The quickstart guide explains how to install and start FEATool Multiphysics as well as describes the modeling and simulation process. Furthermore, step by step instructions how to set up and solve five modeling examples illustrating different techniques and features of FEATool is also included.
1. Introductory example of modeling stresses and strains in a thin plate with a hole.
2. Multiphysics example coupling heat and fluid flow in a heat exchanger.
3. Equation editing example modeling axisymmetric fluid flow in a narrowing pipe.
4. Classic equation example for the Poisson equation on a circle with a point source.
5. Custom equation example for solving the wave equation on a circle.
Prerequisites
The FEATool simulation toolbox is written in the m-script language, which requires either GNU Octave or Matlab to run and interpret the source code. Octave is freely available from the Octave homepage (Octave version 4.0 or later is required for GUI compatibility). Matlab is a commercial alternative to Octave and must be licensed from The Mathworks Inc.
Installation
In order to use the toolbox the FEATool directories must first be added to the Octave and/or Matlab search paths. This can be done permanently by running the install Octave/Matlab m-file script function found in the FEATool installation folder (here it is assumed to be C:\featool)
in Octave or Matlab:
>> cd C:\featool
>> install
or from Windows Explorer
in the FEATool installation folder:
double-click on the "install.bat/install_octave.bat" file icon
Alternatively, instead of setting up the paths permanently it is possible to instead just run addpaths on the command line before starting to use the FEATool functions and subroutines (starting the FEATool GUI by running the featool command also does this automatically).
To verify that the search paths have been installed correctly type
>> path
and check that the following paths are present in the displayed list (with location and directories adjusted for where you have installed FEATool)
c:\featool\util
c:\featool\post
c:\featool\physmodes
c:\featool\lib\uiextras
c:\featool\lib\surfaceintersection
c:\featool\lib\distmesh
c:\featool\lib\findjobj
c:\featool\impexp
c:\featool\gui
c:\featool\grid
c:\featool\geom
c:\featool\featflow
c:\featool\examples
c:\featool\ellib
c:\featool\core
To permanently uninstall (that is to remove the saved paths), run the function uninstall in Octave/Matlab, which also can be found in the FEATool installation folder.
Modeling Process
The typical modeling process in FEATool logically follows the six mode buttons located in the upper portion of the left side toolbar in the main graphical user interface (Gui) window. The modeling steps are thus as follows:
- The first step is to create a geometry to define the domain to be modeled. Geometry objects such as rectangles, circles, blocks, and cylinders can be created and combined to represent complex domains.
- From the geometry a grid or mesh can be generated or imported from an external preprocessing tool or grid generator. The modeled phenomena is to be approximated on each grid cell by a finite element polynomial or shape function. Finer more dense grids and regularly shaped high quality grid cells generally leads to more accurate solutions.
- Equations (PDEs) and model coefficients are then specified in subdomain/equation mode to describe the physical phenomena to be modeled. Equations are pre-defined for many types of physical phenomena like for example heat transfer, structural strains and stresses, and fluid flow. In addition FEATool also allows arbitrary and custom systems of PDE equations to be described and entered.
- Boundary conditions must be prescribed on the geometrical boundaries to account for interactions with the surroundings outside of the modeled domain. For time dependent problems initial conditions must also be given at the start of the simulations.
- After the model problem is fully specified a suitable solver can be employed to compute a solution. FEATool can either use the default Matlab or Octave linear solvers or use specialized external ones, such as the FeatFlow CFD solver, for increased performance.
- Finally, after the problem has been solved, the solution can be visualized, postprocessed, and exported to evaluate the computed results.
Structural Mechanics Example - Thin Plate with Hole
The first example is a well known benchmark test case from structural mechanics, a thin plate with a circular hole in the center is subjected to a load along one axis.
