Particle image velocimetry study of the Xenomystus Nigri

The Xenomystus Nigri, commonly known as the brown African knife-fish, has a unique undulating fin stretching from its gills to its tail, which propels it through the water. Gymnotiformes, or knife fish, are a unique group of fish with undulating fins that allows for rapid acceleration and deceleration both forward and backward, which also posses control over the yaw and pitch. This unique motion of the fish was captured using Particle Image Velocimetry (PIV) to calculate instantaneous velocity and vorticity. These images were analyzed to illistrate velocity and vorticity throughout the motion of the fin and relate it to the vortex shedding structure of the fin. After obtaining the results of the knife fish’s vorticity pattern, it was seen that vorticities are sent away from the fin in alternating pairs on either side of the fish. The vorticitial motion differed when swimming forward or backwards, but the alternating sign pattern was maintained. These results were for comparing stagnant and induced flow propulsion. The results show how the fin of the knife-fish propels the fish through the sinusoidal wave motion. The wave properties show that the fish has control over its fin and can quickly adapt to the surrounding fluid flow. The fish uses different combination of the undulating fin, pectoral fins, body bending, and tail flicking to achieve various propulsive modes.M.S.Includes bibliographical referencesby Thomas T. Reesbec