Elliptic supersonic jet morphology manipulation using sharp-tipped lobes.

Elliptic nozzle geometry is attractive for mixing enhancement of supersonic jets. However, jet dynamics, such as flapping, gives rise to high-intensity tonal sound. We experimentally manipulate the supersonic elliptic jet morphology by using two sharp-tipped lobes. The lobes are placed one on either end of the minor axis in an elliptic nozzle. The design Mach number and the aspect ratio of the elliptic nozzle are 2.0 and 1.65. A two-lobed nozzle with the same exit area and design Mach number as that of the elliptic nozzle is compared when the jet is exhausted to the ambient in an almost perfectly expanded condition. Time-resolved schlieren imaging, longitudinal and cross-sectional planar laser Mie-scattering imaging, planar Particle Image Velocimetry, and near-field microphone measurements are performed. DMD and POD analysis are carried out on the schlieren and the Mie-scattering images. Mixing characteristics are extracted from the Mie-scattering images through the image processing. The flapping elliptic jet consists of two dominant DMD modes, while the lobed nozzle has only one dominant mode, and the flapping is suppressed. The jet column bifurcates in the lobed nozzle enabling closer contact with the ambient fluid and higher mixing rates in the near-field of the nozzle exit. The jet width growth rate of the two-lobed nozzle is about twice as that of the elliptic jet in the near-field, and there is a 40% reduction in the potential core length. PIV contours substantiate the results.