Turbulent flow in a depth‐limited boundary layer

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Abstract
Turbulent flow was investigated at a constant flow Reynolds number in a depth‐limited boundary layer. Relative roughness was 0.12. Roughness density (ratio of plan area of elements to total bed area) was varied to cover the range of flow types described by Morris [1955]. The effect of roughness density on velocity profile shape factors, energy dissipation, and streamwise kinetic energy confirmed the existence of three types of roughness density‐flow interaction, namely, ‘skimming,’ ‘wake interaction,’ and ‘isolated roughness’ flows. The velocity profiles and kinetic energy measures identified three layers in the vertical profile: an outer layer (y/D > 0.35), a wake layer (0.35 > y/D > 0.10), and an inner region (y/D < 0.10). The wake layer is produced by strong turbulent action accompanying vortex shedding at intermediate roughness densities. Spectral measurements and calculations of the turbulent macroscale indicated that the spectral shape is affected by the free surface but that below y/D = 0.5 a parametric model using the turbulent Reynolds number in the spectral energy equation adequately describes the spectral shape for all roughness densities. Implications of these results are discussed for the study of shallow, natural flows with high relative roughness.

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