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Rolling and pitching oscillating foil propulsion in ground effect

机译：轧制和俯仰振荡箔片造成地面效应的推进

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摘要

In this paper, we investigate the effect of operating near a solid boundary on the forces produced by harmonically oscillating thrust-generating foils. A rolling and pitching foil was towed in a freshwater tank in a series of experiments with varying kinematics. Hydrodynamic forces and torques were measured in the freestream and at varying distances from a solid boundary, and changes in mean lift and thrust were found when the foil approached the boundary. The magnitude of this ground effect exhibited a strong nonlinear dependence on the distance between the foil and the boundary. Significant effects were found within three chord lengths of the boundary, and ground effect can be induced at greater distances from the boundary by biasing the tip of the foil toward the boundary. Lift coefficients changed by as much as 0.2 at the closest approach to the ground, with changes > 0.05 for all cases across Strouhal numbers ranging from 0.3 to 0.6, and nominal maximum angle of attack ranging from 20 degrees to 40 degrees. The ubiquity of the ground effect in high thrust kinematics suggests that the ground effect can provide a passive obstacle avoidance capability for foil propelled vehicles. By comparison with previous experimental work, we find that the ground effect experienced by a high-aspect ratio rolling and pitching foil is a fully three-dimensional phenomenon, as it is not accurately predicted when two-dimensional flow and/or two-dimensional kinematics are enforced. While two-dimensional foil kinematics are more easily modeled for numerical studies, three-dimensional foil kinematics may be more practical for real world implementation in underwater vehicles.
机译：在本文中，我们研究了通过谐振推力发电箔产生的力靠近固体边界附近的效果。滚动和抛光箔在淡水罐中牵引，在一系列不同的运动学实验中。在FreeStream中测量流体动力和扭矩，并且从固体边界的变化距离，并且当箔片接近边界时发现平均升力和推力的变化。该地面效果的大小表现出对箔和边界之间的距离的强烈非线性依赖性。在边界的三个弦长内发现显着的效果，并且通过将箔的尖端朝向边界偏置，可以从边界偏差地诱导地面效果。升力系数以最近的地面的最接近的方法更改为0.2，对于跨斯特恒数的速度，跨越0.3至0.6的所有情况，以及标称最大攻角范围为20度至40度。高推力运动学中的地面效应的难以达到地面效应可以为箔的推进车辆提供无源障碍避免能力。通过与先前的实验工作相比，我们发现高纵横比轧制和俯仰箔所经历的地面效应是一个完全三维现象，因为当二维流动和/或二维运动学时不准确地预测。强制执行。虽然二维箔运动学更容易建模用于数值研究，但三维箔运动学可能更加实用，在水下车辆中的现实世界中的实施方面更加实用。

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