Free-surface visualization of streamwise vortices in a channel flow

Visualization of the free surface in an open channel can provide insight into certain characteristics of the flow field, such as the occurrence of low-frequency, large-scale eddies. This paper presents some experimental results obtained in a moving-bed flume constructed for research on free-surface transport without significant shear at the free surface (without wind). Free-surface velocities were measured using particle tracer velocimetry (PTV) at bulk Reynolds numbers (UH/n) between 8500 and 45,500. The PTV streaks were digitized in order to perform a quantitative analysis, allowing determination of spatial velocity autocorrelations. Large, vertical oriented vortices and lateral movement are apparent on the water surface, with a lateral length scale of between 1 and 1.5 times the depth. A relationship between these largest scales of turbulence, large streamwise vortices, and secondary currents in the flow is hypothesized. The large-scale turbulent eddies with streamwise vorticity that form secondary currents across a movable-bed, open-channel flow also exist in a smooth-bed open-channel flow but are not fixed in space. They oscillate laterally at low frequencies and form permanent secondary currents only near the wall, where lateral migration is limited. They appear to be a significant transport mechanism in the outer layer, transporting mass, energy, and outer region bursts to the free surface.