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User define interaction force

Posted Mar 9, 2016, 12:06 p.m. EST Fluid & Heat, Acoustics & Vibrations, Parameters, Variables, & Functions Version 5.2a 2 Replies

Byoung-moo Ann
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Hello

Now I am using charged particle tracing module and ,as you know, the defult interaction among particle set by comsol is Coulomb force.

I want to change Coulomb interaction to other types of interaction, for example, the force inversely proporitonal to particle distance.

I know that I can put the desired expression in "user define force", however, I have no idea to write it down. That is, I dont know the syntax. Whenever I tried, it appeared that the syntax was wrong but I couldn't find any information regarding syntax.

I really look forwad to your helping me. Thanks.
2 Replies Last Post Jul 26, 2016, 6:10 p.m. EDT
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Hello Byoung-moo Ann

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Todd Gillette
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Posted: 8 years ago Jul 25, 2016, 7:01 p.m. EDT
I have a related problem using particle tracing for fluid flow and the acoustophoretic force. I am attempting to implement the secondary particle-particle force which depends on the distance between the particles as well as their relative positions within the acoustic field.

First, I too would like to know how to refer to the distance between particles, as well as the actual particle positions.

In addition, I would like to know how to refer to the velocity and pressure at the particle positions. I know I can use acpr.vx for instance for velocity (and use this to calculate field direction), but I don't know how to get the value at a specific position.
I have a related problem using particle tracing for fluid flow and the acoustophoretic force. I am attempting to implement the secondary particle-particle force which depends on the distance between the particles as well as their relative positions within the acoustic field. First, I too would like to know how to refer to the distance between particles, as well as the actual particle positions. In addition, I would like to know how to refer to the velocity and pressure at the particle positions. I know I can use acpr.vx for instance for velocity (and use this to calculate field direction), but I don't know how to get the value at a specific position.
Todd Gillette
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Posted: 8 years ago Jul 26, 2016, 6:10 p.m. EDT
I was able to track down some of the variables via a combination of the Help documentation and the plot variable selector. For particles, the distance between them is given by fpt.r. The position components of the particle generating a force are qx, qy, qz, while the position components of the particle experiencing the force are dest(qx), dest(qy), dest(qz).

I assume, though I would love to hear if I am wrong, that any fields that are included in the expression are evaluated at the position of the particle producing the force.

As an aside, if your equation is large you might want to break it up and put parts of it (those that can be evaluated outside of the particle-particle calculation) in several variables. It will not only reduce the size of the equation, if there is a problem with units it will make finding the error easier.
I was able to track down some of the variables via a combination of the Help documentation and the plot variable selector. For particles, the distance between them is given by fpt.r. The position components of the particle generating a force are qx, qy, qz, while the position components of the particle experiencing the force are dest(qx), dest(qy), dest(qz). I assume, though I would love to hear if I am wrong, that any fields that are included in the expression are evaluated at the position of the particle producing the force. As an aside, if your equation is large you might want to break it up and put parts of it (those that can be evaluated outside of the particle-particle calculation) in several variables. It will not only reduce the size of the equation, if there is a problem with units it will make finding the error easier.

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

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