20.1 Introduction

The Gravity unit supplied with FLASH4 computes gravitational source terms for the code. These source terms can take the form of the gravitational potential $\phi({\bf x})$ or the gravitational acceleration ${\bf g}({\bf x})$,

$${\bf g}({\bf x}) = -\nabla \phi({\bf x}) .$$ (20.1)

The gravitational field can be externally imposed or self-consistently computed from the gas density via the Poisson equation,

$$\nabla^2\phi({\bf x}) = 4\pi G \rho({\bf x}) ,$$ (20.2)

where $G$ is Newton's gravitational constant. In the latter case, either periodic or isolated boundary conditions can be applied.