The plate is thin enough to satisfy the two dimensional plane stress approximation, and since both the problem and solution will be symmetric around the hole it is enough to model a quarter of the plate. The computational geometry will therefore consist of a 0.05 by 0.05 m square with a quarter of a circle with radius 0.005 m removed from one corner. Due to the symmetry the displacement of the left edge of the plate should be zero in the x-direction, and similarly the y-displacement for the lower edge should also be zero. Furthermore, a horizontal force of 1000 N is applied to the right edge. With a plate thickness of 0.001 m, the resulting load will be 1000/(2*0.05*0.001) N/m^{2}.
Assuming that the plate is made of steel with a Poisson ratio of 0.3 and modulus of elasticity 210*10^{9} Pa then it is expected that the maximum stress in the x-direction will be three times the stress of a plate without a hole, that is = 3*1000/(2*0.05*0.001) Pa = 3*10^{7} Pa.
Thin Plate with Hole using the GUI
This section describes how to set up and solve the thin plate with hole example with the FEATool graphical user interface (GUI).
- Start Octave or Matlab. If you have not run the installation script (which automatically adds the FEATool directory paths at startup) then change your working directory to where your FEATool installation is, for example
cd C:\featool
In the Octave/Matlab main command window type
featool
to start the FEATool graphical user interface (GUI).
- Either select New... from the File menu, or click on the New Model button in the upper horizontal toolbar, to clear all data and start defining a new model.
- In the New Model dialog box, click on the 2D radio button in the Select Space Dimensions frame, and select Plane Stress from the Select Physics drop-down list. Leave the space dimension and dependent variable names to their defaults. Finish and close the dialog box by clicking on the OK button.
- To create the rectangle for the plate, first click on the Create square/rectangle button in the left hand side Tools toolbar frame.
- Then left click in the main plot axes window, hold the mouse button, and move the mouse pointer to draw a rectangle with red outlines.
- Release the button to finalize and create a solid geometry object. The object properties must now be edited to set the correct size and position of the rectangle. To do this, click on the rectangle R1 to select it and highlight it in red. Then click on the Inspect/edit selected geometry object toolbar button
- In the Edit Geometry Object dialog box, change the min and max point coordinates to define a rectangle with length and height 0.05 and lower left corner at the origin. Finish editing the geometry object and close the dialog box by clicking OK.
- To create the circular hole, first left click on the Create circle/ellipse button in the left hand side Tools toolbar frame.
- Then click and hold the left mouse button anywhere in the main plot axes window, move the mouse pointer to show red outlines of a circle or ellipse. Release the button to finalize and a solid ellipse will be created.
- The object properties of the ellipse E1 must be changed to make a circle with radius 0.005 centered at (0, 0). To do this, click on the E1 to select it which also highlights it in red. Then click on the Inspect/edit selected geometry object toolbar button
- In the opened Edit Geometry Object dialog box change the center coordinates to 0 0, and the x and y radii to 0.005 in the corresponding edit fields. Finish editing E1 and close the dialog box by clicking OK (the smaller objects will automatically be selected to be subtracted from the larger).
- To subtract the circle from the rectangle first select both geometry objects by clicking on them so both are highlighted in red. Alternatively, if the circle is obscured by the rectangle they can be selected by holding the Ctrl key while clicking on the labels R1 and E1 in the Selection list box (or simply press Ctrl+a to select all objects). When the geometry objects are selected, press the Subtract geometry objects button to generate the final geometry shape.
- Switch to Grid mode by clicking on the corresponding mode toolbar button.
- Click on the Generate unstructured grid button to call the grid generation function which automatically generates a triangular grid for the geometry.
- The default estimated grid size is sufficient to resolve the rectangle but the hole is not represented well. Try refining the grid by clicking on the Uniform grid refinement button once or twice.
- The uniformly refined grid does not improve resolution of curved boundaries and in this case also creates very distorted cells which leads to poor accuracy. To generate a better grid enter 0.002 in the Grid Size parameter edit field and click on the Generate unstructured grid button again.
- With an improved grid we now can proceed with specifying the equations. Press the mode button in the Mode toolbar to switch from grid mode to physics and equation/subdomain specification mode.
