Author(s): Li Jiaxing; Chen Xin
Linked Author(s):
Keywords: Sheet flow; Sediment transport; Two-phase mixture model; Grain size effect; Oscillatory flow
Abstract: Sheet flow under extreme wave condition leads to intense sediment transport in coastal region. In this paper, a two-phase mixture model is developed for high-efficient simulation of the sediment transport in wave-induced sheet flow. The particle stress representing the particle-particle interactions is introduced into governing equations and closed by the dense granular flow rheology model. The mixture turbulence is closed by km-εm model improved with the turbulence modulations in high-concentrated sediment-laden flow. The sediment diffusion is improved due to the introductions of the particle wake effect in the suspended region and particle self-diffusion in the dense region. The two-phase mixture model is applied to the simulations of the velocity-skewed and acceleration-skewed oscillatory sheet flows (O'Donoghue & Wright, 2004; Li et al., 2008; van der A et al., 2010), as shown in Figure 1. The grain size effect is investigated by the comparisons between the fine sand cases and medium sand cases. The numerical results show satisfactory agreement with the laboratory data, e.g. the temporal and spatial distributions of velocity and sediment concentration, the evolution of sheet flow layer thickness. The model also predicts the net sediment transport rate with a reasonably good accuracy the net sediment transport rate for the velocity-skewed and acceleration-skewed oscillatory sheet flows, as shown in Figure 2. Different transport characteristics are identified between fine sand and medium sand, which are mainly related to the turbulence modulation. Compared to the medium sand cases, much more suspended sediments in the fine sand cases attenuate turbulence intensity and induce lower the boundary layer thickness. Therefore, fine sand cases and medium sands case perform different residual flow. For velocity-skewed oscillatory sheet flows, the negative net transport rate is caused by the notable negative residual flow in fine sand cases, while the medium sand cases perform positive net transport rate due to mobile bed effect and turbulence asymmetry. For acceleration-skewed oscillatory sheet flows, all cases show positive transport, where the positive residual flow strengthens the positive net transport rate in finer sands cases. The grain size influences the sediment flux through the turbulence modulation.
Year: 2024