Abstract This work presents a data-driven approach for the identification of relevant flow features and the breakdown of their contributions to unsteady forces in flapping airfoils. The method exploits the correlation between simultaneous flow-field and force measurements. The Extended Proper Orthogonal Decomposition of flow fields and force is employed to identify relevant flow features and to ascertain their contribution on unsteady aerodynamic loads. The analysis is performed in the wing-fixed non-inertial reference frame to avoid moving boundary effects in the decomposition and to accurately track flow features around the wing. This study discloses new insight into large-oscillation force modelling, allowing linking it to classic unsteady potential theories based on the small oscillations hypothesis. From modal analysis, the force is found to be related to large-scale spatio-temporal structures that model the flow behaviour in the proximity of the wing. Flow features in the mid-far wake are shown to have a negligible effect on the aerodynamic force. Two main spatio-temporal structures are found to be correlated with the loads on the wing. The bulk of chord-normal force oscillations is ascribed to the vorticity production and shedding which models the periodic variation of the circulation around the wing and its release in the wake. An alternating release of vorticity from both sides of the airfoil contributes to the determination of Leading and Trailing Edge Vortices and affects mostly the chord-wise force oscillations. The model retrieved through this approach is closely related to the potential aerodynamic-force models by Theodorsen and Garrick. Chord-normal force can be modelled as a quasi-steady circulatory contribution. The chord-wise force component is modelled by a term that can be assimilated to a non-circulatory force, which is linearly correlated to the airfoil kinematics, and by a term that can be assimilated to the Leading Edge Vortex suction.

中文翻译：

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扑翼中非定常空气动力学现象的数据驱动识别

摘要 这项工作提出了一种数据驱动的方法，用于识别相关的流动特征，并分解它们对扑翼机翼不稳定力的贡献。该方法利用同步流场和力测量之间的相关性。流场和力的扩展正交分解用于识别相关的流动特征并确定它们对非定常气动载荷的贡献。分析是在机翼固定的非惯性参考系中进行的，以避免分解中的移动边界效应并准确跟踪机翼周围的流动特征。这项研究揭示了对大振荡力建模的新见解，允许将其与基于小振荡假设的经典非定常势理论联系起来。从模态分析，发现该力与模拟机翼附近流动行为的大规模时空结构有关。中远尾流中的流动特征对空气动力的影响可以忽略不计。发现两个主要的时空结构与机翼上的载荷相关。大部分的弦法向力振荡归因于涡度的产生和脱落，它模拟了机翼周围环流的周期性变化及其在尾流中的释放。机翼两侧涡量的交替释放有助于确定前缘涡和后缘涡，并且主要影响弦向力振荡。通过这种方法检索到的模型与 Theodorsen 和 Garrick 的潜在空气动力模型密切相关。弦法向力可以建模为准稳态循环贡献。弦向力分量由可同化为与翼型运动学线性相关的非循环力的项以及可同化为前缘涡吸力的项建模。