标题： 非对称模型游泳者在中间雷诺数下 显示英文标题

标题： Nonreciprocal model swimmer at intermediate Reynolds numbers

摘要： 在不同尺度的许多生物中都可以观察到一种称为同步游泳的现象，即肢体以较小的相位滞后依次拍打，但这一现象主要是在高雷诺数或低雷诺数的极限条件下进行研究的。 受卤虫游泳的启发，这是一种在中间雷诺数下运作的中尺度生物，我们计算研究了一个简单的非互易二维模型，该模型能够进行同步游泳。 我们的游泳器由两对以相位差拍打的桨组成，这些桨对称地连接在扁平身体的两侧。 我们数值求解了纳维-斯托克斯方程，并使用浸入边界法来模拟流体与游泳器之间的相互作用。 为了研究惯性和几何形状的影响，我们在范围 $Re = 0.05 - 100$内变化桨的间距和雷诺数进行了模拟。 在所有情况下，我们观察到周期内来回运动，并且在稳态时具有有限的周期平均游泳速度。 有趣的是，我们发现游泳器的游泳速度与 $Re$呈非单调依赖关系，在 $Re\approx1$附近有一个最大值，在 $Re=20-30$之间有一个平坦的最小值，并且在 $Re>35$时最终增加。 为了更深入地了解这种关系背后的机制，我们首先将游泳器的游泳动作以及每个桨的动作分解为推进阶段和恢复阶段，并描述了向前和向后的运动。 与互易游泳器不同，这里在周期的部分时间内，桨的两个动作之间存在竞争——这种平衡导致了净运动。 随后，我们研究了周期平均的、推进阶段平均的和恢复阶段平均的流体流动，并将流体场中的差异与游泳速度的非单调行为联系起来。 我们的结果表明，在中间雷诺数范围内，随着惯性的增加，会出现不同的运动机制。 显示英文摘要

摘要： Metachronal swimming, the sequential beating of limbs with a small phase lag, is observed in many organisms at various scales, but has been studied mostly in the limits of high or low Reynolds numbers. Motivated by the swimming of brine shrimp, a mesoscale organism that operates at intermediate Reynolds numbers, we computationally studied a simple nonreciprocal 2D model that performs metachronal swimming. Our swimmer is composed of two pairs of paddles beating with a phase difference that are symmetrically attached to the sides of a flat body. We numerically solved the Navier-Stokes equations and used the immersed boundary method to model the interactions between the fluid and swimmer. To investigate the effect of inertia and geometry, we performed simulations varying the paddle spacing and the Reynolds numbers in the range $Re = 0.05 - 100$. In all cases, we observed back-and-forth motion during the cycle and a finite cycle-averaged swim speed at steady state. Interestingly, we found that the swim speed of the swimmer has nonmonotonic dependence on $Re$, with a maximum around $Re\approx1$, a flat minimum between $Re=20-30$ and an eventual increase for $Re>35$. To get more insight into the mechanism behind this relationship, we first decomposed the swim stroke of the swimmer and each paddle into power and recovery strokes and characterized the forward and backward motion. Unlike for reciprocal swimmers, here, for parts of the cycle there is competition between the two strokes of the paddles - the balance of which leads to net motion. We then studied the cycle-averaged, as well as, power-stroke-averaged and recovery-stroke-averaged fluid flows, and related differences in the fluid field to the nonmonotonic behavior of the swim speed. Our results suggest the existence of distinct motility mechanisms that develop as inertia increases within the intermediate-Re range.