Learning Forum

[rodriguez]

Hello,

I have to make two simulations of a 2D elbow. One with roughness and one smooth. I am told to make two meshes, one for smooth and one for the rough surface, which I don't understand.

I read about the logarithmic law shifting down in the u+ y+ chart due to the higher wall shear, and saw the problem with the connection of that logarithmic law with the buffer region and the viscous sublayer region.

Based on that, I understand the first cell should always be in the log region, right?. That makes the roughness not being available in the KE model with Enhance Wall Treatment understandable.

Now, if I use SSTkw or the RKE with Scalable Wall Function (recommended wall function), and I am in the log law region with the first cell in the smooth case, why might the mesh not be good enough for the rough case? The only possibility I see is that due to the wall shear increase the y+ will increase and go out of the chart, so to say, being away fromt the log law too. Is that thinking correct?

On top of that, I have seen the "virtual wall shifting" approach that states that y⁺=y⁺ + Kr⁺/2. Now, I don't understand what the difference between the y⁺ on the left side of the equation and the one on the right side is. Is the one at the left the one used by Fluent to calculate the wall shear at the wall and the other one the one with the real y of the mesh?? Also, using wall functions makes it more intuitive, but, what is in general going on in the SSTkw model, where no wall functions apply?

Any help would to help me understand. The Fluent help didn't really help...

Thanks and regards.

[Rob]

There are a couple of courses on turbulence and near wall effects, look under Learning on here. 

The wall roughness models are there to work with the wall functions & turbulence models. You then have to balance the near wall cell height with the roughness: how valid is the assumption if the near wall cell is smaller than the roughness height? The manual may or may not clarify the settings, but as staff we're only able to point you at that. I think the topic is covered in more depth in the Ansys Learning Hub, but you'd need a subscription to access that - have you asked whoever set the exercise for more information?