A low-density gas jet injected into a high-density ambient gas is known to exhibit self-excited global oscillations accompanied by large vortical structures interacting with the flow field. In this study, the formation and evolution of vortices and scalar structure of the flow field are investigated in buoyant helium jets discharged from a vertical tube into quiescent air. This is accomplished by applying the quantitative rainbow schlieren deflectometry technique to optically measure the local helium mole percentage across the whole field. Data were acquired over downstream locations extending from tube exit to about 3.0d (d=31.8 or 19.1 mm is the jet tube inside diameter) at spatial resolution of 0.14 mm and temporal resolution of 16.7 ms. Oscillations at identical frequency were observed throughout the flow field. The evolving flow structure is described by helium mole percentage contours during an oscillation cycle. Instantaneous, mean, and rms concentration profiles are presented to describe interactions of the vortex with the jet flow. Oscillations in a narrow wake region near the jet exit are shown to spread through the jet core near the downstream location of the vortex formation. The effects of jet Richardson number on characteristics of vortex and flow field are investigated and discussed.

Effects of buoyancy on transition from laminar to turbulent flow are presented for momentum-dominated helium jet injected into ambient air. The buoyancy was varied in a 2.2-sec drop tower facility without affecting the remaining operating parameters. The jet flow in Earth gravity and microgravity was visualized using the rainbow schlieren deflectometry apparatus. Results show significant changes in the flow structure and transition behavior in the absence of buoyancy.

Several studies [1][2][3][4][5][6] have focused on the far field behavior of jets. However, the near field flow stability has direct influence on the entrainment and flow pattern in the far field. Self-excited oscillations in the near injector region of low-density gas jets have been studied extensively. Subbarao and Cantwell 7 observed highly periodic oscillations in the near field of helium jets injected into a co-flow air environment for Ri between 0.5 and 6.0. The Strouhal number, St=fd/V correlated with Richardson number for Ri>1.0 indicating buoyancy dependent instability mode. Similar experiments were conducted by Cetegen [8][9][10] for axisymmetric and planar plumes, whereby buoyancy induced toroidal vortical structures contaminating the primary jet flow were observed. Agrawal and coworkers [11][12] conducted experiments to characterize the flow instability in terms of concentration measurements across the whole field of a helium jet injected into quiescent air. Experiments by Yep et al. l1 demonstrated that flow oscillations were absent in the microgravity environment of the 2.2 s drop tower. This paper deals with the computational analysis of buoyancy effects in the near field of an isothermal helium jet injected into quiescent ambient air environment. The transport equations of helium mass fraction coupled with the conservation equations of mixture mass and momentum were solved using a staggered grid finite volume method. Laminar, axisymmetric, unsteady flow conditions were considered for the analysis. An orthogonal system with non-uniform grids was used to capture the instability phenomena. Computations were performed for Earth gravity and during transition from Earth to different gravitational levels. The flow physics was described by simultaneous visualizations of velocity and concentration fields at Earth and microgravity conditions. Computed results were validated by comparing with experimental data substantiating that buoyancy induced global flow oscillations present in Earth gravity are absent in microgravity. The dependence of oscillation frequency and amplitude on gravitational forcing was presented to further quantify the buoyancy effects.Few computational studies have also been a part of low-density jet instability literature. Mell et al. 13performed numerical simulations of helium jet in air and found that the computed oscillation frequency matched with experimentally obtained value by Hamins et al 14 . Recently, Soteriou 15 revealed that the instability characteristics in buoyant planar plumes compared well with experiments, providing insight into the role played by viscous and buoyancy forces in establishing oscillating or non-oscillating modes.

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.Hamins et al.2 studied oscillations in both buoyant helium jets and flames. They used shadowgraph technique to observe the oscillation frequencies of helium jets over a range of Froude number (≈ 0.001 to ≈ 1) which is proportional to 1/Ri and Reynolds number (≈ 1 to ≈ 100). Strouhal number was correlated as a function of inverse Froude number. Oscillations in the flow were not observed until a minimum flow rate was attained. Measured frequency and the location of the vortical structures were dependent on the helium jet exit velocity.

The transition from laminar to turbulent flow in helium jets discharged into air was studied using Rainbow Schlieren Deflectometry technique. In particular, the effects of buoyancy on jet oscillations and flow transition length were considered. Experiments to simulate microgravity were conducted in the 2.2s drop tower at NASA Glenn Research Center. The jet Reynolds numbers varied from 800 to 1200 and the jet Richardson numbers ranged between 0.01 and 0.004. Schlieren images revealed substantial variations in the flow structure during the drop. Fast Fourier Transform (FFT) analysis of the data obtained in Earth gravity experiments revealed the existence of a discrete oscillating frequency in the transition region, which matched the frequency in the upstream laminar regime. In microgravity, the transition occurred farther downstream indicating laminarization of the jet in the absence of buoyancy. The amplitude of jet oscillations was reduced by up to an order of magnitude in microgravity. Results suggest that jet oscillations were buoyancy induced and that the brief microgravity period may not be sufficient for the oscillations to completely subside.