Characteristics of self-oscillating jets in a confined cavity

Abstract

Jets emanating into a confined cavity exhibit self-oscillating behavior. This study is focused on evaluating characteristics of oscillating square and round jets. The jet exits from a submerged square or round nozzle of the same hydraulic diameter into a thin rectangular cavity at a Reynolds number of 54 000 based on the nozzle hydraulic diameter and average jet exit velocity. An investigation of the three-dimensional self-oscillatory flow structures is conducted using the unsteady Reynolds-averaged Navier-Stokes equations with the Reynolds stress turbulence model. Vortex identification using the λ2-criterion is used to investigate the flow dynamics. For the oscillating square jet, vortex rings initially have a square shape near the nozzle exit, before axis-switching and transforming into a circular ring. Upon impact on the walls, two tornado-like vortices are produced. The decay rate of oscillating square and round jets initially shows a trend traditionally noted in the corresponding free jets but changes significantly with distance from the nozzle as the effects of oscillation and confinement begin to dominate. Reynolds stress profiles for both types of jets are qualitatively similar and show two peaks on either side of the centerline, which convert to mild peaks farther downstream. Spread and decay rates of oscillating square jets are higher, while oscillating round jets have higher turbulence intensities near the jet center. Compared to free jets, more uniform Reynolds stresses at farther distances from the jet centerline in oscillating jets will enhance heat transfer over a larger area, making oscillating jets suitable in many cooling applications.