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Results
Numerical Modelling of Breaking Wave Impacts on Seawalls with Recurved Parapets Using qaleFOAM
Li, Qian (School of Science and Technology, City, University of London, London) | Yan, Shiqiang (School of Science and Technology, City, University of London, London) | Zhang, Yi (School of Science and Technology, City, University of London, London) | Zhang, Ningbo (School of Science and Technology, City, University of London, London) | Ma, Qingwei (School of Science and Technology, City, University of London, London) | Xie, Zhihua (School of Engineering, Cardiff University, Cardiff)
_ This paper presents a numerical comparative study on the interaction between breaking waves and vertical seawalls with a recurved parapet attached using the hybrid solver qaleFOAM, which couples a two-phase incompressible Navier–Stokes (NS) solver with the quasi-Lagrangian-Eulerian finite element method solver based on the fully nonlinear potential theory. The focus of the comparison is to reveal the possible scale effect as a result of improper scaling of the viscous effects in the model test adopting the Froude similarity in wave-structure interaction practices. For this purpose, both a 1:8 scale model test and a full-scale experiment with a large prototype model are used in the study. In the NS solver of the qaleFOAM, both a laminar model and k- shear stress transport model are adopted. The wave elevations at different locations and the impact pressure on the seawall are investigated. It is concluded that the performance of the qaleFOAM is similar for both the scaled and full-scale modelling; the viscosity, turbulence, and air compressibility may be critical for the formation of the breaking waves, which eventually affects the prediction of the impact load. Introduction An understanding of the loads induced by large waves on seawalls is important for seawall design and construction, as the occurrence of extreme weather events has become more frequent. Both the maximum wave loading and the runup on the seawall are critical criteria. Compared with increasing the seawall height, installing a recurved parapet on the top of the seawall has a greater benefit on reducing overtopping by diverting the upward water flow back to the sea. Experimental work by Ravindar et al. (2022) has confirmed the effectiveness of the recurved parapet and categorized four types of wave impacts, including (1) the aerated impact, where the wave breaks in front of the wall and hits the wall with aerated water; (2) the air pocket impact, where the wave crest hits the wall, enclosing a thin air bubble; (3) the flip through impact, where the wave crest hits the wall and runs up without trapping any air bubble; and (4) the sloshing impact, where the run-up of the wave is higher than the wave crest so that the wave crest hits the water layer instead of the wall.
- Research Report > Experimental Study (0.94)
- Research Report > New Finding (0.68)
Those tests are usually performed with model tanks at not possible with real fluids, but it can be reached with virtual scale 1: ( 40), filled with water and a mixture of gases chosen ones and numerically tested. If the numerical model only takes into account the density and the compressibility of the fluids, as in order to have the same gas-to-liquid density ratio as on is the case for solvers of the compressible Euler equations like board ships with Natural Gas (NG) and Liquefied Natural Gas SPH-flow, the complete similarity at both scales is reached if and (LNG). Irregular tank motions, calculated at full scale by a seakeeping only if three dimensionless numbers are kept the same at both software, are imposed to the model tank by a six-degreeof-freedom scales: the gas-to liquid density ratio (DR), and both Mach numbers hexapod after having been downscaled according to defined with the speed of sound into the gas and into the Froude similarity. This downscaling simply means that amplitudes liquid, respectively. As the reference velocity to be involved in are divided by and times are divided by . Mach numbers have to be in Froude similarity, this implies that This does not mean that the flow inside the model tank is in speeds of sound into the gas and the liquid at model scale are complete similarity with the full-scale flow. Actually, even disregarding times smaller than the corresponding ones at full scale (Braeunig the biases induced by phenomena that are present at et al., 2009).
