PDESystem

PDESystem is the common symbolic PDE specification for the SciML ecosystem. It is currently being built as a component of the ModelingToolkit ecosystem,

Vision

The vision for the common PDE interface is that a user should only have to specify their PDE once, mathematically, and have instant access to everything as simple as a finite difference method with constant grid spacing, to something as complex as a distributed multi-GPU discontinuous Galerkin method.

The key to the common PDE interface is a separation of the symbolic handling from the numerical world. All the discretizers should not “solve” the PDE, but instead be a conversion of the mathematical specification to a numerical problem. Preferably, the transformation should be to another ModelingToolkit.jl AbstractSystem, but in some cases this cannot be done or will not be performant, so a SciMLProblem is the other choice.

These elementary problems, such as solving linear systems Ax=b, solving nonlinear systems f(x)=0, ODEs, etc. are all defined by SciMLBase.jl, which then numerical solvers can all target these common forms. Thus, someone who works on linear solvers doesn't necessarily need to be working on a discontinuous Galerkin or finite element library, but instead "linear solvers that are good for matrices A with properties ..." which are then accessible by every other discretization method in the common PDE interface.

Similar to the rest of the AbstractSystem types, transformation, and analysis functions will allow for simplifying the PDE before solving it, and constructing block symbolic functions like Jacobians.

Constructors

ModelingToolkit.PDESystem — Type

struct PDESystem <: AbstractMultivariateSystem

A system of partial differential equations.

Fields

eqs: The equations which define the PDE.
bcs: The boundary conditions.
domain: The domain for the independent variables.
ivs: The independent variables.
dvs: The dependent variables.
ps: The parameters.
defaults: The default values to use when initial conditions and/or parameters are not supplied in ODEProblem.

connector_type: Type of the system.

systems: The internal systems. These are required to have unique names.

analytic: A vector of explicit symbolic expressions for the analytic solutions of each dependent variable. e.g. analytic = [u(t, x) ~ a*sin(c*t) * cos(k*x)].

analytic_func: A vector of functions for the analytic solutions of each dependent variable. Will be generated from analytic if not provided. Should have the same argument signature as the variable, and a ps argument as the last argument, which takes an indexable of parameter values in the order you specified them in ps. e.g. analytic_func = [u(t, x) => (ps, t, x) -> ps[1]*sin(ps[2]*t) * cos(ps[3]*x)].

name: The name of the system.

metadata: Metadata for the system, to be used by downstream packages.

gui_metadata: Metadata for MTK GUI.

Example

using ModelingToolkit

@parameters x
@variables t u(..)
Dxx = Differential(x)^2
Dtt = Differential(t)^2
Dt = Differential(t)

#2D PDE
C=1
eq  = Dtt(u(t,x)) ~ C^2*Dxx(u(t,x))

# Initial and boundary conditions
bcs = [u(t,0) ~ 0.,# for all t > 0
       u(t,1) ~ 0.,# for all t > 0
       u(0,x) ~ x*(1. - x), #for all 0 < x < 1
       Dt(u(0,x)) ~ 0. ] #for all  0 < x < 1]

# Space and time domains
domains = [t ∈ (0.0,1.0),
           x ∈ (0.0,1.0)]

@named pde_system = PDESystem(eq,bcs,domains,[t,x],[u])

source

Domains (WIP)

Domains are specifying by saying indepvar in domain, where indepvar is a single or a collection of independent variables, and domain is the chosen domain type. A 2-tuple can be used to indicate an Interval. Thus forms for the indepvar can be like:

t ∈ (0.0, 1.0)
(t, x) ∈ UnitDisk()
[v, w, x, y, z] ∈ VectorUnitBall(5)

Domain Types (WIP)

Interval(a,b): Defines the domain of an interval from a to b (requires explicit import from DomainSets.jl, but a 2-tuple can be used instead)

`discretize` and `symbolic_discretize`

The only functions which act on a PDESystem are the following:

discretize(sys,discretizer): produces the outputted AbstractSystem or SciMLProblem.
symbolic_discretize(sys,discretizer): produces a debugging symbolic description of the discretized problem.

PDESystem

Vision

Constructors

Domains (WIP)

Domain Types (WIP)

`discretize` and `symbolic_discretize`

Boundary Conditions (WIP)

Transformations

Analyses

Discretizer Ecosystem

NeuralPDE.jl: PhysicsInformedNN

MethodOfLines.jl: MOLFiniteDifference