Fig. 5. A low-speed turning maneuver by a sunfish illustrating the propulsive roles of the paired and median fins. In light-video images (A–C), arrows signify the average orientation of jet flow developed by each fin sequentially during the turn. (A) In response to a stimulus issued on the left side of the fish (i.e. at the bottom of the panel), the ipsilateral or ‘strong-side’ pectoral fin abducts, generating a strong laterally oriented wake flow that rotates the body around the center of mass. (B) The contralateral ‘weak-side’ pectoral fin subsequently adducts, creating a posteriorly directed fluid jet that initiates forward translation of the body. Details of wake flow produced by the pectoral fins were presented in an earlier study (Drucker and Lauder, 2001). Near the end of the turn (B,C), the soft dorsal fin abducts independently of body bending and generates an obliquely oriented fluid jet. Flow fields in the wake of the soft dorsal fin during turning are presented in D and E for a frontal-plane laser sheet in position 1 (see Fig.2D). Large solid-line arrows indicate the direction of fin motion. Free-stream velocity of 0.5Ls-1, where L is total body length, or 10.5cms-1, from left to right has been subtracted from each vector. Schematic representations of observed wake structures are shown in F and G. (D) During abduction of the dorsal fin, a free vortex is shed from the trailing edge (shown as counterclockwise flow), while opposite-sign vorticity develops at the fin tip (cf. Fig.4B). (E) Upon return of the fin towards the body midline, bound vorticity is shed into the wake to form a second free vortex (cf. Fig.4C). The momentum of these paired vortices and associated jet flow on the strong side of the body is not balanced by subsequent fin abduction to the opposite side of the body as during steady swimming (Fig.4). The reaction force acting on the soft dorsal fin posterior to the center of mass of the body serves the dual function of slowing initial pectoral-fin-induced body rotation and helping to propel the animal forward away from the stimulus. CM, center of mass of the body (located at 0.36L posterior to the snout) (after Webb and Weihs, 1994). Light-video images in A–C are modified from previous work (Drucker and Lauder, 2001). Scales for D and E: arrow, 10cms-1; bar, 1cm. Vortex labels in F and G are defined in Fig.4.
