On the Nature of Coherent Turbulent Structures in Channel Bends: Burst-Sweep Orientations in Three-Dimensional Flow Fields

It is widely accepted that natural flows are characterized by coherent turbulent structures, known as bursts and sweeps, which transfer fluid momentum across local velocity gradients. While these have been the subject of extensive laboratory and field investigations, no studies have attempted to address their characteristics in channel bends. In part, this is related to the existing analytical frameworks, which are inherently two dimensional. The objectives of this study are to a) develop a fully three-dimensional scheme for analysing coherent turbulent structures; b) investigate the orientation of bursts and sweeps in an experimental channel bend. This involves solving the Reynolds tensor and performing octant analysis in lieu of conventional twodimensional approaches. Results indicate that bursts and sweeps lack a preferred orientation upstream and downstream of the bend. However, flow in the channel bend strongly displays inner-bank oriented bursts and outer-bank oriented sweeps. These findings suggest that burstsweep orientations are controlled by vertical gradient of the cross-stream velocity component and turbulence generated fluctuations in the cross-stream force balance.