- In the Equation Settings dialog box that automatically opens, enter 0.3 for the Poisson ratio and 210e9 for the modulus of elasticity E. The other coefficients can be left to their default values. Press OK to finish and close the dialog box.
- Now we want to add an expression for the load force. We could do it by directly entering the expression in the dialog box, however there are advantages to using the constants and expressions functionality. To use this click on the Model Constants and Expressions button
- Enter a new variable named load with expression 1000*(2*0.05*0.001) (In this case we are just entering constants one can also use complicated formulas involving dependent variables, space coordinates, time, and other expressions). Press OK to finish and close the dialog box.
- Change to boundary condition specification mode by clicking on the mode button.
- In the Boundary Settings dialog box, first select all boundaries and set all conditions to Edge loads with a value of zero, 0.
- Then select the left boundary (number 3 in this case) in the left hand side Boundaries list box and select Fixed displacement, u and zero Edge load, y-dir.. This will fix the x-displacement of this boundary.
- Continue by selecting boundary 5, the bottom boundary, and choose a zero Edge load, x-dir. and Fixed displacement, v boundary conditions. This similarly fixes the lower boundary.
- Lastly, select both Edge load boundary conditions for the right boundary (number 1). Set the edge load in the x-direction on this boundary to load, which will be evaluated from the expression we entered earlier. Finish by clicking the OK button.
- Now that the problem is fully specified, press the mode button to switch to solve mode. Then press the button with an equals sign in the Tools toolbar frame to call the solver with the default solver settings.
- After the problem has been solved FEATool will automatically switch to postprocessing mode and display the computed von Mieses stress. To change the plot, open the postprocessing settings dialog box by clicking on the Postprocessing settings button in the Tools toolbar frame.
- In the Postprocessing settings dialog box choose to plot the Stress, x-component for both the surface and contour plots.
- We can see that the solution for the stress is not smooth. This is due to the fact that stress is a function of derivatives of the displacements (the solution variables) and a linear solution approximation is used per default, this means that stresses will be represented as piecewise constant functions. We can improve on this by going back to the Equation specification mode.
- Select P2/Q2 second order conforming for the finite element shape function specification and Solve the problem again.
- Now we can see that the plotted stress is smooth and the maximum value is slightly above 3*10^{7} which is the expected solution.
- Saving the model can be done from the File menu. Save As... allows you to save the model in binary (.fea) format which can be loaded into the GUI again. Save As M-Script Model... allows you to save all FEATool function commands used to make the model as a m-script file. This file can not be loaded into the GUI but inspected, modified, and run on the command line as a script file.
Thin Plate with Hole using the CLI
The process to set up and solve the thin plate with hole example problem on the command line interface is illustrated in the ex_planestress1 script file which can be found in the examples directory. Moreover, if you export the model using the Save As M-Script Model... option then one can easily see exactly which FEATool functions and commands are used to build the model.
Multiphysics Example - Heat Exchanger
This example illustrates the multiphysics capabilities of FEATool with a simple heat exchanger model featuring both free and forced convection. The model consists of a series of heated pipes around which there is a lower temperature fluid flowing. Two kinds of physics are considered, fluid flow which is modeled by the Navier-Stokes equations and heat transport modeled by a convection and conduction equation for the temperature field. The Boussinesq approximation models the temperature effects on the fluid, and the flow field is coupled to and transports the temperature field. In this way the system is fully two way coupled, the fluid to the temperature and temperature to the fluid.
Due to symmetry it is enough to study a two dimensional slice between the heated pipes. The geometry will therefore consist of a 0.0075 by 0.05 m rectangle with a half circle removed (with radius 0.003 m centered at (0, 0.02)). The mechanism for heating the pipes are not taken in consideration and are thus assumed to be at a fixed temperature of T_{h}=330 K. A cooling fluid flows from the bottom to the top and has an inlet temperature of T_{h}=300 K. The other model parameters can be found in the following model description.
Heat Exchanger using the GUI
This section describes how to set up and solve the heat exchanger example with the FEATool graphical user interface (GUI).
