The stability properties of round variable-density low-Mach-number jets are studied by means of direct numerical simulation (DNS) and linear stability analysis. Fully three-dimensional DNS of variable-density jets, with and without gravity, demonstrate that the presence of buoyancy causes a more abrupt transition to turbulence. This effect helps to explain differences between normal gravity and microgravity jet diffusion flames observed in the laboratory. The complete spectrum of spatial eigenmodes of the linearized low-Mach-number equations is calculated using a global matrix method. Also, an analytic form for the continuous portion of this spectrum is derived, and used to verify the numerical method. The absolute instability of variable-density jets is confirmed using Brigg's method, and a comprehensive parametric study of the strength and frequency of this instability is performed. Effects of Reynolds number, the density ratio of ambient-to-jet fluid (S1), shear-layer thickness and Froude number are considered. Finally, a region of local absolute instability is shown to exist in the near field of the jet by applying linear stability analysis to mean profiles measured from DNS.
Bibliographical noteFunding Information:
This research was supported by NASA Grant NAG3-2517, issued by the NASA Glenn Research Center. Support for the direct numerical simulations was provided by a grant of HPC resources from the Arctic Region Supercomputing Center at the University of Alaska Fairbanks as part of the Department of Defense High Performance Computing Modernization Program. The authors would like to thank John Kramlich and George Kosály for many helpful discussions during the course of this research. We would also like to thank the anonymous reviewers for several helpful suggestions.