What’s Lost Is Found: Wake Energy From Pectoral Fins Can Be Recaptured By Caudal Fin During Fish Swimming

In carangiform swimmers thrust is produced by periodical body-caudal fin (BCF) undulations generated along half of the length of the body. While this locomotor mode is prevalent at swimming speeds that exceed one body length per second, at lower speeds (0.3-0.1BL s^-1), many species display cyclical concurrent abductions and adductions of the pectoral fins that are temporally coordinated with BCF undulations. This gait is believed to mitigate the loss in performance at lower speeds through the following two mechanisms: 1) the sequential deployment of the pectoral fins, in addition to generating thrust can counteract the recoil produced by the high-aspect ratio caudal fin; 2) circulation of vortices produced by the pectoral fins can constructively interact with the caudal fin upon their interception by the latter. High-speed particle image velocimetry (PIV) experiments were performed using a 5L Brett-type swim tunnel to investigate the kinematics of the body and fins of the fish (Eucinostomus argenteus) and to compute PIV-derived vorticity and pressure fields. When specimens rely exclusively on labriform locomotion, the vortices produced by the pectoral fins are immediately shed away from the surface of the body. But when engaging into the pectoral fins-caudal fin gait (PFCF), these vortices travel along the body of the fish and are intercepted by the caudal fin at the end of a stroke cycle. At speeds greater than 1.0BL s^-1, PFCF is substituted for BCF swimming. Implications for the temporal coordination of the motion and gait shifts are discussed.