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Abstract This study presents a three-dimensional numerical simulation of a circular bottom-sitting OWC device for wave energy extraction. The numerical model is based on Reynolds-Averaged Navier-Stokes equations with a modified k- ω turbulence model and a Volume-of-Fluid method to track the air-water surface. The numerical model and setup are validated against a set of wave-flume experimental results. The numerical simulation provides detailed flow-field information, which allows for an analysis of the spatial non-uniformity inside the OWC chamber and the measurement error associated with it. New numerical results show that vortex shedding at the sharp edge of the OWC chamber enhances the spatial non-uniformity inside the OWC chamber through a resonant sloshing mechanism. The enhanced spatial non-uniformity can affect the accuracy of the experimentally-determined quantities such as the extraction efficiency of the OWC chamber and the characteristics of the power take-off device; Suggestions are also provided to improve the accuracy of the experimentally-determined quantities that may be affected by the spatial non-uniformity inside the OWC chamber. [ABSTRACT FROM AUTHOR]

This study examines, both experimentally and numerically, the collapse process of a submerged granular column in loose packing, with an emphasis on the effects of the grain size and the initial aspect ratio of the granular column. A two-phase flow continuum model, developed based on the rheological characteristics of solid-water mixtures, is used to perform the numerical simulations. Two particle sizes representing very fine particles and coarse particles are examined in this study. The two-phase flow model is validated and verified by comparing with the measured characteristics of the collapse process. Compared to the collapse of a granular column of coarse particles, the runout distance of a granular column with very fine particles is longer and the spread velocity is higher. Based on the two-phase flow simulation results supported by the experimental results, critical initial aspect ratios are identified to classify the types of the initial collapse process and the final deposit morphology, and the empirical expressions are provided to describe the changes of the runout distance, collapse duration, final deposition height, and spread velocity with the initial aspect ratio. A phenomenon similar to the so-called Kelvin-Helmholtz instability can be observed in both the experiment and the simulation at the later stage of the collapse, which causes the formation of several vortexes in the two-phase flow during the collapse process. The presence of these vortexes is responsible for the pressure drop on the bed after the passage of the flow front and the wavy feature on the final deposit morphology. [ABSTRACT FROM AUTHOR]

Flow characteristics in the vicinity of a bottom-sitting wave energy converter are important for understanding the wave loading on and the local scour around the structure. Based on three-dimensional numerical simulation results, key features of the vortex flow in the vicinity of a circular oscillating water column (OWC) supported by a C-shaped structure are discussed, including vortices shed from the lower tip of the OWC chamber and the two edges of the C-shaped support structure. Numerical results reveal a time-mean flow pattern in the vicinity of the C-shaped structure. There is a strong connection between the near-field flow pattern and the local morphological change at the C-shaped support structure. [ABSTRACT FROM AUTHOR]

Submarine landslides are one of the major causes of tsunamis but less understood due to complicated dynamics involved and the lack of observational data. In this study, the collapse of a submerged granular column is used as an idealized submarine landslide model. A three-phase (solid–liquid–gas) continuum model is used to study the effect of the column’s initial packing on the collapse process and the resultant waves. Numerical simulations reveal that the initial packing can have significant effect on the duration of the collapse process and the characteristics of the resultant waves. Pore pressure, velocity field of the fluid phase, the volume of the sliding granular mass, and the sliding velocity are calculated and used to understand the effects of initial packing condition. Our results show that it is important to consider the initial packing effects in numerical simulations of submarine landslides and the resultant waves. Our result also show that the large vortex generated by the moving front of the granular flow can affect the form of the waves generated by the landslide. [ABSTRACT FROM AUTHOR]

ABSTRACT Cao, D.P.; Jian, W.; Huang, Z.H., and Lo, E.Y.M., 2018. Prediction of wave runup on columns of two semisubmersibles models. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1031–1035. Coconut Creek (Florida), ISSN 0749-0208. Water wave runup on columns of semi-submersibles has important implications on negative air-gaps created and slamming forces exerted on the underside of such platforms and their safe operations in rough seas. Many prediction formulas have been developed for wave runup on single vertical columns though none could be directly used for runup predictions on the columns of semi-submersibles due to the more complicated geometrical and diffraction effects. In the present study, small-scale experimental tests and limited numerical simulations using the linear diffraction and CFD solvers were conducted on two typical semi-submersible models, a twin-pontoon and a ringpontoon platform, to determine the runup on the columns. The experimental tests have the semi-models tested in a fixed configuration under regular and irregular waves and at different heading angles. Wave gauges and video recordings were used to capture the wave field and the runup on the columns. Key features observed were that sloshing between the front and rear columns created localized breaking over the pontoon, and that the maximum wave runup occurred at the downstream columns. A recently developed predictive runup equation (Cao et al., Coast Eng Jour. 59(3), 2017) is extended through by introducing an equivalent cylinder radius. The extension accounted for possible wave diffraction effects which led to higher runups than seen for single columns. The results from the limited numerical simulations were in quantitative agreement with experiments. [ABSTRACT FROM AUTHOR]

A complicated coupling system performs fuel filling operation of a ship on the sea that involves tank sloshing and filling impact. This research elaborated on the sloshing load and flow characteristics of the variable mass tank and revealed its physical process through numerical simulations to provide safety assurance for fuel filling. First, this work clarified the difference between the numerical theory of variable mass tank sloshing and that of ordinary sloshing. On this foundation, a numerical model was developed for a three-dimensional variable mass tank sloshing. In addition, the model was validated by comparing the results of the previous experiments. Second, the sloshing pressure and internal free surface of the tank were monitored by numerical simulations. At the same time, a series of sensitivity analyses were carried out against the sloshing period, filling rate, and sloshing amplitude. Finally, the baffle was added to the sloshing tank to study the influence of internal barrier structure on the sloshing flow and pressure characteristics. The following conclusions were obtained from this work: Fuel filling caused an increase in liquid mass, suppressing sloshing and increasing the tank load. Overall, the sloshing load increased with the filling rate. Moreover, the impact of internal barrier structure on sloshing is discussed in this study, and it was discovered that the sloshing effect could be efficiently suppressed by the barrier structure. [ABSTRACT FROM AUTHOR]