Beyond Normality: Estimation of Near-Bed Sediment Concentrations Accounting for Asymmetric Distribution and Spatial Influence of Turbulence Coherent Structures

Abstract

Recent experiments have established that the sediment particle motion, especially for particles near the bed, may not follow the normal (Fickian) diffusion behavior. To modify the diffusion equation where the fluctuation velocity is based on the normal distribution, this investigation hypothesizes that the fluctuation velocity based on bivariate probability distributions and particle-bed collision in open channel can provide some physical insight into the particle diffusion behavior. The distribution of fluctuation velocity is obtained using the Gram-Charlier expansion which considers the first four statistical moments of turbulent fluctuation velocity. The correlation between two-dimensional fluctuation velocities is modeled by performing Monte Carlo simulations. Besides, the uniform momentum zones (UMZ) are further identified and consequently the spatial locations of the edges that demarcate UMZs can be estimated. Once UMZs in the turbulent boundary layers can be characterized, the streamwise momentum deficit, and occurrences of ejection events and sweep events in the vicinity of UMZ edges under different Reynolds numbers can be simulated. The spatial influence of turbulent coherent structures on sediment particle trajectory can be demonstrated.