CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > CFX

Y+ for viscous sublayer 2D airfoil

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

LinkBack Thread Tools Search this Thread Display Modes
Old   October 27, 2019, 15:11
Default Y+ for viscous sublayer 2D airfoil
Ferruccio Rossi
Join Date: Jun 2016
Location: Melbourne, FL USA
Posts: 91
Rep Power: 10
frossi is on a distinguished road
Hi all,

I am trying to verify the correctness of my 2D airfoil NACA 0012 simulation by verifying my Y+ value. In particular, I am interested in solving the forces acting on the walls of my airfoil (drag, lift), therefore I am using a k-w SST turbulence model, which the literature says it's ideal for solving forces at the wall in detail.

Since I want a precise result for the forces acting on the airfoil, I want to solve the viscous sublayer, rather than using wall functions. From the literature I was able to find, I understood that by solving the viscous sublayer I can get a more precise result for the forces at the walls compared to using wall functions. Therefore I am aiming for a Y+ < 5. Is this assumption correct for the viscous sublayer? I found cases where the literature was saying that Y+ needs to be around 1, and other cases where the literature says it is acceptable to use Y+ < 5, so I am not really sure which one should I aim for if I want to solve the viscous sublayer.

This said, I refined my structured 2D mesh and ran simulations until I got my Y+ to be between 0.5 and 2 (see picture of Y+ plot), which I believe is sufficient for solving the viscous sublayer.

But I still have the following doubts:
1) See the Y+ plotted along the airfoil chord length (attached picture). Why are there local valleys (local mins) around x=0 and x=0.05? I would expect the plot to be more linear, in the same way as the section from x=0.1 to x=0.2.

2) The residuals are converging down to 1E-06, but as you can see in the attached residuals plot, some of them start oscillating (continuity - black line & y-velocity, green line) even though they don't diverge. Is this acceptable, and if so, what does it mean? I can't figure it out. I don't believe this is an issue, since I think residuals at 1E-06 are sufficiently converged, but please correct me if I am wrong.

3) In light of all these considerations, would you say that my Y+ analysis is correct, and I successfully resolved the viscous sublayer?

I would like to hear your opinions and/or suggestions.
Thank you all for your support!

P.S. I attached pictures of my structured 2D mesh in case you want to have a look.
Attached Images
File Type: png y plus.PNG (25.9 KB, 43 views)
File Type: png residuals.PNG (60.0 KB, 32 views)
File Type: jpg Mesh 1.jpg (77.3 KB, 32 views)
File Type: jpg mesh 2.jpg (141.7 KB, 31 views)
File Type: jpg mesh 3.jpg (86.7 KB, 30 views)
frossi is offline   Reply With Quote

Old   October 27, 2019, 17:12
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,750
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
You appear to be using Fluent and this is the CFX forum. For comments specifically on Fluent you should use that forum, but your question is really a generic CFD question which is relevant to any CFD software.

Firstly, you should not assume that running in the integration to the wall mode (y+ =~1) is more accurate that wall functions. Wall functions work by modelling the effect of the thin layers of the boundary layer without resolving it in the mesh - there is nothing inherently inaccurate about that. If the boundary layer model is accurate then the wall function is accurate.

Rather than target a y+ value you should do a sensitivity analysis to determine if you mesh is fine enough. Not only will this let you know how fine your mesh needs to be to achieve the accuracy you require, but it also gives you an estimate of the error you currently have. To do a sensitivity analysis do another simulation with the mesh significantly changed (I recommend doubling or halving the edge length in all directions) and see what difference it makes to your results. If the difference is an unacceptable error to you then refine the mesh further and continue until you achieve convergence - also known as mesh independence.

Your comment about the convergence flat lining is an FAQ - see:
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote


convergence, mesh, sublayer, viscous, y plus

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
SU2 AOA optimization SU2 9 March 7, 2022 16:17
Ffd_control_point_2d feiyi SU2 4 September 30, 2019 12:42
High drag for airfoil compared to XFOIL and wind tunnel data Ry10 SU2 15 October 30, 2016 17:27
2D FFD Optimization RLangtry SU2 2 August 5, 2014 09:48
Problem with restart solution in robyTKD SU2 Shape Design 21 May 29, 2013 09:26

All times are GMT -4. The time now is 16:03.