An Investigation of Noise Reduction in a Mach 1.3 Round Jet using Tangential Blowing

Abstract

[Abstract] An experimental investigation of the effect of tangential blowing on the shear layer of a round unheated supersonic jet is reported, detailing a reduction in the acoustic energy measured in the far-field. An ideally expanded jet with a Mach number equal to 1.3 and Reynolds number of 1.3x10 6 was used as the primary flow and two methods of introducing tangential flow were tested; one used internal swirl generators upstream of the converging nozzle section and one used discrete tangential jets external to the nozzle. The results confirm earlier data that show a reduction in sound pressure level (SPL) with increased jet swirl. It is also found that a reduction of 2 dB in SPL is possible for a wide range of observer angles using a mass flow ratio of approximately 1% of the primary jet. Nomenclature α = Observer angle, measured from the downstream jet axis D = Primary jet diameter Mj = Mach number of primary jet flow at exhaust conditions Rej = Reynolds number based on primary jet diameter and nominal exhaust velocity St = Strouhal number based on primary jet diameter and nominal exhaust velocity (x, r, �) = Cylindrical co-ordinates with origin at the centre of the primary jet exhaust I. Introduction ircraft jet exhausts are a source of undesirable noise and continue to be an area of investigation driven by increasingly stringent regulation. The noise is produced by the unsteady mixing of the jet with the surrounding air and is dominated by the effects of the shear layer. In the present work unheated jets of Mach 1.3 are used to achieve the same convective Mach number commonly found in civil aircraft jet exhausts 1 and measurements are taken to explore the effect of a tangential component of the flow generated by tangential blowing. The three accepted sources of noise produced from supersonic jet flow are Mach wave radiation, large scale structures and fine scale turbulence 2,3 . Kelvin-Helmholtz instability waves in the shear layer lead to Mach wave