AI 中文总结
研究针对密集均匀粗糙面振荡湍流边界层的矛盾，经PIV记录三重分解分析，明确了受粗糙度控制的薄层结构，揭示离散运动在近床动量传递中的关键作用及对数层的存在机制。
AI 中文摘要
在砾石尺度粗糙度的海岸波浪边界层中，近床轨道偏移量为粗糙度高度的10至100倍，而边界层厚度仅为几个粗糙度高度。若受粗糙度控制的层（RCL）在定常流中的延伸范围为2至5个单元高度，则会没有对数层的空间，但对密集排列大理石的实验却在波峰几毫米范围内恢复了对数剖面。我们通过三重分解重新分析了先前的粒子图像测速（PIV）记录，该分解将锁定于大理石的离散运动与随机湍流分离，覆盖11种波浪、水流及波流共同作用条件。边界层结构分为RCL、过渡区和对数剖面层，其中RCL深度仅为大理石直径的1/10至2/10。这种薄度具有运动学特性：在周期性床面上方，离散场的衰减长度由单元间距决定，因此密集排列使该层厚度限制在直径的一小部分。该层每半个周期会被破坏并重建，准稳态地追踪近床速度，而层内的涡旋仍锁定在波峰间隙。薄度并不意味着强度弱：层内离散动能可与湍流动能相媲美，且离散应力在波峰附近与雷诺应力相当甚至超过后者，表明分离的尾流携带有序动量。对数层得以存在是因为密集排列所施加的衰减长度远小于边界层厚度，该余量由床面几何而非作用力决定。
英文摘要
In coastal wave boundary layers over gravel-scale roughness, with near-bed orbital excursions ten to a hundred times the roughness height, the boundary layer is only a few roughness heights thick. A roughness-controlled layer (RCL) of the steady-flow extent two to five element heights would then leave no room for a logarithmic layer, yet experiments over densely packed marbles recover logarithmic profiles within millimetres of the crests. We resolve this contradiction by re-analysing previous Particle Image Velocimetry (PIV) records with a triple decomposition that separates the marble-locked dispersive motion from the stochastic turbulence, across eleven wave, current and wave-current conditions. The boundary layer organises into an RCL, a transition region, and a logarithmic profile layer, with the RCL only one to two tenths of a marble diameter deep. This thinness is kinematic: above a periodic bed, the dispersive field decays over a length fixed by the element spacing, so close packing caps the layer at a fraction of a diameter. The layer is destroyed and rebuilt every half-cycle, tracking the near-bed velocity quasi-steadily, while the eddies within it stay locked to the inter-crest gap. Thinness does not imply weakness: within the layer, the dispersive kinetic energy rivals the turbulent kinetic energy, and the dispersive stress matches, near the crests exceeds, the Reynolds stress, showing that separated wakes carry organised momentum. The logarithmic layer survives because the decay length imposed by the packing is far smaller than the boundary-layer thickness, a margin set by the bed geometry rather than by the forcing.
