A ‘Turbulent Spot' in An Axisymmetric Free Shear Layer. Part 3. Azimuthal Structure and Initiation Mechanism

The details of a spark-induced 'spot' in an axisymmetric mixing layer of a 12.7 cm diameter (D) free air jet have been deduced in different azimuthal planes and at three streamwise stations. It is shown that 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 boundary-layer spot persists initially in the free shear layer but decays downstream due to the lack of a sustaining mechanism. The mixing-layer spot 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 wave-length. (from authors' abstract)