Flow structure in the near-field of buoyant low-density gas jets

Flow structure of helium jets injected into quiescent air was investigated numerically using finite volume approach. The analysis is based on unsteady, laminar, axisymmetric, isothermal flow in a binary fluid system. The jet Reynolds number, Re was varied from 40 to 150 to encompass steady and oscillating jet flow regimes. Comparison with experimental data revealed excellent agreement, proving reliability of the model in predicting global flow characteristics. At low jet Reynolds numbers, the flow was steady and the concentration shear layer at the tube exit was stratified by mixing between jet and ambient fluids inside the tube. At higher jet Reynolds numbers (Re = 90 and 150), buoyancy induced acceleration contracted the jet core to form a toroidal vortex by entrainment of the ambient fluid. Structural details of the globally unstable flow are discussed using instantaneous, mean and RMS profiles of helium concentration and axial velocity.