y+ for EFM or FVM

suktan · September 12, 2022, 22:53

Dose y+ depend on the choice of using FVM or FEM approach?

bigfootedrockmidget · September 13, 2022, 05:27

y+ is just a nondimensional distance to the wall. It depends on the wall shear stress, so the actual value of y+ depends on the numerical solution. The value of y+ at the first node away from the wall is important because it tells you if your mesh resolves the boundary layer sufficiently.
Here's a nice youtube video for you:
difference between y+ and y*:

https://www.youtube.com/watch?v=nSdVaF3JnI0
and also y+ for wall functions:
https://www.youtube.com/watch?v=fJDYtEGMgzs&t=1132s

sbaffini · September 13, 2022, 09:32

So, formally no, it doesn't depend on the method. Y+ is just a non dimensional quantity involving a length, velocity, density and viscosity. It is a Re number based on viscous scales.

However, in practice, at least for the velocity and length scales you use for it, you have a dependence on the method for the same mesh. Because the location where y+ can be practically computed in FEM is different from the one where it can be computed for FVM, which in turn, for certain cases, can also impact the wall stress computation. Finally, even the wall stress might be computed in a formally different way in FEM and FVM.

In conclusion, y+ obviously is a single concept, but you won't probably get the same y+ for the same mesh with FEM and FVM (besides the other obvious differences between the two methods which, where relevant, would make the numerical solution further different)

September 12, 2022, 22:53	y+ for EFM or FVM	#1
suktan New Member Suk Tan Join Date: Nov 2019 Posts: 19 Rep Power: 6	Dose y+ depend on the choice of using FVM or FEM approach?

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September 13, 2022, 05:27		#2
bigfootedrockmidget Senior Member bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 637 Rep Power: 18	y+ is just a nondimensional distance to the wall. It depends on the wall shear stress, so the actual value of y+ depends on the numerical solution. The value of y+ at the first node away from the wall is important because it tells you if your mesh resolves the boundary layer sufficiently. Here's a nice youtube video for you: difference between y+ and y*: https://www.youtube.com/watch?v=nSdVaF3JnI0 and also y+ for wall functions: https://www.youtube.com/watch?v=fJDYtEGMgzs&t=1132s

September 13, 2022, 09:32		#3
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,191 Blog Entries: 29 Rep Power: 39	So, formally no, it doesn't depend on the method. Y+ is just a non dimensional quantity involving a length, velocity, density and viscosity. It is a Re number based on viscous scales. However, in practice, at least for the velocity and length scales you use for it, you have a dependence on the method for the same mesh. Because the location where y+ can be practically computed in FEM is different from the one where it can be computed for FVM, which in turn, for certain cases, can also impact the wall stress computation. Finally, even the wall stress might be computed in a formally different way in FEM and FVM. In conclusion, y+ obviously is a single concept, but you won't probably get the same y+ for the same mesh with FEM and FVM (besides the other obvious differences between the two methods which, where relevant, would make the numerical solution further different)