The pressure drag reduction effect of tandem swimming by Caranx sexfasciatus and Rhincodon typus

It has been known that small fish often swim in front of large fish, but the reason for this remains unclear. We hypothesized that small fish could swim more efficiently in front of large fish than when swimming singly. Computational fluid dynamical (CFD) analysis was conducted to verify this hypothesis. We calculated and compared the drag that affects small fish (bigeye trevally Caranx sexfasciatus) when swimming alone and that when tandem swimming in front of large fish (whale shark Rhincodon typus); the analysis was performed by changing the distance between the two fish. The results show that the drag coefficient of Caranx sexfasciatus when tandem swimming is smaller than the drag coefficient when swimming singly, and as the distance decreases, so does the drag coefficient. As the mechanism of this phenomenon, the stagnation point located in front of the large fish (Rhincodon typus) reduces the pressure difference between the small fish's (Caranx sexfasciatus) front and rear. The strength of the pressure drag depends on the pressure difference before and after the object. Therefore, we consider it to be the reason for the reduced drag on Caranx sexfasciatus. Moreover, the relationship between distance and drag reduction effect was the exact opposite of a previous study. This is most likely due to the fact that the Reynolds number, body-length ratio, and dimensions of analysis are different from previous studies. This paper suggests that Caranx sexfasciatus swims in front of Rhincodon typus to reduce their travel cost and insights for a more detailed understanding of tandem swimming.

Automated summary

It has been found that small fish swimming in front of large fish can swim more efficiently and reduce drag. This is mainly due to the stagnation point in front of the large fish reducing the pressure difference between the small fish's front and rear, leading to a decrease in drag coefficient. The relationship between distance and drag reduction effect in this tandem swimming scenario is different from previous studies due to variations in Reynolds number, body-length ratio, and dimensions of analysis.