MATH2071: LAB #6: BVP's and PDE's



Our study of the initial value problem for ordinary differential equations has taught us how to estimate the behavior, over time, of a scalar quantity, supposing we have an initial value of the quantity, and a description of how it tends to change over time. This very limited ability can nonetheless sometimes be extended to a few other kinds of problems.

One class of problems is known as boundary value problems. A simple example of such a problem would describe the shape of a chain hanging between two posts. We know the position of the endpoints, and we have a second order differential equation describing the shape. If the two conditions were both given at the left endpoint, we'd know what to do right away. But how do we handle this "slight" variation?

The other sort of problem that we can take a stab at is the solution of certain partial differential equations, particularly equations that describe the flow of heat. We will also show that certain kinds of wave equations can be handled in this way. Really interesting problems require more sophisticated treatment, but at least we can get some flavor of a new field of applications!

Boundary Value Problems

A one-dimensional boundary value problem, or "BVP", is similar to an initial value problem, except that the data we are given isn't conveniently located at a starting time, but rather some is specified at the left end point and some at the right. (We're also usually thinking of the independent variable as representing space, rather than time, in this setting).

Our ideas about solving an initial value problem, or "IVP", have to be modified in some way. One possibility is to stubbornly use the IVP idea, and make up any necessary initial conditions. The other idea might be to somehow think of the solution points of the discretized equation as satisfying a set of coupled linear equations that we can solve.

The clothesline BVP: a rope is stretched between two points. If the rope were weightless, or rigid, it would lie along a straight line; however the rope has a weight and is elastic, so it sags down slightly from its ideal linear shape. Determine the curve described by the rope.

If we define y(x) to be the vertical position of the rope at x, then a simple model for this problem is:

y'' = 0.1
y(0.0) = 0
y(5.0) = 0.5
The number 0.1 is a physical constant that describes how easy it is for the rope to stretch in response to gravity. Different values describe different ropes. Ignoring the boundary conditions, this ODE is linear. This actually implies a lot of things about the existence and uniqueness of solutions.

M FILE: write an M file called rope_ode.m which evaluates the right hand side vector for the first order linear BVP system. (We're not worried about the boundary conditions just yet).

The chemical reaction BVP: the concentration of a certain reactant is supposed to satisfy the following equation:

u'' = lambda * eu
u(0.0) = 0.0
u(1.0) = 0.0
Here lambda is a physical constant to be specified later. This ODE is not linear. A solution of the original ODE is not guaranteed to exist; if a solution exists, it is not guaranteed to be unique. These facts affect the numerical solution of the problem.

M FILE: write an M file called chemical_ode.m which evaluates the right hand side vector for the first order chemical BVP system. Include a line setting lambda to -1, but be prepared to change this value!

Shooting Methods

Since we don't have any idea how to solve a BVP, but we know how to solve an IVP, let's forget the rules and just do what we do best. Taking as our example the rope BVP, how much violence do we have to do in order to make it look like an IVP? Well, we expect to have two conditions at the left point and none at the right point. So let's temporarily consider the related problem, where we have made up an extra boundary condition at our "initial" value of x:

y'' = 0.1
y(0.0) = 0.0
y'(0.0) = ALPHA

COMPUTATION: Some value of ALPHA will probably give us a solution which has the desired value y(5.0)=0.5. Try a few values of ALPHA. I suggest using NSTEP=100, with the RK3 method. The first line lists one result I got:

          Given      Left Guess          Right Error

          Y(0.0)     Y'(0.0) = ALPHA     Y(5.0) - 0.5

          0.0        2.0                 11.25 - 0.5 = 10.75
          0.0        -------------       -------------------
          0.0        -------------       -------------------
This is very much like the "cannon ball" problem you had a few labs ago! The difference is that now we want to figure out a systematic way of having the computer do this problem!

