  Three-dimensional hydrodynamic simulations are commonly used to study the
evolution of the gaseous content in isolated galaxies, besides its connection
with galactic star formation histories. Stellar winds, supernova blasts, and
black hole feedback are mechanisms usually invoked to drive galactic outflows
and decrease the initial galactic gas reservoir. However, any simulation
imposes the need of choosing the limits of the simulated volume, which depends,
for instance, on the size of the galaxy and the required numerical resolution,
besides the available computational capability to perform it. In this work, we
discuss the effects of boundary conditions on the evolution of the gas fraction
in a small-sized galaxy (tidal radius of about 1 kpc), like classical
spheroidal galaxies in the Local Group. We found that open boundaries with
sizes smaller than approximately 10 times the characteristic radius of the
galactic dark-matter halo become unappropriated for this kind of simulation
after about 0.6 Gyr of evolution, since they act as an infinity reservoir of
gas due to dark-matter gravity. We also tested two different boundary
conditions that avoid gas accretion from numerical frontiers: closed and
selective boundary conditions. Our results indicate that the later condition
(that uses a velocity threshold criterion to open or close frontiers) is
preferable since minimizes the number of reversed shocks due to closed
boundaries. Although the strategy of putting computational frontiers as far as
possible from the galaxy itself is always desirable, simulations with selective
boundary condition can lead to similar results at lower computational costs.
