Concentration Measurements in Self-Excited Momentum Dominated Low-Density Gas Jets

Flow structure of self-excited, laminar, axisymmetric, momentum-dominated helium jets discharged vertically into ambient air was investigated using high-speed rainbow schlieren deflectometry technique. Measurements were obtained at temporal resolution of 1 ms and spatial resolution of 0.19 mm for two test cases with Richardson number of 0.034 and 0.018. Power spectra revealed that the oscillation frequency was independent of spatial coordinates, suggesting global oscillations in the flow. Abel inversion algorithm was used to reconstruct the concentration field of helium. Instantaneous concentration contours revealed changes in the flow field and evolution of vortical structures during an oscillation cycle. Temporal evolution plots of helium concentration at different axial locations provided detailed information about the instability in the flow field. Subbarao and Cantwell 1 identified the oscillating and non-oscillating regimes for vertical helium jets in the Reynolds number-Richardson number space using a stroboscopic schlieren technique. Measurements revealed large centerline velocity fluctuations, and early and abrupt breakdown of the potential core. They reported the oscillating behavior of the helium jet at moderate values of Richardson numbers (0.5 < Ri < 6) and stated that this type of flow was subjected to an unusual type of transition to turbulence consisting of a rapid but highly structured and repeatable breakdown and intermingling of the jet with the free stream fluid. The strong dependence of Strouhal number on Richardson number indicated the dominance of buoyancy effect.