Far-Field Noise Control in Supersonic Jets From Conical and Contoured Nozzles

Plasma actuators are used to control far-field noise in Mach 1.65 jets from contoured and conical supersonic axisymmetric nozzles (henceforth, contoured and conical jets, respectively). The contoured nozzle is designed using the method of characteristics for a shock-free jet. The conical nozzle has converging and diverging conical sections with a sharp throat. Eight plasma actuators, distributed uniformly around the nozzle exit, are used and the jet is forced with azimuthal modes $(m)$ 0–3 and $±4$ and forcing Strouhal numbers ranging from 0.09 to 4.0. The far-field acoustic noise is measured by a linear microphone array covering polar angles from 25 deg to 80 deg relative to the jet axis. In both jets, the lower forcing azimuthal modes (⁠$m=0$ and 1) are less effective than the higher modes (⁠$m=2$⁠, 3, and $±4$⁠), which have similar levels of overall sound pressure level (OASPL) reduction. At shallow angles relative to the jet axis, the reduction in OASPL is about 1.6–1.8 dB at low forcing Strouhal numbers in both jets at the most effective forcing mode of $m=3$⁠. However, the OASPL in the sideline direction is only slightly increased (about 1 dB) for both the contoured and conical jets at $m=3$⁠. The reduction at shallow polar angles is related to the decrease in the peak mixing noise level in both jets. The range of forcing Strouhal numbers providing significant noise reduction and the range of polar angles over which the noise is reduced are both much larger in the conical jet compared with the contoured jet. The screech tones are also reduced or suppressed – most likely due to weakening of naturally occurring structures by forcing.