CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

y+ for Airfoil Analysis

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 17, 2007, 05:47
Default y+ for Airfoil Analysis
  #1
asd
Guest
 
Posts: n/a
I am validating airfoils against experimental solutions. I have used the RANS k-w SST model and based on my initial findings, agreement between experimental and CFD data in CL and CD is between 3 and 4% for a NASA Low Speed Airfoil and 4 and 7% for NACA 0012 over an angle of attack range of 0-12 degrees. My test case is based on a chord Reynolds number of 6 million and Mach number of 0.32.

Now I am still very confused on the y+ requirement. It has been suggested that Y+ should be 1 for an enhanced wall function, however my validation for the two airfoils tested indicates Y+ of 50-300, yet I obtain relatively "good results" ( I am not sure whether I got lucky or whether I have a valid grid set up). I have tried to achieve a y+ of 1 through various meshing techniques in GAMBIT, however I have been unsuccessful, as this requires excessive number of grid points on the Airfoil surface to ensure cell aspect ratio is between an allowable limit and not overly stretched.

Based on my operating requirements of Reynolds and Mach number, is a y+ of 50-300 suitable for k-w SST analysis? My results (which are limited only to two airfoils at this stage) would indicate yes, but I am not convinced whether this is in fact a correct methodology.

How can a y+ of 1 be obtained for airfoil test cases for DES analysis (which I intend to use to capture the aerodynamics of high angles of attack) within the boundary layer, whilst considering the aspect ratio of the cells are of an allowable limit for analysis within FLUENT.

Any assistance will be much appreciated.

  Reply With Quote

Old   April 17, 2007, 10:38
Default Re: y+ for Airfoil Analysis
  #2
ag
Guest
 
Posts: n/a
The whole point of using wall functions is to relax the spacing at the wall, so that you can use a y+ of 50-100. If your wall functions require you to use a spacing of y+=1 then your wall functions are not doing you any good. Who has suggested that you need a y+ of 1 with wall functions? Of course all this is predicated on the assumption that the wall functions in your code are working properly - not all wall functions are created equal.
  Reply With Quote

Old   April 18, 2007, 02:07
Default Re: y+ for Airfoil Analysis
  #3
asd
Guest
 
Posts: n/a
Hi, It has been suggested that for Enhanced Wall Functions with the k-w model, it is necessary to formulate the law-of-the-wall as a single wall law for the entire wall region such that the laminar sublayer, buffer region and the fully turbulent region can be represented. The manaul inidcates y+ values between 3-10, yet 1 is suggested as ideal.

Now for an airfoil analysis, trying to maintain a y+ of 1 with aspect ratios within the BL being close to 1 (a maxiumum value of 10 can also be used according to FLUENT), reuslts in excessive number of grid points (in excess of 10,000 on the airfoil surface alone!!!). I am using GAMBIT by specifying the interval size on the airfoil surface such that it produces an AR of 10, yet results in too many cells. Is there an another approach whcih i could consider in trying to duplicate a y+ of 1, whislt ensuring an allowable limit of AR???

thanks for any help!
  Reply With Quote

Old   April 18, 2007, 11:54
Default Re: y+ for Airfoil Analysis
  #4
Jonas Larsson
Guest
 
Posts: n/a
Perhaps you are mixing enhanced wall treatment with the nonequilibrium wall functions in Fluent? The wall functions should not have y+ below 20, whereas the enahanced wall treatment should have y+ around 1 in order to work the best However, the enhanced wall treatment has the ability to switch to wall functions if y+ is too high.
  Reply With Quote

Reply

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
Modelling the cross aisle direction of a steel selective rack for model analysis m8rix ANSYS 0 September 20, 2011 11:28
multifield problems on ia64 zegtuhetmaar ANSYS 7 October 21, 2010 14:18
3D analysis of Ahmed body Irshad22 FLUENT 0 December 17, 2009 05:33
What exactly is a sensitivity analysis? Josh CFX 3 August 19, 2009 10:18
Short Course: Computational Thermal Analysis Dean S. Schrage Main CFD Forum 11 September 27, 2000 18:46


All times are GMT -4. The time now is 01:33.