Reversed flow cyclone
 

I am simulating cyclone and getting reversed flow after about 1000 iterations. The boundary condition at the outlet is "Pressure outlet". Initially, the reversed flow occured at the start of simulation. Then, I extended the outlet pipe and reversed flow was eliminated. But, the reveresed flow started again after 1000 iterations.

My question is: Is it fine to ignore reversed flow? My results are converging properly and the reverse flow is occuring only on 50 faces out of 0.5 million cells.