# Wall function's solving procedure.

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March 5, 2013, 04:15
Wall function's solving procedure.
#1
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Dongyue Li
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Hi guys:

In Wall functions. there are lots of functions as follows.What confused me is how to solve this bunch of functions?I know the main idea of wall functions. but how to solve it? Thanks.
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 1.jpg (8.1 KB, 18 views) 2.jpg (4.8 KB, 11 views) 3.jpg (3.1 KB, 10 views) 4.jpg (5.1 KB, 8 views)

 March 5, 2013, 12:16 #2 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 199 Rep Power: 6 Do you mean how to calculate y+ prior to the simulation of the flow, when you create the mesh? Estimate the shear stress (by calculating the skin friction coefficient with one of the many formulas) then calculate Ut and then you can solve for y+. __________________ Lefteris

March 5, 2013, 19:52
#3
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Dongyue Li
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Quote:
 Originally Posted by Aeronautics El. K. Do you mean how to calculate y+ prior to the simulation of the flow, when you create the mesh? Estimate the shear stress (by calculating the skin friction coefficient with one of the many formulas) then calculate Ut and then you can solve for y+.
Hi Lefteris,

Yes, Are you sure about estimating the shear stress?

If I have the shear stress,I have the friction velocity,but if I want to know y+,I dont know k, how can I get k?

And seems according to this formula,you can caculate y+ only by y and k.

Thanks.
Attached Images
 1.jpg (4.0 KB, 5 views)

 March 6, 2013, 07:30 #4 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 199 Rep Power: 6 Hello Forrest, (Cμ^0.25)*(k^0.5) is the shear velocity. However, I usually use the formula y+=(ρ*Ut*y)/μ which is the same with the one you use but I estimate the shear velocity through the shear stress and therefore I don't have to use the k. __________________ Lefteris

March 6, 2013, 21:44
#5
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Dongyue Li
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Quote:
 Originally Posted by Aeronautics El. K. Do you mean how to calculate y+ prior to the simulation of the flow, when you create the mesh? Estimate the shear stress (by calculating the skin friction coefficient with one of the many formulas) then calculate Ut and then you can solve for y+.
Okay, Thanks, I got it.

First I need to caclulate the skin friction coefficient by moody picture and other things, then I get shear stress. according to shear stress,I have shear velocity,and I have yplus.

After I have yplus I can get k and U ,epsilon in the P node which can be treated as boundary conditions. but how can I get pressure in node P?

I think if I have k,epsilon,U,P,I can integrate the KE model in the main flow and get the solution.

 March 7, 2013, 06:10 #6 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 19 You take all unknown values from the iteration before, or from the initialization. You can not know all of them at the same time, because then you would know the result... __________________ The skeleton ran out of shampoo in the shower.

March 22, 2013, 07:33
#7
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Dongyue Li
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Quote:
 Originally Posted by Aeronautics El. K. Hello Forrest, (Cμ^0.25)*(k^0.5) is the shear velocity. However, I usually use the formula y+=(ρ*Ut*y)/μ which is the same with the one you use but I estimate the shear velocity through the shear stress and therefore I don't have to use the k.
Hi,

I attach my procedure, I dont know if its rite...still I am confused about it.

To Rodriguez:

I dont know where to start my iteration ...
Attached Images
 2.jpg (50.6 KB, 11 views)

 March 22, 2013, 09:33 #8 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 199 Rep Power: 6 Hello Forrest, You don't have to make an assumption for Uτ since you can compute it from the formula Uτ=sqrt(τw/ρ), where ρ is the density and τw the shear stress which can be computed from the formula τw=Cf*(0.5ρU^2). In order to estimate Cf, choose a formula from here: http://www.cfd-online.com/Wiki/Skin_...on_coefficient __________________ Lefteris

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