[maddalena]

Hello everybody,
for the first time I am dealing with pipe flow and I need some ideas and suggestions on how to set up my case properly.

I have a closed loop cooling system, where air passes through some pipes of different diameters: the smallest one has a diameter of some millimeters, while the largest one is of the order of meters, as shown in the attached picture. Air is moved by a fan placed in the mean diameter section. My main objective is to calculate the pressure drop in the system.

Here are my questions:

1. I assume the flow is turbulent, but the local Reynolds number is low due to the smallest pipe. What is the suggested turbulence model in this case? I read somewhere that SpalartAllmaras can be ok, but I never used it so I was thinking to use a low-Re k-epsilon approach, as LaunderSharmaKE. Is it correct?
2. If it is, ok, then I should design my mesh accordingly. Since I want to keep my cell number low, then I was thinking of using the newly nutLowReWallFunction, so I am not forced to have y+<1 as a low-Re mesh usually requires. Thus, I would generate my mesh with a prism boundary layer with y+>1 and use that nutLowReWallFunction. Is it correct?
3. In that case, what wall boundary condition should I use for k and epsilon? zeroGradient, as I usually do for a low-Re y+<1 mesh, or fixedValue $internalValue as I do for a high-Re 30<y+<100 mesh?
4. And what about k and epsilon internal value? Should I use the one that are usually prescribed for the inlet, even if my system is a closed loop?
5. I guess that for fvSchemes and fvSolution I can use what I am used to do:

grad         faceMDLimited Gauss linear 0.5;
div         Gauss linearUpwind cellLimited Gauss linear 1;
laplacian   Gauss linear limited 0.5;

or are there any other suggestions?

These are only of the few questions that are running into my mind... Looking for someone that can shed some light on the subject.
Regards,

mad