Boundary Layer - dimensionless wall distance y+

MartinRei · October 31, 2011, 15:21

Hello everyone,

first of all, if that is the wrong forum for my question, i kindly ask the admins to correct that, because i was not sure where to post it.

I am currently trying to generate a boundary layer mesh for a laminar flow (Re<10000) over a 2D airfoil.

The question is now, how "high" should the first row of boundary layer cells be in this laminar flow? $y_{1}^+ = 1$ or $y_{1}^+ = 30$ ?

Kind regards,
Martin Rei

Sonil · November 2, 2011, 01:10

Martin

Choice of what value of y+ to take depends on how accurately are you trying to capture the flow in laminar sub-layer in a turbulent flow problem. If you make a mesh with y+ 30 then you will have to perform your simulation using a turbulence modelling with wall scales (this will approximated the effects of laminar sub layer and buffer zone numerically) fora turbulent flow. y+=1 or less must be used if you have enough computing power and you need not use wall scale in this case.

In your case of a low reynolds number flow these things may not matter... Still i would suggest mesh as fine as possible, as a high resolution mesh is always preferable if we have enough computing resource.

MartinRei · November 2, 2011, 04:08

Quote:

Originally Posted by Sonil

Martin

[...] in a turbulent flow problem. [...]

Is it different in a laminar problem? Because the problem at hand is laminar.

Sonil · November 2, 2011, 05:57

See it all boils down to resolving your boundary layer...
I would suggest you to calculate B.L. thickness with respect to chord using formula for calculating B.L. thickness for laminar flow over flat plate and try to have minimum of 20 layers inside that length... and keep the growth ratio of the grid inside 1.15 in the B.L.

Far · November 3, 2011, 01:14

Quote:

Originally Posted by MartinRei

Hello everyone,

I am currently trying to generate a boundary layer mesh for a laminar flow (Re<10000) over a 2D airfoil.

The question is now, how "high" should the first row of boundary layer cells be in this laminar flow? $y_{1}^+ = 1$ or $y_{1}^+ = 30$ ?

Kind regards,
Martin Rei

Y+ is not a flow variable, it is by product of turbulence modeling and therefore it is meaning less in laminar flows. However, in laminar flows you should make the mesh independence study.

Quote:

Originally Posted by Sonil

In your case of a low reynolds number flow these things may not matter...

If the flow were turbulent, then in low reynolds number these things matter alot.

ps. If you refine mesh sufficienntly high, then you may reach the DNS with laminar flow solver !!!!

October 31, 2011, 15:21	Boundary Layer - dimensionless wall distance y+	#1
MartinRei New Member Martin R. Join Date: Oct 2011 Posts: 7 Rep Power: 14	Hello everyone, first of all, if that is the wrong forum for my question, i kindly ask the admins to correct that, because i was not sure where to post it. I am currently trying to generate a boundary layer mesh for a laminar flow (Re<10000) over a 2D airfoil. The question is now, how "high" should the first row of boundary layer cells be in this laminar flow? $y_{1}^+ = 1$ or $y_{1}^+ = 30$ ? Kind regards, Martin Rei

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November 2, 2011, 01:10		#2
Sonil New Member Sonil Singh Join Date: Sep 2011 Posts: 5 Rep Power: 14	Martin Choice of what value of y+ to take depends on how accurately are you trying to capture the flow in laminar sub-layer in a turbulent flow problem. If you make a mesh with y+ 30 then you will have to perform your simulation using a turbulence modelling with wall scales (this will approximated the effects of laminar sub layer and buffer zone numerically) fora turbulent flow. y+=1 or less must be used if you have enough computing power and you need not use wall scale in this case. In your case of a low reynolds number flow these things may not matter... Still i would suggest mesh as fine as possible, as a high resolution mesh is always preferable if we have enough computing resource.

November 2, 2011, 05:57		#4
Sonil New Member Sonil Singh Join Date: Sep 2011 Posts: 5 Rep Power: 14	See it all boils down to resolving your boundary layer... I would suggest you to calculate B.L. thickness with respect to chord using formula for calculating B.L. thickness for laminar flow over flat plate and try to have minimum of 20 layers inside that length... and keep the growth ratio of the grid inside 1.15 in the B.L.