Active Noise Control in Jets from Conical and Contoured Supersonic Nozzles with Plasma Actuators

Abstract

An active noise control work in jets from contoured and conical supersonic axisymmetric nozzles (henceforth contoured and conical jets, respectively) with plasma actuators is conducted. The Mach 1.65 contoured nozzle is designed by using the method of characteristics for shock-free jet. The Mach 1.65 conical nozzle has converging and diverging conical sections with a sharp throat. Eight actuators, distributed uniformly around the nozzle exit, are used and the jet is forced with azimuthal modes of 0-3, and ±4 and at forcing Strouhal numbers ranging from 0.18 to 4.0. Far-field acoustic is measured by a linear microphone array covering polar angles from 25 to 80q relative to the jet axis. At a far-field polar angle of 30q, more noise reduction is observed at a higher azimuthal mode of 3 or ±4 in both jets. The maximum reduction in overall sound pressure level is 1.5 and 2.6 dB for the contoured and conical jets, respectively. At a polar angle of 80q (near sideline), the reduction in noise level is negligible. In the conical jet, there is a competition for energy between the naturally-occurring structures and those generated by forcing, similar to the process taking place in forcing an imperfectly-expanded supersonic jet. At moderate forcing Strouhal numbers, the structures generated by forcing prevail and thus the naturally-occurring structures are suppressed or weakened. This seems to be responsible for the reduced or suppressed screech tones in the conical jet.