The effect of rheology on the flow regimes of fine sediment in oscillatory bottom boundary layer - a numerical investigation

Turbulence-resolving numerical simulations of fine sediment transport in oscillatory bottom boundary layers (OBBL) are carried out to study the interplay between turbulence modulation and rheological stress in determining the transport regimes of mud. A recent numerical study (Ozdemir et al. 2010, J. Fluid Mech., 665. 1-45) on fine sediment transport in OBBL reveals the evolution of transport regimes from a well-mixed sediment distribution, to the formation of lutocline and a complete laminarization of wave boundary layer via increasing sediment availability and settling velocity. We are motivated to further study the effect of rheology in determining the transition of flow regimes and hydrodynamic dissipation. A high-accurate numerical scheme based on a hybrid spectral and compact finite difference scheme is developed in order to resolve all the scales of carrier flow turbulence for moderately energetic wave condition. Fourier expansions are adopted in both streamwise and spanwise direction to enforce the periodic boundary condition. To incorporate both the hindered settling effect and various rheology models, a sixth-order compact finite difference based on non-uniform grid is implemented in vertical direction to keep the spectral-like accuracy. By including the rheology, simulation results shows that the increased effective viscosity tends to increase the thickness of bottom viscous sub-layer and hence further reduce the thickness of turbulent layer, which is constraint upward by the lutocline. In selected cases, including the effect of rheology increases the tendency of laminarization. Our numerical model can provide a new tool to interpret field observation of wave-mud interaction.