Turbidity Currents Containing Mixtures of Sand and Mud

We conduct particle resolved direct numerical simulations of turbidity currents propagating over erodible substrates. The sediment in turbidity currents has a wide range of grain sizes, from mud, Ο(10μm), to sand of Ο(1mm) and larger. To address this multi-scale nature we employ a two-level modeling approach in which the much larger sand grains are resolved with the immersed boundary method while the finer sediment is treated using an Eulerian framework. We investigate four classes of substrate and sediment mobility; smooth wall, fixed bed, bed-load transport, and suspended-load transport. We present a detailed analysis on the effects of bed mobility, porosity, and particle-turbulence coupling on the propagation velocity, structure, and turbulence properties of turbidity currents. Finally, we demonstrate a non-monotonic increase in the friction factor as a function of bed mobility.