On coherent structures in a highly excited mixing layer

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Abstract
The flow field in a two-dimensional mixing layer, highly disturbed by a sinusoidally oscillating flap, was mapped in order to estimate the significance of the nonlinear processes associated with the large coherent structures existing in this flow. A mixing layer which does not diverge linearly in the direction of streaming is loosely defined as being highly disturbed. Two velocity components were measured throughout the flow field using a rake of X-wire probes. Streaklines were calculated from the phase-locked measured data and were compared to pictures of smoke injected into the flow, creating a link between flow visualization and quantitative experimental results. The phase-locked vorticity and the Reynolds stresses were calculated from these measurements.It was determined that fluctuations, locked in phase with the disturbance frequency, were not only responsible for the fast initial growth of the mixing layer but also for its contraction farther downstream (the occurrence of regions I and II in the parlance of Oster & Wygnanski 1982). The resumption of the growth of the mixing layer in region III is not controlled by the phase-locked oscillations. The first subharmonic of the imposed frequency was insignificant everywhere, and vortex amalgamation was not observed by visual means.Detailed comparisons between experimental results and theoretical calculations, based on a linear stability model, were carried out. The theory predicted very well the normalized, cross-flow distribution of any quantity that was measured, but failed to predict the amplification rates of these quantities in the direction of streaming.