  We study active agents embedded in bulk or in confinement explicitly
considering hydrodynamics and simulating the swimmers via an implementation
inspired by the squirmer model. We develop a Dissipative Particle Dynamics
scheme for the solvent. This approach allows us to properly deal not only with
hydrodynamics but also with thermal fluctuations. On the other side, this
approach enables us to study active agents with complex shapes, ranging from
spherical colloids to polymers. To start with, we study a simple spherical
colloid. We analyze the features of the velocity fields of the surrounding
solvent, when the colloid is a pusher, a puller or a neutral swimmer either in
bulk or confined in a cylindrical channel. Next, we characterise its dynamical
behaviour by computing the mean square displacement and the long time diffusion
when the active colloid is in bulk or in a channel (varying its radius) and
analyze the orientation autocorrelation function in the latter case. While the
three studied squirmer types are characterised by the same bulk diffusion, the
cylindrical confinement considerably modulates the diffusion and the
orientation autocorrelation function. Finally, we focus our attention on a more
complex shape: an active polymer. We first characterise the structural features
computing its radius of gyration when in bulk or in cylindrical confinement,
and compare to known results obtained without hydrodynamics. Next, we
characterise the dynamical behaviour of the active polymer by computing its
mean square displacement and the long time diffusion. On the one hand, both
diffusion and radius of gyration decrease due to the hydrodynamic interaction
when the system is in bulk. On the other hand, the effect of confinement is to
decrease the radius of gyration, disturbing the motion of the polymer and thus
reducing its diffusion.
