y+ structured vs. unstructed grid
 

I have been playing with a simple 2D duct geometry. After the simulation with air flow I can observe that the y+ plot along the duct is smooth curve in case of structured grid, while the unstructured tetra with inflation layer shows a quite rough curve. The fluctuation is approx. 0.05. What is the cause of this and how to handle it?

There are reasons to get this difference:

1. Friction velocity

2. Distance of first cell

It is difficult to tell that which factor is influencing Y+ in the case of prism mesh. I would suspect first cell distance (dimensional)

If everything else in the simulation was exactly the same and the flow configuration was identical, then it could simply be the calculation of the wall-normal distances.

There might be some information in the ANSYS Fluent guide about the calculation of the first-cell distances and this could be the causing the difference. Additionally, you need to make sure that the transition between the prism layer and the first tetrahedral cell is quite gradual and the volume ratios or growth rates are also comparable to the structured, hexahedral mesh.

Hope that guides the thought-process a little bit better.

Thanks for the hints. The trouble was caused by a large volume gradient between the last prism and first tetra.