Impact of Serrations on the Circumferential Structure of Jet Turbulence and Relation to Jet Noise Reduction

Abstract

The study presented indicates that the impact of nozzle serrations on the azimuthal structure of the jet turbulence is probably not significant enough to justify the noise reduction measured in the far field. This statement is based on the analysis of near pressure field measurements performed with three ring arrays of microphones arranged around the axis of a co-axial subsonic rounded jet at x = 0.5D, 1D and 5D downstream of the nozzle exit. The main observations are the following ones. The serrations have a significant impact on the circumferential structure of the turbulence in the near vicinity of the exit plane. In this region of the jet, the relative amplitude of the low order circumferential modes (m=0 and 1) is reduced at the advantage of the high order circumferential modes. According to Michalke’s theory of jet noise, this modification of the modal content of the jet turbulence should induce a noise reduction because the high order modes are less ecient noise emitters. Unfortunately, this beneficial eect is annihilated by the fact that the absolute amplitude of the pressure fluctuations is increased by the detachment of small axial vortices at the edges of the serrations. The jet noise amplification measured in the far field for high frequencies is probably a consequence of that mechanism. Further downstream, near the end of the potential core, the circumferential modal content measured for the baseline and the configuration with the serrations are almost identical. Fortunately, the amplitude of the pressure fluctuations are smaller due to the mixing enhancement. This could explain why the noise is reduced in the far field at low and medium frequencies.