Streamwise vortex meander in a plane mixing layer

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Abstract
Previous measurements in a two‐stream plane mixing layer have shown that the mean streamwise vorticity decays monotonically with downstream distance such that it is absent in the far‐field (self‐similar) region [J. H. Bell and R. D. Mehta, J. Fluid Mech. 239, 213 (1992); AIAA J. 28, 2034 (1990)]. It was not clear at that time if this measured decay reflected the diffusion of the instantaneous streamwise vorticity or was a result of increasing vortex meander within the mixing layer. The present experimental study was conducted in order to determine the existence of streamwise vortex meander in a mixing layer, and if present, its significance on the measured properties. The dependence of the velocity cross‐correlation on the fixed probe location was shown to be a good indicator of the stationarity of the streamwise vortex location. The cross‐correlation measurements obtained here indicate that spanwise meander is negligible, although transverse apparent meander (normal to the plane of the mixing layer) was indicated. The transverse meander, exemplified by the elliptical shape of the mean streamwise vorticity contours, was expected, since the streamwise vorticity in the braid region is essentially inclined, with respect to the streamwise direction. These conclusions were supported by results of estimated spanwise profiles of the transverse velocity component. The balance of evidence suggests that the measured mean streamwise vorticity decay is representative of the decay of the vorticity rather than an artifact of meander.