Concurrent scale interactions in the far-field of a turbulent mixing layer

Abstract

The interaction between the large- and small-scales in the self-similar region of a nominally two-dimensional planar mixing layer is examined at a centreline Reynolds number Reλ ≈ 260 (where Reλ is the Reynolds number based on Taylor microscale). Particle image velocimetry experiments are performed at two different spatial 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). 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 amplification effect, of the small-scales by the large-scales, is found to increase with the magnitude of the large-scale fluctuations. By drawing an analogy between the two different spatially resolved datasets and a large eddy simulation it is shown that the turbulent kinetic energy flux to the sub-grid-scales (SGS) is highly sensitive to the alignment between the fluctuating velocity vector and the gradient of the mean shear of the flow. When these two large-scale vectors are perpendicular there is a notable increase in the small-scale turbulent kinetic energy (TKE) flux. This small-scale TKE flux was observed to be influenced by this large-scale alignment even for the smallest scales present, where the correlation between u1 and u2 has vanished (SGS scale