  This experimental study investigates the effects of viscosity contrast
between a saltwater jet and its high-viscosity propylene glycol surroundings.
Using density-matched fluids in a gravity-driven flow, Jet Reynolds numbers
(Re) from 1600 to 3400 and ambient-to-jet viscosity ratios (M) from 1 to 50
were examined. Observations indicate that low viscosity ratios lead to
axisymmetric jet breakdown, while higher ratios result in helical modes. The
study employs various diagnostic tools to delineate this transition. Hot film
anemometry reveals a discrete peak in the velocity fluctuation frequency
spectrum, marking the onset of helical modes. This peak shows minimal spatial
variation downstream. Laser-induced fluorescence (LIF) was utilized to
distinguish the jet boundary. High-speed LIF imaging facilitated the
determination of wave growth rates on the jet boundary and oscillation
frequencies of the interface. These frequencies, consistent with self-sustained
oscillation behavior, depend on the viscosity ratio and align with absolutely
unstable modes from spatio-temporal linear stability theory. Spectral Proper
Orthogonal Decomposition of the images identifies several spatial modes,
predominantly a single dominant mode. These findings suggest the presence of a
global mode, likely stemming from absolute instability in velocity and
viscosity profiles near the nozzle exit, providing significant insights into
fluid dynamics at varying viscosity contrasts.
