Coherence between large-scale jet-mixing structure and its pressure field

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Abstract
A schlieren system has been arranged to sense the total fluctuation over a cross-section of the flow and thus becomes very sensitive to large-scale azimuthally coherent structures in the flow. For a natural unexcited jet it is found that there is a concentration of the large-scale structure at a characteristic Strouhal number which is not sensitive to the beam thickness and which reduces progressively with distance from the nozzle. This large-scale structure exhibits a coherence of over 70 % with the near-field pressure and convects at between 75 % and 95 % of the jet velocity. The coherence between the potential core-pressure field and the large-scale structure downstream increases rapidly with distance from the nozzle exit plane, rather limited coherence being found at the exit plane for these observations at a jet-exit Mach number Mj = 0·7. Movement of a central microphone from x = 0 to x = 2D introduced a solid centre body over the first 2·5 diameters of flow and gave rise to a set of discrete components in the flow structure in the range 0.6 < S < 1·4.With harmonic excitation at S = 1·12 a subharmonic at S = 0·55 occurs at x/D = 3 and a second at S = 0.26, x/D = 6. The flow cross-sectional-average sensing thus appears to show up the vortex-pairing mechanism at greater distances from the nozzle than is easily detectable by other means. Under strong impulse excitation a set of discrete components was observed in a transient response extending over times of 400D/Uj. These had a strongest component which decreases more rapidly in Strouhal number with distance than that associated with natural or harmonically excited conditions.