Introduction

This turbulent base flow is extremely susceptible to various kind of disturbances, undergoing transverse oscillations that grow into large-scale coherent motions, as a good number of observations (see illustrations further below) and analyses have shown. As an example for such a generation mechanism, Fig. 1b shows an arbitrary transversely sheared profile u¯(x, y) that can trigger such instabilities characterized by vortical elements of length scale 2D that are much larger than the depth, 2D H. Hence, the structure of these turbulent motions is largely “two-dimensional” with vertically aligned vorticity vectors.