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first node in turbulence model

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Old   August 20, 2001, 04:10
Default first node in turbulence model
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Franco Belosi
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I'm a beginner with CFD (CFX4.3): in dealing with the k-epsilon turbulence model I've to set the first node at y+>11.Since y+ is a function of the unknown friction velocity how can I estimate the first grid position? Are there any empirical relationships which allow a crude estimate of the friction velocity? Thank you in advance for anybody helping me.
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Old   August 20, 2001, 11:17
Default Re: first node in turbulence model
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Jonas Larsson
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Yes, you can estimate the wall-distance you need to get a certain y+. There is even a nice little free online calculator which will do it for you - you can find a link to it in the Resources/Calcs/Misc section of CFD Online.
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Old   August 21, 2001, 08:20
Default Re: first node in turbulence model
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Herve
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In the manual you will see that you can set a USRSCAL (user scalar) to return the y+ values and plot it.

Regarding u*, for open channel for example you can use the crude assumption that a mean u* = sqrt(g*R*So) where g is the gravitational acceleration, R the hydraulic radius and So the energy gradient.
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