CalculustCore.jl

A common interface for PDE solving

PDEs are messy: weird boundary conditions, moving domains, singular operators, you name it! And solvers more so with their discretization schemes, stability charectoristics, and performance tradeoffs. Not to mention the layers of complications added by the software stack, or computer architecture, or pre/post-processing pipelines.

Numerical PDE problems drive high-performance computing. The biggest supercomputers on the planet run physics simulations on millions of MPI ranks over years of wall-time. An optimized workflow is the difference between having a solution, or going home emptyhanded, and every ounce of performance has to be squeezed. As such, highly tuned software packages that specialize on a set class of problems dominate the market. With specializations, however, generalizability and interpoeratibility take a hit.

CalculustCore.jl is written so that package authors won't need to write a method for Δ (Laplacian) every time a new scheme for the gradient ∇ (gradient) comes along. We provides abstractions over components of PDE solvers that reduce the amount of boilerplate code that any new solver would need, and improve interpoeratibility between solvers. Furthermore, a requisite for the developement of Machine Learning based PDE solvers is the ability to mix state of the art discretizations with large neural network models on a AD-compatible, accelerator-friendly test-bench. Julia's multiple-dispatch based programming model allows for abstractly-typed, composable code, that lets packages just plug-and-play. Engineers shouldn't waste their time reinventing the wheel, rather focus on what's new and interesting!

We believe switching from 'Fourier-spectral collocation method' to 'discontinuous Galerkin finite elements' should be a one-line change for the user. And users should not need to deal with inconsistent syntax between solvers for specifying boundary conditions, and forming vector-calculus operators.

Finally, CalculustCore.jl is fully compatible with the SciML.ai ecosystem, so after describing your problem, it should spit out the right BoundaryValuePDEProblem or ODEProblem that you can solve using the relevant DiffEq package.

Abstract Interfaces

CalculustCore.jl contains separate abstract interaces for multidimensional domains, vector-calculus operators, and function spaces. It is general enough that anybody can plug in their discretizations (ie define an inner-product operator, and gradient operator) and start solving boundary value problems, or time-evolution problems.

Once you plug in your discretizations, you can do a lot of cool things like apply any random deformations to the space. CalculustCore.jl translate all your vector calculus operators correctly. That means the same code could solve convection diffusion on a square as well as an annulus with no extra work and basically conserved accuracy.

AbstractDomain interface

concrete types, Boundary tags

• deform - all the mapping stuff
• \otimes

concrete types

• IntervalDomain
• BoxDomain

AbstractSpace interface

define these methods (grad, mass, common functions)

• deform
• \otimes
• transform

AbstractDiscretization interface

• GalerkinProjection
• Collocation

Operator interface - SciMLOperators.jl

BoundaryValueProblem interface

Usually a rank-deficient systems

Associated Packages

• FourierSpaces.jl
• NodalPolynomialSpaces.jl

Roadmap

• AbstractDomain interface

  • Move concrete types to a separate package - PDEDomains.jl
  • Logically rectangular domains
  • Deform domain
  • Boundary tags
  • Interior tags (for multiphase flows, conjugate heat-transfer)
  • Gordon Hall interpolation (transfinite interpolation)
  • meshed domains
  • signed distance geometries
  • Time-varying domains
  • Is it possibe to just use DomainSets.jl and add some metadata info?

• AbstractField <: AbstractVector interface - special array types

  • Spectral polynomial (nothing special needed)
  • transform-based spectral (fourier, cheby) (nothing special needed)
  • Box/ full spectral elements

• Operator interface - moved to SciMLOperators.jl

  • linear algebra operations
  • lazy composition
  • can use array reductions
  • caching
  • Gather-Scatter operator using NNlib
  • General interpolation operator on element-meshes

• Spaces

  • Deformed spaces

  • Tensor product spaces

  • transformed space

  • orthogonal polynomials - NodalPolynomialSpaces.jl

  • Spectral with transforms

    • Fourier - FourierSpaces.jl
    • Cosine/ Sine spaces

  • Box, full spectral elements - create SpectralElementSpaces.jl

• Create a distinction between Space, and Discretization

  • Space is how to represent functions
  • Discretization is how you form operators
  • Rename "discretization" to "scheme" or "solve_scheme" or something. Because discretization is ambiguous. is it referring to spatial discretization? or time discretization? In this package we are using it as a "scheme" to form differential operators on a "discretized" space.
  • Rename Galerkin -> GalerkinProjection for clarity
  • Flux handling in discontinuous galerkin/ finite volume
  • all the shennanigans in stabalized finite elements

• Boundary Condition interface (apply "this" boundary condition based on "that" domain boundary)

  • Dirichlet
  • Neumann
  • Robin

• Problems

  • Problem frontend with ModelingToolkit.jl

  • Boundary Value Problems

    • move boundary information to RHS
    • dispatch to LinearSolve.jl
    • dispatch to NonlinearSolve.jl (after LinearSolve.jl, NonlinearSolve.jl integration)

  • Time-evolution problems

    • play nice with OrdinaryDiffEq
    • for implicit time-steppers, solve a BVP at every time step. impose boundary condition on the operator (wait for SciMLOperators, OrdinaryDiffEq integration)
    • automatically spit out an ODEProblem