Measurements of axial instability waves in the near exit region of a high speed liquid jet

This work presents wavelength measurements of axial instability waves along the surface of a liquid jet operated within the atomization regime. The surface waves appear in the near exit region of the initially laminar jet. The measurements show that the waves stretch as they are convected downstream before the flow becomes turbulent. A set of experiments using three different cylindrical nozzles of length-to-diameter ratios of one, five and ten are performed at jet velocities ranging between 10 and 40 m/s. These sets of tests allow variations in flow Reynolds number, based on the momentum thickness at the exit of the nozzle, between 150 and 750. The measurements provide statistically validated wavelength measurements as a function of the axial coordinate. These measurements are then compared with three stability models and are found to support the hypothesis that the most important mechanism responsible for the presence of the instability waves is a pocket of absolute instability present near the exit of the nozzle. This pocket is responsible for amplifying disturbances of a given frequency as well as feeding them upstream into the flow. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3671733]