Growing supermassive black holes: sub-grid modelling and intermediate-scale processes

The sheer range of scales in the Universe makes it impossible to model all at once. It is necessary, therefore, when conducting numerical experiments, that we employ sub-resolution prescriptions that can represent the scales we are unable to model directly. In this article we present a prescription for black hole growth that incorporates a different accretion regime from the standard approach used in the literature, and discuss the results of dedicated simulations of intermediate processes between small-scale accretion flows and large-scale cosmological volumes that can strongly enhance the accretion rate onto the black hole at the centre of a galaxy.