Simulation of Dynamic Armature Motion in a Railgun with Coupling of Electromagnetic, Thermal and Structural Effects Using Shifted Finite Element Fields

A moving armature railgun code (called HERB) has been developed for coupled electromagnetic, thermal and structural fields using the general purpose code ANSYS as the underlying computational engine. This analysis uses shifted electromagnetic fields to simulate the velocity effects associated with such motion (akin to EMAP3D [1]). Specifically, the velocity effects are produced by the shift of the magnetic vector and scalar potential fields, the temperature and structural fields, and the respective element material attributes. The cylindrical symmetry of the model allows us to employ 3D finite element bricks. These have an advantage over tetrahedral elements by allowing very high aspect ratios to model longer stretches of the problem not in the immediate vicinity of the armature or the end of the rails, with a minimal number of elements, thereby reducing both the model size and the required computational time. The armature motion is calculated self-consistently from the electromagnetic forces on the armature; however, the armature is stationary. Instead the electromagnetic, thermal and structural degrees of freedom, as well as the element material attributes of the non-armature components of the model are shifted to simulate the motion. It is these other non-armature components that move but in the direction opposite to the actual armature motion. The method has been verified for electromagnetic velocity effects using several simple modified textbook cases of closed wire loops moving through either uniform or spatially dependent magnetic fields. In both cases the agreement between the ANSYS and analytical results was very good. In addition, the model has been verified against published EMAP3D coupled electromagnetic railgun results with good agreement. Using ANSYS has many benefits. ANSYS is commercially available, validated and maintained. Further it has fully developed pre and post processors, multi-elements, and multiple field types that can be readily coupled requiring no further user development. Moreover, ANSYS is being continuously improved and updated and comes equipped with full technical support. In addition, the user can add more physics via a relatively straightforward Fortran-like scripting language.