# Manual Partial Differential Equations and Boundary Value Problems

The time mesh you specify is used purely for output purposes, and does not affect the internal time steps taken by the solver. However, the spatial mesh you specify can affect the quality and speed of the solution. After solving an equation, you can use pdeval to evaluate the solution structure returned by pdepe with a different spatial mesh. The goal is to solve for the temperature u x , t. The temperature is initially a nonzero constant, so the initial condition is. Also, the temperature is zero at the left boundary, and nonzero at the right boundary, so the boundary conditions are.

To solve this equation in MATLAB, you need to code the equation, initial conditions, and boundary conditions, then select a suitable solution mesh before calling the solver pdepe. You either can include the required functions as local functions at the end of a file as in this example , or save them as separate, named files in a directory on the MATLAB path. Before you can code the equation, you need to make sure that it is in the form that the pdepe solver expects:.

The value of m is passed as an argument to pdepe , while the other coefficients are encoded in a function for the equation, which is. The initial condition function for the heat equation assigns a constant value for u 0. This function must accept an input for x , even if it is unused. The standard form for the boundary conditions expected by the pdepe solver is. So the values for p and q are. Use a spatial mesh of 20 points and a time mesh of 30 points.

Finally, solve the equation using the symmetry m , the PDE equation, the initial condition, the boundary conditions, and the meshes for x and t. Several available example files serve as excellent starting points for most common 1-D PDE problems. To explore and run examples, use the Differential Equations Examples app.

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To run this app, type. Simple PDE that illustrates the formulation, computation, and plotting of the solution. Solve Single PDE. Solve PDE with Discontinuity. Problem that requires computing values of the partial derivative.

## Boundary value problems and partial differential equations

System of two PDEs whose solution has boundary layers at both ends of the interval and changes rapidly for small t. To compute a solution to Eq. Alternatively, we could now discretize Eqs. For example, if we apply Eq. Equation 48 , the classical Euler's method , can be used to step along the solution of Eq. Application of Eq. In Eq. Note that Eq. We can now consider using Eq. The finite difference form of Eq. We must also specify two boundary conditions BCs for Eq. Equations 49 , 51 , 53 and 55 constitute the full system of equations for the calculation of the numerical solution to Eq.

Note that we have replaced the original PDE, Eq. Also, an analytical solution to Eq. Representative output from this program that compares the numerical solution from Eqs. Additional parameters follow from the values in Table 2. We can note two additional points about these values:. Finally, some descriptive comments about the details of the program in Appendix 1 are given immediately after the program listing. We again have an analytical solution to evaluate the numerical solution.

Since Eq. An important difference between the parabolic problem of Eq.

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Equations 66 , 67 , 68 , 69 , 70 , 71 constitute the complete finite difference approximation of Eqs. A small MATLAB program for this approximation is given in Appendix 2 , including some descriptive comments immediately after the program. The general analytical solution to Eq. The plotted output from the program is given in Figure 1 and includes both the numerical solution of Eqs. We can note the following points about Figure 1 :. Additional parameters follow from the values in Table 3. This is a stringent test of the numerical solution since the curvature of the solution is greatest at these peaks.

Of course, the peak analytical values given by Eq. This explicit finite difference numerical solution also has a stability limit like the preceding parabolic problem. In other words, the parameters of Table 3 were chosen primarily for accuracy and not stability.

Rather, we will convert Eq. The idea then is to integrate Eq. The analytical solution to Eqs. The analytical solution of Eq. To develop a numerical solution to Eq. The output from this program is listed in Table 5. The convergence of the solution of Eq. Additional parameters follow from the values in Table 6. The stability constraint for the 2D problem of Eq. The actual path that the parabolic problem takes to the solution of the elliptic problem is not relevant so long as the parabolic solution converges to the elliptic solution.

## Initial boundary value problems for hyperbolic partial differential equations

Some other trial values indicated that this initial value is not critical but it should be as close to the final value, for example 2. This parametrization is an example of continuation in which the solution is continued from the given assumed starting value of Eq. The concept of continuation has been applied in many forms and not just through the addition of a derivative as in Eq.

In general, the errors in the numerical solution of PDEs can result from the limited accuracy of all of the approximations used in the calculation. For example, the 0. In formulating a numerical method or algorithm for the solution of a PDE problem, it is necessary to balance the discretization errors so that one source of error does not dominate, and generally degrade, the numerical solution. Thus, control of approximation errors is central to the calculation of a numerical solution of acceptable accuracy. In the preceding examples, this control of errors can be accomplished in three ways:.

The three preceding numerical solutions were developed using basic finite differences such as in Eqs. However, many approaches to approximating derivatives in PDEs have been developed and used. Among these are finite elements, finite volumes, weighted residuals , e. Each of these methods has advantages and disadvantages, often according to the characteristics of the problem of interest starting with the parabolic, hyperbolic and elliptic geometric classifications. Thus, an extensive literature for the numerical solution of PDEs is available, and we have only presented here a few basic concepts and examples.

The principal advantage of numerical methods applied to PDEs is that, in principle, PDEs of any number and complexity can be solved which is particularly useful when analytical solutions are not available. As another example, a solution to the Burgers equation could be computed by extending Eq. While Euler's method is general with respect to the form of the initial value integration, it does have two important limitations:. Thus, the Euler method is limited by both accuracy and stability. As might be expected, such higher order methods are more complicated than the Euler method, but fortunately, they have been programmed in library routines that can easily be called and used.

The use of library routines for initial value integration is the basis for much of the work in the numerical method of lines solution of PDEs.

## Boundary value problem - Wikipedia

The use of quality library routines provides an important step in the timely development of computer codes for new applications of PDEs whereby the analyst can take advantage of the work of experts, which is generally much more efficient and reliable than developing codes starting with just a general programming language. Within each output interval 0. The end completes the intermediate loop in i2. Course Information Instructor Dr. Evans, Partial Differential Equations.

### Module Overview

PDEs can be used to describe a wide variety of phenomena such as sound, heat, electrostatics, electrodynamics, fluid flow, elasticity, or quantum mechanics. In this course, we will address many of the mathematical issues on various types of PDEs. We will also discuss boundary value problems on PDE, and related applications in physics and other natural sciences. A brief outline of topics to be covered is as follows. Other selected topics will be covered if time permits. Homework Assignments Homework assignments will be assigned and collected each week.