  We develop a physical and computational model for performing fully coupled,
particle-resolved Direct Numerical Simulations of cohesive sediment, based on
the Immersed Boundary Method. The model distributes the cohesive forces over a
thin shell surrounding each particle, thereby allowing for the spatial and
temporal resolution of the cohesive forces during particle-particle
interactions. The influence of the cohesive forces is captured by a single
dimensionless parameter in the form of a cohesion number, which represents the
ratio of cohesive and gravitational forces acting on a particle. We test and
validate the cohesive force model for binary particle interactions in the
Drafting-Kissing-Tumbling (DKT) configuration. The DKT simulations demonstrate
that cohesive particle pairs settle in a preferred orientation, with particles
of very different sizes preferentially aligning themselves in the vertical
direction, so that the smaller particle is drafted in the wake of the larger
one. To test this mechanism in a system of higher complexity, we perform large
simulations of 1,261 polydisperse settling particles starting from rest. These
simulations reproduce several earlier experimental observations by other
authors, such as the accelerated settling of sand and silt particles due to
particle bonding. The simulations demonstrate that cohesive forces accelerate
the overall settling process primarily because smaller grains attach to larger
ones and settle in their wakes. For the present cohesion number values, we
observe that settling can be accelerated by up to 29%. We propose physically
based parametrization of classical hindered settling functions proposed by
earlier authors, in order to account for cohesive forces. An investigation of
the energy budget shows that the work of the collision forces can substantially
modify the relevant energy conversion processes.
