  A simple model for simulating flows of active suspensions is investigated.
The approach is based on dissipative particle dynamics. While the model is
potentially applicable to a wide range of self-propelled particle systems, the
specific class of self-motile bacterial suspensions is considered as a modeling
scenario. To mimic the rod-like geometry of a bacterium, two dissipative
particle dynamics particles are connected by a stiff harmonic spring to form an
aggregate dissipative particle dynamics molecule. Bacterial motility is modeled
through a constant self-propulsion force applied along the axis of each such
aggregate molecule. The model accounts for hydrodynamic interactions between
self-propelled agents through the pairwise dissipative interactions
conventional to dissipative particle dynamics. Numerical simulations are
performed using a customized version of the open-source LAMMPS (Large-scale
Atomic/Molecular Massively Parallel Simulator) software package. Detailed
studies of the influence of agent concentration, pairwise dissipative
interactions, and Stokes friction on the statistics of the system are provided.
The simulations are used to explore the influence of hydrodynamic interactions
in active suspensions. For high agent concentrations in combination with
dominating pairwise dissipative forces, strongly correlated motion patterns and
a fluid-like spectral distributions of kinetic energy are found. In contrast,
systems dominated by Stokes friction exhibit weaker spatial correlations of the
velocity field. These results indicate that hydrodynamic interactions may play
an important role in the formation of spatially extended structures in active
suspensions.
