Instability and Breakdown of a Low Viscosity Jet

Mixing of a liquid jet into a surrounding fluid of different properties is a commonly encountered phenomenon in natural and engineering systems. Studies in core annular flows have shown that viscosity contrast can lead to absolute instability; however free shear flows with viscosity contrasts have not received significant attention. Here we study the breakdown process of a round jet that is miscible with and has a viscosity at or below that of the ambient fluid. Viscosity ratios M (ambient-to-jet) ranging from 1 to 37 are investigated in a constant density environment for jet Reynolds numbers varying from 500 to 2000. Fluorescent dye imaging and PIV measurements are used to document the jet breakdown process. Consistent with linear stability theory calculations, a configuration with a low viscosity core and high viscosity ambient favors the strong growth of helical modes with wavelengths of the order of the jet diameter. The wavelength of the most unstable mode is reported as a function of the Reynolds number, viscosity ratio and non-dimensional boundary layer thickness at the nozzle exit.