SPH modelling of coherent structures and intermittent events in surf zone turbulence.

Christos Makris, Yannis Krestenitis, Constantine Memos

Monday 29 june 2015

16:00 - 16:15h at Central America (level 0)

Themes: (T) Special session, (ST) Smoothed particle hydrodynamics and other meshfree methods

Parallel session: 3H. Special session: Smoothed Particle Hydrodynamics and other meshfree methods

The prime goal of the study is the detailed numerical simulation of turbulent features in the surf zone, due to weakly plunging breakers on a relatively mild impermeable slope. Specifically, we focus on describing the complex hydrodynamics involved in the breaking process of non-linear monochromatic waves in a numerical tank, simulating recent experiments in a short laboratory flume. In this framework, a modern CFD method is used for the simulation of hydrodynamic free-surface flows, i.e. Smoothed Particle Hydrodynamics (SPH); the academic ‘open-source’ code SPHysics is implemented. Viscosity treatment is based on a Sub-Particle Scale approach for turbulence closure, which consists of a Smagorinsky-type model. The latter treats the energy dissipation of eddies for the unresolved flow scales, similarly to Large Eddy Simulations. We undertake thorough analysis of turbulent eddies’ sizes and thus choose a spatial resolution approaching and marginally outreaching the demarcation point between integral turbulence length scales and Taylor micro-scales. A heuristic ensemble-averaging technique is implemented for the discrimination of turbulent flow components from ordered wave motions. Moreover, Fourier analysis is used to define isotropy or anisotropy of the velocity field over all spectral bands. Furthermore, the reproduction of coherent turbulent structures, based on turbulent kinetic energy (TKE) and vorticity, is pursued on a 2-D vertical cross-sectional plane. The recurring patterns are studied both in Eulerian and Lagrangian perspective, in order to define the topology and quantify the properties of the wave breaking mechanism. Additionally, the cross-shore variation of intermittent turbulence due to wave breaking is investigated through empirical identification of coherent and intense events. The statistics (conditional probabilities) of intermittent events are calculated and correlation patterns between turbulent shear stresses and TKE are detected, allowing us to define possible cross-shore sediment transport patterns related to them. Specifically, coherent events of intermittent turbulence occur 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.