AI 中文总结
本研究通过谱模态与条件分析，发现悬停四旋翼的全局同步涡合并源于四旋翼尾流间的非次谐波相互作用，并验证了该现象在实验复杂性下的鲁棒性。
AI 中文摘要
悬停的全尺寸四旋翼飞行器最具能量的相干结构与其自组织过程相关，在该过程中，各旋翼涡在发生合并事件时同步其频率，从而形成全局相关的结构。我们通过将谱模态分析和条件分析应用于通过Shake-The-Box拉格朗日粒子跟踪获取的同化三维速度数据，识别并表征了这一现象。该数据集捕捉了高雷诺数湍流，该湍流因主动飞行控制导致的时变旋翼转速、低频飞行器漂移、有限时空分辨率及测量不确定性而进一步复杂化。大多数相干结构恢复了已确立的单旋翼特征，如叶尖涡及其次谐波配对。全局同步涡合并表现为旋翼频带下方一个不可通约频率处的谱峰，该现象无法用单旋翼空气动力学、次谐波不稳定性或频带间三波相互作用来解释。相反，条件平均为上述涉及全部四个旋翼尾流的间歇性、明显非次谐波的涡合并过程提供了证据。确定该现象是否在不同飞行条件和配置下均被观察到仍属推测；然而，尽管存在实验复杂性（尤其是旋翼转速变化），通过互补的频域和时域分析对全局同步涡合并的一致表征证明了其鲁棒性。
英文摘要
The most energetic coherent structure of a hovering full-scale quadcopter is associated with a self-organizing process in which the individual rotor vortices synchronize their frequencies while undergoing merging events, yielding a globally correlated structure. We identify and characterize this phenomenon by applying spectral modal and conditional analyses to assimilated three-dimensional velocity data acquired via Shake-The-Box Lagrangian particle tracking. The dataset captures a high-Re, turbulent flow further complicated by time-varying rotor speeds stemming from active flight control, low-frequency vehicle drift, finite spatio-temporal resolution, and measurement uncertainty. Most coherent structures recover established single-rotor features such as tip vortices and their subharmonic pairing. The globally synchronized vortex merging manifests as a spectral peak at an incommensurate frequency below the rotor band, which cannot be explained by single-rotor aerodynamics, subharmonic instabilities, or band-to-band triadic interactions. Instead, conditional averaging provides evidence of the aforementioned intermittent, distinctly non-subharmonic vortex-merging process involving all four rotor wakes. Establishing whether or not this phenomenon is observed across different flight conditions and configurations remains speculative; however, the consistent characterization of the globally synchronized vortex merging using complementary frequency- and time-domain analyses despite experimental complexities, in particular rotor speed variations, demonstrates its robustness.
