3

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

Nut

U_Centerline

Geometry

Best Regards, Gesetzt

Gesetzt
  • 93
  • 4

2 Answers2

1

I solved my problem. I refined the mesh. Part of the problem was the high aspect ratio of my cells, because my tube is very thin in comparison to the length. I have solved the problem laminar and then take this as a starting value for my turbulence model. My results are physical now and the flow is fully developed some time after the expansion. Thanks to everyone who has contributed.

Gesetzt
  • 93
  • 4
0

It is likely, in my opinion, that you cannot clearly observe any vortex due to the RANS nature of your simulation. You are seeing a time averaged view of the flow across a single time step. I like this description of RANS in layman's terms:

enter image description here

Let us think about a concept of a time-averaged field calculated by RANS. Imagine a lot of people crossing a diagonal crosswalk in central city at a busy commute time. It may be hard to follow the movement of each person if you look down from the top of a building, but you can tell that most people are walking from the subway station towards the sidewalk on the opposite side. This general view is the “time-averaged field”. That is to say, in RANS, you would not be conscious of detailed movements or routes of each individual; instead, you would calculate how many and in which direction people cross in a fixed amount of time. Nevertheless, time-averaged fields are affected by turbulence as the nature of flow.

You are seeing a general view of your flow field (influenced by turbulence and therefore by your choice of turbulence model), if you want to be sure of capturing the detailed movements, such as a vortex, you may wish to try URANS (although be careful to chose a total simulation time that is long enough to capture everything - URANS still uses time averaging in a given time step) or if you have a very nice PC (and lots of time) LES would do a nice job - maybe overkill for this problem though.

Petrichor
  • 518
  • 1
  • 8
  • 22