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how to calculate y+ in Natural convection??

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Old   February 11, 2014, 00:52
Question how to calculate y+ in Natural convection??
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teguh hady
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Dear all,
I've project in simulation of Natural Convection on Parallel Vertical Plate. On the reference, Y+ value for Natural Convection is 1. My problem is how to set first cell height based on that Y+? Could you tell me Equation of Y+ based on Rayleigh number? Because Most of y+ calculation is based on Reynolds Number.
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Old   February 11, 2014, 04:31
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Hi teguh,

Y+ = 1 means that you will solve up to the laminar region of your BL (no wall functions would be applied). Be sure that you are using the right turbulence model (kind of LowRe).

Regarding equations for Re:

external flow
cell_size_yplus = yplus*viscosity/(sqrt((0.5*0.058*(Re^(-0.2))*velocity^2)));

internal flow
cell_size_yplus = yplus*viscosity/(sqrt((0.5*0.079*(Re^(-0.25))*velocity^2)));

If you assume an very low & reasonably velocity you'll manage to find reasonable value. You can try a sensitivity study to try to understand how critical is your velocity to match the right y+. Bear in mind that you can go over y+1, eventhough solving the laminar BL.

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Old   February 12, 2014, 03:43
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Hello cfdivan,
Thank you for your reply. Could you tell me the reference?
Thank you so much
Best regards
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