Cd or drag coeeficient converges and then diverges
 

Hello All,
Please find below the plot for Cd for Flow past cylinder (RE=3900, RANS type SST k omega), it seems to stay steady for a while and then starts diverging and oscillating. My residual limits are low and I am PimpleFOAM as a solver. Has anyone experienced anythign like this before.

Looking forward to some comments.

As far as I am corcened, at that Reynolds number you should be obtaining an unsteady solution. Did you try to run your simulation for longer times? How is your mesh and y+?

Correction - Re= 3900 and I have a decent mesh or I belive..my Yplus obtaine dfrom Yplus RAS is less than 6 and I am not using any wall functions.
I have already run this simulation for about 60 seconds. I am sorry but when you say unsteady is it okj to assume that these fluctuations are normal i.e is it ok to see that divergence but again it is not about a mean value you see.

Your solution is not stable yet. You should have one solution like the one I have in attachment (forget about the cd because it's for another Re, but the development for your case will be similar)

So I would say:

Let it go....

on.

Continue the test.