addressed comments

include/aspect/material_model/rheology/composite_visco_plastic.h

@@ -38,6 +38,31 @@ namespace aspect
 
  namespace Rheology
  {
+ /**
+ * A namespace for selecting how to average the viscosity
+ * over multiple chemical compositions.
+ *
+ * Current options are:
+ * 'isostress': Reuss (harmonic) averaging of viscosities.
+ * 'isostrain': Voigt (arithmetic) averaging of viscosities.
+ */
+ namespace ViscosityAveraging
+ {
+ enum Kind
+ {
+ isostress,
+ isostrain
+ };
+
+ /**
+ * Read the lithosphere age model from the parameter file,
+ * using the parameter name given in @p parameter_name, and return the
+ * enum that corresponds to this operation.
+ */
+ ViscosityAveraging::Kind
+ parse (const std::string &parameter_name,
+ const ParameterHandler &prm);
+ }
 
  template <int dim>
  class CompositeViscoPlastic : public ::aspect::SimulatorAccess<dim>
@@ -83,6 +108,7 @@ namespace aspect
  const std::vector<double> &phase_function_values = std::vector<double>(),
  const std::vector<unsigned int> &n_phase_transitions_per_composition = std::vector<unsigned int>()) const;
 
+ private:
  /**
  * Compute the isostress viscosity over all compositional fields
  * based on the composite viscous creep law.
@@ -104,15 +130,34 @@ namespace aspect
  /**
  * Compute the total strain rate and the first derivative of log strain rate
  * with respect to log viscoplastic stress from individual log strain rate components
- * over all compositional fields. Also updates the partial_strain_rates vector
+ * over all compositional fields. Also updates the partial_strain_rates vector.
  */
  std::pair<double, double>
  calculate_isostress_log_strain_rate_and_derivative(const std::vector<std::array<std::pair<double, double>, 4>> &logarithmic_strain_rates_and_stress_derivatives,
  const double viscoplastic_stress,
  std::vector<double> &partial_strain_rates) const;
 
  /**
- * Compute the compositional field viscosity
+ * Compute the isostrain viscosity over all compositional fields
+ * based on the composite viscous creep law.
+ * If @p expected_n_phases_per_composition points to a vector of
+ * unsigned integers this is considered the number of phase transitions
+ * for each compositional field and viscosity will be first computed on
+ * each phase and then averaged for each compositional field.
+ */
+ double
+ compute_isostrain_viscosity (const double pressure,
+ const double temperature,
+ const double grain_size,
+ const std::vector<double> &volume_fractions,
+ const SymmetricTensor<2,dim> &strain_rate,
+ std::vector<double> &partial_strain_rates,
+ const std::vector<double> &phase_function_values = std::vector<double>(),
+ const std::vector<unsigned int> &n_phase_transitions_per_composition = std::vector<unsigned int>()) const;
+
+
+ /**
+ * Compute the viscosity for a single compositional field
  * based on the composite viscous creep law.
  * If @p n_phase_transitions_per_composition points to a vector of
  * unsigned integers this is considered the number of phase transitions
@@ -131,37 +176,41 @@ namespace aspect
 
  /**
  * Compute the total strain rate and the first derivative of log strain rate
- * with respect to log viscoplastic stress from individual log strain rate components.
- * Also updates the partial_strain_rates vector
+ * with respect to log viscoplastic stress from individual log strain rate components
+ * for a single compositional field.
+ * Also updates the partial_strain_rates vector.
  */
  std::pair<double, double>
  calculate_composition_log_strain_rate_and_derivative(const std::array<std::pair<double, double>, 4> &logarithmic_strain_rates_and_stress_derivatives,
  const double viscoplastic_stress,
  std::vector<double> &partial_strain_rates) const;
 
- private:
  /**
  * Enumeration for selecting which type of viscosity averaging to use.
  */
- enum ViscosityAveragingScheme
- {
- isostrain,
- isostress
- } viscosity_averaging_scheme;
+ ViscosityAveraging::Kind viscosity_averaging_scheme;
 
  /**
- * Whether to use different deformation mechanisms
+ * Whether to use different deformation mechanisms.
  */
  bool use_diffusion_creep;
  bool use_dislocation_creep;
  bool use_peierls_creep;
  bool use_drucker_prager;
 
  /**
- * Vector storing which flow mechanisms are active
+ * Vector storing which flow mechanisms are active.
  */
  std::vector<unsigned int> active_flow_mechanisms;
 
+ /**
+ * Total number of decomposed strain rates. This includes the maximum
+ * viscosity limiter but not the minimum viscosity limiter,
+ * which is arranged in parallel with the viscoplastic elements and
+ * therefore does not contribute to the total strain rate.
+ */
+ static constexpr unsigned int n_decomposed_strain_rates = 5;
+
  /**
  * Pointers to objects for computing deformation mechanism
  * strain rates and effective viscosities.
@@ -178,7 +227,7 @@ namespace aspect
 
  /**
  * The maximum viscosity, imposed via an isoviscous damper
- * in series with the composite viscoplastic element
+ * in series with the composite viscoplastic element.
  */
  double maximum_viscosity;
 
@@ -221,7 +270,7 @@ namespace aspect
  unsigned int stress_max_iteration_number;
 
  /**
- * Useful number to aid in adding together exponentials
+ * Useful number to aid in adding together exponentials.
  */
  const double logmin = std::log(std::numeric_limits<double>::min());
  };