Convince yourself that:

Now the easiest way to set up a solution scheme for this problem would be to get two solutions to start with, and then use the secant method to try to locate a good solution. So our algorithm might be:

  1. For ALPHA = 0.0, compute y(5.0), and then f(0.0) = y(5.0) - 0.5;
  2. For ALPHA = 1.0, compute y(5.0), and then f(1.0) = y(5.0) - 0.5;
  3. Use the secant method and the previous two values of ALPHA and f(ALPHA) to compute the next value of ALPHA to try.
  4. Evaluate f(ALPHA). If small enough, return the value of ALPHA.
  5. If we've taken more than 20 steps, print an error message and give up.
  6. Go back to step 3.
Where we write y(5.0), we should really write y(5.0,ALPHA), to emphasize that the solution depends on the parameter.

Things you should think about: What is it about the bisection method that made me not choose that as my nonlinear equation solver? What disadvantage can you see to using Newton's method? Can you think of at least two ways in which the secant method could break down, even if there really was a solution to the problem?

M FILE: write an M file called rope_shoot.m which accepts values of ALPHA and NSTEP and evaluates f(ALPHA), for the rope BVP. Your code should have the form:

function f = rope_shoot ( alpha )
and this code should do the following:

COMPUTATION: Copy the code secant.m from the web page. (This version of the secant method has been rewritten from what we used last term.) Use this secant method, and the ideas discussed above, to seek a solution of the rope BVP. What value of ALPHA did your algorithm discover? Plot your solution to see if the rope looks like it is hanging properly, and has the correct position at the right endpoint.

Discretizing a BVP

For the rope problem, we know that the ODE determines the position of the rope. Rather than think of this as happening by means of a formula, it's better to think of the ODE as reporting, at each point x, that the "curvature" y'' of the rope balances the force 0.1. (The second derivative is similar to curvature, but it is a mathematical sin to pretend that it is the same thing).

Keep this idea in mind. Assume that we have divided the interval up into N-1 equal intervals of width dx determined by N points, and now use the standard central difference approximation to the second derivative. The difference equation that corresponds to our ODE is then:

( yi-1 - 2 yi + yi+1 ) / dx2 = 0.1
We can associate this equation with the solution value at yi, except for i=1 and i=N. But magically, those just happen to be the points at which we have boundary conditions specified.

In particular, let us look at approximating our rope BVP at 6 points. We set up the ODE at points 2, 3, 4 and 5, and associate the boundary conditions with the first and last solution values. I also multiplied through by the divisor to make things look nicer:

        u1 = 0.0
        u1 - 2 u2 + u3 = 0.1 * dx2
        u2 - 2 u3 + u4 = 0.1 * dx2
        u3 - 2 u4 + u5 = 0.1 * dx2
        u4 - 2 u5 + u6 = 0.1 * dx2
        u6 = 0.5

As a sanity check, what are these equations saying? The boundary conditions "nail down" the ends of the rope. But what is this relationship between sets of three successive points? Well, ignore the right hand side, and ask yourself, what is the typical value of yi-1-2yi+yi+1 for points on a constant function, a linear function, a quadratic function? You should see that this quantity (divided by dx2) will exactly compute the second derivative of functions up to a quadratic. And conversely, if you know the second derivative, you expect a particular relationship between consecutive triples.

By discretizing the differential equations we have created a set of linear alebraic equations, which have the symbolic form A*u=b. (Officially, we don't have any idea what linear equations are, because that's in the next chapter of the book. But unofficially, let's keep going.) To set up and solve these equations in MATLAB, we could type:

          n = 6;
          x = linspace ( 0.0, 5.0, n );

          A = [  1  0  0  0  0  0;
                 1 -2  1  0  0  0;
                 0  1 -2  1  0  0;
                 0  0  1 -2  1  0;
                 0  0  0  1 -2  1;
                 0  0  0  0  0  1];

          dx = 5.0 / ( n - 1 );
          rhs = 0.1 * dx^2;
          b = [ 0.0; rhs; rhs; rhs; rhs; 0.5 ];  (a column vector!)
          u = A \ b
You probably recognize everything that's happening here except for the last bizarre line. That line essentially tells MATLAB to solve the equation A*u=b. We'll have to wait til the next chapter to really talk about that!

Computation: Try to solve the rope BVP using this method. Print out the vector b and verify by hand at least one of the linear equations. Then plot x versus u.

