On axisymmetric deformations of the electric arc column in an accelerating flow

A review of axial flow arcs which exhibit well defined varicose oscillations suggests that, although these have frequently been attributed to shear instabilities, their observed phase speeds are an order of magnitude less than those expected for shear driven phenomena. It is suggested that the coupling between the energy balance of arc and the cold flow conditions the response of the arc column to oscillatory forcing, delaying the onset of shear instability. A linearized mathematical analysis of the thermal coupling alone confirms that, in common with many of the observations, such disturbances travel at the cold flow speed.