Stability Characteristics of a Low-Viscosity Axisymmetric Jet

Abstract

Studies of low-density jets indicate the existence of instabilities that may lead to self-sustained oscillations, which are characterized by a spectacular breakdown of the jet with rapid mixing. However, most studies do not account for viscous effects that can dramatically alter the flow stability. Understanding how viscosity stratification influences jet instability can help design systems with control over the mixing characteristics. Hence, experiments are performed with a low viscosity, density-matched jet issued into an ambient fluid with higher viscosity. These types of systems are often found in engineering applications such as plasma torches, sewage discharges, jet exhausts, etc., and to some extent, in naturally occurring phenomena such as glacial and lava flows. A study of instabilities and the subsequent breakdown of an axisymmetric jet was carried out over Reynolds numbers, Re, ranging from 400 to 3300, for a range of jet-to-ambient viscosity ratio, M, ranging from 1 to 45. Flow visualization results indicate that viscosity stratification leads to helical modes and a transition to axisymmetric modes is observed. Hotwire anemometry results indicate the existence of sharp peaks in the frequency spectrum that vary as a function of Re and M. The flow visualization and hotwire anemometry results are used as markers to identify the possible existence of a global mode.