Swimming freely near the ground leads to flow-mediated equilibrium altitudes
Swimming freely near the ground leads to flow-mediated equilibrium altitudes
复制标题
DOI：
10.1017/jfm.2019.540
复制
发表时间：
2019-07
影响因子：
3.7
通讯作者：
Melike Kurt;Jackson Cochran-Carney;Q. Zhong;A. Mivehchi;D. Quinn;K. Moored
中科院分区：
文献类型：
--
作者：
Melike Kurt;Jackson Cochran-Carney;Q. Zhong;A. Mivehchi;D. Quinn;K. Moored
本文给出了翼型绕其前缘在平面固体边界附近俯仰运动的实验和计算结果。当翼片在空间中受到约束时，以及当翼片不受约束或在横流方向上自由游动时，对翼片进行检查。人们发现，翼片有稳定的平衡高度：在一定高度上，时间平均升力为零，并使翼片返回到这些平衡状态。这些稳定的平衡高度存在于受约束和自由游动的翼片中，并且与翼片的初始条件无关。在所有情况下，当Strouhal数增加或降低频率时，平衡高度移动远离地面。位流模拟预测平衡高度在3%-11%之间，表明平衡高度主要是由于无粘机制。事实上，稳定的平衡高度是由三种时间平均力的相互作用引起的：负的射流偏转循环力、正的准静态循环力和负的附加质量力。在平衡状态下，与远离地面的俯仰翼相比，翼表现出偏转的尾流，推力增加4%-17%，而效率没有损失。这些新发现的横向稳定性特征表明，非定常地面效应可能在近边界鱼和鱼启发机器人的控制策略中发挥作用。
Experiments and computations are presented for a foil pitching about its leading edge near a planar, solid boundary. The foil is examined when it is constrained in space and when it is unconstrained or freely swimming in the cross-stream direction. It was found that the foil has stable equilibrium altitudes: the time-averaged lift is zero at certain altitudes and acts to return the foil to these equilibria. These stable equilibrium altitudes exist for both constrained and freely swimming foils and are independent of the initial conditions of the foil. In all cases, the equilibrium altitudes move farther from the ground when the Strouhal number is increased or the reduced frequency is decreased. Potential flow simulations predict the equilibrium altitudes to within 3 %–11 %, indicating that the equilibrium altitudes are primarily due to inviscid mechanisms. In fact, it is determined that stable equilibrium altitudes arise from an interplay among three time-averaged forces: a negative jet deflection circulatory force, a positive quasistatic circulatory force and a negative added mass force. At equilibrium, the foil exhibits a deflected wake and experiences a thrust enhancement of 4 %–17 % with no penalty in efficiency as compared to a pitching foil far from the ground. These newfound lateral stability characteristics suggest that unsteady ground effect may play a role in the control strategies of near-boundary fish and fish-inspired robots.