Wave Equation on Square Domain

This example shows how to solve the wave equation using the solvepde function.

The standard second-order wave equation is

2ut2-u=0.

To express this in toolbox form, note that the solvepde function solves problems of the form

m2ut2-(cu)+au=f.

So the standard wave equation has coefficients m=1, c=1, a=0, and f=0.

c = 1;
a = 0;
f = 0;
m = 1;

Solve the problem on a square domain. The squareg function describes this geometry. Create a model object and include the geometry. Plot the geometry and view the edge labels.

numberOfPDE = 1;
model = createpde(numberOfPDE);
geometryFromEdges(model,@squareg);
pdegplot(model,'EdgeLabels','on'); 
ylim([-1.1 1.1]);
axis equal
title 'Geometry With Edge Labels Displayed';
xlabel x
ylabel y

Specify PDE coefficients.

specifyCoefficients(model,'m',m,'d',0,'c',c,'a',a,'f',f);

Set zero Dirichlet boundary conditions on the left (edge 4) and right (edge 2) and zero Neumann boundary conditions on the top (edge 1) and bottom (edge 3).

applyBoundaryCondition(model,'dirichlet','Edge',[2,4],'u',0);
applyBoundaryCondition(model,'neumann','Edge',([1 3]),'g',0);

Create and view a finite element mesh for the problem.

generateMesh(model);
figure
pdemesh(model);
ylim([-1.1 1.1]);
axis equal
xlabel x
ylabel y

Set the following initial conditions:

  • u(x,0)=arctan(cos(πx2)).

  • ut|t=0=3sin(πx)exp(sin(πy2)).

u0 = @(location) atan(cos(pi/2*location.x));
ut0 = @(location) 3*sin(pi*location.x).*exp(sin(pi/2*location.y));
setInitialConditions(model,u0,ut0);

This choice avoids putting energy into the higher vibration modes and permits a reasonable time step size.

Specify the solution times as 31 equally-spaced points in time from 0 to 5.

n = 31;
tlist = linspace(0,5,n);

Set the SolverOptions.ReportStatistics of model to 'on'.

model.SolverOptions.ReportStatistics ='on';
result = solvepde(model,tlist);
457 successful steps
38 failed attempts
992 function evaluations
1 partial derivatives
112 LU decompositions
991 solutions of linear systems
u = result.NodalSolution;

Create an animation to visualize the solution for all time steps. Keep a fixed vertical scale by first calculating the maximum and minimum values of u over all times, and scale all plots to use those z-axis limits.

figure
umax = max(max(u));
umin = min(min(u));
for i = 1:n
    pdeplot(model,'XYData',u(:,i),'ZData',u(:,i),'ZStyle','continuous',...
                  'Mesh','off','XYGrid','on','ColorBar','off');
    axis([-1 1 -1 1 umin umax]); 
    caxis([umin umax]);
    xlabel x
    ylabel y
    zlabel u
    M(i) = getframe;
end

To play the animation, use the movie(M) command.