# Discussion Forum

## Uniform Magnetic field?

 Topics: Physics, Results and visualization, 4.2a
RSS feed   |   Turn on email notifications   |   5 Replies   Last post: May 3, 2013 2:59pm UTC

Merlin Wang

October 16, 2009 10:12pm UTC

Uniform Magnetic field?

Hi everyone,
I am wondering how I can generate a uniform magnetic field in Comsol. Basically, I want to simulate a metal particle moving in this uniform field. Now I am thinking put all the particles inside a very big magnet. The field is almost uniform, but I don't think it is the best way!

Anyone any ideas?

Thanks for helping!

Ivar Kjelberg

October 17, 2009 8:36pm UTC in response to Merlin Wang

Re: Uniform Magnetic field?

Hi

can you not try out by going to the physics settings of your surrounding subdoman and in the "constitutive relation" select either B=mu0*mur*H+Br or B=mu0*H+mu0*M and define a Br or a M in the desired direction and of the desired strength ?

By the way, be carefull with interpreting the Maxwell stress tensor values, check that they seem coherent as they are rather mesh dependent because they rely on a good precise and coherent boundary integration

Good luck
Ivar

Juan Parra-Robles

October 22, 2009 7:51am UTC in response to Merlin Wang

Re: Uniform Magnetic field?

Define the space where your particles will move (e.g. a cube in 3D) and specify the field intensity at the boundaries, that way you don't need to worry about the field sources .

May 8, 2012 2:44pm UTC in response to Juan Parra-Robles

Re: Uniform Magnetic field?

Setting the field at all boundaries works for creating a uniform field region once you realize that the "Magnetic Field" boundary condition specifies the "tangential" component of the field in absolute coordinates -- a somewhat non-intuitive convention (at least to my mind).

So, for example, if you want 1 A/m field in the z direction, 4 sides of your boundary cube will specify 1 A/m z component, wheras two of you cube sides will specify zero z component!!!!!

Mithlesh Kumar

April 15, 2013 9:54am UTC in response to Ivar Kjelberg

Re: Uniform Magnetic field?

Hi,
I am using comsol 4.2a. I am trying to simulate the hartmann flow problem. I have two parallel plates and a conducting liquid is flowing through it, in the presence of uniform constant magnetic field. I do not know how to produce this type of magnetic field. I am working with Laminar flow(spf) and Magnetic fields(mf) in 2d geometry.
Can you please tell me how to produce the uniform magnetic field.
Thanks
Mithlesh

Ivar Kjelberg

May 3, 2013 2:59pm UTC in response to Mithlesh Kumar

Re: Uniform Magnetic field?

Hi

There are several options, but first of all so you really need MEF and solving for V ? (haven't checked the application though) as its far heavier than MF and often MF is enough at least to debug and start with. (in MEF all materials must have a conductivity different from zero, except if defined as a Ampere laws domain)

One way is to make the surrounding a cylinder (add a cylindrical coordinates along with the cylinder axis to define the current more easily) and set a phi current flow in the cylinder shell, another way is to set a top and bottom. But because of the vector field behaviour of the Maxwell equations you will get non uniformities along all sharp edges, so often its worth to cut out the outer tube in the vicinity of your current shell (add a 10% radius layer and hide the external tube part in the view hide entities)

But this solenoid type fields is probably not "uniform enough (but its rather realistic of what one achieve from an experimental "uniform" B field)

So to get it "more linear", try adding a "Perfect magnetic conductor" on the flat top and bottom surfaces of the cylinder
remains to ask yourself how "physical" this is, I leave that to you

the attached file is 3D and v4.3a.161
--
Good luck
Ivar

Rules and guidelines