Flow field and acoustic properties of a Mach number 0·9 jet at a low Reynolds number

Abstract

An experimental study of the flow field and acoustic properties of a low Reynolds number ( Re ≅ 3600), M = 0·9 jet has been performed in our low pressure anechoic test chamber. The mean flow field was surveyed with a conventional Pitot pressure probe and flow fluctuations were detected with a normal hot wire probe. Also, condenser microphone measurements were made in the acoustic field. The major goal of the study was to develop a better understanding of the noise generation mechanisms of subsonic jets. The flow fluctuations within the jet were found to be dominated initially by a relatively discrete, large-scale, wave-like instability centered around a Strouhal number of 0·44. The axial wavelength of this instability was determined to be 1·45 jet diameters and its azimuthal character includes the n = 0 and n = ± 1 modes. The growth of this instability coupled with its non-linear breakdown are major contributors to the termination of the potential core region of the jet. The acoustic field of the jet, in contrast to the flow field, has a broad frequency spectrum with a peak amplitude near a Strouhal number of St = 0·2. The results indicate that a non-linear mechanism involving the large scale flow instability is responsible for a dominant portion of the noise generated from this jet.