stepped pipe flow
 

I am using Floworks to model what seems like a relatively simple problem: flow through a pipe that suddenly increases in diameter. The flow is from the small diameter section to the large. I know there is reverse flow, the amount and velocity of which depends on the combination of diameters. Floworks gives data for net flow, but I want to know what proportion of the flow is in the reverse direction as well as the velocity of that flow.

Can anyone offer suggestions about how I might solve this problem?

Thanks,

Tom