Example: Electrodynamics Problem
This example illustrates the solution of a system of partial differential equations. The problem is taken from electrodynamics. It has boundary layers at both ends of the interval, and the solution changes rapidly for small .
The PDEs are
. The equations hold on an interval for times .
The solution satisfies the initial conditions
and boundary conditions
The demo |
pdex4 contains the complete code for this example. The demo uses subfunctions to place all required functions in a single M-file. To run this example type
pdex4 at the command line. See PDE Solver Basic Syntax and Solving PDE Problems for more information.
- Rewrite the PDE. In the form expected by
pdepe, the equations are
- The boundary conditions on the partial derivatives of have to be written in terms of the flux. In the form expected by
pdepe, the left boundary condition is
and the right boundary condition is
- Code the PDE. After you rewrite the PDE in the form shown above, you can code it as a function that
pdepe can use. The function must be of the form
[c,f,s] = pdefun(x,t,u,dudx)
s correspond to the , , and terms in Equation 14-4.
function [c,f,s] = pdex4pde(x,t,u,DuDx)
c = [1; 1];
f = [0.024; 0.17] .* DuDx;
y = u(1) - u(2);
F = exp(5.73*y)-exp(-11.47*y);
s = [-F; F];
- Code the initial conditions function. The initial conditions function must be of the form
- The code below represents the initial conditions in the function
- Code the boundary conditions function. The boundary conditions functions must be of the form
[pl,ql,pr,qr] = bcfun(xl,ul,xr,ur,t)
- The code below evaluates the components and (Equation 14-6) of the boundary conditions in the function
function [pl,ql,pr,qr] = pdex4bc(xl,ul,xr,ur,t)
pl = [0; ul(2)];
ql = [1; 0];
pr = [ur(1)-1; 0];
qr = [0; 1];
Apply the PDE solver. The example calls
- Select mesh points for the solution. The solution changes rapidly for small . The program selects the step size in time to resolve this sharp change, but to see this behavior in the plots, output times must be selected accordingly. There are boundary layers in the solution at both ends of [0,1], so mesh points must be placed there to resolve these sharp changes. Often some experimentation is needed to select the mesh that reveals the behavior of the solution.
m = 0, the functions pdex4pde, pdex4ic, and pdex4bc, and the mesh defined by x and t at which
pdepe is to evaluate the solution. The
pdepe function returns the numerical solution in a three-dimensional array
sol(i,j,k) approximates the
kth component of the solution, , evaluated at
View the results. The surface plots show the behavior of the solution components.
m = 0;
sol = pdepe(m,@pdex4pde,@pdex4ic,@pdex4bc,x,t);
u1 = sol(:,:,1);
u2 = sol(:,:,2);
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