École polytechnique fédérale de Lausanne, Institute of Mechanical Engineering, Unsteady flow diagnostics laboratory, 1015 Lausanne, Switzerland.
Bioinspir Biomim. 2021 Jan 29;16(2). doi: 10.1088/1748-3190/abd012.
Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping wing flight are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency with the help of an evolutionary algorithm andforce and torque measurements at the wing root. Additional flow field measurements are conducted to link the vortical flow structures to the aerodynamic performance for the Pareto-optimal kinematics. The optimised pitch angle profiles yielding maximum stroke-average lift coefficients have trapezoidal shapes and high average angles of attack. These kinematics create strong leading-edge vortices early in the cycle which enhance the force production on the wing. The most efficient pitch angle kinematics resemble sinusoidal evolutions and have lower average angles of attack. The leading-edge vortex grows slower and stays close-bound to the wing throughout the majority of the stroke-cycle. This requires less aerodynamic power and increases the hovering efficiency by 93% but sacrifices 43% of the maximum lift in the process. In all cases, a leading-edge vortex is fed by vorticity through the leading edge shear layer which makes the shear layer velocity a good indicator for the growth of the vortex and its impact on the aerodynamic forces. We estimate the shear layer velocity at the leading edge solely from the input kinematics and use it to scale the average and the time-resolved evolution of the circulation and the aerodynamic forces. The experimental data agree well with the shear layer velocity prediction, making it a promising metric to quantify and predict the aerodynamic performance of the flapping wing hovering motion.
生物扑翼飞行器在广泛的飞行条件下高效稳定地运行，为工程师提供了极大的灵感。扑翼飞行的非定常空气动力学主要由大尺度涡结构主导，这些结构增强了空气动力学性能，但对机翼激励的微小变化敏感。我们通过进化算法和在机翼根部的力和扭矩测量，实验优化了悬停中扑翼系统的俯仰角运动学，以最大化冲程平均升力和悬停效率。进行了附加的流场测量，将涡流动结构与帕累托最优运动学的空气动力学性能联系起来。产生最大冲程平均升力系数的优化俯仰角轮廓呈梯形形状，具有较高的平均迎角。这些运动学在循环早期产生强大的前缘涡，从而增强机翼的力产生。最有效的俯仰角运动学类似于正弦演化，具有较低的平均迎角。前缘涡在整个冲程周期中生长较慢，并保持与机翼紧密结合。这需要较少的空气动力功率，并将悬停效率提高 93%，但在此过程中牺牲了 43%的最大升力。在所有情况下，前缘涡通过前缘剪切层中的涡量供应来供给，这使得剪切层速度成为涡的生长及其对空气动力的影响的良好指标。我们仅从输入运动学估计前缘处的剪切层速度，并使用它来缩放环流和空气动力的平均和时间分辨演化。实验数据与剪切层速度预测吻合良好，这使其成为量化和预测扑翼悬停运动空气动力学性能的有前途的指标。