CFD Simulations of Wave Impact Loads on a Truncated Circular Cylinder by Breaking Waves
Ha, Yoon-Jin (Korea Research Institute of Ships and Ocean Engineering) | Nam, Bo Woo (Seoul National University) | Kim, Kyong-Hwan (Korea Research Institute of Ships and Ocean Engineering) | Hong, Sa Young (Korea Research Institute of Ships and Ocean Engineering)
In this study, a numerical study was carried out on the wave impact problems between a truncated circular cylinder and breaking waves by using computational fluid dynamics (CFD) simulations. The target problem, which has been experimentally examined in the model test of Ha et al. (2018), is the wave impact problem between the cylinder and three different focusing waves (i.e., steep, spilling, and plunging waves). First, wave generation performance is checked by applying the present CFD simulation. In this case, the measured stroke signals of the wave maker in the test were directly used as input to the velocity boundary condition of the numerical calculation. Then, a series of CFD simulations were performed to estimate the local wave impact forces acting on the cylinder. For the validation, the numerical simulation results were directly compared with the model test data. Introduction The accurate estimation of wave impact force is significant for the survivability of the offshore platform under harsh environmental conditions. Moan (2005) reported accident rates for floating and fixed platforms in severe weather conditions. He explained that severe weather conditions would greatly affect structure damage of floating and fixed platforms. He also showed that the structural damages of various accidents cause a low fatigue life. Rosenthal et al. (2007) reported on the Draupner wave in the North Sea through the MaxWave Project. They showed the existence of high waves that were more than two times higher than the significant wave height—so-called rogue waves. It is known that severe sea conditions have caused the loss of more than 200 carriers.
- Asia > Japan (0.47)
- Asia > South Korea (0.46)
- Europe > United Kingdom > North Sea (0.24)
- (3 more...)
In recent years, interest has grown in studying impact loads by extreme waves when designing offshore structures. To date, the estimation of extreme wave and impact loads has been performed mostly through model tests. Since extreme waves are highly nonlinear, lengthy programs for model tests are required to properly estimate the level of impact load. In this sense, many attempts have been made to see results within relatively shorter durations. An example is numerical analysis based on computational fluid dynamics (CFD) as an alternative to a model test. However, the reliability of CFD simulation for extreme waves and induced impact load has not yet been properly proven. For better understanding of extreme waves, this paper studied the realization of focusing waves and associated impact loads on a fixed cylinder. CFD was used to consider the inherent nonlinearity in focusing waves. The focusing waves were generated by a numerical wavemaker, and the motion of the wavemaker was reconstructed from time series in model tests. A numerical scheme was calibrated with a combination of mesh size and time step to properly generate the focusing wave. Impact loads on a fixed cylinder were then calculated with calibrated parameters. Comparison was made for three different types of focusing waves: plunging, spilling, and steep waves. The effect on the sensor's mesh and air compressibility were also discussed. Introduction Most offshore structures are required to operate for more than 20 years in the harsh environmental conditions on site, requiring a complete structural integrity that can be achieved by predicting the extreme wave loads of these conditions. Thus, designing offshore structures presents a significant challenge. In order to properly predict design loads, irregular sea states need to be considered in long-term statistics. This requires a great amount of computation time and cost. Hence, as an alternative approach, the focusing wave is being widely employed on the basis of nonlinear wave-wave interaction.
- Asia (0.47)
- North America (0.46)
- Europe (0.29)
CFD Modeling of Arctic Coastal Erosion due to Breaking Waves
Ahmad, Nadeem (Norwegian University of Science and Technology (NTNU)) | Bihs, Hans (Norwegian University of Science and Technology (NTNU)) | Chella, Mayilvahanan Alagan (Norwegian University of Science and Technology (NTNU)) | Kamath, Arun (Norwegian University of Science and Technology (NTNU)) | Arntsen, Øivind A. (Norwegian University of Science and Technology (NTNU))
Computational fluid dynamics (CFD) modeling of breaking waves over a slope and the resulting erosion in the case of an Arctic coastline is presented in this study. The study is performed with the open-source numerical model REEF3D. First, the numerical model is validated for the simulation of incident waves, wave breaking on a slope, and the sediment transport process. The numerical results show good agreement with wave theory and experimental data. The validated numerical model for the hydrodynamics and the sediment transport process is then used to simulate the coastal erosion process under the breaking wave impact on a vertical bluff. An Arctic coastline at Bjørndalen region at Isfjorden, Svalbard, is chosen, where a significant coastal erosion was observed during a storm event in September 2015. Introduction Most of the Arctic coastline is susceptible to climate change. Because of global warming and the transfer of additional heat fluxes, the frozen period of the upper active layers in the Arctic coastline is reduced. Consequently, coastline stability decreases during the extended warmer period. The average thickness of the active sediment layer in Svalbard, Norway, varies between 1.0 and 10.0 m and consists of coarse-grained sandy soil (Fromreide, 2014). Climate change can affect this Arctic coastline in two ways. First, the extended warmer period results in the formation of deeper and weaker active sediment layers (IPCC, 2007). Second, the melting of the Arctic ice sheets increases the sea level, resulting in higher tides. In combination, the higher tides approach the Arctic coastline (Thompson et al., 2016) and erode the weaker active sediment layer. A recent example of this change has been experienced in the Bjørndalen region in Isfjorden, Svalbard, where significant coastal erosion occurred during a storm event in September 2015. The waves reached the cabins built near Isfjorden and resulted in an almost 1.0-m-deep scour hole (Barstein, 2015). Therefore, in order to better understand the coastal erosion phenomenon in the Arctic regions, the processes of wave breaking and the resulting sediment transport have to be investigated in detail. The study is also important for the design of new coastal structures and suitable mitigation measures at the Arctic coastline.
