Pulsatile flow for global mass conservation
 

I have a question for pulsatile flow.

In pulsatile flow, negative inflow is generated during some period.

To conserve global mass conservation, mass flux should be also negative in the outlet side.

And I use convective boundary condition in the outlet side.

dui/dt + cdui/dx = 0

After applying to convective boundary condition, following equation is used.

ui=ui*Qin/Qout

At this procedure, some problems occurs.

If Qin is zero, ui becomes zero.

I think this flow is non-physical.

Do you have any ideas to fix this problem?