The oscillatory instability of plane variable-density jets

Abstract

The results of an experimental investigation of the instability of variable-density plane jets issuing into ambient air are reported. When the jet to ambient fluid density ratio S(S=ρj/ρ∞) is less than a critical value, an intense oscillating instability is observed. This instability is characterized by sharp peaks in the power spectral density measured in the near field of the jet. The effects of the control parameters S, Re, and H/θ (jet width to exit momentum thickness) on the instability regime are determined. It is shown that Re is a better scaling parameter than H/θ. The Strouhal number of the dominant mode StH increases with S and Re up to a constant value of 0.25, which is in rather good agreement with the theory and the experiments of Yu and Monkewitz [Phys. Fluids A 2, 1175 (1990); J. Fluid Mech. 255, 323 (1993)]. In the present experiments the critical value Sc above which the oscillating regime disappears is an increasing function of Re and Sc seems to reach a limiting value in the neighborhood of 0.7, which does not agree well either with the theory or with the experiments of Yu and Monkewitz [Phys. Fluids A 2, 1175 (1990); J. Fluid Mech. 255, 323 (1993)]. This difference is in qualitative agreement with the results of linear stability computations, also reported in the paper, which take into account differences in shape and relative positions of the density and velocity profiles.