- Start Octave/Matlab, and if you have not run the installation script (which automatically adds the FEATool directory paths at startup) then change your working directory to where your FEATool installation is, for example
cd C:\featool
In the command window type
featool
to start the graphical user interface (GUI).
- Either select New... from the File menu, or click on the New Model button in the upper horizontal toolbar, to clear all data and start defining a new model.
- In the opened New Model dialog box, click on the 2D radio button in the Select Space Dimensions frame, and select Navier-Stokes Equations from the Select Physics drop-down list. Leave the space dimension and dependent variable names to their default values. Finish and close the dialog box by clicking on the OK button.
- To define the geometry, first create a rectangle by clicking on the Create square/rectangle button in the left hand side Tools toolbar frame.
- Then left click in the main plot axes window, hold the mouse button, and move the mouse pointer to draw a rectangle.
- Release the mouse button to finalize and create a solid geometry object. The object properties must now be edited to set the correct size and position of the rectangle. To do this, click on the rectangle R1 to select it and highlight it in red. Then click on the Inspect/edit selected geometry object toolbar button
- Change the minimum and maximum x-coordinates to 0 and 0.0075, respectively. Also change the y-dimensions to span between 0 and 0.05. Press OK to finish editing the rectangle properties.
- In a similar way, create a circle centered at (0, 0.02) with a radius of 0.003.
- To create the final geometry select both the rectangle and circle so they are highlighted in red (either by directly clicking on them or selecting them in the Selection list box). Then click on the Subtract geometry objects button to subtract the smaller circle from the larger rectangle.
- Switch to Grid mode by clicking on the corresponding mode toolbar button. Enter 0.001 in the Grid Size parameter edit field and click on the Generate unstructured grid button to automatically generate a grid.
- Press the mode button in the Mode toolbar to change to physics and equation/subdomain specification mode. In the Equation Settings dialog box that automatically opens enter the following coefficients, rho for the density, mu for the viscosity, and alpha*g*rho*(T-Tc) for the volume force in the y-direction.
- We now have to add a heat transfer physics mode. To access the multiphysics selection and add another physics mode press the plus + tab and select Heat Transfer from the Select Physics drop down list. Add the selection by pressing the Add Physics >>> button.
- In the Equation Settings ht tab, set the density , specific heat , and heat conductivity to rho, cp and k, respectively. The convective velocities should be coupled from the Navier-Stokes equations physics mode, to do this enter u and v in the corresponding edit fields (as these are the default names of the dependent variables for the velocities). Press OK to finish with the equation specifications.
- The values of the specified coefficients must now be prescribed. Click on the Model Constants and Expressions button to open the corresponding dialog box.
- Enter the values shown below in the Model Constants and Expressions dialog box. Space for more constants can be made by clicking to the Add Row button. Press OK to finish.
- Switch to boundary condition specification mode by clicking on the mode button. First select the ns tab, which corresponds to the boundary conditions prescribed to the Navier-Stokes equations physics mode. Then select all vertical boundaries (here 2, 4, and 7). Choose Symmetry/slip, x-direction from the drop down box. Switch to the heat transfer physics mode by selecting clicking on the ht tab and select Thermal insulation/symmetry boundary conditions.
- Now continue with the top boundary (number 3) which is the outflow. Select Outflow/pressure for the Navier-Stokes physics mode and Convective flux/outflow for the heat transfer mode.
- The bottom boundary (number 1) is the inflow and should be prescribed with the constant velocity uin in the y-direction by using the Inlet/velocity condition. The Temperature should here be fixed to Tc, the lower temperature.
- Lastly, the boundaries on the cylinder (5 and 6) are walls and should be prescribed with Wall/no-slip boundary conditions for the velocity. For the Temperature a constant high temperature of Th should be prescribed. Press OK to finish prescribing boundary conditions.
- Now that the problem is fully specified, press the mode button to switch to solve mode. Then press the button with an equals sign to start the solution process.
