Periodic boundary in unsteady flow
 

I want to simulate unsteady flow in a channel. The flow rate increases steadily in time. However, if you take a long enough channel, the flow is still fully developed unsteady flow, i.e. at every point in space, the velocity profile is the same as long as the pipe is long enough.

To do this I can either use a channel that is 40 diameters long or use a periodic boundary enabling me to use a small section of channel - so that the inlet and outlet are coupled. However, when I try to use periodic boundaries by /grid/modify-zones make periodic, the only option open to me is a constant mass flow - I want to specify a changing mass flow.

Without using periodic boundaries, I can do this by define/boundary condition/inlet then next to velocity I can input a specified profile. Is it possible to do the same for periodic boundaries?

Many thanks for any help with this.