Modelling a force as a momentum source term - HELP

Rick

Hi, I'm a 3rd year Manchester uni Aerospace student, a (near) complete fluent virgin, and i need some help! My dissertation involves modelling the ionised flows in MagnetoHydroDynamic power generation. Im obviously doing this at its very simplest level. What im trying to do at the moment is model the lorentz force (JxB). Ive worked out what forces this would apply to a charged particle in three dimensions, i would like to apply this as a body force to the flow. I can only do this in a discrete phase model, with a few inonised particles in a laminar flow, my problem is the whole flow is ionised. Im now considering modelling it as a momentum source term, which i believe would be very simple, if i had ANY experience of UDF's or programming in C!! Can anyone help out a rather bemused and slightly panicking student?! Any help at all with the UDF code, or even just pointers - am i going about this the right way? how do i convert a vector force written on my pad of paper to a momentum source term?

HELP!