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?

[![Flow Field](https://i.stack.imgur.com/RRBdu.png)](https://i.stack.imgur.com/RRBdu.png)

[![Nut](https://i.stack.imgur.com/36pRm.png)](https://i.stack.imgur.com/36pRm.png)

[![U_Centerline](https://i.stack.imgur.com/gxtpq.png)](https://i.stack.imgur.com/gxtpq.png)

[![Geometry](https://i.stack.imgur.com/QnO2o.png)](https://i.stack.imgur.com/QnO2o.png)

Best Regards,
Gesetzt