AI 中文总结
本研究基于展向壁面方波激励减阻实验，分析不同流向波长下的湍流结构，揭示后最优减阻下小尺度猝发导致的湍流恢复机制，证实该调制具有尺度选择性。
AI 中文摘要
基于Knoop等人（《物理评论·流体》，第10卷，2025年）的实验，通过展向壁面速度的稳态方波激励实现减阻。针对未激励的参考工况以及激励振幅$A^+ = 12$（$+$表示粘性标度）下的三种流向激励波长工况，分析了粒子追踪测速数据，这三种波长分别对应次优（$Λ_x^+ = 471$）、近最优（$Λ_x^+ = 942$）和后最优（$Λ_x^+ = 1884$）减阻条件。对湍动能大尺度猝发的条件平均场表明，所有工况下激励均抑制了近壁喷发，而外层扫掠的抑制作用随$Λ_x^+$增大而增强。后最优激励下湍流呈现流向周期性的衰减与恢复。这种恢复现象由远小于壁湍流中典型含能尺度的极小尺度增强所致，且与小尺度猝发事件的出现相关。尽管这些小尺度猝发在未激励和次优工况中统计意义不显著，但在近最优到后最优工况之间，其发生频率提升了4倍。小尺度猝发呈现出一致的特征和强度，暗示恢复现象可能具有普遍性。小波分析表明，在后最优工况中，壁面速度保持恒定的区域内，极小尺度沿流向逐渐增长，驱动了小尺度的重新赋能与抑制的循环模式，这与早期的统计分析结果一致。这种湍流调制机制具有尺度选择性：尽管小尺度结构周期性出现，但随着激励波长增加，大尺度运动受到更有效的抑制。
英文摘要
Drag reduction is achieved by a steady square-wave forcing of the spanwise wall velocity, based on the experiments in Knoop et al. (Phys. Rev. Fluids, 10, 2025). Particle tracking velocimetry data are analyzed for a non-actuated reference case and actuation at forcing amplitude $A^+ = 12$ ($+$ denotes viscous scaling) for three streamwise forcing wavelengths, which correspond to sub-optimal ($Λ_x^+ = 471$), near-optimal ($Λ_x^+ = 942$), and post-optimal ($Λ_x^+ = 1884$) drag reduction conditions. Conditionally averaged fields on large-scale bursts of turbulent kinetic energy show that forcing suppresses near-wall ejections across all cases, while outer-layer sweep suppression strengthens with $Λ_x^+$. Post-optimal forcing exhibits streamwise-periodic attenuation and recovery of turbulence. The recovery phenomenon is caused by an enhancement of very small scales, significantly smaller than those typically energetic in wall turbulence, and is linked to the emergence of small-scale bursting events. While these small-scale bursts are statistically insignificant in the non-actuated and sub-optimal cases, their frequency increases by a factor of four between the near-optimal and post-optimal cases. The small-scale bursts exhibit uniform signatures and intensities, hinting at the possible universality of the recovery phenomenon. A wavelet analysis shows that, in the post-optimal case, very small scales increase progressively in the streamwise direction in regions where the wall velocity remains constant, driving a cyclic pattern of small-scale re-energization and suppression consistent with earlier statistical analysis. This turbulence modulation mechanism is scale-selective: while small-scale structures emerge periodically, large-scale motions are more effectively suppressed as the forcing wavelength increases.
