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Oliver R.H. Buxton
Department of Aeronautics, Imperial College London, Prince Consort Road, London, U.K.

Bharathram Ganapathisubramani
Engineering and the Environment University of Southampton Tizard, Highfield, Southampton, SO17 1BJ, UK


The interaction between the large and small scales in the self-similar region of a nominally two-dimensional planar mixing layer is examined at two different Reynolds numbers, Reλ ~ 260 and Reλ ~ 470 (where Reλ is the Reynolds number based on Taylor microscale). Particle image velocimetry experiments were performed at two different resolutions, one that captures the range from integral scale (L) to Taylor microscale (λ) and the other that captures the range from Taylor microscale to the Kolmogorov length scale (η), simultaneously. It is found that the amplitude of the small-scale fluctuations (scales < λ) is modulated by the large-scale velocity fluctuations (scale > λ). Negative large-scale fluctuations (i.e. large-scale fluctuations that are less than the local mean) are found to coincide with regions where an increase in the amplitude of the small-scale fluctuations is found. This magnitude amplifying effect, of the small scales by the large scales, is found to increase with the magnitude of the large-scale fluctuations. It is also observed that there is a phase lag in the amplification of the small scale fluctuations by the larger scales. This phase lag can be interpreted to be the effect of large-scale spanwise roller-type structures that leave a wake of the finest scales behind them. The amplitude modulation, which is representative of the interaction between the large and the small scales, is shown to be Reynolds number dependent with the interaction marginally increasing in the Reynolds number range examined.