adding to the challenge. I'm also uncertain about the necessary steps for conducting a decent grid test analysis for LES simulation in OpenFOAM.
I would highly appreciate your assistance. If anyone knows of a good resource such as a video tutorial or thesis on this topic, it would be greatly beneficial.
Thank you all so much for your help.
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Old   May 7, 2024, 04:14
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Grid sensitivity studies are indeed more complex for LES studies since changing the grid size also changes the time step and therefore temporal and frequency resolution, and also the magnitude of the resolved scales and the balance between resolved/SGS energy. Be careful with the latter to ensure that you still maintain adequate/suitable conditions for LES.

I don't have any examples of previous LES grid refinement studies - apologies & good luck.
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