# PyLith Development Plan, April 2019

## Version 3.0 (June 2019)

* Multiphysics ![expert](images/expert.png) [30%]
  * Implement modular approach for specifying governing equations and computing residuals and Jacobians. ![expert](images/expert.png)
  * Incompressible elasticity via a pressure field  ![intermediate](images/intermediate.png) [20%]

* Higher order basis functions ![difficult](images/difficult.png) [100%]

  Allow user to select order of basis functions independent of the mesh (which defines the geometry). This permits higher resolution for a given mesh.

* Switch to using PETSc time-stepping (TS) algorithms. ![intermediate](images/intermediate.png) [75%]

  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.

* Modular approach for initial conditions ![intermediate](images/intermediate.png) 

* Testing via Method of Manufactured Solutions ![difficult](images/difficult.png) 

## Version 3.1

* Improve fault formulation for spontaneous rupture ![expert](images/expert.png) [10%]

  Removes inner solve associated with updating Lagrange multipliers. This will significantly accelerate the nonlinear solve.

* Add additional multiphysics implementations and rheologies ![intermediate](images/intermediate.png)

  * Poroelasticity  [5%]

## Version 3.2

* Convert to Python 3 ![difficult](images/difficult.png) 
* Drucker-Prager bulk rheology with relaxation to yield surface ![intermediate](images/intermediate.png) 
* Elasticity + heat flow  ![intermediate](images/intermediate.png) 
* Reorganize output for time-dependent Green's functions and adjoints ![intermediate](images/intermediate.png)  
* Multilevel nonlinear solve  ![expert](images/expert.png) 
* Radial basis functions for spatial databases  ![difficult](images/difficult.png) 

## Version 4.0

* Earthquake cycle modeling 

  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. 

* Moment tensor point sources  [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 

      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 implementation to support inclusion of data assimilation 

* Minor features
  * Begin implementation of data assimilation capabilities via adjoint equation.
  * Combined prescribed slip / spontaneous rupture fault condition 

    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.