Is it really important Y+??

Freeman

Hi all,

I am trying to understand how really affects the mesh in the solution for a well-known problem like the flow past a sphere (D=0.1m) I am performing series of simulations, all at Re=1.4e4 and where the Cd should be 0.4, according to several experimental data also performed at Re=1e4.

The fact is that I am not able to see a logical correlation between the Y+ and the accuracy in the solution depending on the turbulence model deployed in the simulation. Let me show you my little results:

===Simulation 1===

-Velocity inlet: v=1m/s

-Turbulence Int=0.1%

-Hydraulic diam=4m

-Mesh: boundary layer composed of 4 prisms, first row is 3mm thick and the other 3 grow geometrically at a ratio of 1.2

-Solutions:

k-e Realizable, QUICK, Std wall functions -> Cd=0.303

k-w SST, QUICK -> Cd=0.357

Y+ in both cases goes from 2 to 18, which is not suitable for Std. wall functions, as showed in Fluent manuals (well-known recommendation of 30<Y+<300)

===Simulation 2===

-Velocity inlet: v=1m/s

-Turbulence Int=0.1%

-Hydraulic diam=4m

-Mesh: boundary layer composed of 10 prisms, first row is 0.5mm thick and the other 9 grow lineally with a slope of 0.5 up to a thickness of 15mm

-Solutions:

k-e Realizable, QUICK, Std wall functions -> Cd=0.363

k-e Realizable, QUICK, Non-equilibrium wall functions -> Cd=0.392 (convergence is difficult)

k-w SST, QUICK -> Cd=0.51

Y+ in both cases goes from 0.5 to 2.5

I don't really see that following the criteria of 30<Y+<300 the results would be the best. What do you think?

My headache and worry now is to understand and be able to make simulations that are mesh-independent. How can I rely on my mesh? Do you know literature on the internet or book where I can read about this issue of mesh-independency solutions?

Thanks a lot for your opinions in advance!