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The outer zone of turbulent boundary layers

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sing data from new simulations of the turbulent boundary layer, we describe several features of the outer zone of wall-bounded flow at relatively high Reynolds numbers (Reth=6600). For example, using two-point velocity correlations, we show that the size and inclination of the different velocity components are different, and interpret this as representing different moments in their evolution. Other features are not so well understood. E.g., the transverse components (v and w) are similar in boundary layers and channels, but the streamwise velocity is much longer in the latter, and the outer zone of boundary layers is characterized by a large-scale oblique organization of w, of unknown origin, but reminiscent of some of the transitional structures observed in extended Couette flow.

We next discuss the turbulent/nonturbulent interface, detailing with some care the effect of how the interface is defined, including the variable being thresholded, the thresholding details, and the thresholding level. We note that the definition of distance between a point and a surface has a profound effect on the definition of the interface and, particularly, on the conditional analysis of the flow in its neighbourhood. We introduce an appropriate distance and show that the interface undergoes a topological transition as the threshold is increased and the interface enters the turbulent core. This can be used to define a ‘natural’ threshold, and we show that this definition clarifies, at least for this flow, some of the hypotheses that have been advanced about the interface structure. A simple structure is proposed which roughly agrees with some of the older proposals by Townsend and others.

This talk is part of the Fluid Mechanics (CUED) series.

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