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Particle energy distribution

Posted 10.06.2013, 11:02 GMT-4 Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3 5 Replies

Florian Kroll
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Hey Comsol gurus,

I am trying to simulate complex particle distributions in electric and magnetic fields. The mf and es parts are working quite nicely but I am experiencing some difficulties setting up my particle source. I want to have complete controll over the particles energy/velocity distribution. I tried to use an analytic function to describe the energy spectrum but this (within the Inlet condition of cpt) worked only in dependancy on x,y,z - and I dont want to have that. As an example, it would be helpful if someone told me how I can set up a distribution with (for example) exponential energy spectrum homogeneous over a certain surface - ideally without using a parametric sweep.

I hope you guys can help me and I am thanking you very much in advance,

Florian
5 Replies Last Post 04.03.2014, 17:15 GMT-5
Daniel Smith COMSOL Employee

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Posted: 1 decade ago 10.06.2013, 13:32 GMT-4
Hi, you would need to use some combination of the random(...) and randomnormal(...) functions to construct a custom velocity distribution function. You can use the "particleindex" as the input argument.

For example, try entering an inlet velocity like:

randomnormal(particleindex,t)^2

or

random(particleindex,t)

then create a histogram of the resulting velocity to see the distribution. By using combinations of the uniform and normal distribution functions you should be able to construct a custom distribution.
Hi, you would need to use some combination of the random(...) and randomnormal(...) functions to construct a custom velocity distribution function. You can use the "particleindex" as the input argument. For example, try entering an inlet velocity like: randomnormal(particleindex,t)^2 or random(particleindex,t) then create a histogram of the resulting velocity to see the distribution. By using combinations of the uniform and normal distribution functions you should be able to construct a custom distribution.
Florian Kroll
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Posted: 1 decade ago 12.06.2013, 04:41 GMT-4
Hey Daniel,

thank you very much for your quick response. It worked fine for me.
Now I want to apply a certain binning in particle energy, so for example
allow only integere numbers as particle energies. Is there a rounding
function that I can applie after the random?

Is there somewhere a list with all the Comsol functions and internal
variables? I think this could be very helpful to a lot of Comsol users.

Best regards,
Florian
Hey Daniel, thank you very much for your quick response. It worked fine for me. Now I want to apply a certain binning in particle energy, so for example allow only integere numbers as particle energies. Is there a rounding function that I can applie after the random? Is there somewhere a list with all the Comsol functions and internal variables? I think this could be very helpful to a lot of Comsol users. Best regards, Florian
Daniel Smith COMSOL Employee

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Posted: 1 decade ago 12.06.2013, 08:39 GMT-4
Hi, random(...) generates numbers between -0.5 and 0.5, so you to create random integers between say 0 and 100 you would need to enter something like:

round(100*(0.5+random(particleindex,t)))

Note that you can't use only energy as an initial condition directly. There are 6 initial conditions required for each particle, position (qx,qy,qz) and velocity (d(qx,t), d(qy,t), d(qz,t)). Position plus energy is only 4 so you would need to add two conditions for, say, the initial angle as well. To do this you would have to implement the transform between velocity space and energy/angular space.

A list of reserved names and functions can be found in the Introduction to COMSOL Multiphysics booklet:
www.comsol.com/shared/downloads/IntroductionToCOMSOLMultiphysics.pdf

randomnormal(...) is just a fast way creating a normally distributed random number using Defintions>Functions and particleindex is just a quick way of referencing the postprocessing variable cpt.pidx.
Hi, random(...) generates numbers between -0.5 and 0.5, so you to create random integers between say 0 and 100 you would need to enter something like: round(100*(0.5+random(particleindex,t))) Note that you can't use only energy as an initial condition directly. There are 6 initial conditions required for each particle, position (qx,qy,qz) and velocity (d(qx,t), d(qy,t), d(qz,t)). Position plus energy is only 4 so you would need to add two conditions for, say, the initial angle as well. To do this you would have to implement the transform between velocity space and energy/angular space. A list of reserved names and functions can be found in the Introduction to COMSOL Multiphysics booklet: http://www.comsol.com/shared/downloads/IntroductionToCOMSOLMultiphysics.pdf randomnormal(...) is just a fast way creating a normally distributed random number using Defintions>Functions and particleindex is just a quick way of referencing the postprocessing variable cpt.pidx.
Jason Amsden
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Posted: 1 decade ago 11.07.2013, 14:15 GMT-4
I have a related question. Suppose I have a file with a list of initial poisitions and velocities. Is there a way to use the values in that file as the inlet for the particle tracking?

Thank you,

Jason
I have a related question. Suppose I have a file with a list of initial poisitions and velocities. Is there a way to use the values in that file as the inlet for the particle tracking? Thank you, Jason
Yelena Bagdasarova
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Posted: 1 decade ago 04.03.2014, 17:15 GMT-5
Jason, did you ever find out how to do this?

Thanks,
Yelena
Jason, did you ever find out how to do this? Thanks, Yelena

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