# Turbulence model for low reynolds problem

I'm simulating the flow in a capillary with a sudden expansion. The small capillary has a diamater of 250 µm. The big capillar has a diamater of 1000 µm. My flow rate ist $1\frac{ml}{min}$. Using standard properties of water I have a Reynolds number of 85 in my small pipe. For my mesh I choose one similar to this case: https://openfoamwiki.net/index.php/Sig_Turbulence_/_Dellenback_Abrupt_Expansion. I am using a low-Re mesh. I simulated it laminar first, but it seems unphysical to me, because I couldn't see any vortex, but because of the high expansion ratio one would expect a vortex after the sudden diameter change. Now I'm using k-omega-SST and getting similar results (no vortex at all). My simulation is converging well.

Do I have to use a special turbulence model for these situation? Or am I wrong in expecting a vortex in this situation?

Best Regards, Gesetzt

• When I used Phoenics, there were several turbulence models - so you have to find out if one that Opefoam has is relevant - I have not used openfoam... – Solar Mike Jan 30 '18 at 8:37
• First up, what type of CFD analysis are you doing, RANS? URANS? Given your description it isn't LES. Inferring that this is either RANS or URANS, do you understand the difference between each case? – Petrichor Jan 30 '18 at 8:42
• I'm doing it with RANS, I'm using the simpleFoam-Solver. I know that URANS is for unsteady simulation. Do you think I would need URANS in my case? – Gesetzt Jan 30 '18 at 9:28
• If you want to be sure that you are capturing unsteady flow phenomena you need to run some sort of transient simulation, not RANS. URANS can do this, providing the time period for your simulation is long enough to capture the effects, this time period is often not easy to determine. I would argue URANS is not truly unsteady, since it is still using time averaging, but the time averaged quantities vary across time steps. LES and DNS are truly unsteady, but are too computationally expensive for many scenarios. – Petrichor Feb 1 '18 at 17:07
• @Gesetzt from how I understand your set-up I think RANS is sufficient. I suspect you want to see a kind of annular vortex after the diameter-jump. similar to a backward facing step, right?! In order to help you it would be great if you could show some screenshots. If possible I would like to see: 1) your setup, 2) a stream-line-plot, 3) a plot of the turbulent viscosity – rul30 Feb 2 '18 at 13:17