Fig. 14. Schematic of the primary flow features constituting the mechanisms of surge and heave force generation. (A) Surge: streamwise jet with the associated vortex rings. Position of vortex rings do not necessarily follow a regular pattern. See also Fig. 6C and Fig. 7. (B) Vortex tubes created along the edge of the ribbon fin shown in Fig. 8. Yellow and blue indicate the sign of vorticity, which alternates according to the direction of the fin tip movement.