AI 中文总结
本文通过直接数值模拟研究$Re_b=4900$下受控管流的减阻与亚临界湍流，发现受控管流可层流化，其局地化湍流 puff 性质特殊，为层流控制策略提供了新视角。
AI 中文摘要
采用直接数值模拟方法，在体积雷诺数$Re_b=4900$的条件下，研究由周向壁面速度的流向行波控制的管流。对减阻的综合分析表明，该流动的响应与槽道流存在根本差异：在合适的控制作用下，管流可实现层流化，而槽道流无法达到该状态。根据控制参数的不同，流动会呈现出转捩管流特有的空间局地化湍流状态，其中湍流 puff 可在体积雷诺数达到未受控情况的3倍时仍持续存在。仅体积雷诺数无法确定局地化的 onset（起始）；此外，减阻会改变体积速度与摩擦速度之间的自然关系，不足以识别通用的 onset 判据。基于截面湍动能空间方差的间歇性指标，将局地化湍流的出现与控制产生的低壁面摩擦相关联，尽管二者并非一一对应。尽管受控 puff 与典型湍流 puff 在性质上具有定性相似性，但仍存在显著差异，例如其前沿的传播速度可能快于体积流速。总体而言，本研究全面表征了受控管流中的亚临界湍流状态及其湍流 puff，并为旨在实现流动层流化的控制策略提供了新视角。
英文摘要
Pipe flow controlled by streamwise-travelling waves of azimuthal wall velocity is studied using direct numerical simulations at a bulk Reynolds number $Re_b=4900$. A comprehensive analysis of drag reduction shows that the flow response differs fundamentally from that of channel flow. Under suitable forcing, pipe flow relaminarizes, whereas channel flow does not. Depending on the control parameters, the flow exhibits the spatially localized turbulent state characteristic of transitional pipe flow, with turbulent puffs persisting at bulk Reynolds numbers up to three times higher than in the uncontrolled case. The bulk Reynolds number alone does not determine the onset of localization. Moreover, drag reduction, which alters the natural relation between bulk and friction velocities, is insufficient to identify a universal onset criterion. An intermittency indicator based on the spatial variance of the cross-sectional turbulent kinetic energy relates the emergence of localized turbulence to the low wall friction produced by the control, although the correspondence is not one-to-one. Despite their qualitative resemblance to canonical turbulent puffs, the controlled puffs exhibit distinct properties; for example, their fronts may propagate faster than the bulk flow. Overall, this work provides a comprehensive characterization of the subcritical turbulent state and its turbulent puffs in controlled pipe flow, and offers a new perspective on control strategies that aim at flow relaminarization.
