  We execute a numerical simulation on active nematics with particles
interacting by an excluded volume effect. The systems with rhomboidal particles
and that with elliptical particles are considered in order to investigate the
effect of the direct contact of particles. In our simulation, the void regions,
where the local number density is almost zero, appear in both systems when the
aspect ratio of the particles is high. We focused on the relationship between
the void regions and the particle orientation of the bulk. The particle number
density, particle orientation, topological defects, and void regions are
analyzed for different aspect ratios in both systems. The systems with
rhomboidal particles have characteristic void sizes, which increase with an
increase in the aspect ratio. In contrast, the distribution of the void-region
size in the systems with elliptical particles is broad. The present results
suggest that the void size in the systems with rhomboidal particles is
determined by the correlation length of the particle orientational field around
the void regions, while that might be determined by the system size in the
systems with elliptical particles.
