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Discrepancy between particle trajectory and streamline?

Posted 27.03.2013, 15:11 GMT-4 Charged Particle Tracing, Fluid & Heat, Microfluidics, Particle Tracing for Fluid Flow Version 5.2 3 Replies

Alisha Schor
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Hello,

I'm trying to teach myself how to use the particle tracing module, so I'm modeling a simple T-junction microfluidic channel. I get an unexpected result, however, when I trace the particles. Since it is a microchannel, I expect the particles to follow streamlines. However, they do not (see attached). I've tried both the Newtonian and massless configurations. This is a 2D, incompressible model, assuming a shallow channel of 30 um. Could anyone lend any insight into what's happening or which results I should trust?

Thanks,
Alisha
3 Replies Last Post 04.05.2016, 09:15 GMT-4
Daniel Smith COMSOL Employee

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Posted: 1 decade ago 28.03.2013, 08:56 GMT-4
Hi Alisha, you will only see trajectories similar to streamlines if you add a Drag Force feature to the model and set the fluid viscosity and density appropriately. See the example:

Model Library>Particle Tracing Module>Fluid Flow>laminar_mixer_particle

which shows how to do this.

~ Dan
Hi Alisha, you will only see trajectories similar to streamlines if you add a Drag Force feature to the model and set the fluid viscosity and density appropriately. See the example: Model Library>Particle Tracing Module>Fluid Flow>laminar_mixer_particle which shows how to do this. ~ Dan
Alisha Schor
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Posted: 1 decade ago 28.03.2013, 09:52 GMT-4
Wow, I do not know how I missed that.... so what's governing the particle motion now? Just initial conditions and inertia?

Thanks!
Wow, I do not know how I missed that.... so what's governing the particle motion now? Just initial conditions and inertia? Thanks!
Signe Engelsholm
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Posted: 8 years ago 04.05.2016, 09:15 GMT-4

Hi Alisha, you will only see trajectories similar to streamlines if you add a Drag Force feature to the model and set the fluid viscosity and density appropriately. See the example:

Model Library>Particle Tracing Module>Fluid Flow>laminar_mixer_particle

which shows how to do this.

~ Dan


Hi Dan

I am currently having the same problem.

It says in the documentation (Particle Tracing Module User's guide, p. 46) "The
Massless formulation means that the particles follow streamlines of the particle velocity
expression". When the particle velocity expression is set to a previously computed velocity field, then the particles should follow the streamlines of that same velocity field, right?

Also, I have experienced something odd, in my own model which is a periodic structere (spline) where I have mapped the velocity field solution from one unit onto the other units in the geometry. I then used this mapped solution in a particle trace study with massless particles. In the first loop/arch of the spline, the particle trajectories are jagged and weird, while in the next, they are nice and smooth (see the attached snapshot), even though the fluid fields they move in, and the time steps used in the solver, are completely identical. What is happening here?

I hope someone can help me.

Best regards
Signe
[QUOTE] Hi Alisha, you will only see trajectories similar to streamlines if you add a Drag Force feature to the model and set the fluid viscosity and density appropriately. See the example: Model Library>Particle Tracing Module>Fluid Flow>laminar_mixer_particle which shows how to do this. ~ Dan [/QUOTE] Hi Dan I am currently having the same problem. It says in the documentation (Particle Tracing Module User's guide, p. 46) "The Massless formulation means that the particles follow streamlines of the particle velocity expression". When the particle velocity expression is set to a previously computed velocity field, then the particles should follow the streamlines of that same velocity field, right? Also, I have experienced something odd, in my own model which is a periodic structere (spline) where I have mapped the velocity field solution from one unit onto the other units in the geometry. I then used this mapped solution in a particle trace study with massless particles. In the first loop/arch of the spline, the particle trajectories are jagged and weird, while in the next, they are nice and smooth (see the attached snapshot), even though the fluid fields they move in, and the time steps used in the solver, are completely identical. What is happening here? I hope someone can help me. Best regards Signe

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