[Rischa]

Hello Guys,


I try to simulate a simple pipe flow with SST in CFX.
My problem is: I want to achieve YPlus around 1, but until now I can't achieve any value below 10, even if I lower the first layer hight about an magnitude.
I calculated the first layer with 9e-06 m and lowered it down to 1e-07 m (tryed higher values too). At this point my solution stops to converge at about 1e-04 (conv. criteria at 1e-06) and the YPlus value stays at 10-12.
I also tryed structured and unstructured meshing with element size of 1 mm.
My tube has a diameter of 0.02 m and a length of 2 m at an Inlet velocity of 2 m/s. I use water at 20 °C. Sand grain roughness is 0.175 mm.

I get this values only if I use the Ansys Post YPlus value. When I calculate it by my self (as expression) with:


For n I used both Wall Distance and my specified first prism layer hight (0.000009 m).
With this I get values about 0.5 to 1.

Is this a well known problem and I am dumb don't knowing it or has anyone an idea what's the problem in this case?

Thanks for your help.