What y+ value shall we choose for laminar flow?
Hi,
May I ask what y+ value shall we choose for laminar flow? Is that the same as for turbulent flow? Could I still use the calculator given by this forum? (http://www.cfdonline.com/Tools/yplus.php) Thank you very much. lnk 
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viscoussublayer
The y+ value is important for the turbulentmodel. It defines the height of the viscoussublayer where a laminar flow near the wall in turbulent flows occurs. I'm not a specialist but i would say it doesn't matter how big the height of your first cell is because there isn't a turbulence model.
regards 
hi the y+ is only important for turbulent flow because it used in turbulent flow.
boundary layer is three region in turbulent flow they are sub layer , buffer layer & log layer. velocity is linear in sub layer and it's logarithmic for log layer. if y+ was below 5 then the first cell is in sub layer. velocity for first cell obtain linearly if y+ was larger than 30 the the first cell is in log layer . velocity for first cell obtain logarithmic. however boundary layer of laminar flow is one region and it's velocity is defined we do'nt need y+ for laminar flow 
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Consider the example of the plane channel flow. Then,
y+ = u_tau *y/ni = (u_tau *H/ni) * (y/H) = Re_tau* y' being y' the nondimensional position along the vertical direction. Assuming that y'=1 in the halfheigh of the channel, as in laminar flow one assumes Re_tau=O(1), you see that y+ will be very very small close to wall and will be O(1) in the halfheight. 
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Thanks for your answer. But in order to resolve laminar boundary layer well, we still need a lowest wall cell width criteria, right? For example, what wall cell width would you choose for laminar pipe flow? 
calculate first cell height based on boundary condition

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Could you briefly give a short example of how? Thank you very much. 
You can calculate the maximum displacement of the boundary layer from the Blasius solution and then you can figure out how many points you need in order to cover this (with a geometric distribution let's say) after you decide your initial ds. Usually, the initial ds should be small enough, let's say of order 10^4.

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I usually take 3 orders of magnitude less the boundary layer thickness and then take half that height if I want to be conservative. El. K. suggested 10^4 which should also work. You just need to estimate the boundary layer thickness somehow (can always resort to Blasius solution). 
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Laminar flows have no turbulence models and hence have none of these constraints on y+ for the first cell. What is required for the first cell and every other cell in the grid is that the gradients in the flow are adequately resolved. This is a function of the discretisation scheme and how well the gradients need to be resolved at the cell location. For example, a fully developed flow in a pipe may require only 1 cell to be fully resolved with a higher order numerical scheme or tens of cells for a low order scheme. A resolution criteria based on y+ is going to be rather limited in applicability compared to a normal one based on gradients of the solution variables. 
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