A ‘turbulent spot’ in an axisymmetric free shear layer. Part 3. Azimuthal structure and initiation mechanism

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Abstract
The details of a spark-induced ‘spot’ in an axisymmetric mixing layer of a 12·7 cm diameter (D) free air jet have been educed in different azimuthal planes and at three streamwise stations corresponding to x/D = 1·5, 3·0, 4·5. The measurement technique and the spot properties in the plane of the spark at the three stations were discussed in parts 1 and 2 (Sokolov et al. 1980 and Hussain, Kleis & Sokolov 1980, hereinafter referred to as I and II, respectively). The present part describes the azimuthal structure of the spot and its initiation mechanism.It is shown that the distributions of phase-average longitudinal and lateral velocities, the intermittency and the coherent Reynolds stress within the spot are essentially the same in various azimuthal planes at each streamwise location. The spark induces a local boundary-layer spot on the nozzle wall and simultaneously triggers the instability of the free shear layer downstream from the lip. The boundarylayer spot persists initially in the free shear layer but decays downstream due to the lack of a sustaining mechanism. The mixing-layer spot – the result of a roll-up of a natural instability mode triggered in the free shear layer by the acoustic disturbance radiated from the spark – grows downstream and undergoes intense interactions, remaining essentially axisymmetric and travelling at about 60% of the core fluid velocity. Velocity signals in different azimuthal planes of the free shear layer show that the natural instability of the jet occurs axisymmetrically on an instantaneous basis even though the jet diameter is considerably larger than the instability wavelength. The natural instability is amplitude-modulated in a random manner; this modulation is also essentially axisymmetric.The large-scale coherent structures produced by the intense localized spark are not only axisymmetric on the phase-average basis, but are also individually axisymmetric in the laminar instability region.