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Y+ model and turbulence models

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Old   August 1, 2021, 14:22
Default Y+ model and turbulence models
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Brett
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Hi all,

This has probably been asked before.

I understand what the y+ value is, but what I'm hoping to get some clarity on is how does turbulence model selection affect the required y+ value of the first cell?
Building on that, what are the different types of wall functions and when are they applied? to my knowledge wall functions are applied to model, not resolve, the physics below the first cell. So if you're first cell is in the buffer layer then a wall function could model the flow closer to the wall even though you don't have cells close enough to resolve that.

Any thoughts most welcome.

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Old   August 1, 2021, 17:59
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Quote:
Originally Posted by Bdew8556 View Post
I understand what the y+ value is, but what I'm hoping to get some clarity on is how does turbulence model selection affect the required y+ value of the first cell?
The issue at hand is rather historic and not relevant to modern CFD. So-called high reynolds numbers models simply don't support the viscosity affected region and you should never go there. Then there are low-Re models which are only good in the viscosity affected region, and you should stay there and never ever try to use them far from walls. In this low-Re case, your first cell y+ is not restricted, your greatest y+ is. The reason it's not relevant anymore is that you actually can rarely use either of these models nowadays unless you write your own CFD code. There's also models like k-omega which was always designed from the start to require small y+ (if you've read more than 6 pages of Wilcox's book, he makes it very clear). The popular implementations of turbulence models nowadays are good everywhere, all the time. Hence, the general meta nowadays is go to for small y+'s.

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Originally Posted by Bdew8556 View Post
to my knowledge wall functions are applied to model, not resolve, the physics below the first cell.
Regardless of what you are doing, every transport equation needs boundary conditions (at walls, duh). Wall functions are just really really fancy boundary conditions for your turbulence transport variable (e.g. wall boundary conditions for k and epsilon).

Wall functions never resolve anything, they're merely boundary conditions. The physics are dictated by the physics equations (the navier-stokes equations and the turbulence model being used). Whether or not the physics are resolved depends on those equations and whether the grid provides the resolution needed to resolve them. Wall functions always model what happens underneath the first cell; because, the physics certainly can't, there's no grid there! It's inaccurate to say the wall function can model the flow near the wall below the first cell, it MUST or you will have the wrong solution.

You can't really talk about a wall function without first declaring what turbulence transport equation you have that it is meant to provide boundary conditions for.
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