Spatial and temporal structure of shear layer turbulence at a stream confluence

The shear layer that develops at the interface between converging flows is a prominent but poorly understood hydrodynamic feature at stream confluences. To examine the spatial and temporal characteristics of turbulence and coherent turbulent structures within a shear layer, three-dimensional velocity measurements were obtained at a small stream confluence in Illinois using two acoustic Doppler velocimeters spaced at various separation distances. Results indicate that coherent structures can be readily identified through cross-correlation analysis of the velocity time series for different measurement locations. Maximum correlations decrease relatively slowly over short separation distances but decrease rapidly for separation distances greater than about 2 times the flow depth. Convective velocities of coherent structures derived from the correlation analysis differ from local mean velocities and from spatially averaged mean velocities, indicating that coherent structures evolve as they are transported along the shear layer, a finding consistent with dye-tracing experiments. Despite this evolution, correlation and spectral analyses indicate that the frozen turbulence hypothesis holds for frequencies corresponding to length scales of at least some shear-generated coherent vortices. A domain of quasi-two-dimensional turbulence energy at low frequencies appears to be associated with large-scale intrusions of fluid from one stream into the other along the mixing interface.