Abstract This study examines the pronounced periodicity of large-scale coherent structures in turbulent boundary layers, which are of the order of the boundary layer thickness (δ) and reside in the logarithmic and wake regions. To this end, a series of multi-camera planar particle image velocimetry (PIV) measurements are conducted in a streamwise/spanwise and streamwise/wall-normal planes at a friction Reynolds number of Reτ ≈ 2500. The experiments are configured to capture a large field-of-view with velocity fields that cover a streamwise extent in excess of 15δ. The resulting vector fields reveal large-scale streamwise and spanwise organisation instantaneously, which is often lost when only examining mean statistics. By extracting the dominant streamwise and spanwise Fourier modes of the large-scale motions, a clearer picture of these structural organisations and coherence is presented. A targeted inspection of these dominant modes reveal that these features remain coherent over a significant fraction of the boundary layer thickness in the wall-normal direction, but only a fraction of them have coherence that extends to the wall (wall-coherent). Further, the spatial extents and the population density of these wall-coherent and wall-incoherent modes are characterised, with the former conforming to the attached eddy arguments of Townsend (1976) and the subsequent attached eddy models. Collectively, through the evidence gathered here, we provide a conceptual picture of the representative large-scale structures in turbulent boundary layers, which are likely to have implications on the type of representative structures to be used in structure-based models for these flows.

中文翻译：

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湍流边界层中大尺度相干的周期性

摘要 本研究考察了湍流边界层中大尺度相干结构的显着周期性，这些结构与边界层厚度 (δ) 数量级相同，位于对数区和尾流区。为此，以 Reτ≈2500 的摩擦雷诺数在流向/展向和流向/壁法向平面中进行了一系列多相机平面粒子图像测速 (PIV) 测量。具有覆盖超过 15δ 的流向范围的速度场的视场。由此产生的向量场瞬间揭示了大规模的流向和展向组织，这在仅检查均值统计数据时通常会丢失。通过提取大尺度运动的主要流向和展向傅立叶模式，更清晰地描述了这些结构性组织和连贯性。对这些主要模式的有针对性的检查表明，这些特征在壁法线方向的边界层厚度的很大一部分上保持连贯，但只有一小部分具有延伸到壁的连贯性（壁相干）。此外，对这些壁相干和壁非相干模式的空间范围和人口密度进行了表征，前者符合 Townsend (1976) 的附加涡流论证和随后附加的涡流模型。总的来说，通过这里收集的证据，我们提供了湍流边界层中具有代表性的大尺度结构的概念图，