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Mixing and Turbulence Structure in a Stratified Fluid Due to Surface Shear Stress

Author(s): Yarko Nino; Laura Reyes

Linked Author(s): Yarko Niño

Keywords: Stratified flows; Interface tilting; Wind-induced mixing; Turbulent mixing; Reynolds stress; Laboratory experiments

Abstract: Laboratory experiments are used to study the velocity field and mixing processes induced by surface shear stress in a two-layer stratified water volume. This study aims at providing results of basic research for the analysis and modeling of wind effects on the hydrodynamics of water bodies such as lakes and reservoirs. The experiments were conducted in a tank provided with a conveyor belt to exert the surface shear stress. An experimental technique, based on the analysis of video images, was developed in order to measure instantaneous two-dimensional velocity and density fields of the flow. The results obtained showed that the surface shear stress induces tilting of the density interface and mixing at the base of the surface layer. Its induces also a two-cell circulation inside the tank which is restricted mainly to the surface layer. The bottom region is always basically stagnant. The rms longitudinal velocity is maximum near the free surface, but it has a second peak at a location coincident with that where the velocity of the return current is maximum. The rms vertical velocity tends to zero near the free surface and has a maximum at about the same location as the second peak of the longitudinal rms velocity. The total longitudinal shear stress, including viscous and Reynolds stresses, is maximum near the free surface and has a second peak at the location of the maximum negative flow velocity. Initially, it also has a negative peak in the zone of the density interface, and then decreases to zero in the bottom region of the flow. This explains the observed large tilting of the density interface. As the mixing proceeds in time, the shear stress at the deepening density interface becomes negligible.


Year: 2001

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