Discussion Forum

Anthony,

instead of using a number in the material property or in the user setting in Ampere's law you can use a function of T. E.g. something like mur(T) = mu_ref*exp(-(T - 273.12)). This would be a function that provides the value mu_ref at 0 °C and falls exponentially to zero for high temperature. In case you have tabulated data define an interpolation function.

Cheers Edgar

Hello Edgar,

Thank you replying to my question and giving some pointers in COMSOL, it is most appreciated. It take it for mu_ref, it would be called under parameters and given a value of like 1, would the Temperature variable raised to the exponenet also need to be called under parameters ?

Kind Regards,

Anthony Crawford

Hi Anthony,

no, T is the dependent variable in your Heat Transfer physics. It is already declared in the model and you can use it in expressions throughout the model.

Cheers Edgar

I've called the function under variables and mu_ref under parameters. The model runs, but when I plot the temperature data against time, it just shows a temperature of just above 20 degrees, whiltst the 3D temeprature revolution is showing that the object is heated to over 1000 degrees. Have I set up the function correctly ?

Kind Regards,

Anthony Crawford

I didn't find the definition of the perm()-function. I think you can use the variable mur_steel, it seems to be set up properly.

The variable mur_steel is the function, I have set up and have been using, with the value of mu_ref being set to 1 under parameters. Where else could I set up and call this function besides under variables ?

mur_steel is set up as a variable, not a function. You can use mur_steel in all entities you defined it in and in any expression. You may also set it up globally if no spatial dependencies are needed. Functions are defined in a more generic way and you call them in the form f(something), where the function definition uses a generic independent variable, e.g. x. In the model x can be replaced by anything. I recommend to test things like this in very simple examples that solve in a few seconds to gain a better understanding of it, before you spend much time with it in the complex model.

In my case, how would I set it up as a function then ?

Look in Definitions -> Functions either in global definitions or in the component definitions.

Unsure what kind of function it would be as, I don't have tabulated data, so that rules out interpolation and piecewise, leaving only step and analytic

Well, of course you need to know what the temperature dependance of µr in your material is. This may be available from the material manufacturer or from literature sources. In many cases material data like this is not available and you need to measure it by yourself or a suitable service provider.