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March 23, 2007, 06:33 
model particle movement under magnetic force

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Dear CFD experts:
I am wondering whether anyone here had done anything like this and can offer some suggestion: Problem descriptions: iron particles suspended in water inside a cup. The dimension of the cup is on the order of 1cm x 2cm x 10cm, wheras the diameter of the iron particles are on the order of 2 micronmeter. There are thousands of iron particles in the domain, but, we will only track 1 iron particle to simplfy the problem. At time 0+, this cup is exposed to a permanent magnet, locaed on one side of the cup. Hence, magnetic force will track the iron particle toward the magnet. The difficulty lies in computing magnetic force exerts on the iron particle. Any suggestion? phsieh2005 

March 23, 2007, 08:30 
Re: model particle movement under magnetic force

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Did you try looking for papers in the field of magnetoydrodynamics. Also books on astrophysics will have some chapters on the topic.


March 23, 2007, 09:57 
Re: model particle movement under magnetic force

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Hi, Harish,
Thanks for the suggestion. I never thought about looking into Magnetohydrodynamics. I will look into it. phsieh2005 

March 23, 2007, 10:19 
Re: model particle movement under magnetic force

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Dear phsieh,
If your problem does not include applied electric field, and the magnetic field is steady, you can model the problem as a magnetostatics problem. That is, using Ampere's law curl H = 0 div B = 0 B = mu * H You can simplify the problem by introducing a magnetic scalar potential, ie. H = grad Phi, then Ampere's law is automatically satisfied, and div (mu grad Phi) = 0 must be solved for Phi. The force on the particle due to electromagnetic effects includes several contributions: electrostatics (Coulomb), magnetohydrodynamic (Lorentz), magnetostatic/phoretic (Kelvin), etc.. In your case, I think that only the Kelvin force is needed: F_mag = mu_o * M dot grad H Just add this force to your particle force balance. You should search for information on Ferrohydrodynamics. Magnetohydrodynamics is for conducting fluids. Either way, you will learn a lot of by searching on both. Opaque 

March 23, 2007, 11:25 
Re: model particle movement under magnetic force

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Dear Opaque:
Thanks a LOT for the reply! The past month, I have learned a lot about magnetostatics. I have computed the magnetic flux density field using GetDP (I have tried both scalar potential and vector potential). It looks like F_mag = mu_o * M dot gradH is what I need. But, what is M? I just realized another problem I am facing: Although the magnetic field is static in general, when the iron particle moves to a new location, I need to remesh, recalculate the magnetic flux density and magnetic force. Because the length scale of the particle is 3 order of magnitude smaller than the fluid domain, it will be a big job to remesh the magnetic field, especially for a 3D problem. Is it possible to calculate the magnetic flux density field "without" the iron particle. Then, somehow estimate the "pull" force on the particle based on this static magnetic flux? The iron particle is very small compared to the fluid domain. phsieh 

March 23, 2007, 17:33 
Re: model particle movement under magnetic force

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Dear Opaque:
Could you please post the reference that showed the Kelvin force equation (that is: "F_mag = mu_o * M dot grad H")? I did searches for Kelvin force and was not able to find a good source for it. Thanks! phsieh 

March 27, 2007, 09:28 
Re: model particle movement under magnetic force

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Dear phsieh,
M stands for Magnetization.. It is a material property, and there are different ways of how to model it as well. For a reference on the topic try the book, Ferrohydrodynamics R. E. Rosensweig Dover Publications Inc.,1985. Also, you could try the following link http://web.mit.edu/nse/nanofluids/events/pdfs/Zahn.pdf Hope this helps, Opaque 

March 27, 2007, 11:35 
Re: model particle movement under magnetic force

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Dear phsieh
Here is another link that you may like. http://www2.mic.dtu.dk/research/MIFT...thesisCMIK.pdf Enjoy, Opaque 

March 28, 2007, 07:12 
Re: model particle movement under magnetic force

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Thanks a lot Mr. Opaque!
I quickly read through the abstract. This thesis is very helpful to me. Best Regards, phsieh 

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