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February 18, 2000, 15:37 
The length of the first node

#1 
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Hi! guys,
Could you tell me how to define the length of the first node to the wall, if the flow is laminar or turbulent? Thanks. Jennie 

February 21, 2000, 11:29 
Re: The length of the first node

#2 
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Hi Jennie, What you mean by the "length", I think you mean the length normal to the wall. If this is true, in the laminar flow problems this length is not important but in turbulent flow problems it is very important and it is a different story. Good luck =Ahmed=


February 22, 2000, 09:24 
Re: The length of the first node

#3 
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Hi Jenny,
I think your question is concerning the grid size of first cell adjacent to the wall? Turbulent flows: So for turbulent flow you should have a look to the definition of the y+=(density*u_tau*yp)/dyn.viscosity. u_tau is the so called friction velocity defined by sqrt(tau/density). For a pipe flow there is a relationship for u_tau/u=6.99(u_tau*R/kin. viscosity)^1/7 (R=pipe Radius, u=mean velocity) 1/7Power law which can be generalized to a 1/nlaw for different Renumbers (e.g. see Blevins "Handbook of apllied fluid dynamics" for the relations between the friction coeff. lambda (or. s.t. called f) and n=1/sqrt(f), f=0.316/Re^0.25. So solve for u_tau and put it into the def. of y+. Then you you get an equation for y+ as function of y which is the distance of the cell center (!!!) from the wall. Then you are able to estimate y+ even for nonpipe flows. Just exchange the Radius with the half of the hydr. diameter of your problem. It works fine. The y+value which suits you fine depends of course on the turbulent model and wall model you want to use. The y+ value is correct only for th full developed flow region. Laminar flows: have a look to the Fluent User's Guide p514 (fluent 5) or 523 (1996). For a fully developed pipe flow with Radius R choose: y/R<=0.1, for a flow between two plates choose y/H<=0.05 in order to resolve the flow profile correctly, that means that the stepwise linear approximation is not to far from reality. Unless the pressure loss due to shear stresses (or perpendicular momentum transport) will not be ok. Good luck 

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