Computation: To get more accuracy, we want to increase N, the number of points. Consider the problem of typing in the matrix A if N is 101! Since the matrix is so simple, we can use the MATLAB diag command to set up the "1 -2 1" diagonals, although we will have to adjust the first and last row. Copy the file dif2.m from the web page. Type the command A = dif2 ( 6 ). Now correct the first and last rows of A, namely, the entries A(1,1), A(1,2), A(N,N-1) and A(N,N), to get the matrix we had in the previous problem.

Got a minute for a puzzle? The function ones(m,n) returns an m by n matrix of 1's. The function diag(v,d) returns a matrix whose d-th diagonal contains the values in the vector v. Using these ideas, can you figure out how dif2.m can compute the difference matrix in a single line? OK, you can look now!

Computation: Now we are ready to solve the big problem! Repeat the previous calculation, but now with N=101. Use the dif2 function to set up the matrix A, and correct the first and last rows. You must also figure out a clever way of entering the 101 entries of b. Can you see how the ones function can help you again? Now solve the linear system, and plot x versus u.

If you call your problem variables u2 and x2, you can compare the plots of (x,u) and (x2,u2) and see how refining the mesh has changed the solution.

A Heat Equation

A partial differential equation or "PDE" involves derivatives of a function U which depends on more than one independent variable. One of the classic PDE's is the one-dimensional heat equation, which describes the behavior of the temperature U(x,t) of a rod.

Ut = Uxx + f(x,t)
We also specify an initial condition, that is, the temperature of every point on the rod at the starting time,
U(x,0) = g(x)
and boundary conditions, describing the behavior of the solution at points which are exposed to the outside:
U(0,t) = 0,
U(1,t) = 0
The boundary conditions could be more interesting, changing with time, or describing the behavior of the derivative Ux.

The Method of Lines

To find a numerical approximation to the solution of the heat equation, one approach is to focus attention on a finite grid of equally spaced points, (x1,...,xN). Having discretized space in this way, we know that at any fixed time t, the solution function will now be a vector of values (U(x1,t),...,U(xN,t)) associated with these grid points. We must replace our differential equation by an appropriate equation involving spatial differences of these solution values, which we still think of as having a continuous dependence on time.

Without yet discretizing time, our ODE has become the following semi-discretized system for (U(x1,t),...,U(xN,t)).:

(d/dt) U(xi,t) = (U(xi-1,t)-2*U(xi,t)+U(xi+1,t))/dx2 + f(xi,t)
These equations may be set up for i = 2 through N-1. Our first and last equations will use the boundary conditions, but can you see how the first equation, U(x1,t)=0, can be made to look like a differential equation? The right hand boundary condition can be treated this way as well. So now we have N ordinary differential equations.

Example: Consider the PDE:

Ut = Uxx + x*t
U(x,0) = sin(pi*x)
U(0,t) = 0, U(1,t) = 0
If we discretize space using just 4 points, and replace the differential operator by a discretized version, and simplify notation by writing U(i,t) when we really mean U(xi,t), we get:
d/dt U(1,t) = 0
d/dt U(2,t) = (U(1,t)-2*U(2,t)+U(3,t)) / (1/9) + 0.33 * t
d/dt U(3,t) = (U(2,t)-2*U(3,t)+U(4,t)) / (1/9) + 0.67 * t
d/dt U(4,t) = 0
with initial conditions:
U(1,0.0) = 0.0
U(2,0.0) = 0.5 * sqrt ( 3.0 )
U(3,0.0) = 0.5 * sqrt ( 3.0 )
U(4,0.0) = 0.0
and we know how to solve a system of ODE's like this! This method of discretizing in space to turn a PDE into a set of coupled ODE's is called the method of lines. It is only applicable to certain classes of PDE's. The stepsize dt used in the time discretization often must satisfy a certain stability test defined by the spatial discretization size dx. In short, once you've picked your spatial discretization, there is a limit on the size of your time step.

If we are using Euler's method for the time integration, and if our spatial stepsize is dx, then stability suggests using a time step of 0.5*dx2. If we do so, then we may expect that each time we halve dx, the error will go down by a factor of 4. We take the error at the midpoint as representative of the total error.

