Compositional fields

Compositional fields#

Subsection: Compositional fields#

Parameter: Compositional field methods

Default value:

Documentation: A comma separated list denoting the solution method of each compositional field. Each entry of the list must be one of the currently implemented field methods.

These choices correspond to the following methods by which compositional fields gain their values:* “field”: If a compositional field is marked with this method, then its values are computed in each time step by advecting along the values of the previous time step using the velocity field, and applying reaction rates to it. In other words, this corresponds to the usual notion of a composition field as mentioned in Compositional fields.

“particles”: If a compositional field is marked with this method, then its values are obtained in each time step by interpolating the corresponding properties from the particles located on each cell. The time evolution therefore happens because particles move along with the velocity field, and particle properties can react with each other as well. See Particles for more information about how particles behave.
“volume of fluid“: If a compositional field is marked with this method, then its values are obtained in each timestep by reconstructing a polynomial finite element approximation on each cell from a volume of fluid interface tracking method, which is used to compute the advection updates.
“static”: If a compositional field is marked this way, then it does not evolve at all. Its values are simply set to the initial conditions, and will then never change.
“melt field”: If a compositional field is marked with this method, then its values are computed in each time step by advecting along the values of the previous time step using the melt velocity, and applying reaction rates to it. In other words, this corresponds to the usual notion of a composition field as mentioned in Compositional fields, except that it is advected with the melt velocity instead of the solid velocity. This method can only be chosen if melt transport is active in the model.
“darcy field”: If a compositional field is marked with this method, then its values are computed in each time step by advecting along the values of the previous time step using the fluid velocity prescribed by Darcy’s Law, and applying reaction rates to it. In other words this corresponds to the usual notion of a composition field as mentioned in Compositional fields, except that it is advected with the Darcy velocity instead of the solid velocity. This method requires there to be a compositional field named porosity that is advected the darcy field method. We calculate the fluid velocity \(u_f\) using an approximation of Darcy’s Law: \(u_f = u_s - K_D / \phi * (rho_s * g - rho_f * g)\).
“prescribed field”: The value of these fields is determined in each time step from the material model. If a compositional field is marked with this method, then the value of a specific additional material model output, called the ‘PrescribedFieldOutputs’ is interpolated onto the field. This field does not change otherwise, it is not advected with the flow.
“prescribed field with diffusion”: If a compositional field is marked this way, the value of a specific additional material model output, called the ‘PrescribedFieldOutputs’ is interpolated onto the field, as in the “prescribed field” method. Afterwards, the field is diffused based on a solver parameter, the diffusion length scale, smoothing the field. Specifically, the field is updated by solving the equation \((I-l^2 \Delta) C_\text{smoothed} = C_\text{prescribed}\), where \(l\) is the diffusion length scale. Note that this means that the amount of diffusion is independent of the time step size, and that the field is not advected with the flow.

Parameter: Types of fields

Default value: unspecified

Documentation: A comma separated list denoting a “type” for each of the compositional fields requested. ASPECT uses these types to determine how fields are handled when evaluating the material model and when solving the equations as described below.

Each entry of the list must be one of several recognized types: * “chemical composition”: This type of field represents the bulk composition of the modeled material. This type of field is generally considered by the material model to determine the equation of state, rheology, and reactions.

“stress”: This type of field represents stress in the material. Whether the fields represents a scalar stress invariant or tensor components, and which type of stress is represented depends on the interpretation of the material model.
“strain”: This type of field represents accumulated strain. It behaves similar to the type “stress” discussed above except tracking the accumulated strain.
“grain size”: This type of field represents an average mineral grain size of the material. It will only be considered in material models that include models for grain size evolution.
“porosity”: This type of field represents porosity in a two-phase flow or Darcy flow system. Note that setting the type of a compositional field to “porosity” does not automatically enable melt transport, which is done with the parameter “Melt settings/Include melt transport”.
“density”: This type of field is a finite-element field representation of the density in the model. This field type is not usually used except for the projected density approximation of the compressible Stokes equations, which uses this field type to compute gradients and time-derivatives of the density.
“entropy”: This type of field represents entropy. If one or more entropy fields are found in a model, they automatically replace temperature as the main thermodynamic state variable in the model. The temperature equation is then automatically changed to a pure diffusion equation, which is coupled to the entropy advection equation as described in the paper [Dannberg et al., 2022].
“generic”: The generic type is intended to be a placeholder type that is not used by any component of ASPECT unless in user-provided source code.
“unspecified”: The unspecified type is intended to tell ASPECT that the user has not explicitly indicated the type of this field. ASPECT will then try to detect the type automatically based on the name, but will default to “chemical composition” if the name does not correspond to a known type.

Note that while ASPECT’s functionality can make use of the field types, not all of the code will make use of it. It is the user’s responsibility to check that the chosen material model and other plugins interpret the compositional fields as intended.

Compositional fields

Contents

Compositional fields#

Subsection: Compositional fields#