# solving partial differential equations with boundary conditions

This process is experimental and the keywords may be updated as the learning algorithm improves. Not logged in This is nothing but a system of ordinary differential equations in \(N-1\) unknowns \(u_{1}(t),\ldots,u_{N-1}(t)\)! Not affiliated If these boundary conditions and \(\sigma\)do not depend on time, the temperature within the rod ultimately settles to the solution of the steady-state equation: \[ \frac{d^2 T}{dx^2}(x) = b(x), \; \; \; b(x) = -\sigma(x)/\alpha.\] In the examples below, we solve this equation with … In the literature, this strategy is called the method of lines. b_j \\ Other video formats, such as MP4, WebM, and Ogg can also be produced: Unstable simulation of the temperature in a rod. To make a Flash video movie.flv, run. If we look back at equation (31), we have in full generality: If we set \(T_0=1\), this equation becomes: We observe that compared to our previous setup, the left-hand side has not changed. By B. Knaepen & Y. Velizhanina These plots can be combined to ordinary video files. PARTIAL DIFFERENTIAL EQUATIONS Math 124A { Fall 2010 « Viktor Grigoryan grigoryan@math.ucsb.edu Department of Mathematics University of California, Santa Barbara These lecture notes arose from the course \Partial Di erential Equations" { Math 124A taught by the author in the Department of Mathematics at UCSB in the fall quarters of 2009 and 2010. One can observe (and also mathematically prove) that the solution \(u(x,t)\) of the problem in Exercise 5.6 is symmetric around x = 0: \(u(-x,t)=u(x,t)\). Commonly used boundary conditions are. Trying out some simple ones first, like, The simplest implicit method is the Backward Euler scheme, which puts no restrictions on, $$\frac{u^{n+1}-u^{n}}{\Delta t}=f(u^{n+1},t_{n+1})\thinspace.$$, In our case, we have a system of linear ODEs (, $$\frac{u_{0}^{n+1}-u_{0}^{n}}{\Delta t} =s^{\prime}(t_{n+1}),$$, $$\frac{u_{i}^{n+1}-u_{i}^{n}}{\Delta t} =\frac{\beta}{\Delta x^{2}}(u_{i+1}^{n+1}-2u_{i}^{n+1}+u_{i-1}^{n+1})+g_{i}(t_{n+1}),$$, $$\frac{u_{N}^{n+1}-u_{N}^{n}}{\Delta t} =\frac{2\beta}{\Delta x^{2}}(u_{N-1}^{n+1}-u_{N}^{n+1})+g_{i}(t_{n+1})\thinspace.$$, $$u_{0}^{n+1} =u_{0}^{n}+\Delta t\,s^{\prime}(t_{n+1}),$$, $$u_{1}^{n+1}-\Delta t\frac{\beta}{\Delta x^{2}}(u_{2}^{n+1}-2u_{1}^{n+1}+u_{0}^{n+1}) =u_{1}^{n}+\Delta t\,g_{1}(t_{n+1}),$$, $$u_{2}^{n+1}-\Delta t\frac{2\beta}{\Delta x^{2}}(u_{1}^{n+1}-u_{2}^{n+1}) =u_{2}^{n}+\Delta t\,g_{2}(t_{n+1})\thinspace.$$, A system of linear equations like this, is usually written on matrix form, $$A=\left(\begin{array}[]{ccc}1&0&0\\ -\Delta t\frac{\beta}{\Delta x^{2}}&1+2\Delta t\frac{\beta}{\Delta x^{2}}&-\Delta t\frac{\beta}{\Delta x^{2}}\\ 0&-\Delta t\frac{2\beta}{\Delta x^{2}}&1+\Delta t\frac{2\beta}{\Delta x^{2}}\end{array}\right)$$, $$A_{i,i-1} =-\Delta t\frac{\beta}{\Delta x^{2}},\quad i=2,\ldots,N-1$$, $$A_{i,i+1} =-\Delta t\frac{\beta}{\Delta x^{2}},\quad i=2,\ldots,N-1$$, $$A_{i,i} =1+2\Delta t\frac{\beta}{\Delta x^{2}},\quad i=2,\ldots,N-1$$, $$A_{N,N-1} =-\Delta t\frac{2\beta}{\Delta x^{2}}$$, $$A_{N,N} =1+\Delta t\frac{2\beta}{\Delta x^{2}}$$, If we want to apply general methods for systems of ODEs on the form, $$K_{i,i-1} =\frac{\beta}{\Delta x^{2}},\quad i=2,\ldots,N-1$$, $$K_{i,i+1} =\frac{\beta}{\Delta x^{2}},\quad i=2,\ldots,N-1$$, $$K_{i,i} =-\frac{2\beta}{\Delta x^{2}},\quad i=2,\ldots,N-1$$, $$K_{N,N-1} =\frac{2\beta}{\Delta x^{2}}$$, $$K_{N,N} =-\frac{2\beta}{\Delta x^{2}}$$, $$u(0,t)=T_{0}+T_{a}\sin\left(\frac{2\pi}{P}t\right),$$, Show that the present problem has an analytical solution of the form, An equally stable, but more accurate method than the Backward Euler scheme, is the so-called 2-step backward scheme, which for an ODE, $$\frac{3u^{n+1}-4u^{n}+u^{n-1}}{2\Delta t}=f(u^{n+1},t_{n+1})\thinspace.$$, We consider the same problem as in Exercise, $$E=\sqrt{\Delta x\Delta t\sum_{i}\sum_{n}(U_{i}^{n}-u_{i}^{n})^{2}}\thinspace.