NDSolve for Laplace equation on disk is not working

Question

I want to solve the laplace equation on the disk using NDSolve. Particularly I want to solve the problem $$\frac{\partial^2 u }{\partial r^2 } +r^{-1}\frac{\partial u }{\partial r }+r^{-2}\frac{\partial^2 u }{\partial \theta^2 }=0$$ with $$u(a,\theta )=f(\theta )$$ and $0<\theta <2 \pi $.

Also set $a=3$ and $f(\theta )=\begin{cases} 1 & 0 \leq \theta \leq \pi \\ \sin^2 \theta & \pi < \theta < 2 \pi \end{cases}$.

So my attemped code is

Clear["Global`*"]
a = 3;
f[theta_] := 
 Piecewise[{{1, 0 <= theta <= π}, {(Sin[theta])^2, π < theta < 2*π}}]
PDE = (D[u[r, theta], {r, 2}]) + r^-1(D[u[r, theta], {r, 1}]) + 
    r^-2(D[u[r, theta], {theta, 2}]) == 0;
BC = u[r, theta /. theta -> 0] == u[r, theta /. theta -> 2*π]
IC = u[r /. r -> a, theta] == f[theta];
sol = NDSolve[{PDE, BC, IC }, u, {r, theta} ∈ Disk[]];
Plot3D[u[r, theta], {r, 0, a}, {theta, 0, 2*π}]

However the code is not working, I can't figure out why. Can you explain my mistake?

Answer 1

3 issues here.

1. You've mixed up polar coordinates and Cartesian coordinates. Since the equation is defined in polar coordinates, the shape of domain of definition is no longer a Disk but a rectangle. (Even if it's a disk, it should be Disk[{0, 0}, a] rather than Disk[]. )

2. Plot3D[u[r, theta], …] is obviously wrong, you should call the sol with ReplaceAll (/.), or use NDSolveValue instead of NDSolve.

3. There's an issue related to periodic b.c. of FiniteElement method, which is discussed in this post. To obtain the desired solution, we need to set 2 PeriodicBoundaryConditions and a ghost layer.

The following is the fixed code:

fem[measure_: Automatic] := {"FiniteElement", 
     "MeshOptions" -> MaxCellMeasure -> measure};
a = 3; eps = 10^-5;
f[theta_] := Piecewise[{{1, 0 <= theta <= π}}, (Sin[theta])^2]
PDE = (D[u[r, theta], {r, 2}]) + r^-1(D[u[r, theta], {r, 1}]) + 
    r^-2(D[u[r, theta], {theta, 2}]) == 0;
BC = {PeriodicBoundaryCondition[u[r, theta], theta == -Pi && r < a, 
    TranslationTransform[{0, 2 π}]],
   PeriodicBoundaryCondition[u[r, theta], theta == Pi + eps && r < a, 
    TranslationTransform[{0, -2 π}]]};
IC = u[a, theta] == f[theta];
sol = NDSolve[{PDE, BC, IC}, u, {r, 0, a}, {theta, -Pi, π + eps}, 
   Method -> fem[10^-3/2]];
RevolutionPlot3D[u[r, th] /. sol // Evaluate, {r, 0, a}, {th, -Pi, Pi}, 
 PlotRange -> All]

But since setting PeriodicBoundaryCondition is so troublesome, why not staying in Cartesian coordinates?:

sol2 = NDSolveValue[{Laplacian[u[x, y], {x, y}] == 0, 
   DirichletCondition[u[x, y] == f[ArcTan[x, y]], True]}, 
  u, {x, y} ∈ Disk[{0, 0}, a], Method -> fem[10^-3/2]]
With[{ϵ = 10^-6}, 
 Manipulate[Plot[{u[Sqrt[x^2 + y^2], ArcTan[x, y]] /. sol[[1]], sol2[x, y]} // 
    Evaluate, {x, -Sqrt[a^2 - y^2], Sqrt[a^2 - y^2]}, 
   PlotStyle -> {Automatic, Dashed}, PlotRange -> {0, 1}], {y, -a + ϵ, a - ϵ}]]

Remark

1. I haven't set "MeshElementType" -> "TriangleElement" instead of the ghost layer, because this trick isn't working well in this case. This might be related to the removable singularity at $r=0$.

2. I've moved the domain of $\theta$ from $[0,2\pi]$ to $[-\pi,\pi]$ to make it consistent with range of 2-argument arctangent.

3. If you check the last animation carefully, you'll find small but obvious discrepancy between the 2 solutions. Further comparing them with the analytic solution (you can find it in Nasser's answer), you'll see the solution in Cartesian coordinates is more accurate. Setting a even smaller option value for MaxCellMeasure in polar coordinates does help, but the convergence speed is not great. OK, now we have another reason for staying in Cartesian coordinates.

Answer 2

It looks like the problem was with the periodic BC and JM's above showed how to handle this periodic BC.

This below shows that DSolve can also solve this analytically and compares the output from the numerical solution by JM and the analytical solution.

note: used $-\pi\dots\pi$ and not $0\dots 2\pi$. Also note the periodic boundary conditions syntax for the analytical solution. This now gives same result as the numerical solution.

Analytical solution

Clear["Global`*"]
a = 3;
f[r_, θ_] :=  Piecewise[{{1, 0 <= θ <= π}, {Sin[θ]^2, -π < θ < 0}}]
BC1 = u[a, θ] == f[a, θ]
BC2 = {u[r, -Pi]==u[r, Pi], (D[u[r, θ], θ]/. θ -> -Pi)==(D[u[r, θ], θ] /. θ -> Pi)};
sol = DSolveValue[{PDE, BC1, BC2}, u, {r, θ}]

ParametricPlot3D[{r Sin[θ], r Cos[θ], sol[r, θ]}, 
     {θ, -π, π}, {r, 0, a}, AspectRatio -> Automatic,PlotRange -> All]

Numerical solution

Clear["Global`*"]
a = 3;
PDE = Laplacian[u[r, θ], {r, θ}, "Polar"] == 0;
f[r_, θ_] := Piecewise[{{1, 0 <= θ <= π}, {Sin[θ]^2, -π < θ < 0}}]
BC1 = DirichletCondition[u[r, θ] == f[r, θ], r == a && 0 < θ < 2 π];
BC2 = PeriodicBoundaryCondition[u[r, θ], θ == 0, TranslationTransform[{0, 2 π}]];
solN = NDSolveValue[{PDE, BC1, BC2}, u, {r, 0, a}, {θ, 0, 2 π}];
ParametricPlot3D[{r Sin[θ], r Cos[θ], solN[r, θ]}, 
      {θ, 0, 2 π}, {r, 0, a}, AspectRatio -> Automatic]