Fig. 8. Summary of the empirically determined hydrodynamic forces created by chub
mackerel Scomber japonicus swimming steadily at 1.2 FL
s-1. Flow fields and forces are depicted in the vertical
(XY, top) and horizontal (XZ, bottom) planes. Vortex rings
are shown in blue; ring dimensions and orientation represent those determined
empirically in this paper (Table
1). Thick black arrows represent central jet flow. Red arrows
represent the reaction force on the caudal fin; in the vertical plane,
reaction force is directed behind the body, so the red arrow is broken.
Thrust, lift and lateral force values are the mean reaction forces averaged
over a stroke period. S. japonicus swims with its body slightly
tilted down (-3° on average); the jet is tilted down -3° in the
opposite direction. Thus, the reacion force at the caudal fin has a lift
component that acts over the lever arm of body length to the center of mass
(black-and-white checkered circle) to rotate the head down (green arrow). Lift
generated by abducted pectoral fins (P), such as has been observed in previous
kinematic studies, could counterbalance lift generated at the caudal fin.