- After the problem has been solved FEATool will automatically switch to postprocessing mode and display the computed solution. We can see the the velocity will be accelerated when passing between the cylinders. Open the postprocessing settings dialog box by clicking on the Postprocessing settings button and select to view the Temperature, T.
- We can clearly see how the fluid is heated around the hot cylinder and follows the flow upwards. FEATool Multiphysics and Professional also allows advanced postprocessing such as boundary integration. By integrating the expression T/w (where w is the width 0.0075 of the domain) we effectively calculate the mean temperature and can see that at the outflow the temperature has risen by about 1.5 degrees.
Heat Exchanger using the CLI
The process to set up and solve the heat exchanger multiphysics problem on the command line interface is illustrated in the ex_heat_exhanger1 script file which can be found in the examples directory. Moreover, you can also export the model using the Save As M-Script Model... option and see exactly which FEATool commands are used in building the model.
Equation Editing Example - Axisymmetric Fluid Flow
FEATool is designed to be able to perform complex Octave and Matlab multiphysics simulations in arbitrary dimensions (1D, 2D, and 3D). However, running full 3D simulations often requires a significant amount of computational resources in the form of memory and simulation time. It is therefore desirable to find simplifications to reduce simulations to two or even one dimension if possible.
Problems which feature cylindrical and rotationally symmetric geometries and solutions can be reduced to two dimensions through a cylindrical or axisymmetric coordinate transformation (also referred to 2.5D). A symmetry axis, usually r=0, is taken as reference around which the coordinates and PDE operators (gradient and divergence) are transformed. In this way the governing equations will be reduced to 2D while representing a rotationally symmetric three dimensional problem.
This example models fluid flow in a narrowing pipe section. The constriction of the pipe will accelerate the flow according to the venturi effect. As the fluid is assumed to be both laminar and isothermal the problem is governed by the incompressible Navier-Stokes equations. For scalar equations like the convection and diffusion, and heat transfer equations axisymmetric transformation simply results in a multiplication of the equation with the radial coordinate. In this case the vector valued equations results in additional terms compared to the usual Cartesian case
In addition to modifying the equations an appropriate boundary condition for the symmetry boundary must be chosen. A homogeneous Neumann insulation/symmetry condition is typically employed for scalar equations, but in the case of fluid flow a slip condition preventing any radial velocity u(r=0) while allowing axial velocity is appropriate.
The geometry of the problem considers a 2:1 constriction with an initial pipe diameter of 2. The inlet velocity is assumed to be uniform v(z=0)=1 and the fluid has a density of and viscosity . This results in a laminar Reynolds number of .
Axisymmetric Fluid Flow using the GUI
This section describes how to set up and solve the axisymmetric flow problem with the FEATool graphical user interface (GUI). Although FEATool does not feature predefined physics modes for axisymmetric coordinate systems, this example shows how one can use the edit equations feature to modify the built in equations and accommodate these transformations.
- Start Octave/Matlab, and if you have not run the installation script (which automatically adds the FEATool directory paths at startup) then change your working directory to where your FEATool installation is, for example
cd C:\featool
In the command window type
featool
to start the graphical user interface (GUI).
- Either select New... from the File menu, or click on the New Model button in the upper horizontal toolbar, to clear all data and start defining a new model.
- In the opened New Model dialog box, click on the 2D radio button in the Select Space Dimensions frame, and select Navier-Stokes Equations from the Select Physics drop-down list. Change the space dimension names to r and z but leave the dependent variable names to their default values. Finish and close the dialog box by clicking on the OK button.
- Use the Create square/rectangle button in lower left toolbar to create two rectangles. Put one on top of the other with their left edges aligned with the z-axis (r=0). The lower one should have dimensions 1 x 2 and the upper one 0.5 x 1. Also create a circle with radius 0.5 centered at (1, 2).
- To create the compound geometry, select Combine Objects... from the Geometry menu. Enter the formula R1 + R2 - E1 in the edit field of the Combine Geometry Objects dialog box and press OK.
