Why does a flying fish taxi on sea surface before take-off? A hydrodynamic interpretation

Abstract

Flying fish have been observed jumping out of warm ocean waters worldwide. Before take-off, the flying fish are seen to taxi on the water surface by rapidly beating their semi-submerged tail fins, which process may help them airborne with enough speed to glide over a long distance. To understand the underlying physical mechanisms, here, we study a flying fish, 0.25 m in length and 0.191 kg in weight, considering both its underwater swimming and surface taxiing locomotion. Its hydrodynamic characteristics are numerically studied by computational fluid dynamics (CFD). Underwater, the fish is assumed to swim at a constant speed of 10 m s-1. Different critical frequencies are identified for various maximum deflected angles, ranging from{theta} 0 = 10{degrees} to 30{degrees}, at which the fish reaches cruising states, when the horizontal forces are balanced. The corresponding minimum power required for cruising swimming is 350 W, obtained at a deflected angle of 10{degrees} and a critical frequency of 145 Hz. In contrast, in the taxiing stage, the minimum power required for a stead-state locomotion at 10 m s-1 is 36 W, occurring at a deflected angle of 15{degrees} and a frequency of 50 Hz. We note that the power is significantly smaller than the swimming locomotion. Further, by increasing the flapping power, we find that larger speeds can be achieved. In specific, when the power is brought up to 350 W, it can reach a speed of 16.5 m s-1. Clearly, from the direct comparison between the two locomotive modes, it is apparently evidenced that the flying fish can be further accelerated by taxiing along the water surface.