Coherent structures in oscillatory flows within the laminar-to-turbulent transition regime for smooth and rough walls

We investigate coherent structures present in oscillatory boundary layers over smooth and rough beds for Reynolds numbers between 103 and 104, in the transition to turbulence regime. A two-camera 2D- particle image velocimetry (PIV) system was used to visualize coherent structures in an oscillatory-flow tunnel. The obtained results show that smooth-bed flow is populated by vortex-tube structures that are formed due to the Kelvin–Helmholtz instability. Three types of coherent structures are observed in the rough-wall experiments: vortices randomly distributed in space; turbulent bursts – spatial structures without a clear shape compared to vortices but with a longer life than the former; and shear layers of vortices originating due to flow separation from some of the grains on the bed. The reported study contributes to the description of coherent structures in oscillatory flows that are captured with the PIV technique, particularly a new structure in flows over rough beds in the transition to turbulence regime.

本研究针对雷诺数介于10³至10⁴区间、处于转捩至湍流态过程中的光滑与粗糙床面振荡边界层内的相干结构开展探究。研究采用双相机二维粒子图像测速（Particle Image Velocimetry，PIV）系统，于振荡流隧道中对相干结构进行可视化观测。所得结果显示，光滑床面流场中存在由开尔文-亥姆霍兹不稳定性诱发形成的涡管结构。粗糙壁面实验中则观测到三类相干结构：一是空间随机分布的涡旋；二是湍流猝发——此类空间结构虽无明确涡旋形态，但其存续时长长于前者；三是因床面部分颗粒诱发流动分离而形成的涡剪切层。本研究借助PIV技术捕获振荡流场内的相干结构，完善了相关流场的描述体系，尤其揭示了转捩至湍流态的粗糙床面流场中一类新型相干结构。