Directional Suppression of Noise from a High-Speed Jet

Abstract

Experiments demonstrate directional suppression of noise from a high-speed jet using an asymmetric parallel secondary stream. The secondary stream attenuates Mach wave radiation in the lower hemisphere of the acoustic far eeld, leaving unaltered the upward-propagated Mach waves. An eccentric nozzle arrangement with a Mach 1.5, 700-m/s inner stream and a Mach 1.0, 360-m/s outer stream produces noise reduction superior to that from concentric arrangements or from the fully mixed equivalent jet. The angle of peak perceived noise shifts from the aft quadrant to the lateral direction. The beneet of the eccentric arrangement is attributed to its shorter potential core relative to a concentric jet. The experiments also reveal emission of strong crackle from the untreated jet, a noise component arising from the nonlinearity of Mach waves. The secondary eow suppresses crackle.