Hi,

I am modeling a flow through a pipe with a constriction and then a expansion. The Re at the inlet is around 400 and at the constriction, around 4000. It returns back to around 1000 after the expansion. The pressure drop in the flow according to the experiments is around 180, whereas Fluent simulations give a value of around 350 Pa. There is a flow separtion in the pipe during the expansion and the recirculation region in the pipe is around 2.4 cms wheeas experiments indicate a length of around 8 cms.

I am using a k-e model for the simulation.

Is there any way to model such kind of flows where the Reynolds number varies considerably? Any suggestions on what is to be done for this case?

Thanks Murali.

Hi Murali,

I am far from being an expert in turbulence models but my opinion is that your k-e is probably not very well suited to the Reynolds numbers encountered in your study.

Since the transition Reynolds number in pipe is around 2300 and taking into account the fact that the k-e model gives poor predictions at average Reynolds numbers, this could possibly explain the large discrepancy raised by your numerical simulations when compared with the experimental results. What do you think about that point ?

What I know about k-e model is that it gives good predictions when the Reynolds number exceeds 10^5.

Hope it can help,

Regards,

Anthony

How about try to make another run with finer grid, especially around the constriction? In my experience, insufficient number of grid gave me a poor result.

If it doesn't make any difference in the prediction of pressure difference, try simulations with another turbulence models.

Seong Hoon

Hi Anthony,

Thanks for your comments. I tried other turbulence models too, k-w, RSM and realisable k-e, but the results does not seem to vary much. Probably I will try using LES to capture time varying phenomenon in flow.

Regards Murali.

Hi Seong,

Thanks for your suggestions. I refined the mesh once and found that the values do not vary much compared to the coarser one. I will try refining the mesh much more and try it .

Regards Murali.

Hi Murali,

your case is interesting........ I am also analysing nearly similar model ..........but between a range of Reynolds number between 10^5 to 10^7. My model do consists of constrictions ........in the flow field but the intensity is not very much (because of the high flow speed) and ........it is periodic. Thats why I took the .......average diameter of the pipe and used ......same Reynolds number through out the flow. I too like to know, if it is possible .........to model different Reynolds numbers at inlet and outlet and etc.....

paparao madasu.