THERMOMECHANICAL AND MAGNETOHYDRODYNAMIC STABILITY OF ELONGATING PLASTIC JETS

In this study the stability characteristics of uniformly elongating pl astic jets exposed to axial electric currents are investigated. The ob jective of this study is to expand the results of previous analyses by Littlefield [''The effect of electromagnetic fields on the stability of a uniformly elongating plastic jet,'' Phys. Fluids A 2, 2240 (1990) ; ''Finite conductivity effects on the MHD instabilities in uniformly elongating plastic jets,'' ibid. 3, 166 (1991); ''Enhancement of stabi lity in uniformly elongating plastic jets with electromagnetic fields, '' ibid. 3, 2927 (1991)] to include high levels of electric current, w here thermal energy effects must be included. Coupling of the magnetoh ydrodynamic and thermal characteristics of the flow is accomplished th rough the variation of mechanical, thermal, and electrical properties with temperature. Phase change effects are also considered. The jet is assumed incompressible and perfectly plastic, with the Levy-von Mises criterion imposed to limit the effective stress. Solutions to the app ropriate base flow are subjected to small axisymmetric disturbances, a nd linear perturbation theory is employed to determine the time evolut ion of these disturbances. Perturbations that grow the fastest in magn itude as time progresses are identified as the most unstable. Results of the analysis indicate that thermal effects can dramatically alter b oth the base and perturbed flow fields, as well as the growth rate of perturbations.