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asd April 17, 2007 04:47

y+ for Airfoil Analysis
 
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.


ag April 17, 2007 09:38

Re: y+ for Airfoil Analysis
 
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.

asd April 18, 2007 01:07

Re: y+ for Airfoil Analysis
 
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!

Jonas Larsson April 18, 2007 10:54

Re: y+ for Airfoil Analysis
 
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.


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