Unforced Jet Research Articles

Influence of Mach number on coherent structure relevant to jet noise

The cross-spectral densities of turbulent pressure and its azimuthal constituents have been obtained experimentally using condenser microphones in a round jet for the entire region between !<*//)< 12 and 0 </•//)< 2, and for range of Mach numbers 0.30 <M0< 0.70. The coherent structures which were thus determined appear not to be strongly influenced by Mach number. A comparison of our measured phase velocities, as calculated from the axial distributions of phase of cross-spectral density, with those of other authors suggests a difference in eddy-wave behavior between acoustically-forced and unforced jets.

Correlation techniques and modal decomposition analysis for the detection of azimuthally coherent structures in jet flows

In recent years researchers in jet turbulence and jet noise have become increasingly interested in what is termed “large scale coherent jet structures”. There is now considerable evidence that azimuthally coherent structures can be generated by acoustically forcing a jet from upstream. However, the evidence for such structures in unforced jets, except close to the nozzle at low Reynolds numbers, is, at best, circumstantial. The role of such structures in subsonic jet noise production is also completely unproven. In an attempt to establish a link between azimuthally coherent structures and the jet noise field a number of experimenters have recently made azimuthal cross-correlation measurements of either the near field pressure or far field noise, and used the observed coherence to infer the existence of an azimuthally coherent source field. The term azimuthally coherent is used here to infer that the source region is dominated by low order azimuthal components, with relatively little contribution coming from the higher azimuthal components. The purpose of this paper is to question the interpretation of that data. Specifically the sound field generated by a simple ring source with various types of azimuthal coherence is considered theoretically. It is shown that the azimuthal coherence of both the near and far field pressures is principally a function of the Helmholtz number and in many cases of practical interest is relatively insensitive to any coherent structure of the source.