[SCG65-P08] Development Processes of Turbidity Currents Toward the Equilibrium State: Examination by Numerical Simulation
Keywords:turbidity current, numerical simulation, development processes
Therefore, this study focuses on the processes of both temporal and spatial developments of turbidity currents to become the equilibrium state. This study conducted the two-dimensional numerical simulations using computational fluid dynamics software FLOW-3D in order to obtain spatio-temporal change of flow properties of turbidity currents in both vertical and flow-parallel directions. The simulation was conducted under the condition at which the turbidity current continued flowing from the upstream end of the computational domain at constant rates of velocity and sediment concentration for a given time. The computational domain was 200 m long and 30 m deep, and the computational grid size was 5 cm for both vertical and horizontal directions. The flow velocity and height at the upstream boundary were respectively fixed to the values 1 m/s and 0.5 m, and the experimental duration was set to 1800 seconds. As a result of simulation, we obtained the following findings: (1) the turbidity current reached the steady state about several minutes after the beginning of simulation, (2) the height of the horizon showing the maximum velocity was constant in the region about 4 m from the inlet to the downstream end, (3) the maximum velocity converged to the constant value at about 150 m from the inlet, and (4) the flow height defined by the inflection point of the flow velocity profile continuously increased downstream. These results suggest that the lower part of the turbidity current reached the equilibrium state within about 150 meters at the given experimental condition, whereas the upper part of the flow remained non-uniform because of entrainment of the ambient water. Although further numerical simulations at various experimental conditions are required to conclude, we tentatively infer that the lower part of turbidity currents at natural scale can easily reach the equilibrium state and the upper part continues being rarified. In the future, this study will lead to the development of a new layer-averaged model of two-layered turbidity currents which can solve the large-scale morphodynamic problems.
Luchi, R., Parker, G., Balachandar, S., and Naito, K. (2015). Mechanism governing continuous long-runout turbidity currents (in Japanese with English abstract). Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 71.