Longitudinal dispersion of multiple Microcystis patches in a turbulent open-channel flow

摘要

Longitudinal dispersion of multiple Microcystis patches is significant for understanding the mechanisms of Microcystis blooms in ambient waters. Presented in this paper is an analysis of multiple Microcystis patches in a turbulent open channel flow, based on the concentration transport equation. Results show that the total amount of Microcystis on a streamline, centre-of-mass motion, as well as longitudinal dispersion finally stabilise for multiple Microcystis patches as they do for a single Microcystis patch. The centre-of-mass velocity exhibits a weak oscillation with a small amplitude in the vertical direction for multiple Microcystis patches, but it can reach the same migration velocity for long time evolution. The large Pe(z), results in an asymptotic increase of centre-of-mass velocity towards a positive value except for very early time for the Microcystis patches in close proximity to the free surface, and an initial reduction to zero, subsequent increase to a maximum, and final decrease to a constant for the bed bottom, where Pe(z) represents the relative strength of vertical swimming and the total effective diffusion in the vertical direction. For the case of large Pe(z), the longitudinal dispersion coefficient first increases to a maximum, then decrease to a minimum, and finally increases to a constant for the Microcystis patches near the free surface. The effects of initial distribution on the evolution of the concentration of Microcystis patch mainly appear during the initial stage and can be negligible for the long time evolution. The Microcystis patches tend to accumulate near the free surface due to the vertical migration velocity caused by density differences between the Microcystis and the ambient water.