Inflows/outflows driven particle dynamics in an idealised lake

This paper considers fluid mixing driven by inflows connected to a circular shallow lake using a numerical framework consisting of a shallow water hydrodynamic model and a passive particle-tracking model. With the flow field driven by alternate inflows predicted by a shallow water model, particle trajectories are traced out using a particle tracking model. The horizontal fluid mixing dynamics are then interpreted using dynamics system analysis approaches including finite-time Lyapunov exponent (FTLE) and Lagrangian coherent structure (LCS). From the simulation results, it is confirmed that periodic inflows are able to create a weak dynamic system in an idealised circular lake, with the particle dynamics controlled by a single dimensionless parameter associated with the inflow duration. The mixing and transport property of the lake changes from regular to chaotic as the value of the dimensionless parameter increases until global chaotic particle dynamics is achieved. By further analysing the advection of particles injected continuously to the inflows (freshwater), the fate of “freshwater” particles in a “polluted” lake is tracked and revealed. The results provide useful guidance for engineering applications, i.e., transferring freshwater from rivers to improve the water quality in polluted water bodies such as lakes. The presented approach will be able to facilitate the design of ‘optimised’ schemes for such engineering implementation.