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September 30, 2008, 03:45 
how to model boundary layer

#1 
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hi people, i am a beginner in cfd. i was given a problem where i have to capture the Boundary Layer effects. prob: flow through pipe. i was given inlet velocity and properties of fluid. i was told to calculate y+ and accordingly mesh near wall. i dont know what is this y+?
In meshing software which i am using, there r attributes like initial size, growth factor, no of layers....etc. i want to know what r these attributes, what do they mean, how to determine these values, what is the procedure i need to follow... can anyone help me out plz... 

September 30, 2008, 08:25 
Re: how to model boundary layer

#2 
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given the velocity > get the Reynolds number > then, get your boundary layer thickness. The boundary layer thickness will help you determine the attributes of your mesh.
You can do a search for 'y+' on here. There has been a lot of discussion of it. From what I am familiar with: initial size is the initial dimension of your grid growth factor is how much this initial dimension changes at each step..ie..1.2x or 1.4x etc no of layers should be how far the growth factor is applied (not sure on that one) 

October 2, 2008, 05:12 
Re: how to model boundary layer

#3 
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thank u very much for that... Mine is a 2d pipe flow. As per Re no, it is coming as Turbulent flow. As i am going to use 'enhanced wall treatment', i have chosen y+ value near to 1. Now, using y+ = y*Ut/nu, i need to calculate y, which is the first grid point location. nu is known from properties. and Ut=sqrt(Tw/rho). But how to estimate this Tw(wall shear stress).......
Am i going in the right direction. If so, plz tell me how to determine the wall shear stress(Tw) 

October 2, 2008, 16:13 
Re: how to model boundary layer

#4 
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for a *very* rudimentary guess at the wall shear stress you can calculate the local Reynold's number at a certain xdistance, get the boundary layer thickness there, and use that as the 'dy', and use the free stream velocity as the 'du'. Just fill those into the shear stress equation. That will be a very rough estimate, but you can at get started from there. This is assuming a linear velocity profile from the wall to the freestream  which is not correct. Still, if I remember correctly, there are better ways to get y+.


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