Bandyopadhyay Promode R
Naval Undersea Warfare Center, Newport, RI, 02841, USA.
, Middletown, RI, 02842, USA.
Sci Rep. 2023 Aug 24;13(1):13820. doi: 10.1038/s41598-023-40986-w.
When a sailfish circles to corral a school of flying fish in a vortex near the ocean surface, a tiny patch of arced surface waves confined to oppositely placed 70° sectors appears dispersing coherently, but why? It is modeled that, when the fish motions stop suddenly, the corralled school compacts, the tail shed propulsion vortices touch, break and radiate the pressure released from the centrifugal vortex rotation creating an acoustic monopole. The surface-wave patch is a section of the sphere of radiation. The oppositely placed curved bodies of the sailfish and the flying fish act as concave acoustic mirrors about the monopole creating a reverberating bell-shaped cloak in between which vibrates the ear bones and bladders of the flying fish disorienting them. A cup of water firmly struck on a table induces a similar vibration of a purely radial mode. The sailfish circles around the school at a depth where the wind induced underwater toroidal motion in the vertical plane becomes negligible such that the flying fish is unable to sense the tailwind direction above, limiting the ability to swim up and emerge in the right direction to glide. Experiments confirm that the flying fish tail rigidity is too low for a quick ballistic exit, which is not called for either.
当旗鱼在海面附近的漩涡中盘旋以围捕一群飞鱼时，一小片局限于相对位置的70°扇形区域的弧形表面波似乎在相干地分散，但这是为什么呢？据模型推测，当鱼的动作突然停止时，被围捕的鱼群会紧凑起来，尾部脱落的推进涡流相互接触、破裂，并辐射出离心涡旋旋转释放的压力，从而产生一个声学单极子。表面波斑块是辐射球体的一部分。旗鱼和飞鱼相对放置的弯曲身体充当围绕单极子的凹面声镜，在它们之间形成一个回荡的钟形“斗篷”，使飞鱼的耳骨和鱼鳔振动，从而使其迷失方向。一杯水用力敲击桌面会引发类似的纯径向模式振动。旗鱼在一个深度围绕鱼群盘旋，在这个深度，风在垂直平面上引起的水下环形运动变得可以忽略不计，这样飞鱼就无法感知上方的顺风方向，限制了它们向上游动并以正确方向浮出水面滑翔的能力。实验证实，飞鱼的尾巴刚性过低，无法实现快速的弹道式逃离，而且这也并非必要。
