A Magnetofluid Mechanical Model to Describe Rail–Armature Interface Phenomena

When an armature slides between a pair of rails under the influence of Lorentz forces, the material at the interface will experience steep increase in temperature due to frictional and Joule heating. Experiments show condensed metal deposits on the rail, which indicates melting of the interface layer. As the temperature increases, the interface material progressively changes phase from solid to liquid and later ionizes to become a plasma. The charge transport from the rail to the armature has to occur through this layer and, hence, the magnetofluid mechanics (MFM) of this layer will affect essential railgun parameters, such as the inductance gradient. Initially, the armature will translate as a solid body. As an interface layer liquefies (and later becomes plasma), it will translate with the armature; it will also experience rotational motion through shear at the wall and through magnetic stirring by the jtimesB forces. The velocity field will influence the railgun through viscous dissipation and through VtimesB fields. In order to analyze and understand these phenomena, a model is proposed here. The armature is assumed to translate with an interface layer composed of aluminum liquid. A 1-D diffusing field with closed-form expressions for the magnetic potentials, fields, and current densities is imposed, enabling calculation of the accelerations and velocities of the armature. The secondary flow induced in the interface layer composed of liquid aluminum due to shear at the wall is calculated using MFM equations. The viscous dissipations are calculated and compared to Joule heating