Plunging solitary waves: a 3D numerical investigation
Date
2014
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
A 3D numerical investigation based on large-eddy simulation (LES) is carried out to study a plunging solitary wave over a slope. This study is motivated by recent field surveys of the aftermath of several tsunami disasters where significant amount of soil erosion and foundation failure seem to occur during the drawdown stage of the tsunami impact. The mathematical formulation of the model is based on 3D filtered Navier-Stokes equations with a dynamic Smagorinsky closure. The model is solved numerically using on an open source CFD library of solvers called OpenFOAM, which was previously validated for a spilling solitary wave in a laboratory wave flume ( Sangermano, [2013]). In this study, the numerical model is further validat-ed with laboratory experiments of Sumer et al. [2011] and Synolakis [1986] for plunging wave condition. After validation with laboratory data of Sumer et al. [2011], simulation re-sults are further analyzed to understand the structures of flow velocity and turbu-lence during the run-up and drawdown stages of the plunging solitary wave and the resulting bottom stress and near bed flow acceleration. During run-up, wave breaking turbulence is not generated until the horizontal 2D rollers degenerated into smaller 3D structures due to the collapse of plunging breaker onto the bed. During the drawdown stage, flow landward of the initial shoreline is dominated by boundary layer process, similar to those reported in the swash zone. However, at later stage of the drawdown process, hydraulic jump is observe in the laboratory experiment, which is also well-captured by the present numerical simulation. More detailed analysis on the simulation results further reveal the existence of boundary layer separation under the hydraulic jump. The separation coincides with the location of strong adverse pressure gradient, reversal of bottom shear stress, and intensive turbulence generation. Peak near bed flow acceleration can reach as high as 30 m/s2 which occurs at the initial shoreline during the impingement of plunging solitary wave. This peak acceleration is mainly associated with the mean component of the accelera-tion while the turbulent fluctuating components only contribute a minor portion. During drawdown stage, flow acceleration can also exceed 10 m/s 2 , which is as-sociated with the moving hydraulic jump and the corresponding flow separation. In this case, the fluctuating component contributes more to the total acceleration than the mean component. High flow acceleration increases the possibility on the occurrence of plug flow and the present simulation results suggest plug flow is likely to occur during the plunging of the solitary wave over a slope. Examining simulation results of Synolakis, [1986] also indicates similar occurrence of boundary layer separation under the hydraulic jump, suggesting that boundary layer separation may be common for plunging solitary wave. Future work should extend the existing simulation with sediment transport capability.