Author(s): Shengtao Du; Chaolin Wang; Zhiyong Zhang; Guoxiang Wu; David Z. Zhu; Bingchen Liang
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Keywords: Local scour; Circular-crossing pile; Square-crossing pile; Waves and current; SST k–ω model
Abstract: Experimental tests of local scour around a circular-crossing pile (CCP) and a square-crossing pile (SCP) were carried out to study the effects of superimposed waves upon current in the local scour process and the differences in characteristics in the two shaped piles. Non-uniform sediment with a gradation of 1.6 was used in the tests. Stokes waves of 0.11 m in height and 1.6 s in period were generated in a 50 cm water depth flume. The detailed flow turbulence characteristics of two submerged piles are determined by solving the Reynolds-averaged Navier–Stokes equations with the SST k–ω turbulence model. The model is verified by comparing experimental and numerical results for hydrodynamic parameters with the previous literature, both for square-crossing piles (SCP) and circular-crossing piles (CCP). The original topographies of a flat bed and a scoured bed (i.e., the initial and equilibrium scouring stages) are based on experimental results obtained by the authors in the present paper. Both SCP and CCP flow features in the scouring process are discussed. The temporal bed elevation profiles and scour depth development were very similar between the SCP and CCP, although the scour hole and sand dunes in the former were individually deeper and larger than the latter. The maximum scour depth in the SCP was nearly equal to that of the CCP in the waves-current condition, but it was much larger in the current-only condition. Both the time-averaged drag and the rms of the lift coefficient increase linearly during the scouring process in the SCP case, while in the CCP case, the rms of the lift coefficient increases first and then decreases to nearly the same value as during in the initial scouring stage. The minimum pressure coefficient is always located in the upstream corners in the SCP case. In the CCP case, it moves from 73.5° to 79° when the scour hole is fully developed. Downward flow behind the pile, which is generated by separated boundary layers above the top face of the pile, can reach the sand bed and turn the separated shear layers into patches of small vortices in the near-wake regions. The high shear stress zones are mainly at the scour edges under scoured bed conditions.
Year: 2023