[JoSchl]

Hello,

I read about the Yplus value and its meaning, but now I have no idea how I can calculate my first layer thickness which I have to set for my meshing.
I am working with Ansys AIM and doing a steady fluid flow car simulation (Picture). I am using the SST turbulence model, so my y+ needs to be <1.
I already found the https://www.cfd-online.com/Tools/yplus.php calculator but I do not know the valuesof the "Boundary layer length".
So my question is, how I can estimate a value for the "boundary layer length". for the upper calculation. My windtunnel is 25*5*7[m] with symmetry, the car inside is 4.5*1.3[m].
Furthermore, which growth rate and maximum number of layers are recommended for such a simulation?

Thank you.

[dinhanh]

Hi,
You can use this web for pre-estimate y+ value.
https://www.computationalfluiddynami...t-cell-height/

Also, for SST k-omega model, you dont need y+<1. Look at the Menter's paper about this turbulent model.

[plucas]

Y+ does not need to be 1 at all; and if so, your first cell height would be tiny. You also should check y+ after the simulation as the value will change across the car's surface. Not sure if you can post-process y+ in aim as I'm a Fluent user

[piu58]

The dimensionless wall distance y+ results from the boundary layer theory of Ludwig Prandtl. According this theory, the flow in the near of a surface is similar at every boundary layer. It gets (nearly) equal (instead of only similar) if you transform the wall distance and therewith the velocity profile to a dimensionless unit system. The system depends on the free flow velocity and the fluid properties (rho and nu).

Being (nearly) equal a boundary layer is an ideal place for modelling it instead of simulating the details in the boundary layer (which would / should lead to a result well known).

[JoSchl]

Thank you all for your replys and help!

Ok so when y+ not needs to be <+1, I would work with default settings for my boundary layers, that means "smooth transition with maximum 10 layers and 1.2 growth rate (pic attached).
In Ansys AIM it is possible to get a yplus contour, but I am not sure how to evaluate the result and if I need a more tiny first layer. I attached a picture of YPlus contour, any statements?