Instability analysis and suppression of instability in low-density gas jets

Abstract

A numerical study is conducted to understand the global instability of very low-density jets (as encountered in thermal plasmas). The simulations have been carried out for different parameters of density ratios, S = ƒÏj /ƒÏ‡ ranging from 0.5 to 0.03, different Reynolds numbers ranging from 500 to 4000 and different momentum thickness obtained by different extension tube length ranging from 3 to 6 diameter long. The flow parameters and vortex structures has been visualized to understand the details of the evolution of the flow field. The axisymmetric shear layer rolls up in the near field of the jet forming vortex rings. The rings merged, formed secondary mode of instability in the near field before they interacts with each other to form turbulence. Spectra results show that a global instability exists in the range of density ratios investigated. An envelope of the absolutely globally unstable region has been defined in a 3D space of Reynolds number, density ratio and momentum thickness space. A strategy for reducing the instability by altering the density profile of the surrounding gas stream is explored. Specifically, a ramp density profile or a step shape with an outward offset was explored, and it was observed that there was a reduction in the instability amplitude with the modified density profiles. Such a lowering in the instability fluctuations can be beneficial in stabilizing the thermal plasma behavior.