ABSTRACT
This paper reports on the results of large-eddy simulations of turbulent flow and scalar transport in open channel flows over smooth and rough beds. The rough bed is accomplished artificially through a layer of equidistantly-spaced cubes. The computational setup and boundary conditions of the LES are chosen to correspond to the experimental studies of Rahman and Webster (2005), the results of which are used to validate the simulations. Based on the high-resolution data obtained from the LES, three-dimensional turbulent vortical structures in the flow are recognized, the scalar transport characteristics are quantified statistically, and the role of turbulence in the dispersion of a passive scalar is investigated. In particular, three features of turbulence predominant in channel flows are identified, i.e. a) alternating narrow streaks of low-and high-speed fluid, b) sweeps and ejections associated with the bursting phenomenon, and c) three-dimensional vortical structures, and their influence on scalar dispersion is demonstrated.
