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Secondary Currents and Coherent Structures, Their Distribution Across a Cross Section and Their Relation to Surface Boils in Turbulent Gravel-Bed Open-Channel Flow

Author(s): I. Albayrak

Linked Author(s): Ismail Albayrak

Keywords: Secondary currents; Coherent structures; Surface boils; Large Scale PIV

Abstract: Secondary currents, coherent structures and surface boils and their interaction in openchannel flow have been investigated experimentally in order to understand and quantify their contribution to transport and mixing throughout the water column. ADVP, surface-PIV and hot-film measurements were conducted in a gravel-bed open-channel with high aspect ratio (12. 2) in three different sets of experiments at high, moderate and low Reynolds numbers. The results of ADVP and PIV measurements have showed patterns of secondary currents in the long-term average flow structure. Upwelling regions are identified by lower mean longitudinal velocity and higher vorticity. Streaks of higher mean longitudinal velocity and lower vorticity are observed in downwelling regions. Hot-film bottom shear stress measurements and Reynolds stress distribution in the water column have shown that bed shear stresses in downwelling areas are higher than those found in upwelling areas and the undulation of bed shear stress and Reynolds stress across the channel is affected by the existence of secondary currents. Visualization of the water surface with PIV measurements indicated that vortex boils mostly occur in the upwelling areas with high vorticity concentration whereas downwelling areas show lower vorticity. A quadrant analysis was carried out in the upwelling and downwelling areas in order to determine the relation between coherent structures and secondary currents. This analysis revealed that in the wall region of downwelling areas, sweep events are dominant and in the region close to the free surface, ejection events dominate over the sweep events. The dominance of ejection events at the free surface explains the occurrence of a large number of surface boils observed in the upwelling regions. The measurements have shown that secondary currents, surface boils and coherent structures are correlated, thus producing 3D flow structures. The observed mean transversal surface currents between upwelling and downwelling and the surface boils contribute to surface renewal and gas transfer at the free surface. Downwelling regions play a key role for the mass transport from the free surface into the water column. Additionally, the dispersion and mixing due to secondary currents provide for rapid 3D distribution in the entire water column.


Year: 2007

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