The flowrate should be the same - if you pump in 1 l/s into your domain, 1 l/s should come out. The velocity does not need to be the same everywhere. (of course, if inlet and outlet have equal cross-section, the average velocity will be equal - but not necessarily locally there can be differences)

you mean if the average velocity at the outlet is the same as inlet it will be ok although some points at the outlet have a lower velocity than that at the inlet

what else can i do other than increasing the downstream length?

thanks

why do you want the outlet velocity to equal the inlet velocity everywhere? since you put a perturbation in the flow, that's not going to happen for any realistic scenario; you'll need an enormous downstream domain.

ok i got it thank you