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mesh quality checking for large internal flow (yStar)

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Old   November 21, 2014, 08:49
Default mesh quality checking for large internal flow (yStar)
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Hi foamers, I'm working on a large internal flow case. I'm walking step by step on preliminary models: a simplified geometry, no account for temperature and so on. I've adopted the simpleFoam solver and the \epsilon-k model with near-wall treatment. I'd like to check the quality of my mesh, so I'm back to the very old and debated question about y+ and y*. I've been far from OpenFOAM in the last two years, maybe I'm not updated about last changes and enhancements.

1. I set the nutkWallFunction. In the nutkWallFunctionFvPatchScalarField.C file I read
Code:
yPlus = Cmu25*y[faceI]*sqrt(k[faceCellI])/nuw[faceI]
and that defines y*.

2. The yPlusRAS utility outputs the min, max and average y* value on the wall patches.

3. y* is different from y+, but in the inner boundary layer log-law region they are similar and I can use the standard OpenFOAM utility.

4. Anyway, I read from FLUENT Theory Guide that
Quote:
The lower limit always lies in the order of y*~15. Below this limit, wall functions will typically deteriorate and the accuracy of the solutions cannot be maintained. The upper limit depends strongly on the Reynolds number. For very high Reynolds numbers (for example, ship, airplanes), the logarithmic layer can extend to values as high as several thousands, whereas for low Reynolds number flows (for example, turbine blades, etc.) the upper limit can be as small as 100. For these low Reynolds number flows, the entire boundary layers is frequently only of the order of a few hundred y* units.
I wonder - is there any experienced user in large internal flow? - if the y* range of my simulation is good enough. I'm analyzing the air flow and pressure distribution in a large geometry, including a ship engine room with funnel and other connected space (the blockMesh background mesh is about 30 m x 25 m x 30 m). I must meet the meshing requirements and my hardware configuration limit and I obtained a 4-4.5M cells mesh with
Code:
Patch 12 named domain y+ : min: 22.09039 max: 657.0444 average: 197.6188
In your opinion, is the mesh good enough?

Thanks for sharing your experience.
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