  In this paper, fluid-particle interactions in lock-exchange turbidity
currents (TCs) over a flat bed are investigated using a model combining LES and
DEM (Large-Eddy Simulation and Discrete Element Method). The reliability of
this model is demonstrated via comparing the numerical solutions with
measurements of the front positions, fluid velocity profile, and particle
concentration profile of lock-exchange TCs. The following physical
understandings are obtained. The vorticity field plays an important role for
the current evolution by affecting the fluid lift force (i.e., in the direction
normal to the fluid-particle slip velocity) acting on the particles. At the
very beginning, a longitudinal positive lift force due to strong positive
vorticity promotes longitudinal particle transport. Afterwards, the
longitudinal lift force decreases and eventually becomes negative, with a
magnitude that even exceeds that of the positive longitudinal drag force,
because more and more of the settling particles are affected by the negative
vorticity near the bottom wall caused by surface friction. Interestingly, in
spite of the complex behavior of the fluid-particle interaction forces and
their role in TC evolution, only a very small fraction of the initial particle
gravitational potential energy is actually transformed into TC kinetic energy
(both particle and fluid).
