Coherent structures induced by two simultaneous sparks in an axisymmetric jet

The axisymmetric mixing layer of a 12.7-cm-diam circular air jet at the exit speed of 20 m/s has been excited by two sparks fired simultaneously at diametrically opposite points near the lip. The resulting large-scale coherent structure-buried in the large-amplitude turbulent fluctuations in the mixing layer-has been educed at three streamwise stations and along three azimuthal planes. Spatial distributions of the coherent structure velocity perturbations, coherent Reynolds stress, vorticity, pseudo-streamfunctions, intermittency, as well as phase averages of the background fluctuation intensities and Reynolds stress have been deduced. The coherent Reynolds stress is considerably larger than the background turbulence Reynolds stress and the coherent peak vorticity is noticeably larger than time-mean peak vorticity. The coherent structure becomes progressively weaker with increasing x. The azimuthal variations of the educed contours suggest that the induced structure is a naturally occurring structure triggered by the sparks and is essentially axisymmetric; it undergoes a rapid change in its geometry, dynamics, and convection velocity as it travels downstream.