I want to test how different loads affect the boundary conditions., for example, a pressure that increases over time and then remains constant. Can PyFR achieve this？
Yes, as per the documentation:
Parameterises constant, or if available space (x, y, [z]) and time (t) dependent, boundary condition labelled name in the .pyfrm file with […]
and so BC values are allowed to be arbitrary expressions of both space and time. All of the usual mathematical functions are available; see:
Regards, Freddie.