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henry May 24, 2002 08:14

y+ calculations

Does any one now how to calculate the distance a near wall cell should be ,if I know the flow details, and I want to maintain the y+ between 30 and 100 as per CFX.

please help.


cfd guy May 24, 2002 08:41

Re: y+ calculations
In TASCflow and CFX-5 the yplus values at walls are stored by default in the results file. In CFX-4 you only need to add three lines in your command file to see these values under the post-processor.
Regards, cfd guy

Robin May 24, 2002 10:36

Re: y+ calculations
Actually, TASCflow will only output yplus values if the BCINFO = t flag is set.

As an initial guess before you run for the first time, there are correlations for boundary layer development over a flat plate. Look up section 4.4.4 of the TASCflow doc for details, or Mathmatical Models and Solution Algorithms > Modelling Flow Near the Wall > Guidelines for Grid Generation section of the CFX-5.5 doc (see math_models.pdf).

Regards, Robin

Michael Bo May 27, 2002 08:45

Re: y+ calculations

henry May 27, 2002 08:47

Re: y+ calculations
hi Thanks for the info,

but I wanted the formula to calculate the Y+

from the flow variables like Re ,etc


henry May 27, 2002 09:01

Re: y+ calculations

Could some one give the formula please.


Patrice Jaumouillie May 28, 2002 08:41

Re: y+ calculations
y+, the dimensionless distance from the wall, can be calculated from the formula :


where rho and mu are respectively the density and the dinamic viscosity of the fluid, y is the distance from the wall and u* is the shear velocity.

The three regions are (may vary from different authors) :

The viscous sublayer 0<y+<5 The intermediate zone : 5<y+<30 The turbulent zone : 30<y+<500


Robin May 30, 2002 16:43

Re: y+ calculations
Have a look at the documentation, it's in there.


henry June 3, 2002 03:38

Re: y+ calculations

Patrice thanks soo much ,

do you know how the shear velocity is calculated.

thanks again


Min-Hua Wang June 27, 2002 20:48

Re: y+ calculations
Hello, All:

I am very interested in this topic. I am using CFD-4 now. I am trying to export y+ data from the simulation.

It seems to me that one can obtain some meanfull y+ values as the wall is still (not moving). Do I still expect the correct y+ values from the simulation with a standard wall function if the wall is in moving ( bassically the wall is rotating in my study)?

Min-Hua Wang

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