- Press the mode button in the Mode toolbar to switch from geometry mode to grid generation mode. To change the target grid size, enter 0.1 in the Grid Size edit field. Then click on the Generate unstructured grid button to call the automatic grid generation function.
- Change to physics and equation/subdomain specification mode by pressing the button. In the Equation Settings dialog box that automatically opens, set the density to 1, viscosity to 5e-2, and the source terms to zero.
The equations must now be changed from a Cartesian to cylindrical coordinate system. To do this press the edit eqn button next to the equation description. This will bring up the Edit Equations dialog box and show the currently defined partial differential equations. Change the equations to the following
- r*miu_ns*(2*ur_r + uz_z + vr_z) + r*rho_ns*(u*ur_t + v*uz_t) + r*p_r + 2*miu_ns*u/r - p = 0
- r*miu_ns*(vr_r + uz_r + 2*vz_z) + r*rho_ns*(u*vr_t + v*vz_t) + r*p_z = 0
u_t + r*ur_t + r*vz_t = 0
Press OK to finish with the equation and subdomain specifications.
- Switch to boundary condition specification mode by clicking on the mode button. In the Boundary Settings dialog box, first select all boundaries in the left hand side Boundaries list box and choose the Wall/no-slip boundary conditions from the drop-down list. Now select the lower inflow boundary (here number 1) in the left hand side Boundaries list box and choose the Inlet/velocity boundary condition from the drop-down list. Enter of 1 in the edit field for the z-velocity coefficient .
- Select the top outflow boundary (number 5) in the left hand side Boundaries list box and choose the Neutral outflow/stress boundary boundary condition from the drop-down list (alternatively one can also prescribe a pressure with the Outflow/pressure condition).
- Lastly, select the left side boundaries (here number 3 and 6) and select the Symmetry/slip, r-direction boundary condition from the drop-down list which will prevent flow in the radial direction while allowing it in the axial direction. Finish by clicking the OK button.
- Now that the problem has been set up, press the mode button to go to solve mode. Then press the Solver Settings button . Increase Maximum non-linear iterations to 100 and set the Non-linear relaxation parameter to 0.8 in the Non-Linear Solver Settings frame to relax the convergence of the solver. Press Apply and then Solve to start the solution process.
- After the problem has been solved FEATool will automatically switch to postprocessing mode and display the computed solution. We can see that the velocity field is significantly accelerated by the pipe constriction.
- It is also possible to study a section of the velocity profile by using the Point/Line Evaluation... feature from the Post menu (available with FEATool Multiphysics and Professional licenses). By entering a series of coordinates to examine we get both the values and a cross section plot of the evaluation expression. In this case the velocity profile close to the outlet at z=2.8 is starting to shift from parabolic to a more square profile indicating a higher velocity flow and we might need to study a longer outflow section to recover the expected parabolic laminar flow profile.
Axisymmetric Fluid Flow using the CLI
The process to set up and solve this fluid flow problem on the command line interface is illustrated in the ex_navierstokes8 script file which can be found in the examples directory. Alternatively, one can also use the Save As M-Script Model... feature to get an equivalent Matlab script file for all the corresponding CLI commands that has been executed by the GUI.
Classic Equation Example - Poisson Equation with a Point Source
The classic Poisson equation is one of the most fundamental partial differential equations (PDEs). Although one of the simplest equations, it is a very good model for the process of diffusion and comes up again and again in many applications such as in fluid flow, heat transfer, and chemical transport.
This example shows how to up and solve the Poisson equation
for a scalar field on a circle with radius in two dimensions. The diffusion coefficient and right hand side source term which prescribes a point source at the center. The Poisson problem is also considered stationary meaning the time dependent term can be neglected. With these assumptions equation (1) simplifies to
Moreover, homogeneous Dirichlet boundary conditions are prescribed on all boundaries of the domain, that is . The exact solution for this problem is which can be used to measure the accuracy of the computed solution.
