How to Develop a Model Using

COMSOL Multiphysics

Tom Dougherty

Introduction

COMSOL Multiphysics is a finite element analysis solver that is widely used in engineering and

physics disciplines. The software was developed in 1986 by a pair of Swedish graduate students

attending the Royal Institute of Technology, and since then numerous yearly updated versions

have been released. The most current version, and the one that is used in this instruction set, is

COMSOL 4.4. This program is available at select computer lab locations across college

campuses, or it can be downloaded online after the purchase of a renewable COMSOL license at

the following website:

http://www.comsol.com/product-download/4.4/windows (Windows version)

This instruction set is designed to introduce the reader to the COMSOL Multiphysics program by

proceeding step-by-step through a simple example model. The example model that has been

chosen is the thermal ablation of a hepatic tumor. This is an innovate new cancer treatment

procedure that uses heated probes to kill tumors in the liver as well as in other parts of the body.

The key objective in this method is to heat the tumor up to a high temperature so that the tumor

can no longer survive while leaving the healthy tissue unaffected.

In order to model this procedure (and every other potential procedure), several simplifications

must be made. For this model, the skin surface is represented by a simple rectangular shape with

certain material properties (thermal conductivity, density, and heat capacity). The rectangle is set

to an initial temperature representing the skin temperature under normal, non-heating conditions.

Instead of going through the complex procedure of modelling the heating probe, a simple

boundary condition is implemented which sets the temperature at the left edge of the rectangle at

the high temperature of the probe. After all of the simplifications have been set, the model is

tested under the specified physical conditions and results are obtained. The interface of the tumor

and healthy tissue is chosen at a specified point on the rectangle and, using a temperature plot, it

can be analyzed whether or not any healthy tissue has become damaged in the process.

It should be noted that the exact specifications and simplifications relevant to this specific model

do not need to be understood; however, the reader should use this instruction set to become more

familiar with the overall COMSOL interface. This is merely one of a myriad of potential models

that can be designed using this program. This model was specifically chosen for its simplicity

and functional design.

This example model should take approximately 25-30 minutes to complete, assuming that the

user has a general knowledge of how to operate a standard computer program in addition to some

knowledge of basic mathematical concepts.

Materials

The only required material is a working computer with COMSOL 4.4 installed. Newer

computers are preferred since computation times for solving models can sometimes be lengthy.

Developing the Model

1. Open a new COMSOL model. Simply click on the “Model Wizard” icon to start a new

COMSOL model. This option is made available as soon as the program has finished

loading.

2. Specify the problem type. First select the space dimension in which the model resides.

For this example, the model exists in a 2-dimensional space.

The next page will provide a listing of various physics types that can be applied to a

given model. Since we are modelling the thermal ablation of a tumor, heat transfer

through a solid will be the governing physics.

The remaining step before fully defining the problem is to select the study type. Heat that

is transferred through a solid object changes with respect to time, so the study is time-

dependent.

3. Create the geometry. It is during this step that the actual component is built. To begin

construction, first right-click on “Geometry” located in the Model Builder toolbar on the

left-hand side of the COMSOL interface. Then scroll down the pop-up window and select

“Rectangle”.

Specify the dimensions of the shape. The proper width and height of the rectangle are

0.06 meters and 0.02 meters, respectively. Select “Build Selected” to build the given

component.

The rectangle component should appear in the Graphics toolbar on the far right-hand side

of the COMSOL interface (should appear similar to below image).

4. Define material properties. Right click on the “Material” icon under the Model Builder

toolbar and select “New Material”.

Specify the important material parameters thermal conductivity, density, and heat

capacity at constant pressure. In order, these values are 0.512, 1060, and 3600 (units and

other information are automatically included). Make sure that the green checkmarks

appear next to each property (below), since a full set of these marks indicates that the

material is fully defined.

5. Define initial conditions. Click on “Initial Values 1” under the Model Builder toolbar.

Set the temperature, T, to 310 degrees K. This value represents the skin temperature of

the skin (rectangle) prior to the external influence of heat transfer.

6. Set boundary conditions. Right click on “Heat Transfer in Solids” under the Model

Builder toolbar and then click on the “Temperature” boundary condition.

Under the Graphics toolbar, click on the left side of the rectangle. This side should turn

blue to indicate that it has been selected.

Set the temperature, T0, of this boundary to 363 degrees K. This value represents the

temperature at the interface where the heating device is applied.

After specifying the boundary condition, the left side of the rectangle which was

previously highlighted blue should now be green to indicate a correct interface boundary

condition.

7. Specify solver parameters. Click on “Step 1: Time Dependent” under the Model Builder

toolbar.

Fill in the “Times” field with “range(0,1,180)”. This step sets the time that the heat

transfer is applied at 180 seconds in intervals of 1 second apiece.

8. Solve the Model. Click on “Study 1” under the Model Builder toolbar.

Select the “Compute” icon.

After roughly one minute (depending on the computer), COMSOL will output the results

of the model with the applied physics for the specified duration of 180 seconds. Under

the Graphics toolbar, a temperature graph will be displayed representing the temperature

distribution of the skin following the heat transfer.

9. Save and Exit. After the resulting graph has been analyzed and extracted, make sure to

save the model for future study and then exit the COMSOL interface.

Troubleshooting

If an error occurs and the model cannot be solved, an error message will explain exactly

what issues are preventing the output of a completed solution

Aside from the first two steps above (opening a new COMSOL model and specifying the

problem type) no particular order must be followed to obtain the solution

As mentioned in the above steps, make sure that green checkmarks appear next to the

material parameters to ensure that they are accepted values

There is a model library feature of COMSOL that includes a wide variety of correct

example models which can be used to compare with models that do not function properly