# Linear advection equation with variable coefficients

This tutorial is concerned with the linear advection equation

\begin{aligned} \partial_t u(t,x) + \partial_x (a(x) u(t,x)) &= 0, && t \in (0,T), x \in (x_{min}, x_{max}), \\ u(0,x) &= u_0(x), && x \in (x_{min}, x_{max}), \\ \text{boundary conditions}, &&& x \in \partial (x_{min}, x_{max}) \end{aligned}

with variable coefficient $a$.

The boundary conditions depend on the sign of the transport velocity $a$ at the boundary. In particular, specifying a Dirichlet type boundary condition is only allowed for inflow boundaries, e.g. $a(x_{min}) > 0$ at $x = x_{min}$.

SummationByPartsOperators.jl includes a pre-built semidiscretization of this equation: VariableLinearAdvectionNonperiodicSemidiscretization. Have a look at the source code if you want to dig deeper. Below is an example demonstrating how to use this semidiscretization.

using SummationByPartsOperators, OrdinaryDiffEq
using LaTeXStrings; using Plots: Plots, plot, plot!, savefig

# general parameters
xmin = -1.
xmax = +1.
tspan = (0., 8.0)
afunc(x) = one(x)
u0func(x) = sinpi(x)
# Dirichlet type boundary conditions; they are used only at inflow boundaries
left_bc(t) = t >= 3 ? sinpi(t) : zero(t)
right_bc(t) = zero(t)

# discretization parameters
interior_order = 4
N = 101
# whether a split form should be applied or not
split_form = Val(false)

# setup spatial semidiscretization
D = derivative_operator(MattssonSvärdShoeybi2008(), 1, interior_order, xmin, xmax, N)
# whether or not artificial dissipation should be applied: nothing, dissipation_operator(D)
Di = nothing
semi = VariableLinearAdvectionNonperiodicSemidiscretization(D, Di, afunc, split_form, left_bc, right_bc)
ode = semidiscretize(u0func, semi, tspan)

# solve ODE
sol = solve(ode, SSPRK104(), dt=D.Δx, adaptive=false,
save_everystep=false)

# visualise the result
plot(xguide=L"x", yguide=L"u")
plot!(evaluate_coefficients(sol, semi), label=L"u_0")
plot!(evaluate_coefficients(sol[end], semi), label=L"u_\mathrm{numerical}")
savefig("example_linear_advection.png"); 