Response of flow and saltating particle characteristics to bed roughness and particle spatial density

With the goal to explore the effects of natural bed roughness on bedload transport, numerical simulations of flow and particle saltation are carried out with varying bed roughness and particle spatial density. A combination of Eulerian and Lagrangian point-particle methods is applied to solve the equations of motion of the fluid and the particles within the large-eddy simulation framework. Flows over smooth and rough beds with four particle densities are considered. As the bed roughness increases, there is a leftward shift of the double-averaged streamwise velocity profiles against dimensionless vertical distance, which is scaled with the bed roughness height, an upward shift of the peak values of double-averaged Reynolds stresses, and a fragmentation and disappearance of coherent structures in the form of high-speed and low-speed near-bed streaks. These observations are consistent with those of previous studies. As the bed roughness increases, the mean resting time of saltating particles increases, however the particles' saltation length, velocity, and angular velocity decrease, while their saltation height remains almost unchanged. Saltation height, saltation length, particle angular velocity, and resting time exhibit linear, gamma, normal, and exponential distributions, respectively. Further, as the bed roughness increases, the kurtosis and skewness of some particle parameters vary, and the particle velocity shifts from a symmetrical normal to an asymmetrical gamma distribution.