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defining output variable of an interface as dependent variable in global equation

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I am trying to simulate surface deformation due to magnetic field. My model consists of a cylinder filled with conductive fluid surrounded by a single coil (approximated by a line element). I coupled the magnetic field, moving mesh, and global ODEs interface.

I want to use the magnetic field density, B, calculated from the mf interface and plug it in an equation for the surface displacement, h:
h =- B^2 / (4*rho*g_const*mu)

However, I don't know how to specify that I only need the B values along a particular line (i.e. the free surface/top surface of the fluid). I tried defining a local variable, B_surf, along the boundary I am interested using the mf.Bz values (i.e. mf interface output variable,), hoping that it would give me a set of B values only along that boundary. I, then, used B_surf in my h equation instead of using mf.Bz.

The error message says: failed to evaluate variable B_surf

My equation might not be in the right format. I entered it in the ODE table as: h*4*rho*g_const*mu/ (- B_surf^2) to solve for h and then I entered h as the prescibed displacement in the ale interface.


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