how to correct backflow in modelling cylinder?
 

Hi,

It can be a model problem, a numerical problem or a mesh problem.
Please could you describe better your simulation, like reynolds number, fluid proprieties, and geometry?

thank you for your attention,
that's a simple grid model, a simple cylinder with an inlet and outlet, but I want to model two fluids at once, one at the right hand of the piston and the other at the left hand of piston, oil as the first fluid at the right and air at the right hand as the second fluid.
I have some experiences in modeling cylinders and a piston pump, but I want to ask about any information or ebook ... to help me know further about backflow, what would cause backflow and how to correct backflow.
this cylinder modelling that I described at first is so important for me, please try to help me, thanks a lot.

Backflow is not a real problem in simulations, depend on the phenomena involved. Example if there is a circulation zone at the outlet, there will be a a reverse flow in some cells at outlet. A way to fix this is increase de outflow lenght, but it is no necessary if you accept the backflow in this area.
I think there is a recirculation zone near to your outflow.

my dear,
but when I display velocity vector results, although there are perfect results that are expected but all the vectors are reversed, and no vector is displayed so that passing out of the boundary.
what would cause the problem?