Intense turbulent flows and vorticity associated with breaking waves make the surf zone of special significance within near-shore dynamics and costal sediment transport. Even if been investigated since long, they are still far from being completely elucidated. Several lab, field and numerical studies have demonstrated that the turbulent flow due to wave breaking is characterized by the formation of large-scale vortex structures rotating relative to each other and of any shape (Farahani et al., 2015; Zhou et al. 2014; Ting, 2008; Kimmoun and Branger, 2007; Lubin and Glockner, 2015). Their identification, based on vorticity or other turbulent features, is difficult as well as the description of intermittent events of turbulent nature, which primarily control sediment movements and cross-shore morphodynamic evolution.Many methods have been used, such as wavelets, quadrant analysis, ensemble averaging to discriminate turbulent fluctuations from the ordered wave motion (Zhou et al., 2014). Anyway, a thorough knowledge of the formation and spreading of these coherent structures along the water column in the surf zone is more expected, to better explain their interactions with bottom sediments and effects on tracers’ diffusion (Ting, 2008). In previous studies, the spilling wave case has been primarily investigated for analogy with bores and jumps, where coherent eddies were firstly documented, or for the solitary wave case, being more feasible to study separately the turbulent velocity field from the effects of return flow and residual turbulence. Few experimental studies have been conducted to examine the behavior of these swirling eddies under plunging waves (Na et al., 2016), which are characterized by higher turbulence due to the presence of the overturning jet and the splash-up cycle. In the present research, we investigate both a spilling and a plunging regular wave on a fixed and impermeable slope, as reproduced in the laboratory of the Department of Civil, Land, Building Engineering and Chemistry of the Polytechnic University of Bari (Italy). Measurements are carried out by means of a Laser Doppler Anemometer in some selected channel sections, located in the outer and inner surf zone. Following Cox and Kobayashi (2000), our analysis focuses on the instantaneous turbulent events in comparison to the phase-averaged turbulent quantities such as Reynold stresses and turbulent kinetic energy. Furtherly, the quadrat analysis method is used to identify coherent motions as a function of wave phase and quantify the bursting processes. Useful hints to turbulence spreading and transport are obtained.
Exploring turbulence and coherent events in laboratory breaking waves / De Serio, Francesca; Mossa, Michele. - ELETTRONICO. - (2018). (Intervento presentato al convegno The 8th International Symposium on Environmental Hydraulics tenutosi a University of Notre Dame, Indiana, USA nel June 4-7, 2018).
Exploring turbulence and coherent events in laboratory breaking waves
De Serio, Francesca;Mossa, Michele
2018-01-01
Abstract
Intense turbulent flows and vorticity associated with breaking waves make the surf zone of special significance within near-shore dynamics and costal sediment transport. Even if been investigated since long, they are still far from being completely elucidated. Several lab, field and numerical studies have demonstrated that the turbulent flow due to wave breaking is characterized by the formation of large-scale vortex structures rotating relative to each other and of any shape (Farahani et al., 2015; Zhou et al. 2014; Ting, 2008; Kimmoun and Branger, 2007; Lubin and Glockner, 2015). Their identification, based on vorticity or other turbulent features, is difficult as well as the description of intermittent events of turbulent nature, which primarily control sediment movements and cross-shore morphodynamic evolution.Many methods have been used, such as wavelets, quadrant analysis, ensemble averaging to discriminate turbulent fluctuations from the ordered wave motion (Zhou et al., 2014). Anyway, a thorough knowledge of the formation and spreading of these coherent structures along the water column in the surf zone is more expected, to better explain their interactions with bottom sediments and effects on tracers’ diffusion (Ting, 2008). In previous studies, the spilling wave case has been primarily investigated for analogy with bores and jumps, where coherent eddies were firstly documented, or for the solitary wave case, being more feasible to study separately the turbulent velocity field from the effects of return flow and residual turbulence. Few experimental studies have been conducted to examine the behavior of these swirling eddies under plunging waves (Na et al., 2016), which are characterized by higher turbulence due to the presence of the overturning jet and the splash-up cycle. In the present research, we investigate both a spilling and a plunging regular wave on a fixed and impermeable slope, as reproduced in the laboratory of the Department of Civil, Land, Building Engineering and Chemistry of the Polytechnic University of Bari (Italy). Measurements are carried out by means of a Laser Doppler Anemometer in some selected channel sections, located in the outer and inner surf zone. Following Cox and Kobayashi (2000), our analysis focuses on the instantaneous turbulent events in comparison to the phase-averaged turbulent quantities such as Reynold stresses and turbulent kinetic energy. Furtherly, the quadrat analysis method is used to identify coherent motions as a function of wave phase and quantify the bursting processes. Useful hints to turbulence spreading and transport are obtained.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