Poisson Equation with a Point Source using the GUI
This section describes how to set up and solve the Poisson equation (1) with the FEATool graphical user interface (GUI) which is available when using FEATool together with Octave version 4.0 or later and Matlab.
- Start Octave/Matlab, and if you have not run the installation script (which automatically adds the FEATool directory paths at startup) then change your working directory to where your FEATool installation is, for example
cd C:\featool
In the command window type
featool
to start the graphical user interface (GUI).
- Either select New... from the File menu, or click on the New Model button in the upper horizontal toolbar, to clear all data and start defining a new model.
- In the opened New Model dialog box, click on the 2D radio button in the Select Space Dimensions frame, and select Poisson Equation from the Select Physics drop-down list. Leave the space dimension and dependent variable names to their default values. Finish and close the dialog box by clicking on the OK button.
- To create a circle, first left click on the Create circle/ellipse button in the left hand side Tools toolbar frame.
- Then click and hold the left mouse button anywhere in the main plot axes window, and move the mouse pointer to show red outlines of a circle or ellipse. Release the button to finalize and create a solid geometry object.
- The object properties must be changed to make a circle with radius 1 centered at the origin. To do this, click on the ellipse E1 to select it which also highlights it in red (alternatively you select it by clicking on R1 in the selection list box under the left side toolbar buttons). Then click on the Inspect/edit selected geometry object toolbar button
- In the opened Geometry Object dialog box change the center coordinates edit field 0 0, and the x radius and y radius to 1 in the corresponding fields. Finish editing the geometry object and close the dialog box by clicking OK.
- To define a point select Add Point... from the Geometry menu. This opens a dialog box where you can define a new point. Enter 0 0 in the Point Coordinates edit field and press OK to add the point P1 to the center. This point will ensure that we will have a grid point in the center where constraints can be prescribed.
- Press the mode button in the Mode toolbar to switch from geometry mode to grid generation mode.
- Click on the Generate unstructured grid button to call the grid generation function which automatically generates a grid of triangles for the circle.
- Press the mode button in the Mode toolbar to switch from grid mode to physics and equation/subdomain specification mode.
- In the Equation Settings dialog box that automatically opens, set the diffusion coefficient to 1 and source term coefficient to 0 in the corresponding edit fields. All other coefficients can be left to their default values. Press OK to finish and close the dialog box.
- To add the point source select Add Point Sources... the from the Equation menu, and enter 1 in the corresponding edit field for the point in the center (in this case point number 3). Press OK to finish.
- Switch to boundary condition specification mode by clicking on the mode button.
- In the Boundary Settings dialog box, select all boundaries in the left hand side Boundaries list box and choose Dirichlet boundary condition in the drop-down list. Set the Dirichlet boundary coefficient r equal to 0 in the Boundary Coefficients frame and finish by clicking on OK.
- Now that the problem is fully specified, press the mode button to switch to solve mode. Then press the button with an equals sign in the Tools toolbar frame to call the solver with the default solver settings.
- After the problem has been solved FEATool will automatically switch to postprocessing mode and display the computed solution. To change the plot, open the postprocessing settings dialog box by clicking on the Postprocessing settings button in the Tools toolbar frame.
- Activate Height plot by marking the corresponding check boxes and press OK or Apply to show how the solution looks like in three dimensions. We can clearly see how the central point source results in a spike in the solution.
- By returning to the Postprocessing settings dialog box and entering the expression u+1/2/pi*sqrt(x^2+y^2) in the Surface Plot expression edit field it is possible to plot and visualize the difference between the computed and exact reference solution. We can see that the largest errors are found in the center due to the coarse grid. To improve the accuracy one should ideally create a grid that is locally refined around the central point.
Poisson Equation with a Point Source using the CLI
The process to set up and solve this Poisson problem on the command line interface is illustrated in the ex_poisson7 script file which can be found in the examples directory.
