Title: Dynamic stall reattachment revisited Show Chinese title

Title: 动态失速再附着的重新审视

Abstract: Dynamic stall on airfoils is an undesirable and potentially dangerous phenomenon. The motto for aerodynamic systems with unsteadily moving wings, such as helicopters or wind turbines, is that prevention beats recovery. In case prevention fails or is not feasible, we need to know when recovery starts, how long it takes, and how we can improve it. This study revisits dynamic stall reattachment to identify the sequence of events during flow and load recovery and to characterise key observable features in the pressure, force, and flow field. Our analysis is based on time-resolved velocity field and surface pressure data obtained experimentally for a two-dimensional, sinusoidally pitching thin airfoil. Stall recovery is a transient process that does not start immediately when the angle of attack falls below the critical stall angle. The onset of recovery is delayed to angles below the critical stall angle and the duration of the reattachment delay decreases with increasing unsteadiness of the pitching motion. An angle of attack below the critical angle is a necessary, but not sufficient condition to initiate the stall recovery process. We identified a critical value of the leading-edge suction parameter, independent of the pitch rate, that is a threshold beyond which reattachment consistently initiates. Based on prominent changes in the evolution of the shear layer, the leading-edge suction, and the lift deficit due to stall, we divided the reattachment process into three stages: the reaction delay, wave propagation, and the relaxation stage, and extracted the characteristic features and time-scales for each stage. Show Chinese abstract

Abstract: 机翼上的动态失速是一种不希望发生且可能危险的现象。 对于具有非定常运动机翼的气动系统，如直升机或风力涡轮机，其原则是预防胜于恢复。 如果预防失败或不可行，我们需要知道恢复何时开始、需要多长时间以及如何改善恢复过程。 本研究重新审视动态失速再附着，以确定流动和载荷恢复期间的事件序列，并表征压力、力和流动场中的关键可观察特征。 我们的分析基于对二维正弦俯仰薄机翼进行实验获得的时间分辨速度场和表面压力数据。 失速恢复是一个瞬态过程，在攻角降至临界失速角以下时不会立即开始。 恢复的开始被延迟到低于临界失速角的角度，再附着延迟的持续时间随着俯仰运动的非定常性增加而减少。 攻角低于临界角度是启动失速恢复过程的必要条件，但不是充分条件。 我们确定了一个独立于俯仰速率的前缘吸力参数的关键值，这是再附着一致开始的阈值。 基于剪切层演变、前缘吸力和由于失速导致的升力缺陷的显著变化，我们将再附着过程分为三个阶段：反应延迟阶段、波传播阶段和松弛阶段，并提取了每个阶段的特征特性和时间尺度。