Free and enclosed inertial active gas

In this work, the free expansion of an inertial active gas in three dimensions made of spherical non-interactive active Brownian particles with both translational and rotational inertia (IABPs) is studied. After elucidating the active particles' orientational correlation in three dimensions by employing a Fokker-Planck formalism, the diffusion, mean-square speed, persistence length, reorientation time, Swim and Reynolds pressures and total pressure of this system, are obtained theoretically and corroborated by performing Langevin dynamics simulations. Afterwards, a numerical study on particles' distribution and the mechanical pressure exerted by the active gas enclosed in a cubic box and its dependence on inertia is also carried out. This experiment highlights two important observations: first, as inertia in the system grows while fixing activity, a more uniform particle distribution within the box is achieved. In other words, the classical accumulation of active particles at the walls is seen to be suppressed by inertia. Second, an active gas with translational and rotational inertiae and made of spherical particles still has a state equation which is offered here. This is supported by the fact that both the mechanical pressure definition and the bulk pressure definition as the trace of the swim and Reynolds stress tensors, coincide in the thermodynamic limit.