Aeroacoustics of sonic ESTS lobed nozzles*

Elliptic Sharp Tipped Shallow (ESTS) lobed nozzles are established as a potential candidate for mixing in supersonic flows. These nozzles have the ability to reduce centre-line total pressure loss, increase jet spreading through efficient streamwise vortices generation, and promote far-field centre-line velocity decay. However, the aeroacoustic characteristics of these nozzles are not available in the open literature. Hence, a rigorous experimental campaign is undertaken to study the ESTS lobed nozzles' aeroacoustic characteristics in an anechoic environment. The study uses different lobe numbers (n = 3, 4, 5, 6, 7, 8) and nozzle pressure ratios (ζ = 1.5 to 6.5). Preliminary microphone measurements taken at an angle of φ = 135° from the jet axis are done for all the cases at varying ζ. A circular sonic jet is kept as the baseline case for comparison. High-speed Schlieren imaging resolving apparent streamwise density gradient is used to identify the spatiotemporal modes using modal analysis. The findings from both experiments reveal the presence of different jet oscillation modes at varying intensities across the ESTS lobed nozzles. Particularly, the difference between the overall sound pressure level (η) for one of the ESTS lobed nozzles (n=8) is ∼12 dB less than the baseline case. Moreover, increments in the lobe numbers cull the variations in η across different ζ.