Quasi-periodic structure of a turbulent jet

Abstract

The instantaneous near field pressure fluctuations of an axisymmetric subsonic jet were measured by using a longitudinal and an azimuthal microphone arrays in order to qualitatively determine the behaviors of the quasi-periodic structure within the flow. Statistical analysis is used to explain the characteristic of the pressure signals. In addition to the information obtained by forming the power spectral density, auto- and cross-correlation functions, two types of signals are extracted through a conditional probability analysis to represent the quasi-periodic and the random fine structures within the turbulent jet. The quasi-periodic structure first appears as a rolling up of the mixing layer flow within one nozzle diameter downstream of the exit, then becomes fully developed at approximatelt 3 nozzle diamters downstream with a preferred Strouhal number range 0·3–0·4, and finally disappears beyond the end of potential core. This behavior is also reflected in the variation of the convection velocity. The maxima of the overall sound pressure level and the peak pressure level occur at approximately 1 nozzle diameter from the exit, indicating that the early stage is of significant contribution to the vortex pairing and instability waves.