The dynamic interplay between surface and subsurface flow in the presence of a permeable boundary was investigated using low and high frame-rate particle-image velocimetry measurements in a refractive-index-matching flow environment. Two idealized permeable wall models were considered. Both models contained five layers of cubically packed spheres, but one exhibited a smooth interface with the flow, while the other embodied a hemispherical surface topography. The relationship between the large-scale turbulent motions overlying the permeable walls and the small-scale turbulence just above, and within, the walls was explored using instantaneous and statistical analyses. Although previous studies have indirectly identified the potential existence of amplitude modulation in permeable-wall turbulence (a phenomenon identified in impermeable-wall turbulence whereby the outer large scales modulate the intensity of the near-wall, small-scale turbulence), the present effort provides direct evidence of its existence in flow over both permeable walls considered. The spatio-temporal signatures of amplitude modulation were also characterized using conditional averaging based on zero-crossing events. This analysis highlights the connection between large-scale regions of high/low streamwise momentum in the surface flow, downwelling/upwelling across the permeable interface and enhancement/suppression of small-scale turbulence, respectively, just above and within the permeable walls. The presence of bed roughness is found to intensify the strength and penetration of flow into the permeable bed modulated by large-scale structures in the surface flow, and linked to possible roughness-formed channelling effects and shedding of smaller-scale flow structures from the roughness elements.

中文翻译：

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渗透壁湍流中调幅的实验证据

在折射率匹配的流动环境中使用低和高帧速率粒子图像测速测量研究了存在可渗透边界的表面和地下流动之间的动态相互作用。考虑了两种理想化的渗透墙模型。两种模型都包含五层立方体填充的球体，但其中一个表现出与流动的平滑界面，而另一个则体现了半球形表面形貌。使用瞬时和统计分析探索了渗透壁上方的大规模湍流运动与壁上方和内部的小规模湍流之间的关系。尽管以前的研究间接确定了可渗透壁湍流中幅度调制的潜在存在（在不可渗透壁湍流中发现的一种现象，即外部大尺度调节近壁小尺度湍流的强度），但目前的努力提供考虑到它存在于两个可渗透壁的流动中的直接证据。还使用基于零交叉事件的条件平均来表征幅度调制的时空特征。该分析突出了地表流中高/低流向动量的大尺度区域、穿过渗透界面的下涌/上升流以及渗透壁上方和内部的小尺度湍流的增强/抑制之间的联系。