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Title

EXPERIMENTAL STUDY OF FLOW AROUND A SPUR DIKE WITH SIDE SLOPE

Pages

 Start Page 1 | End Page 10

Abstract

 Spur dikes are man-made transverse river structures. They divert the high velocity to the channel center and prevent undesirable bank erosion. In this paper, flow structure around a single spur dike with SIDE SLOPE is investigated experimentally. Three dimensional FLOW FIELD around a single spurdike with 75o SIDE SLOPE located in a flat-bed rectangular laboratory flume has been measured using the Acoustic Doppler Velocimeter (ADV) and mean flow and TURBULENCE PARAMETERS are investigated. Downstream of the spur dike due to the decrease of the flow velocity, a recirculation zone forms, while the flow accelerates at the middle zone of the channel. Upstream of the tested spur dike, flow is divided into two parts. A part of the approaching flow is diverted to the channel bed and the remaining form a bow wave near the water surface. Downward deflected flow interacts with the near bed approach flow and results in the horseshoe vortex. The horseshoe vortex is the main responsible for scouring at the upstream of the spur dike. Dye visualizations showed that due to the upstream SIDE SLOPE of the spur dike, the down flow is weaker than the spur dike with vertical SIDE SLOPE. Along the main channel two distinctly velocity amplification zones form. A velocity amplification along the outer boundary of the shear layer, downstream of the spur dike forms due to the local effects of the spur dike. Due to the spur dike constriction imposed to the channel, another high velocity zone forms. The second velocity amplification zone forms along the right channel wall. Flow velocity in the first amplification zone is higher than the second zone. The horse shoe vortex is strong near the lower layers and consequently the velocity amplification along the shear layer at the near bed layers is higher than the near water surface layers. By going from near bed layers to the water surface layers the extent of the second amplification zone increases. Flow streamlines at the near bed plane shows development of some oblique streamlines, after the reattachment zone. This stream lines are attributed to the coherent flow structures reported in the literature. The near bed streamlines are more diverted to the opposite channel wall, compared to the near water surface layers. By going from the near bed layers to the water surface layer the center of the recirculating zone moves downstream. The mean flow kinetic energy is amplified 2.5 times of the approach flow. The maximum mean flow amplification occurs at the central zone of the channel, while the maximum turbulent kinetic energy is measured along the outer boundary of the separation zone. The higher turbulent kinetic energy along the shear layer is responsible for the pickup and movement of the sediment particles at low mean velocity zones. Distribution of the Reynolds shear stresses show that the maximum-`pu’v’ stress occurs along the shear layer bounding the separation zone. Minus values of the `pv’w’ and `pv’w’ stresses shows that the sedimentation will occur at the downstream zone of the spur dike.

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