Learning Forum

[soloviev]

Hello,

The Fluent Guide for the MHD Add-On Module states that one limitation of the Fluent MHD model is that it assumes a sufficiently conductive fluid so that the charge density and displacement current terms can be neglected, which may not be valid for marginally conductive fluids like the air. Would it be applicable to seawater? The conductivity of seawater is 4 S/m.

Thank you,

Alex

[Konstantine Kourbatski]

The full electric displacement in Maxwell's equations is:

D = eps * E + P

where P is the polarization current density which comes from the change in polarization of the individual molecules of the dielectric material. Fluent's MHD drops the P term and defines D as:

D = eps * E

So the question is if seawater is a conductor enough at a given salinity that the polarization current density can be neglected. Depending on your needs and fidelity of results sought, P in sea water may or may not be a factor. 

This is a CFD forum, and my knowledge of Electromagnetics is somewhat rusty. I suggest you post on the Emag forum, I recall people working with our HFSS have looked the effects of salinity on sea water polarization, and they may give a better guidance.

[soloviev]

I did post it originally to Emag Forum but it was moved to CFD Forum, probably by the moderator.

I am not very concerned about the displacement term. My question actually should be formulated as follows: Does Fluent MHD use any approximations like a low magnetic Reynolds number or high magnetic Reynolds number?

Thanks,

Alex

 

[Konstantine Kourbatski]

No it doesn't approximate based on the magnetic Reynolds number. It solved the magnetic induction equation (magnetic induction method) or electric potential equation (electric potential method), as shown in the MHD manual, and then applies calculated induced field or potential to compute Lorentz force and Joule heating. Lorentz force effect on the fluid flow is accounted for only when the magnetic induction method is used. It is ignored in the electric potential method which applies Lorentz force only to DPM particles.