# Having trouble with understanding of what is y+

 Register Blogs Members List Search Today's Posts Mark Forums Read

November 12, 2015, 10:43
Having trouble with understanding of what is y+
#1
New Member

Anton
Join Date: Oct 2015
Posts: 13
Rep Power: 7
Hi,
I have read lots of things about y+, including ANSYS help, this forum and other CFD-dedicated sites.
But I still have a mess up in my head regarding that value.

"Officially" y+ is a non-dimensional (regarding velocity and viscosity) distance to the wall.
y+ tells me, in which region of the near wall layer I put my first node, if I choose a certain cell size.

So why in Ansys Turbogrid do I set y+ value?

I should calculate an y+ value and then choose a cell size, depending on what I want to model, isn't it?

Or the question here
Quote:
 "Today we will consider three critical questions that are often asked by CFD engineers when developing or refining a CFD simulation: Do I have an appropriate Y+ value and a sufficient number of inflation layers?"

must be reformulated to "Do I have an appropriate 1st cell size with respect to my y+ value?"
Because for the flow with fixed viscosity, Reynolds, velocity, etc.
y+ is also fixed. Is that right?

It looks like some people define y+ as a dimensionless distance, while others (incl. ansys) treat y+ as a 1st cell size.

November 12, 2015, 14:00
#2
Senior Member

Join Date: Feb 2011
Posts: 493
Rep Power: 15
Quote:
 Originally Posted by AntonZ Hi, I have read lots of things about y+, including ANSYS help, this forum and other CFD-dedicated sites. But I still have a mess up in my head regarding that value. "Officially" y+ is a non-dimensional (regarding velocity and viscosity) distance to the wall. y+ tells me, in which region of the near wall layer I put my first node, if I choose a certain cell size. So why in Ansys Turbogrid do I set y+ value? I should calculate an y+ value and then choose a cell size, depending on what I want to model, isn't it? Or the question here must be reformulated to "Do I have an appropriate 1st cell size with respect to my y+ value?" Because for the flow with fixed viscosity, Reynolds, velocity, etc. y+ is also fixed. Is that right? It looks like some people define y+ as a dimensionless distance, while others (incl. ansys) treat y+ as a 1st cell size.
Your choice of y+ value usually depends on turbulence model choosen. Turbulence models have recommended y+ ranges. Next, y+ is related to y (dimensional distance of first mesh node from wall) and flow properties (I think you can easily find formulas). So after you choosed y+ and determined characteristic flow properties you can evaluate y to create your mesh properly.

 November 13, 2015, 01:49 #3 New Member   Anton Join Date: Oct 2015 Posts: 13 Rep Power: 7 OK, lets be more specific. I have a centrifugal compressor, RANS + SST. The initial meshing was done with y+ 200 (Re 1.4e+06). In real calculation in post processing CFX Post shows me y+ = 830. Which mesh size should I set next (in a refined grid), if I don't need viscous sublayer. Should I refine my mesh at all? Last edited by AntonZ; November 13, 2015 at 03:33.

 November 13, 2015, 05:11 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 16,666 Rep Power: 130 You need to do a mesh refinement study and find out. All simulations are different. Some will require a finer mesh, and some will be just fine with large y+ like that.

November 13, 2015, 13:35
#5
Senior Member

Join Date: Feb 2011
Posts: 493
Rep Power: 15
Quote:
 Originally Posted by AntonZ OK, lets be more specific. I have a centrifugal compressor, RANS + SST. The initial meshing was done with y+ 200 (Re 1.4e+06). In real calculation in post processing CFX Post shows me y+ = 830. Which mesh size should I set next (in a refined grid), if I don't need viscous sublayer. Should I refine my mesh at all?
I think that if you don't need viscous sublayer your y+ shouldn't be less than 11. Better >30.

 Tags cfx, turbogrid, wall model, yplus