first node in turbulence model

August 20, 2001, 04:10

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.

August 20, 2001, 11:17

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.

August 21, 2001, 08:20

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.

August 20, 2001, 04:10	first node in turbulence model	#1
Franco Belosi Guest Posts: n/a	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.

August 20, 2001, 11:17	Re: first node in turbulence model	#2
Jonas Larsson Guest Posts: n/a	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.

August 21, 2001, 08:20	Re: first node in turbulence model	#3
Herve Guest Posts: n/a	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(gRSo) where g is the gravitational acceleration, R the hydraulic radius and So the energy gradient.

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