Abstract: To study the formation and mechanism of fluid thrust and drag on a fish during its steady swimming, this paper presents an analysis on the characteristics of pressure distribution and fluid forces around a juvenile grass carp (Ctenopharyngodon idellus), using the particle image velocimetry (PIV) technology for velocity measurement. The results show that during the movement of the carp, the thrust of surrounding fluid on its body and its time variation depend on the positive pressure distribution on its body, while the negative pressure distribution dominates fluid drag formation. The experimental data reveal the proportions of thrust generated by the positive and negative pressure on the fish are 59.0% and 41.0% respectively. The swimming efficiency is (80.2±17.5)% based on positive pressure and (58.8±25.8)% based on negative pressure, and the drag proportion from negative pressure is as high as 81.9%. The thrust proportions on the fish’s head, middle and tail is 13.7%, 38.5% and 47.8% respectively; the drag proportion on its middle part reached up to 72.8%. Its whole body contributes to thrust generation, while the tail is the main source of thrust and the highest swimming efficiency is (77.2±12.1)%. The results of this study helps deepen the understanding of fish swimming dynamics and mechanism and is useful in fish bionics application and ecological protection.

Key words: steady swimming, PIV, pressure field, thrust, drag, swimming efficiency