Outlet Backflow problem in simulating pulsatile flow
 

Dear All,
I am simulating a pulsatile blood flow through a complex tortuous vascular system. I have the inlet BC as a velocity wave form and outlet boundary condition to be zero pressure. For the first cardiac cycle I do not have any problem although the results are not stable and physically meaningful, and when I repeat the solution over some same cycles I got an error that a wall placed on my outlet and see that I have backflow through my outlet!! I am sure that physically I should not have any backflow through my out put so I used an outlet BC instead of opening one. I should also note that following some suggestions I extended my outlet boundary to a further place to prevent backflow but it did not work in my case. Does anybody have any suggestion for my case?
Sincerely,
Ali