  This study was inspired from the swimming habits of red nose tetra fish that
prefer side-by-side configurations and exhibit changes to their synchronization
mid-swimming. Using numerical simulations, alterations to the unsteady wake
dynamics imposed by abrupt changes in the phase angle between two pitching
side-by-side foils were examined at Reynolds number of 4;000 and Strouhal
number of 0:50. Four hybrid modes were considered in thisstudy with two modes
representing an abrupt phase change by pi during the 20th cycle. The other two
modes represented a simplified case of burst-and-coast swimming, in which there
was a brief (2 oscillation periods) suspension of oscillations before imposing
a change in phase angle. In all cases, the foils initially performed
out-of-phase pitching, and then they started their in-phase motion by either
performing the upstroke or down-stroke first. This kinematic change resulted in
the formation and growth of a secondary vortex street in between two primary
streets, which enabled and maintained a split wake configuration. Furthermore,
the phase switching altered the pressure levels on the top and bottom surfaces
of both foils to almost similar levels, which attributed to a reduction in the
side-force. The growth rate of the secondary vortex street remained consistent
for all four hybrid modes.
