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Time- and Space-Dependent Current Density

Posted 14 juil. 2016, 09:31 UTC−4 Materials Version 5.2, Version 5.2a 5 Replies

Mattia Mantovani
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Hi everyone,

I would like to compute the radiated power from a distribution of current which varies in time and space. I was using the RF-module, using a 'External Current Density'. I tried the 'Frequency Domain' study type, so that I can implement Perfectly Matched Layers, but if I try to put the following expression in the J_e box (external current)

J0*exp(-abs(sqrt(x^2+y^2+z^2)))*sin(2*pi*c_const/lambda*t)

it says that there are undefined variables (t, x, y, z...).
J0 and lambda are user-defined parameters. The units of measurement are correct.
Should I try the 'Time Explicit' module and the 'Time Dependent' study? But how can I setup Perfectly Matched Layes or a similar boundary condition?

Thanks,
Mattia
5 Replies Last Post 4 nov. 2016, 16:46 UTC−4
Ross Wakelin
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Posted: 8 years ago 15 juil. 2016, 03:58 UTC−4
Following this, as I am having a similar problem - want to define soil porosity that varies with (x,y) depending on distance under a sloping surface. x and y are said to be undefined variables, don't know how to make the geometrical coordinates available in the material property functions.

one thing I discovered along the way, that might or might not help your situation
- when inserting a function in physics settings if I used the "insert expression" icon instead of typing in the function it took the parameters with it. That is, when I typed just pw1 it didn't work, but when I selected the function from the insert expression tree it came with the parameters specified, ie pw1(t).

Hope someone can answer about the making the geometric coordinates available to functions... it must be a simple thing...
Following this, as I am having a similar problem - want to define soil porosity that varies with (x,y) depending on distance under a sloping surface. x and y are said to be undefined variables, don't know how to make the geometrical coordinates available in the material property functions. one thing I discovered along the way, that might or might not help your situation - when inserting a function in physics settings if I used the "insert expression" icon instead of typing in the function it took the parameters with it. That is, when I typed just pw1 it didn't work, but when I selected the function from the insert expression tree it came with the parameters specified, ie pw1(t). Hope someone can answer about the making the geometric coordinates available to functions... it must be a simple thing...
Magnus Ringh COMSOL Employee

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Posted: 8 years ago 15 juil. 2016, 05:09 UTC−4
Hi,

The expression that you are using

J0*exp(-abs(sqrt(x^2+y^2+z^2)))*sin(2*pi*c_const/lambda*t)

should work in a 3D geometry with the default spatial coordinates x, y, and z. The time t, though, is undefined in a frequency domain study, so you need to define it as a constant. Also, you may want to "dedimensionalize" the x, y, and z variables in that expression (they have a unit of meters when using SI units).

Best regards,
Magnus Ringh, COMSOL
Hi, The expression that you are using J0*exp(-abs(sqrt(x^2+y^2+z^2)))*sin(2*pi*c_const/lambda*t) should work in a 3D geometry with the default spatial coordinates x, y, and z. The time t, though, is undefined in a frequency domain study, so you need to define it as a constant. Also, you may want to "dedimensionalize" the x, y, and z variables in that expression (they have a unit of meters when using SI units). Best regards, Magnus Ringh, COMSOL
Ross Wakelin
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Posted: 8 years ago 15 juil. 2016, 05:26 UTC−4
de-dimensionalising means writing variable x as x/1[m] each place it occurs, etc?

Is there any other trick for getting x, y, z available as variables in material property functions? I am still not succeeding in getting x and y to be recognised as variables....
de-dimensionalising means writing variable x as x/1[m] each place it occurs, etc? Is there any other trick for getting x, y, z available as variables in material property functions? I am still not succeeding in getting x and y to be recognised as variables....
Mattia Mantovani
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Posted: 8 years ago 15 juil. 2016, 06:58 UTC−4
Dear Magnus Ringh,

You're right about the X, y, z dimensions, I actually did that but forgot to mention. In fact the formula dimensionality is correct (A/m^2).

Regarding the time dependence, do you mean that I have to define "t" as a variable in the definitions, and then do a Parameter Sweep study in the time range I desire? Will this work as a "time dependent" simulation?

Br,
MM
Dear Magnus Ringh, You're right about the X, y, z dimensions, I actually did that but forgot to mention. In fact the formula dimensionality is correct (A/m^2). Regarding the time dependence, do you mean that I have to define "t" as a variable in the definitions, and then do a Parameter Sweep study in the time range I desire? Will this work as a "time dependent" simulation? Br, MM
David Vieyra
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Posted: 8 years ago 4 nov. 2016, 16:46 UTC−4
Dear Magnus,

I am trying to implement a temperature boundary condition, function of the spacial coordinates on a 2D Symmetric modeling ( r , z ) coordinates system. My question is how I can write the formula.

This is : 77 + 216.15 *( z - 0.2)

How I must right the Z coordinate?

Thank you.


Christian.
Dear Magnus, I am trying to implement a temperature boundary condition, function of the spacial coordinates on a 2D Symmetric modeling ( r , z ) coordinates system. My question is how I can write the formula. This is : 77 + 216.15 *( z - 0.2) How I must right the Z coordinate? Thank you. Christian.

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