Using passive and active compositional fields

One frequently wants to track where material goes, either because one simply wants to see where stuff ends up (e.g., to determine if a particular model yields mixing between the lower and upper mantle) or because the material model in fact depends not only pressure, temperature and location but also on the mass fractions of certain chemical or other species. We will refer to the first case as passive and the latter as active to indicate the role of the additional quantities whose distribution we want to track. We refer to the whole process as compositional since we consider quantities that have the flavor of something that denotes the composition of the material at any given point. There are basically two ways to achieve this: one can advect a set of particles (“tracers”) along with the velocity field, or one can advect along a field. In the first case, where the closest particle came from indicates the value of the concentration at any given position. In the latter case, the concentration(s) at any given position is simply given by the value of the field(s) at this location. ASPECT implements both strategies, at least to a certain degree. In this cookbook, we will follow the route of advected fields.

• The passive case.
• The active case.
• The active case with reactions.