The impact of a deep-water plunging breaker on a wall with its bottom edge close to the mean water surface

Abstract

The impact of a deep-water plunging breaker on a finite height two-dimensional structure with a vertical front face is studied experimentally. The structure is located at a fixed horizontal position relative to a wave maker and the structure’s bottom surface is located at a range of vertical positions close to the undisturbed water surface. Measurements of the water surface profile history and the pressure distribution on the front surface of the structure are performed. As the vertical position, $z_{b}$ (the $z$ axis is positive up and $z=0$ is the mean water level), of the structure’s bottom surface is varied from one experimental run to another, the water surface evolution during impact can be categorized into three classes of behaviour. In class I, with $z_{b}$ in a range of values near $-0.1\unicode[STIX]{x1D706}_{0}$, where $\unicode[STIX]{x1D706}_{0}$ is the nominal wavelength of the breaker, the behaviour of the water surface is similar to the flip-through phenomena first described in studies with shallow water and a structure mounted on the sea bed. In the present work, it is found that the water surface between the front face of the structure and the wave crest is well fitted by arcs of circles with a decreasing radius and downward moving centre as the impact proceeds. A spatially and temporally localized high-pressure region was found on the impact surface of the structure and existing theory is used to explore the physics of this phenomenon. In class II, with $z_{b}$ in a range of values near the mean water level, the bottom of the structure exits and re-enters the water phase at least once during the impact process. These air–water transitions generate large-amplitude ripple packets that propagate to the wave crest and modify its behaviour significantly. At $z_{b}=0$, all sensors submerged during the impact record a nearly in-phase high-frequency pressure oscillation indicating possible air entrainment. In class III, with $z_{b}$ in a range of values near $0.03\unicode[STIX]{x1D706}_{0}$, the bottom of the structure remains in air before the main crest hits the bottom corner of the structure. The subsequent free surface behaviour is strongly influenced by the instantaneous momentum of the local flow just before impact and the highest wall pressures of all experimental conditions are found.

Breaking-wave induced pressure and acceleration on a clifftop boulder

“…Throughout the breaking x f range, pressure oscillations following the second pressure maximum can be seen in figure 4. These oscillations become more pronounced from test 6 to test 20 and increase in frequency from 200 Hz to 600 Hz giving bubble radii of between 5.6 mm and 15.5 mm as calculated from (4.5) in Wang et al (2018). The three-dimensionality of these experiments allows air to escape along the sides of the boulder following bubble formation, possibly leading to faster oscillation decay than in vertical wall impact experiments (Bogaert et al 2010;Hattori et al 1994).…”

“…Previous experiments on flip-through-type impacts have shown how maximum pressure values can be concentrated in very small areas (Lugni et al 2006;Wang et al 2018;Chan & Melville 1988;Cooker 2002). Throughout this experimental campaign, pressure on the boulder was measured at the boulder's front face via a single transducer with a 5.54 mm active area diameter.…”

“…Pressure oscillations are a common feature of bubble formation during impact and increase in frequency as entrapped air decreases (Hattori, Arami & Yui 1994). The bubble's natural frequency has been related to its equilibrium radius by Minnaert (1933); Plesset & Prosperetti (1977) and used in previous experiments on plunging breaking waves to estimate bubble size from pressure oscillations (Wang et al 2018). When wave run-up is higher than the impacted structure, overtopping, including green-water (non-aerated) overtopping, will occur.…”

On Local Singularities in Ideal Potential Flows with Free Surface

“…Strong forces and very large accelerations can be produced as a sheet of water "flips through" the trough and generates a powerful upward jet of fluid. For discussion of the flip-through phenomenon and related experiments see [53,4,58].…”

“…Accuracy check. We checked the accuracy of the numerical scheme for a Dirichlet ellipse as described in section 5.2 above, with initial data for (58) given by…”