Learning Forum

[Bo Raadam]

I am trying to model the flow of a liquid metal through a circular pipe with an applied magnetic field. For a simple comparison, I have made the magnetic field constant and 1D (B = <0,By,0>). I have run Fluent with the MHD module and been able to calculate the current density due to the fluid moving in the magnetic field as well as the resulting force on the fluid. I have then export the data for the velocity, magnetic field, current density, and force vectors along a radial line in the fully developed flow region. The current density should be given by j = conductivity*(v x B) but when I use the velocity and magnetic field data I exported, the current density tends to be off by 3-4 orders of magnitude. I know that the user guide says current density is calculated by j = (1/permeability)*curl b but when I have spoken to professors who's work include modeling magnetic fields, they said either equation should be correct for calculating the current density. I have also checked the force using F = j x B with the j and B vectors Fluent gives and in some cases this lines up with the reported force but in about half the cases it is 1/2 the force reported by Fluent. I have tried adjusting both velocity and magnetic field strength and in the main directions (for current density, orthogonal to magnetic field primary direction and pipe axis, for force parallel to fluid axis) the response of current density and force due vary linearly with v and B which is predicted by j = conductivity*(v x B) though not by F = j x B (F should vary linearly with B^2 not B). Please advise on why the current density is so different from that given by j = conductivity*(v x B) and why the force reported by Fluent is sometimes double F = j x B. Thank you

[Nikolas Perantakos]

 

Hello Bo,

 

Thank you for contacting us.

The MHD module also calculates the potential, which, in turn, determines another current. These two currents are then added together, so they can nearly cancel out.

Additionally, the outcome depends heavily on whether the boundary is insulated or conducting.

All these can significantly affect the force (F = J × B).

 

Best,

Nikolas