Learning Forum

[dippa399]

I have been trying to verify and compare my analytical and numerically predicted y+ values for turbulent fluid flow through a 2D backward-facing step problem (Re_hydraulic_diameter ≈ 1e5) based on the wall modelling approach of using a wall function such that my first grid cell lies in the range: 30 < y+ < 300. Based on the formula provided in the ANSYS Customer Training Material for Lecture 6 Turbulence Modeling, released around December 2010, the calculation of first cell height is based on the normal distance from the wall, almost equal to twice the cell-centroid position, however, the same formula has been used in different calculators to estimate the near wall cell size based on the cell-centroid position. Mathematically, the difference in estimation of y+ calculations can be expressed below as:

As per Lecture 6 Turbulence Modeling of ANSYS Customer Training Material:

[Karthik Remella]

Hi I'd stick to the formula shared in the post. And, the y value you are looking for the cell-centroid position (your first cell height would be twice this obtained value). The formulation takes into account the cell centroid height as we generally use the Finite Volume Method in CFD. If you are attempting to resolve the viscous sub-layer, your y+ value should be close to 1. If you intend to use the wall function approach, then you can have a larger y+ value (y+ ~ 30).
Hope this clarifies your question.
Karthik

[dippa399]

Hi Karthik Thanks for replying.
So if I am getting your statement correctly, you are suggesting the y+ version of the formula for the cell-centroid position over the y+ for the first layer height, right?
Kind Regards Diptesh Pawa

[Karthik Remella]

Yes, it calculates the height of the cell-centroid.
Karthik

[dippa399]

Thank you so much Karthik for the clarification. I also understand your justification in the previous comment now and had a similar thought process. However, due to the many different representations of the same formula, I was a bit confused.