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Sph Modelling of Coherent Structures and Intermittent Events in the Surf Zone of Weak Plungers

Author(s): Christos V. Makris; Yannis N. Krestenitis; Constantine D. Memos

Linked Author(s): Constantine Memos

Keywords: Surf zone; SPH; Coherent structures; Intermittent events; Weak plunging breakers

Abstract: The prime goal of the present study is the detailed numerical simulation of surf zone hydrodynamics, due to weakly plunging breakers on a relatively mild impermeable slope. Smoothed Particle Hydrodynamics method is used in combination with a Smagorinsky-type model for the sub-particle scales of the flow, similarly to a Large Eddy Simulations’approach; hence the academic‘open-source’code SPHysics is implemented. We undertake thorough pre-processing analysis of experimental eddy sizes in regular wave breaking and thus choose a spatial resolution approaching the demarcation point between integral turbulence length scales and Taylor micro-scales. Moreover, a combination of heuristic techniques for the discrimination of turbulent fluctuations from the ordered wave motion is proposed as a postprocessing manipulation of SPH Lagrange-type data. The model is validated against experimental data, concerning several wave features. Spectral analysis of the fluctuating components for the velocity field is used to investigate its isotropic substance. Results in the surf zone concern wave-induced mean flows, dynamic characteristics of weak plungers and consequent bores, coherent structures on vertical cross-sections, and intermittent events at fixed gauges. The recurring patterns of vorticity are studied both in Eulerian and Lagrangian perspective, and reveal plausible reproduction of classic coherent vortical structures, which define the topology and properties of the wave breaking mechanism. The latter is supported by the derivation of coherent structures of turbulent kinetic energy (TKE) and Reynolds stresses, too. Additionally, the cross-shore variation of intermittent fluctuations due to wave breaking is investigated by empirical identification of coherent and intense events. The statistics of intermittency agree with experimental data; e. g. coherent intermittent events occur only for a small portion (< 20%) of the TKE time-series, but contribute significantly to its magnitude by containing a significant amount (> 50%) of all motions. Correlations between shear stresses and TKE are detected, allowing us to define possible cross-shore sediment transport patterns related to them.


Year: 2015

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