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Jason Mc Beth January 16, 2008 15:52

Boundary Layer question
I was wondering whats the difference betweem first cell height and wall function when meshing the boundary layer. I am only 16 and CFD is my passion. Looking forward to some replies which could help me considerably.

Thanks, Jason

David Stanbridge January 16, 2008 16:52

Re: Boundary Layer question

The first cell height will determine the y+ that you have close to the wall that then has an impact on the wall functions that you use. Some require down to a y+ of one whereas others will be more lenient upto a y+ of 300. y+ is a function also of the fluid properties you have as well as the speed of the fluid over the domain. In effect these should be considered when you are meshing the model.

Jason Mc Beth January 17, 2008 05:51

Re: Boundary Layer question
hey thanks for the reply. How do I know which model needs which boundary layer setup?

David Stanbridge January 17, 2008 06:44

Re: Boundary Layer question
This is contained within the Fluent help files. Do you have these? If not send me your mail address and I will forward some of the info to you.

Jason Mc Beth January 17, 2008 09:18

Re: Boundary Layer question
Thanks mate, my email is jasonmcbeth at

So to get this right I must first know what Re I am operating in and then decide on the turb model. The reason I am asking is that I am trying to mesh a front wing of a race car (made by myself) but I am having trouble with the mesh and fluent bit. Unfortunately my math skills arent there yet to understand partial differential equations, but it would be nice to know how some of you guys would tackle something like that? By the way then is y+ dependent on Reynolds? Cause this would justify the fact that along the chord of my profile y+ increases. Why is that? Only on Reynolds or is there something else?

Anyways, thanks David. ANy help I can get is truely appreciated as I am a total novice, but CFD and aero is my passion.

Sham January 18, 2008 21:19

Re: Boundary Layer question
Yes y+ value is dependent of Re. The higher the Re, generally the higher the y+ value would be, hence you need a finer mesh.

If you would like to measure the forces along your front wing, I'd suggest that you need a fine mesh along the wing profile. How fine is fine? A proper CFD research would do a mesh sensitivity studies on that. To get better results, sensitivities should also be done on time step size, residual values, and also the turbulence models to make sure that your model converges properly.


Jason Mc Beth January 23, 2008 06:11

Re: Boundary Layer question
So what is the difference between y* and y+? Furthermore, can anyone please explain to me what happens when a rotating cylinder is in ground effect? I have been studying stationary cylinder in ground effect, but what about rotating cylinder in ground effect? could anyone enlighten me?

Thanks in advance. Jason.

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