$$, The Crank-Nicolson method for ODEs is very popular when combined with diffusion equations. Our setting of parameters required finding three physical properties of a certain material. The images or other third party material in this chapter are included in the work’s Creative Commons license, unless indicated otherwise in the credit line; if such material is not included in the work’s Creative Commons license and the respective action is not permitted by statutory regulation, users will need to obtain permission from the license holder to duplicate, adapt or reproduce the material. The ODE system above cannot be used for \(u_{0}^{\prime}\) since that equation involves some quantity \(u_{-1}^{\prime}\) outside the domain. Reformulate the problem in Exercise 5.6 such that we compute only for \(x\in[0,1]\). # The source term at grid nodes 1 and nx-2 needs to be modified. The relevant object name is ThetaRule: Consider the physical application from Sect. Here, we will limit our attention to moderately sized matrices and rely on a scipy routine for matrix inversion - inv (available in the linalg submodule). You can then compare the number of time steps with what is required by the other methods. Use these values to construct a test function for checking that the implementation is correct. # We set the boundary value at the left boundary node based on the Neumann, # We set the boundary value at the right boundary node based on non-homoge-, \(\displaystyle T_{exact}=-\frac12(x^2-4x+1)\), 'Heat equation - Mixed boundary conditions', Numerical methods for partial differential equations, 2. Demonstrate, by running a program, that you can take one large time step with the Backward Euler scheme and compute the solution of (5.38). 0 & 0 & 0 & 0 & \dots & 1 & -2 & 1 & 0 & 0 \\ The initial and boundary conditions are extremely important. If we denote respectively by \(T_i\) and \(b_i\) the values of \(T\) and \(b\) at the grid nodes, our discretized equation reads: where \(A_{ij}\) is the discretized differential operator \(\frac{d^2}{dx^2}\). Solve Differential Equation with Condition. These keywords were added by machine and not by the authors. Partial Diﬀerential Equations: Graduate Level Problems and Solutions Igor Yanovsky 1. One such class is partial differential equations (PDEs). \begin{pmatrix} Consider the problem given by (5.9), (5.10) and (5.14). The first-order wave equation 2. Solving Partial Differential Equations with Python Despite having a plan in mind on the subjects of these posts, I tend to write them based on what is going on at the moment rather than sticking to the original schedule. \end{pmatrix}.\end{split}\], \[ \frac{- 2T_1+T_2}{\Delta x^2} = b_1.\], \[ \frac{T_1 - 2T_2 + T_3}{\Delta x^2} = b_2.\], \[ \frac{T_{nx-4} - 2T_{nx-3} + T_{nx-2}}{\Delta x^2} = b_{nx-3}.\], \[ \frac{T_{nx-3} - 2T_{nx-2}}{\Delta x^2} = b_{nx-2}.\], \[\begin{split}\frac{1}{\Delta x^2} We also have briefly discussed the usage of two functions from scipy and numpy to respectively invert matrices and perform array multiplications. Solve the partial differential equation with periodic boundary conditions where the solution from the left-hand side is mapped to the right-hand side of the region. \vdots \\ Report what you see. Without them, the solution is not unique, and no numerical method will work. Boundary and initial conditions are needed! 'Heat equation - Mixed Dirichlet boundary conditions'. Implement the θ rule with aid of the Odespy package. $$\Delta t\leq\frac{\Delta x^{2}}{2\beta}\thinspace.$$. \end{pmatrix} We may also make an animation on the screen to see how \(u(x,t)\) develops in time (see file rod_FE.m ): The plotting statements update the \(u(x,t)\) curve on the screen. Also, at x = 0 and x = 1, the solution satisfies the boundary conditions . The solution of the equation is not unique unless we also prescribe initial and boundary conditions. The surface temperature at the ground shows daily and seasonal oscillations. Enough in the box to type in your equation, denoting an apostrophe ' derivative of the function and press "Solve the equation". We can then simplify the setting of physical parameters by scaling the problem. The present problem involves a loop for computing the right-hand side: This loop can be replaced by a vectorized expression with the following reasoning. The type and number of such conditions depend on the type of equation. We remark that a separate ODE for the (known) boundary condition \(u_{0}=s(t)\) is not strictly needed. To proceed, the equation is discretized on a numerical grid containing \(nx\) grid points, and the second-order derivative is computed using the centered second-order accurate finite-difference formula derived in the previous notebook. We shall now construct a numerical method for the diffusion equation. It reads: The next equation - around grid node 2 - reads: For this one, there is nothing to change. \frac{-\frac23 T_1 + \frac 23 T_2}{\Delta x^2} = b_1 + \frac{4}{3 \Delta x}.