Discussion Forum

Hi,

streamlines can be a little tricky. It is important to choose good start points. In the magnitude controlled way COMSOL sets many start points where the field magnitude is high.
Even if your model is symmetric, the mesh may be not, if you are using the automatic meshing procedures. The small mesh related asymmetries may spoil the streamline picture.
You can get around that by defining your own start points in a symmetric way.

Cheers
Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com

Thank you!

I have defined a start point right in the centre of the two electrodes and my mesh size is normal, i could make it finer. Even with a central start point i get asymmetries.

What do you define as symmetric start points? Can I define more that one start point?

-Shagun

You can define as many points as you want. One point gives one streamline. Check the options of the streamline node. You will have to play a little with it. Also check the advanced options to control length and precision of the streamlines.

Cheers
Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com

Hi!

I tried the "on selected boundaries option" and i get a nice symmetric plot.

I have some more questions:

Physically what makes more sense: magnitude controlled or on selected boundaries?
How is 'on selected boundaries' different from 'uniform density' option?
When we define uniform density and the spacing, how is that different from giving a density value to a magnitude controlled plot?

Thank you so much for your quick replies! It has helped me a lot!
I am just trying to understand this better.

Best,
Shagun

Physically what makes more sense: magnitude controlled or on selected boundaries?
How is 'on selected boundaries' different from 'uniform density' option?
When we define uniform density and the spacing, how is that different from giving a density value to a magnitude controlled plot?

Magnitude controlled or selected boundaries give different views of the field. They are both physically correct in some sense.

On selected boundaries results in the streamlines being uniform distributed on they boundary where they begin. Uniform density results in streamlines being added until the density is approximately the same everywhere.

Okay.
Thank you.

But say for two parallel plate electrodes, the electric field is strongest between the plates but when i simulate it using COMSOL, the streamlines don't show the strength of the field, they show the field lines even behind the two plates in more density if not equal. So how do i get a more physical representation of the electric field?

Is there any plot i could do that would, say give me magnitudes of the electric field along different planes in my model?
or something to give a more physically accurate description?
I am attaching a picture of the parallel plate electrode problem.

Streamlines are not well suited to show the field strength. You have to realize that they show the direction of the field vector, not the magnitude.
It is possible to visualize field strength by choosing a high density of start points where the field strength is high.

If you want to visualize the electric field I would recommend a surface or contour plot of the field, not the potential. Your plot is a surface and contour plot of the potential, not the field.

I guess you don't find streamlines between the plates because you chose the start points outside the capacitor. The streamlines then loop from one exterior plate surface to the other arond the capacitor. They cannot penetrate the plate because there is no field inside the conductive plate.

Almost all about the correct use of streamlines is about the choice of the start points. If you chosse them in a proper way they are nice for qualitative visualization, but they are mostly useless for quantitative purposes.


--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com