Near field dynamics of subsonic free square jets. A computational and experimental study

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Abstract
Results of a combined numerical and experimental investigation of the near field of low-subsonic air square jets are presented. The study focuses on examining the role of initial conditions and other features of the jet dynamics in determining the nature and frequency of occurrence of axis switching and the related mechanisms which enhance entrainment, mixing, and turbulence production. Three different experimental square jet facilities were utilized, including orifice jets with low and high initial turbulence level, and pipe jets. Unsteady, spatially developing jets were investigated computationally using direct and monotonically-integrated large-eddy simulation approaches, and appropriate inflow/outflow boundary conditions. Insight on the axis-switching process was obtained using the detailed database from the simulations to investigate how the unsteady vorticity dynamics reflects on the time-averaged properties of the jet cross sections. The different experimental jets were chosen such that important parameters affecting the initial conditions could be tested. Depending on the particular initial conditions of the subsonic jets studied, several or no axis switchings were observed in the first few diameters of jet development. Observed trends reported, include the effects of initial conditions such as, ratio of equivalent diameter to characteristic-momentum-thickness, turbulence level, nonuniform azimuthal momentum-thickness distributions, and Reynolds number.

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