\], # right-hand side vector at the grid points, Constructs the centered second-order accurate second-order derivative for, matrix to compute the centered second-order accurate first-order deri-, vative with Dirichlet boundary conditions on both side of the interval. T_2 \\ For convenience, we start with importing some modules needed below: Let’s consider a rod made of heat conducting material. Matrix and modified wavenumber stability analysis 3. The reason for including the boundary values in the ODE system is that the solution of the system is then the complete solution at all mesh points, which is convenient, since special treatment of the boundary values is then avoided. Here we are going to set this flux at the left boundary node and assign a specific temperature at the right boundary node: The condition at the right boundary node is treated in the same way as in the previous section. In addition, we save a fraction of the plots to files tmp_0000.png, tmp_0001.png, tmp_0002.png, and so on. In mathematics, in the field of differential equations, a boundary value problem is a differential equation together with a set of additional constraints, called the boundary conditions. A nice feature with having a problem defined as a system of ODEs is that we have a rich set of numerical methods available. Therefore, most of the entries are zeroes. We now turn to the solving of differential equations in which the solution is a function that depends on several independent variables. for solving partial differential equations. However, since we have reduced the problem to one dimension, we do not need this physical boundary condition in our mathematical model. To implement the Backward Euler scheme, we can either fill a matrix and call a linear solver, or we can apply Odespy. Instead, we use the equation \(u_{0}^{\prime}(t)=s^{\prime}(t)\) derived from the boundary condition. The heat equation around grid node \(1\) is then modified as: The effect of the Neumann boundary condition is two-fold: it modifies the left-hand side matrix coefficients and the right-hand side source term. This operation is performed with the help of the numpy.dot function that allows many sorts of vector and matrix multiplications. If everything went how we expected, \(T\) now contains the approximate solution. Solving Partial Differential Equations In a partial differential equation (PDE), the function being solved for depends on several variables, and the differential equation can include partial derivatives taken with respect to each of the variables. A complete code is found in the file rod_FE_vec.m . The notebook introduces finite element method concepts for solving partial differential equations (PDEs). The focuses are the stability and convergence theory. Often, we are more interested in how the shape of \(u(x,t)\) develops, than in the actual u, x, and t values for a specific material. b_1 \\ One dimensional heat equation 4. 0 & 0 & 0 & 0 & \dots & 0 & 0 & 1 & -2 & 1 \\ 0 & \dots & \dots & \dots & \dots & \dots & \dots & \dots & \dots & 0 \\ The type and number of such conditions depend on the type of equation. PDEs and Boundary Conditions New methods have been implemented for solving partial differential equations with boundary condition (PDE and BC) problems. We can run it with any \(\Delta t\) we want, its size just impacts the accuracy of the first steps. In a later chapter of this course we will explain how to obtain approximate inverses for large systems. Also note the remarks in Exercise 5.6 about the constant area under the \(u(x,t)\) curve: here, the area is 0.5 and \(u\rightarrow 0.5\) as \(t\rightarrow 0.5\) (if the mesh is sufficiently fine – one will get convergence to smaller values for small σ if the mesh is not fine enough to properly resolve a thin-shaped initial condition). These programs take the same type of command-line options. 0 & 0 & 0 & 0 & \dots & 0 & 1 & -2 & 1 & 0 \\ In 2D and 3D problems, where the CPU time to compute a solution of PDE can be hours and days, it is very important to utilize symmetry as we do above to reduce the size of the problem. The unknown in the diffusion equation is a function \(u(x,t)\) of space and time. The surface corresponds to x = 0 and the x axis point downwards into the ground. Free ordinary differential equations (ODE) calculator - solve ordinary differential equations (ODE) step-by-step This website uses cookies to ensure you get the best experience. We can compare it with the exact solution \(T(x)=\displaystyle\frac{1}{2}x(1-x)\), which obviously satisfies the required boundary conditions. If present, the latter effect requires an extra term in the equation (known as an advection or convection term). In these cases, the boundary conditions will represent things like the temperature at either end of