Learning Forum

[NickFL]

 

Now you understand why I was encouraging you to not have so many iterations per timestep ;) 😉

The truth is there is no real solution here. The CFL condition, in theory, provides an “easy” way to select a time step. But reality is not that easy. We have a fine grid near the wall and if we use this small y dimension, the time step is tiny. We can have large aspect ratio elements, as the elements can be stretched in the flow direction and we can try and use that dimension. But as the velocity increases, the time step limit shrinks. Then your mesh isn’t always aligned with the flow field, so trying to figure out the velocity vector in terms of your mesh orientation just adds complexity. (I will say, based upon the velocity picture on the other thread, it seems you created a very good mesh–well done)

Somewhere on the internet there is a Best Practices for LES by Florian Mentor (ANSYS Germany). I do not think it is on the ANSYS site (it may be in the Customer Portal somewhere), but if poke around on the internet I am sure you will find it. I would recommend reading that. It will explain the difference between LES and the DDES that the previous study used and how they navigated this roadblock by not doing a full LES. Ideally, we would love to have a CFL under 1, but in practice is just isn’t realistic for real-world problems.

Sorry I do not have better advice.