  Accurate numerical-relativity simulations are essential to study the rich
phenomenology of binary neutron star systems. In this work, we focus on the
material that is dynamically ejected during the merger process and on the
kilonova transient it produces. Typically, radiative transfer simulations of
kilonova light curves from ejecta make the assumption of homologous expansion,
but this condition might not always be met at the end of usually very short
numerical-relativity simulations. In this article, we adjust the infrastructure
of the BAM code to enable longer simulations of the dynamical ejecta with the
aim of investigating when the condition of homologous expansion is satisfied.
In fact, we observe that the deviations from a perfect homologous expansion are
about 30% at roughly 100ms after the merger. To determine how these deviations
might affect the calculation of kilonova light curves, we extract the ejecta
data for different reference times and use them as input for radiative transfer
simulations. Our results show that the light curves for extraction times later
than 80ms after the merger deviate by less than 0.4mag and are mostly
consistent with numerical noise. Accordingly, deviations from the homologous
expansion for the dynamical ejecta component are negligible for the purpose of
kilonova modelling.
