Fig. 14. Proposed dynamic stall vortex pairs shown in color in the sunfish pectoral fin formed during outstroke when the fin is undergoing `cupping' motion. The fin picture is from Lauder et al. (Lauder et al., 2007). The total fish length is 17 cm and the stream speed is 8.5 cm s<sup>–1</sup>. The fish is maintaining its station in a `uniform' stream, shown by the vertical arrow, while the fin is turning upstream and the spanwise edges are curling inward. The stall vortices locally augment the pressure difference across the two leading edges and could help cancel perturbations in the vertical directions. The co-flowing jets on both sides of the fish could be vectored appropriately to hold station laterally and provide some thrust. The vertical arrow shows the stream direction.