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====== PyLith Development Plans, Mar 2015 ======

Priorities for PyLith software development, such as new features and enhancements. This a draft for community comment (Mar 6, 2015).

This plan attempts to balance meeting short-term objectives of delivering high priority, new features and meeting long-term objectives of extending the code to solve a broader range of scientific problems.

Current development has slowed because Brad Aagaard (who is not funded by CIG) is unable to devote much time to PyLith development as a result of a substantial increase of other obligations.

===== Version 2.2 (Target: Jun 2015) =====

  - Improve fault formulation for spontaneous rupture {{intermediate.png}} [10%]
    * Removes inner solve associated with updating Lagrange multipliers. This will significantly accelerate the nonlinear solve.
  - Add viscoelastic Drucker-Prager bulk rheology

===== Version 2.3 (late 2015) =====

  - Higher order basis functions {{expert.png}} [0%]
    * Allow user to select order of basis functions independent of the mesh (which defines the geometry). This permits higher resolution for a given mesh.
  - Reorganize top-level code to conform to layout needed for multiphysics {{difficult.png}} [0%]
    * Setup modular approach for specifying governing equations and computing residuals and Jacobians.
  - Reorganize top-level code to allow different startup cases {{intermediate.png}} [0%]
    * Elastic prestep
    * User-specified initial solution
    * Checkpoint via special spatial database?
  - Add multigrid nonlinear solver 
  - Radial basis functions for spatial databases {{intermediate.png}} [0%]

===== Version 3.0 (Spring 2016) =====

  - Multiphysics
    - Incompressible elasticity via a pressure field {{difficult.png}}
    - Elasticity + heat flow  {{difficult.png}}
    - Elasticity + fluid flow  {{difficult.png}}
  - GUI interface for specifying parameters
  - Switch to using PETSc time-stepping (TS) algorithms. {{intermediate.png}} [0%]
    * Replace simple Python-based time-stepping implementations with PETSc time-stepping algorithms that provide support for higher order discretization in time and real adaptive time stepping.
  - Multilevel nonlinear solve

===== Version 3.1 (Fall 2016) =====

  - Earthquake cycle modeling {{difficult.png}}
    * Same mesh for dynamic and quasi-static parts (dynamic -> quasi-static, quasi-static -> dynamic, complete cycle)
  - Create strain hardening/softening 2-D and 3-D Drucker-Prager elastoplastic models. {{intermediate.png}}
  - Moment tensor point sources via equivalent body forces {{difficult.png}} [5%]
    * Moment tensor point sources provide a mesh independent deformation source that is better suited for Green's function calculations than slip on a fault surface via cohesive cells.

===== Features for Future Releases =====

  * Major features
    - Earthquake Cycle Modeling
      * Different meshes for dynamic and quasi-static parts {{expert.png}}
        * Requires interpolation of fields between different meshes/discretizations and may require extrapolation of solutions when quasi-static problems span a larger domain than the dynamic problems.
    - Data assimilation
      * Use flexibility of multiphysics organization to support inclusion of data assimilation {{expert.png}}

  * Minor features
    - Use KD tree search algorithm to allow output of time histories at an arbitrary location  {{difficult.png}}
    - Combined prescribed slip / spontaneous rupture fault condition {{difficult.png}}
      * Use fault constitutive model to control slip on fault except during episodes of prescribed slip. Need some way to describe when to turn on/off prescribed slip.
    - Use threading to accelerate integrations on multi-core machines. {{difficult.png}}
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