Author(s): I. Albayrak; U. Lemmin
Keywords: Large Scale PIV; Turbulent open-channel flow; Secondary currents; Surface boils
Abstract: This paper presents results of an experimental investigation of flow features at the free surface in turbulent open channel flow by means of flow visualization and velocity measurements using surface Particle Image Velocimetry measurements. The experiments were carried out in a gravel bed open channel with high aspect ratio (12. 2) in the Reynolds number range from 29. 5 to 106. 8 103. The results indicate that large streamwise vortices scale with water depth, result in stable secondary current cells and create upwelling and downwelling motions at the free surface. Lower longitudinal mean surface velocities are observed in the upwelling zones. Downwelling regions are identified as streaks of higher longitudinal mean surface velocity and lower vorticity on the surface. The transversal velocity progressively decreases in magnitude from the sidewall to the channel center. For a constant aspect ratio, the number of secondary cells is independent of the Reynolds number. However, it is found that bottom roughness affects the number of cells. Surface boils and eddies with vertical axes are primarily detected in the upwelling zones. Vortex size does not exceed water depth. Our measurements have shown that up- and downwelling as well as surface boils occur over a wide range of Reynolds numbers and can therefore be considered important processes in river dynamics, because they play a key role in surface renewal, gas exchange and transport between the surface and the pelagic zone.