Custom PDEs

Macchiato.jl is not limited to the built-in physics models — you can define and solve any PDE using the same infrastructure. This tutorial walks through solving the Poisson equation on a unit square with a manufactured solution.

Differential operators come from RadialBasisFunctions.jl, and Macchiato re-exports the operator-building surface (laplacian, partial, mixed_partial, gradient, the @operator macro, the PHS/IMQ/Gaussian bases, and the weights accessor) — so using Macchiato is all you need.

The Problem

We solve the 2D Poisson equation:

with Dirichlet boundary conditions   on .

We use a manufactured solution to verify correctness. Choose an exact solution and derive the source term and BCs from it:

Since   on all edges of the unit square, the Dirichlet BCs are   everywhere on the boundary.

Step 1: Define the Model

Create a model struct that subtypes AbstractModel and implement two required methods:

using Macchiato

struct PoissonModel{F} <: AbstractModel
    source::F  # source term f(x, t) -> value
end

# Number of solution variables (1 for scalar PDE)
Macchiato.num_vars(::PoissonModel, _) = 1

# Assemble the linear system for steady-state
function Macchiato.make_system(model::PoissonModel, domain; kwargs...)
    x = node_coordinates(domain)
    ∇² = laplacian(x; k = 40, kwargs...)
    A = weights(∇²)

    # Evaluate source term at each point
    b = [model.source(xᵢ, 0.0) for xᵢ in x]

    return A, b
end

That's it — just a struct and two methods. The key points:

• num_vars returns the number of unknowns per point (1 for scalar, dim for vector)

• make_system builds the system matrix A and right-hand side b; Macchiato handles BC application and solving

• node_coordinates returns the cloud's coordinates unit-stripped, ready for the operator constructors

• weights is the supported accessor for an operator's sparse weight matrix

• k is the stencil size. If omitted, RadialBasisFunctions.jl picks the minimum the basis needs (12 here), which can produce singular stencils near boundaries where neighbors are nearly collinear — pass a larger value; the built-in Macchiato models use 40

Step 2: Solve and Verify

Boundary conditions use the generic constructors directly — no aliases or trait definitions needed:

using WhatsThePoint
using Unitful: m, °, ustrip

# Manufactured solution and source term
u_exact(x) = sin(π * x[1]) * sin(π * x[2])
f_source(x, t) = -2π^2 * sin(π * x[1]) * sin(π * x[2])

# Geometry: unit square point cloud
part = PointBoundary(rectangle(1m, 1m)...)
split_surface!(part, 75°)
dx = 1/33 * m
cloud = discretize(part, ConstantSpacing(dx))

# Model
model = PoissonModel(f_source)

# BCs: use generic constructors directly
bcs = Dict(
    :surface1 => PrescribedValue(0.0),
    :surface2 => PrescribedValue(0.0),
    :surface3 => PrescribedValue(0.0),
    :surface4 => PrescribedValue(0.0),
)

# Solve
domain = Domain(cloud, bcs, model)
sim = Simulation(domain)
run!(sim)

# Verify against exact solution
# For custom models, access the solution vector via the public accessor
u_numerical = solution(sim)
u_exact_vals = u_exact.(node_coordinates(domain))
error = maximum(abs.(u_numerical .- u_exact_vals))
println("Max error: $error")

┌ Warning: Only FornbergFlyer algorithm is implemented for 2D point clouds. Using it.
└ @ WhatsThePoint ~/.julia/packages/WhatsThePoint/2HBRO/src/discretization/discretization.jl:45

⠋ generating nodes    Time: 0:00:00
✓ generating nodes    Time: 0:00:00
Max error: 0.0022068782040292056

With 33 points per side you should see a max error on the order of or better.

Multi-Term Operators: the @operator Macro

The Poisson example needs only a bare Laplacian, but most PDEs combine several terms. Rather than building each operator and summing weight matrices by hand, write the operator in mathematical notation with @operator — the whole expression is fused into a single weight matrix:

struct AdvectionDiffusion{T, V, S} <: AbstractModel
    ν::T          # diffusivity
    c::V          # advection velocity vector
    source::S     # source term f(x, t) -> value
end

Macchiato.num_vars(::AdvectionDiffusion, _) = 1

# ν∇²u − c·∇u = f, assembled in one fused weights build
function Macchiato.make_system(model::AdvectionDiffusion, domain; kwargs...)
    (; ν, c) = model
    x = node_coordinates(domain)
    op = (@operator ν * ∇² - c ⋅ ∇)(x; k = 40, kwargs...)
    b = [model.source(xᵢ, 0.0) for xᵢ in x]
    return weights(op), b
end

Recognized symbols include ∇²/Δ, ∂(dim), ∂²(dim), ∂(i, j) (mixed partials), ∇ ⋅ (κ * ∇) (diffusion), c ⋅ ∇ (advection), and f/I (identity), plus scalar coefficients. Built operators also compose directly — α * op and op₁ + op₂ combine the existing weight matrices without re-collocation, which is how the built-in LinearElasticity model assembles its blocks. See the Building PDE Operators guide in RadialBasisFunctions.jl for the full vocabulary.

Optional: Named BC Aliases

For readability, you can define named aliases that wrap the generic constructors:

PoissonValue(value) = PrescribedValue(value)
PoissonFlux(flux) = PrescribedFlux(flux)
PoissonZeroFlux() = ZeroFlux()

PoissonZeroFlux (generic function with 1 method)

These are purely syntactic sugar — they construct the same generic types (PrescribedValue, PrescribedFlux, ZeroFlux) that power all built-in BCs.

Key Takeaways

2. Any PDE works. Define an AbstractModel subtype and implement num_vars and make_system. That's all Macchiato needs.

3. No boilerplate traits required. Generic BC types (PrescribedValue, PrescribedFlux, ZeroFlux) dispatch on the mathematical hierarchy (Dirichlet/Neumann/Robin), so they work with any model — no equation-type trait needed.

4. Operators come from RadialBasisFunctions.jl and are re-exported. Use laplacian, partial, mixed_partial, gradient, or compose multi-term operators with @operator — using Macchiato brings them all in. Access an operator's sparse matrix with weights(op), never the internal field.

5. Generic BC types work directly. PrescribedValue(value), PrescribedFlux(flux), and ZeroFlux() work out of the box for custom PDEs. Named aliases like Temperature are just convenience wrappers used by the built-in models.

6. Transient support. For time-dependent PDEs, implement make_f(model, domain) instead of make_system to return an ODE right-hand side f(du, u, p, t). Macchiato integrates it with OrdinaryDiffEq.jl automatically. See the Package Design page for details on the transient path. One caveat: in the transient path only Dirichlet surfaces are enforced by the framework — Neumann/Robin flux surfaces must be folded into your operator inside make_f via Macchiato.build_neumann_diffusion(domain; k, operator = ...), or they are silently ignored. The operator keyword accepts a builder closure for non-Laplacian operators; examples/niederer_benchmark/niederer_benchmark.jl is the worked anisotropic example.