Abstract

Perching and hovering are two bio-inspired flight maneuvers that have relevance in engineering, especially for small-scale uncrewed air vehicles. In a perching maneuver, the vehicle decelerates to zero velocity while pitching or plunging, and in hovering the pitch and plunge motion kinematics are used to generate fluid dynamic forces even when the vehicle velocity is zero. Even for an airfoil, the fluid dynamics of such maneuvers pose challenges for low-order modeling because of the time-varying freestream velocity, high amplitudes and rates of the motion kinematics, intermittent formation and shedding of the leading-edge vortex (LEV), and the strong effects of the shed vorticity on the loads. In an earlier work by the authors, a leading-edge suction parameter (LESP) was developed to predict intermittent LEV formation for round-leading-edge airfoils undergoing arbitrary variation in pitch and plunge at a constant freestream velocity. In this research, the LESP criterion is extended to situations where the freestream velocity is varying or zero. A discrete vortex method based on this criterion is developed and the results are compared against those from a computational fluid dynamics (CFD) method. Abstractions of perching and hovering maneuvers are used to validate the predictions in highly unsteady vortex-dominated flows, where the time-varying freestream/translational velocity is small in magnitude compared to other contributions to the velocity experienced by the airfoil. Time instants of LEV formation, flow features, and force coefficient histories for the various motion kinematics from the method and CFD are obtained and compared. The LESP criterion is seen to be successful in predicting the start of LEV formation, and the discrete vortex method is effective in modeling the flow development and forces on the airfoil.

Graphical abstract

中文翻译：

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栖息和悬停机动的离散涡流建模

摘要

栖息和悬停是两种受生物启发的飞行动作，与工程相关，尤其是对于小型无人驾驶飞行器。在停靠机动中，车辆在俯仰或俯冲时减速至零速度，而在悬停时，即使车辆速度为零，俯仰和俯冲运动运动学也用于产生流体动力。即使对于机翼，由于随时间变化的自由流速度、运动运动学的高振幅和速率、前缘涡流 (LEV) 的间歇形成和脱落，此类机动的流体动力学对低阶建模提出了挑战，以及脱落涡度对载荷的强烈影响。在作者的早期作品中，开发了前缘吸力参数 (LESP) 来预测圆形前缘翼型的间歇性 LEV 形成，这些翼型在俯仰和以恒定的自由流速度下发生任意变化。在这项研究中，LESP 准则被扩展到自由流速度变化或为零的情况。开发了基于此标准的离散涡流方法，并将结果与​​计算流体动力学 (CFD) 方法的结果进行了比较。栖息和悬停机动的抽象用于验证高度不稳定的涡流中的预测，其中时变自由流/平移速度与机翼所经历的速度的其他贡献相比在幅度上很小。LEV 形成的时刻、流动特征、获得并比较了该方法和 CFD 的各种运动运动学的力系数历史记录。LESP 准则被认为在预测 LEV 形成的开始方面是成功的，而离散涡流方法在模拟流动发展和翼型上的力方面是有效的。

图形概要