From the analysis above, particle behavior in the rotary drum repeats
periodically. And the periodic time is mainly made up of two
parts: the rise time (time for particles to move with the drum and
the flights from the bottom half to the top half) and the fall time
(time for particles to fall downto the surface of the bed.) The fall time
is found to be small compared to the rise time with video photography,
and the fall time does not change obviously with rotational
velocity, which is consistent with the finding of Ref. [22]. While
the rise time decreases with the increasing of rotational velocity.
Therefore, the periodic time of particle behavior in the transverse
section decreases with the increasing of rotational velocity.