  In contrast with a gas at thermodynamic equilibrium, the mean force exerted
on a wall by a gas of active particles usually depends on the confining
potential, thereby preventing a proper definition of mechanical pressure. In
this paper, we investigate numerically the properties of a gas of underdamped
self-propelled dumbbells subject to Brownian noise of increasing intensity, in
order to understand how the notion of pressure is recovered as noise
progressively masks the effects of self-propulsion and the system approaches
thermodynamic equilibrium. The simulations performed for a mobile asymmetric
wall separating two chambers containing an equal number of active dumbbells
highlight some subtle and unexpected properties of the system. First, Brownian
noise of moderate intensity is sufficient to let mean forces equilibrate for
small values of the damping coefficient, while much stronger noise is required
for larger values of the damping coefficient. Moreover, the displacement of the
mean position of the wall upon increase of the intensity of the noise is not
necessarily monotonous and may instead display changes of direction. Both facts
actually reflect the existence of several mechanisms leading to the rupture of
force balance, which tend to displace the mean position of the wall towards
different directions and display different robustness against an increase of
the intensity of Brownian noise. This work therefore provides a clear
illustration of the fact that driving an autonomous system towards (or away
from) thermodynamic equilibrium may not be a straightforward process, but may
instead proceed through the variations of the relative weights of several
conflicting mechanisms.
