Spatiotemporal Analysis of a Low-Viscosity Jet

The effect of viscosity contrast between a jet and its surrounding is experimentally investigated using density-matched fluids. A gravity-driven flow is established, with a saltwater jet of relatively low-viscosity emerging into an ambient composed of high-viscosity propylene glycol. Jet Reynolds numbers, Re, ranging from 1600 to 3400 are studied for ambient-to-jet viscosity ratios, M, ranging from 1 to 45. Flow visualization suggests that at low values of viscosity ratios, the jet breakdown is axisymmetric, while helical modes develop at high values of M. High-speed imaging with a fluorescent dye in the jet is used to enhance contrast and identify the interface. Temporal analysis is performed to determine the frequency of the jet while spatial analysis yields the wavelength of perturbations. Proper Orthogonal Decomposition (POD) is performed to find the eigenvalues associated with axisymmetric and helical modes.