Custom Equation Example - Wave Equation on a Circle
This section explains how to set up and solve a generalized wave equation model. The wave equation is a hyperbolic partial differential equation (PDE) of the form
where is a constant defining the propagation speed of waves, and is a source term. This equation cannot be solved as it reads due to the second order time derivative. However, the problem can be transformed by reformulating the wave equation as two coupled parabolic PDEs, that is
This dual coupled problem can easily be implemented in FEATool with the custom equation feature. An example of the wave equation on a unit circle, with zero boundary conditions, constant , source term , and initial condition is described in the following
Wave Equation using the GUI
This section describes how to set up and solve the wave equation with the FEATool graphical user interface (GUI) which is available when using FEATool together with Octave version 4.0 or later and Matlab.
- Start Octave/Matlab, and if you have not run the installation script (which automatically adds the FEATool directory paths at startup) then change your working directory to where your FEATool installation is, for example
cd C:\featool
In the command window type
featool
to start the graphical user interface (GUI).
- Click on the New Problem button in the upper horizontal toolbar to clear all data and start defining a new problem.
- In the opened New Problem dialog box, click on the 2D radio button in the Select Space Dimensions frame, and select Custom Equation from the Select Physics drop-down list. Leave the space dimension as it is but change the dependent variable names to u v (The custom equation physics mode allows for entering an arbitrary number of dependent variables through the use of a space separated list). This will add two equations for u and v, respectively. Finish and close the dialog box by clicking on the OK button.
- To create a circle, first left click on the Create circle/ellipse button in the left hand side Tools toolbar frame.
- Then click and hold the left mouse button anywhere in the main plot axes window, and move the mouse pointer to show red outlines of a circle or ellipse. Release the button to finalize and create a solid geometry object.
- The object properties must be changed to make a circle with radius 1 centered at the origin. To do this, click on the ellipse E1 to select it which also highlights it in red. (Alternatively you select it by clicking on R1 in the selection list box under the left side toolbar buttons.)Then click on the Inspect/edit selected geometry object toolbar button
- In the opened Geometry Object dialog box change the center coordinates edit field 0 0, and the x radius and y radius to 1 in the corresponding fields. Finish editing the geometry object and close the dialog box by clicking OK.
- Press the mode button in the Mode toolbar to switch from geometry mode to grid generation mode.
- Click on the Generate unstructured grid button to call the grid generation function which automatically generates a grid of triangles for the circle.
- Press the mode button in the Mode toolbar to switch from grid mode to physics and equation/subdomain specification mode.
- An Equation Settings dialog box will now automatically open. Set the initial condition for u, to 1-(x^2+y^2). Then click on the edit eqn button.
- In the Edit Equations dialog box enter the equations u' - v_t = 0 and v' + c*(ux_x + uy_y) = 0 in the corresponding edit fields for u and v. Here u and v are the dependent variables, u'/v' denote the corresponding time derivative, and an underscore will treat it implicitly in the weak finite element formulation (for example v_t corresponds to v multiplied with the test function for u, and ux_x is analogous to du/dx*dv_t/dx). Note, that the first equation could also be implemented as u' = v but then v would be evaluated explicitly in the right hand side, and by transferring it to the implicit left hand side matrix we will get a linear problem which is more efficient to solve. Press OK and close the equation settings dialog boxes.
- Click on the Model Constants and Expressions button and enter a new constant named c with value 1 (This is the constant used in diffusion term of the v equation). Press OK to finish.
- Switch to Boundary condition specification mode and set Dirichlet conditions with prescribed value 0 on all boundaries.
- Change to Solve mode and open the Solver Settings dialog box. Choose the Time-Dependent solver and also set the time step to 0.05. Press Apply and Solve to start the solution process.
- Once the solver has finished the solution at the final time step will be displayed. In the Postprocessing Settings dialog box it is also possible to select and visualize the solution at different times.
Wave Equation using the CLI
The process to set up and solve the wave equation problem on the command line interface is illustrated in the ex_waveequation1 script file which can be found in the examples directory.
More Examples and Information
Additional m-script command line examples and can also be found in the featool/examples directory. Also bookmark the FEATool news and blog page where tips and tutorials are posted.