PhysicalFDM.jl

Finite Differencing Method

WARNING: This package is under development!!!

Installation

]add PhysicalFDM

or

using Pkg; Pkg.add("PhysicalFDM")

or

using Pkg; Pkg.add("https://github.com/JuliaAstroSim/PhysicalFDM.jl")

To test the Package:

]test PhysicalFDM

User Guide

This package is extracted from AstroNbodySim.jl. You may find more advanced examples there.

Differencing

Central differencing scheme is defined as

$$ \left(\frac{\partial u}{\partial x}\right){i, j}=\frac{1}{2 \Delta x}\left(u{i+1, j}-u_{i-1, j}\right)+O(\Delta x) $$

Suppose we have an 1D data, and $\delta x = 5$:

julia> d1 = [1,2,1]
3-element Vector{Int64}:
 1
 2
 1

julia> grad_central(5, d1)
3-element Vector{Float64}:
  0.2
  0.0
 -0.2

2D example, where $(\nabla_x(d2), \nabla_y(d2))$ is returned as Tuple:

julia> d2 = [1 1 1; 1 2 1; 1 1 1]
3×3 Matrix{Int64}:
 1  1  1
 1  2  1
 1  1  1

julia> grad_central(5, 5, d2)
([0.1 0.2 0.1; 0.0 0.0 0.0; -0.1 -0.2 -0.1], [0.1 0.0 -0.1; 0.2 0.0 -0.2; 0.1 0.0 -0.1])

3D example

julia> d3 = ones(3,3,3);
       d3[2,2,2] = 2;

julia> grad_central(1, 1, 1, d3)
([0.5 0.5 0.5; 0.0 0.0 0.0; -0.5 -0.5 -0.5;;; 0.5 1.0 0.5; 0.0 0.0 0.0; -0.5 -1.0 -0.5;;; 0.5 0.5 0.5; 0.0 0.0 0.0; -0.5 -0.5 -0.5], [0.5 0.0 -0.5; 0.5 0.0 -0.5; 0.5 0.0 -0.5;;; 0.5 0.0 -0.5; 1.0 0.0 -1.0; 0.5 0.0 -0.5;;; 0.5 0.0 -0.5; 0.5 0.0 -0.5; 0.5 0.0 -0.5], [0.5 0.5 0.5; 0.5 1.0 0.5; 0.5 0.5 0.5;;; 0.0 0.0 0.0; 0.0 0.0 0.0; 0.0 0.0 0.0;;; -0.5 -0.5 -0.5; -0.5 -1.0 -0.5; -0.5 -0.5 -0.5])

FDM solver

Poisson equation

$$\Delta u = f$$

can be discretized to

$$\frac{u_{i+1}-2 u_{i}+u_{i-1}}{\Delta x^{2}}=f_{i}, i = 1, 2, \cdots, N$$

In Dirichlet boundary conditions,

$$ \frac{1}{\Delta x^2} \begin{pmatrix} -2 & 1 & 0 & \cdots & \cdots & 0 \\ 1 & -2 & 1 & 0 & \cdots & \vdots \\ 0 & 1 & -2 & 1 & \cdots & \vdots \\ \vdots & \ddots & \ddots & \ddots & \ddots & \vdots \\ \vdots & \ddots & 1 & -2 & 1 & 0 \\ \vdots & \ddots & 0 & 1 & -2 & 1 \\ 0 & \cdots & \cdots & 0 & 1 & -2 \end{pmatrix} \cdot \begin{pmatrix} u_1 \ u_2 \ \vdots \ \vdots \ \vdots \ u_{N-1} \ u_N \end{pmatrix} = \begin{pmatrix} f_1 \ f_2 \ \vdots \ \vdots \ \vdots \ f_{N-1} \ f_N \end{pmatrix} $$

The Poisson problem is converted to solving a matrix equation

$$\mathbf{A} \cdot \mathbf{u} = \mathbf{f}$$

PhysicalFDM.jl is here to provide user-friendly tools to generate \mathbf{A} matrix.

For example, suppose we have a 1D mesh with 3 points, and for the Poisson problem we have a 2nd-order differencing operator:

julia> A = diff_mat(3,2)
3×3 Matrix{Float64}:
 -2.0   1.0   0.0
  1.0  -2.0   1.0
  0.0   1.0  -2.0

Here's a MWE:

using PhysicalFDM
f = [0, 1, 0]
A = diff_mat(3,2)
u = f \ A

Package ecosystem

• Basic data structure: PhysicalParticles.jl
• File I/O: AstroIO.jl
• Initial Condition: AstroIC.jl
• Parallelism: ParallelOperations.jl
• Trees: PhysicalTrees.jl
• Meshes: PhysicalMeshes.jl
• Plotting: AstroPlot.jl
• Simulation: ISLENT