  When an object is accelerated in a fluid, a primary vortex is formed through
the roll-up of a shear layer. This primary vortex does not grow indefinitely
and will reach a limiting size and strength. Additional vorticity beyond the
critical limit will end up in a trailing shear layer and accumulate into
secondary vortices. The secondary vortices are typically considerably smaller
than the primary vortex. Here, we focus on the formation, shedding, and
trajectory of secondary vortices generated by a rotating rectangular plate in a
quiescent fluid using time-resolved particle image velocimetry. The Reynolds
number (Re) is varied from 840 to 11150. At low Re, the shear layer is a
continuous uninterrupted layer of vorticity that rolls up into a single
coherent primary vortex. At Re=1955, the shear layer becomes unstable. For
Re>4000, secondary vortices are discretely released from the plate tip. First,
we demonstrate that the roll-up of the shear layer, the trajectory of the
primary vortex, and the path of secondary vortices can be predicted by a
modified Kaden spiral. Second, the timing of the secondary vortex shedding is
analysed. The time interval between the release of successive secondary
vortices is not constant during the rotation but increases the more vortices
have been shed. The shedding time interval also increases with decreasing
Reynolds number. The increased time interval under both conditions is due to a
reduced circulation feeding rate.