Consider the following PDE:

Ut = Uxx + (pi2-0.1) * exp (-t/10) * sin(pi*x)
U(x,0) = sin(pi*x)
U(0,t) = 0, U(1,t) = 0
which has the exact solution:
U(x,t) = exp(-t/10) * sin(pi*x)

Computation: Copy the file method_of_lines.m from the web page. Type help method_of_lines to see how to use it. Use the method of lines with various values of nx, and nt, to compute a table of values of the PDE solution U for 0.0 <= x <= 1.0 and 0.0 <= t <=5.0.

          NX         NT       Computed       Exact
        (space)    (time)    U(0.5,5.0)    U(0.5,5.0)       Error
           4        160      __________    __________    __________
           8        640      __________    __________    __________
          16       1280      __________    __________    __________
(If your PC can't handle it, you can skip the last computation. But if you can solve it, what goes wrong? How can we fix it?) When we write the functional form U(0.5,5.0), where do we actually need to look in the MATLAB variable U(?,?).

To see a snapshot of the solution at a timestep j, use the command

plot ( x, u(:,j) )
To see the time evolution of the solution value at a fixed point i, use the command
plot ( t, u(i,:) )
If you're using MATLAB, one way to get a nice plot of the entire solution is to type
mesh ( u )

A Wave Equation

PDE's describing the behavior of waves are superficially similar to those describing heat flow, but the properties of the equation and its solution are usually very different. We do not have time to worry about these matters; we will simply run an example in which a version of the method of lines is used successfully to solve a wave equation.

Suppose we are observing the behavior of a one-dimensional ocean. Waves may arise, or move to the left or right. We keep track of the quantity u(x), the height of the water surface, at each point. In the absence of any other disturbances, we know that waves will move about in more complicated way than heat does. One model for this behavior is the Korteweg-deVries equation or KdV equation:

du/dt = 6 * u * du/dx - d3u/dx3.

As with the heat equation, we are going to discretize space, then discretize the spatial differentiation operators, and come up with a semi-discrete coupled system of differential equations. There are a couple of differences you should note. The second derivative is easy to approximate with a second difference that is symmetric. But the commonly used approximations to first and third derivatives are not symmetric. In order to avoid adding a directional bias in the equation, we will go out of our way to use symmetric approximations. Secondly, instead of using the Euler method to do the time integration, we will use a version of the midpoint method.

If you are running MATLAB, copy the M files kdv_mid.m and kdv_movie.m. Otherwise, copy just the first file. Look at how the calculation in kdv_mid is set up. Be able to answer the following:

If you are running MATLAB, run kdv_movie, otherwise run kdv_mid. For a change, don't bother thinking. Just enjoy looking at the movie.

If you were able to run the movie, then you will see a simulation of the behavior of a very special wave. It starts out looking like a single wave, but it is actually composed of 2 solitons, that is, 2 waves that tend to maintain their own identity. These waves each have a characteristic shape and velocity. They can pass through each other, temporarily making what looks like a single wave, but they then separate again. In this simulation, we are watching the waves separate.

For special initial conditions, you get this nice picture. If you want to see 3 solitons, look at the code and change the appropriate number. If the initial condition does not have the right value, you'll get a more complicated wave pattern. Note also that when the waves begin to interact with the boundary, we get jaggy lines. This is because the soliton solution really only works on an infinite domain. Our region is too small, and our treatment of the boundary conditions too simple, to avoid this problem.


COMPUTATION: Some nonlinear BVP's have multiple solutions. Solve the chemical BVP using the shooting method. Use the value lambda = - 1. Record the value of ALPHA that the shooting method returns. There are actually two different solutions. To find the second one, you must vary the starting values that you give to the secant method. I will tell you that each pair of starting values for the secant method can be of the form (N,N+1) where N is an integer between 0 and 15.

           Secant starting values     ALPHA         Y(1.0)
          __________    __________    __________    __________
          __________    __________    __________    __________
Mail the results to me.

Last revised on 17 February 2000.