Simulation of Near-bed and Pore-space Turbulent Flows in Gravel Riverbed Matrix Systems Using the LES code Hydro3D

Computational models are often employed to simulate large-scale affects of numerous environmental challenges including pollutant dispersion, flooding and hydro-peaking events. Such challenges are greatly affected by turbulent flows which drive many sediment, nutrient and pollutant transport mechanisms. Until recently however, most definitions of turbulence, specifically near-bed turbulence, were based on empirically derived roughness functions which poorly describe highly turbulent near-bed flow [1]. It is only with recent advances in technology that it has been possible to work towards a physically based quantitative definition resulting in something of a resurgence in interest in exploring near-bed turbulence. It is well established that near-bed turbulence drives pore-space microscopic turbulent flow [1-3]. Therefore, the entire system of the riverbed matrix must be investigated to gain a full understanding of both near-bed and pore-space turbulence. However, many studies [4-14] have used simplified roughness geometry for ease of numerical representation and thus, reduced computational demand by numerical simulations. With advances in computing hardware, Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES) methods have made great strides in approximating turbulent flows using simplified roughness geometry [1]. The next evolution in this field of study is to employ more realistic roughness geometry comparable to a typical natural gravel riverbed.

The present study aims to further understanding of near-bed and pore-space turbulent flows by simulating an artificial representation of a gravel riverbed [15] using the finite volume Large-Eddy Simulation (LES) code Hydro3D. The approximation of a gravel riverbed matrix used in this study is comparable to a natural gravel riverbed with low surface roughness and reduced particle size variance, yet typical particle distribution and porosity [15]. The numerical simulations are verified and validated using experimentally derived data obtained in conjunction with a novel study using a physical model of a gravel riverbed matrix [15] in Cardiff University’s Hydraulics Laboratory narrow flume.

Through application of this research, the aim is not only to improve understanding of riverbed turbulence, but to apply that knowledge to wider investigation into sediment, nutrient and pollutant transport mechanisms and their affects on riverine ecosystems at large.