a bar, or the heat flow into/out of … At the boundary x = 0 we need an ODE in our ODE system, which must come from the boundary condition at this point. (The Mathe- matica function NDSolve, on the other hand, is a general numerical differential equation solver.) T_{j-1}\\ In particular, we may use the Forward Euler method as implemented in the general function ode_FE from Sect. In such a case, we can split the domain in two and compute u in only one half, \([-1,0]\) or \([0,1]\). The physical significance of u depends on what type of process that is described by the diffusion equation. Identify the linear system to be solved. A major problem with the stability criterion (5.15) is that the time step becomes very small if \(\Delta x\) is small. It takes some time before the temperature rises down in the ground. # Return the final array divided by the grid spacing **2. In two- and three-dimensional PDE problems, however, one cannot afford dense square matrices. The power of scaling is to reduce the number of physical parameters in a problem, and in the present case, we found one single problem that is independent of the material (β) and the geometry (L). Since Python and Matlab have very similar syntax for the type of programming encountered when using Odespy, it should not be a big step for Matlab/Octave users to utilize Odespy. Consistency and monotonicity of the scheme are discussed. Solve non-linear differential equation with boundary conditions 2 How do I solve a 3D poisson equation with mixed neumann and periodic boundary conditions numerically? T_{nx-2} In fact, a large part of the solution process there will be in dealing with the solution to the BVP. For this particular equation we also need to make sure the initial condition is \(u_{0}(0)=s(0)\) (otherwise nothing will happen: we get u = 283 K forever). The vectorized loop can therefore be written in terms of slices: This rewrite speeds up the code by about a factor of 10. The matrix \(\tilde A_{ij}\) on the left-hand side has dimensions \((nx-2)\times(nx-2)\). Once again, the computed solution behaves appropriately! So the effect of applying a non-homogeneous Dirichlet boundary condition amounts to changing the right-hand side of our equation. Solve the equation with the initial condition y(0) == 2. Mathematically, (with the temperature in Kelvin) this example has \(I(x)=283\) K, except at the end point: \(I(0)=323\) K, \(s(t)=323\) K, and g = 0. You should have a look at its documentation page. Vectorize the implementation of the function for computing the area of a polygon in Exercise 2.6. Problems involving the wave equation, … This is yet another example why one needs implicit methods like the Backward Euler scheme. the values are set to \(0\)). The aim of this tutorial is to give an introductory overview of the finite element method (FEM) as it is implemented in NDSolve. system of ordinary differential equations. Indeed, in many problems, the loss of accuracy used for the boundary conditions would degrade the accuracy of the solution throughout the domain. To avoid oscillations in the solutions when using the RKFehlberg method, the rtol and atol parameters to RKFFehlberg must be set no larger than 0.001 and 0.0001, respectively. We can just work with the ODE system for \(u_{1},\ldots,u_{N}\), and in the ODE for u 0, replace \(u_{0}(t)\) by \(s(t)\). We now write a Python code to solve our problem with a very simple term on the right-hand side: As in the previous notebook, we rely on the diags routine to build our matrix. The solution of the equation is not unique unless we also prescribe initial and boundary conditions. We can find proper values for these physical quantities in the case of aluminum alloy 6082: \(\varrho=2.7\cdot 10^{3}\hbox{ kg/m}^{3}\), \(\kappa=200\,\,\frac{\hbox{W}}{\hbox{mK}}\), \(c=900\,\,\frac{\hbox{J}}{\hbox{Kkg}}\). 4.3.6). 3. Open Access This chapter is distributed under the terms of the Creative Commons Attribution‐NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/), which permits any noncommercial use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, a link is provided to the Creative Commons license and any changes made are indicated. To summarize, the partial differential equation with initial and boundary conditions reads, $$\frac{\partial u(x,t)}{\partial t} =\beta\frac{\partial^{2}u(x,t)}{\partial x^{2}}+g(x,t), x\in\left(0,L\right), t\in(0,T],$$, $$\frac{\partial}{\partial x}u(L,t) =0, t\in(0,T],$$, $$u(x,0) =I(x), x\in\left[0,L\right]\thinspace.$$, $$x_{0}=0

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