Near-field structure of underexpanded elliptic jets

Abstract

Quantitative measurements of velocity for a low aspect ratio underexpanded elliptical jet are presented. Four jets at nozzle pressure ratios NPR = [2.2, 2.6, 3.4, 4.2] are studied for a smoothly contoured elliptical nozzle with an aspect ratio a/b = 2. High-resolution planar particle image velocimetry is used to extract information about the velocity fields and turbulent statistics. All four jets display the expected axis switching phenomenon, with the axis switching gaining strength with increasing pressure ratio. Evidence of regular reflection is seen for jets at pressure ratios of 2.6 and above. Measurements of fluctuating velocity indicate an oscillatory flapping mode in the minor axis plane for all but the highest pressure ratio. At the highest pressure ratio of NPR = 4.2, there is evidence of a cut-and-connect vortex bifurcation previously only observed for jets with higher aspect ratio.