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Y+ value for logarithmic region

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Old   January 15, 2020, 17:44
Default Y+ value for logarithmic region
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Hi all. I am doing simple CFD simulation flow over spherical body to understand the concept of y+ and boundary layer.

I have a simple question related to log-law layer. I read that for this region y+ value should be between 30 to 300 range. Now, how I would know that for my current simulation y+ value 35 is better or 50 or something higher value?
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Old   January 15, 2020, 18:04
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First, y+ is a fancy coordinate system. Do not confuse this with the more popular wall y+, which specifically refers to the y+ of the cell centroid of the wall adjacent cell. This wall y+ is the y+ of the cell nearest the wall.

The log-layer is indeed the region where the y+ is roughly between 30 and a few hundred. For just about any wall-bounded flow with a boundary layer, you can find this log region no matter if the BL is only a few millimeters thick, a kilometers thick, or millions of kilometers thick.

So given any flowfield, you (in theory) should be able to calculate the y+ field. That means you have already solved your problem (i.e. run your simulation first). You can then go to the cells nearest the wall, and find the y+ of just those cells. Then you can inspect the value if it is 30, 50, 100, or whatever. y+ is determined a posteriori, not a priori.
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