Influence of Ullage Pressure on Wave Impacts Induced by Solitary Waves in a Flume Tank

We study experimentally the wave impacts generated by solitary waves and a bathymetry profile as a function of the ullage pressure within the range p_u∈ [35, 7990] mbar. The experiments are carried out in the Atmosphere (ATM) facility (MARIN) by using water as the liquid phase, a mixture of Nitrogen and water vapor as the gaseous phase and for a nearly constant temperature of 20 °C. As a consequence, we obtain a variation of the gas-to-liquid density ratio corresponding to DR ∈ [3.20 × 10 ^-5, 9.04 × 10^-3]. Our experiments show that the resulting breaking waves are repeatable and can be put in the context of Elementary Loading Processes (ELPs). Moreover, when the ullage pressure is p_u∈ [35, 1005] mbar, the global wave shape upon impact (GWS) remains nearly the same. For p_u > 1005 mbar however, the waves show a "backwards inclination" effect which leads to GWS upon impact with smaller gas pockets as pu increases. Remarkably however, we observe that for the lowest ullage pressure explored (p_u= 35 mbar), the entrapped gas pocket upon impact does not oscillate and instead, exhibits a near collapse – a clear signature of phase transition effects. For the case of p_u∈ [100, 1005] mbar, where the GWS is nearly the same, we perform a quantitative analysis of the impact pressures at both the crest level and inside the gas pocket. Regarding the wave crest, we find that the mean pressure (although only obtained with five repetitions) hints towards a decreasing trend with increasing p_u (or DR). Similarly, when investigating the pressures in the gas pocket, we find that the mean pressure and the associated dominant frequency of oscillation increase with p_u (or DR). Finally, we compare the pressure inside the gas pocket with the Bagnold model, where we use the p_u = 1005 mbar case as a reference in order to obtain the Bagnold parameters for the remaining p_u. By doing so, we find that – during the first compression cycle – the model is able to reproduce the experimental data for the case of p_u = 378 mbar, which gives a good estimate of both the rise time and the maximum pressure. In contrast, at p_u = 100 mbar, the model only reproduces the rise time and underestimates the maximum pressure. We elucidate that this effect could be related to the presence of phase transition in the experiments at p_u = 100 mbar.

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