- Europe (1.00)
- North America > United States > California (0.46)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Military > Army (0.69)
- Government > Regional Government > North America Government > United States Government (0.47)
This contribution addresses the applicability of an efficient lattice Boltzmann-based single-phase free-surface model for the simulation of wave impact on the side walls of 2-D containers. The computational efficiency of the method is known to allow for very short turnaround times, but wave impact simulations have not been investigated in detail yet. Results for a selected wave impact case are discussed, the convergence behavior in space and time is analyzed, and limitations of the single-phase free-surface model are revealed. The results show that lattice Boltzmann method (LBM)-based single-phase free-surface models are a viable tool for predicting the impact wave behavior, but the quality of the pressure signal is limited, because of the absence of air in the simulations. Introduction The efficient numerical simulation of violent tank sloshing and wave impact is important to many different fields of engineering. Besides the numerical accuracy of the employed solvers, the computational efficiency and the time to solution are of interest as well, as even two-dimensional simulations of tank sloshing require a substantial amount of computational time. In this context, a very efficient numerical methodology based on the lattice Boltzmann method (LBM) is assessed in this paper. The LBM is an alternative to conventional methods on the basis of the Navier-Stokes equations that offers solver-specific advantages in terms of data locality and parallel computing. The LBM usually operates on a finite difference grid, is explicit in time, and requires only next neighbor interaction. It is very suitable for implementation on graphics processing units (GPUs) and other high-performance computing (HPC) hardware. Recently published LB results comprise laminar and turbulent bulk flows, multiphase flows, and free-surface flow applications. For all applications, a comparably high computational performance on both CPU- and GPU-based parallel architectures is reported. In the scope of this contribution, the applicability of the LBM to tank sloshing and wave impact is analyzed. Emphasis is given to the actual result accuracy, times to solution, and potential problems of the free-surface model. First, a short description of the LBM for bulk flows and the employed LBM free-surface model is given before addressing the violent tank sloshing case. Finally, conclusions are drawn and future perspectives are discussed.
- Information Technology > Hardware (0.89)
- Information Technology > Graphics (0.55)
- Information Technology > Scientific Computing (0.54)
In this paper, the non-similarity sloshing-induced slamming phenomena in geometrically similar Gaztransport & Technigaz (GTT) membrane-type floating liquefied natural gas (FLNG) facilities are investigated. The experiments were conducted in three scaled prismatic tanks at scales of 1=20, 1=40, and 1=60 with roll excitation, and the impact pressure was measured. The experimental results demonstrate that the kinematic free-surface behavior, time traces, and spatial distribution of the impact pressure during a single roll wave impact exhibit non-similarity in the three geometrically similar tanks. Furthermore, the quantitative statistical analysis of the impact pressure and impact rise time indicates that Froude's law yields conservative estimates for the slamming force in the tank by using the same experimental media. Introduction The challenge of designing floating liquefied natural gas (FLNG) facilities has attracted considerable attention from both the offshore gas exploration industry and academia. Floating above the deep water or marginal gas fields, the FLNG system is designed to produce, liquefy, store, and transfer liquefied natural gas (LNG). Consequently, it needs large volume tanks, a large amount of deck space, and easy maintenance. Gaztransport & Technigaz (GTT) membrane-type cargo containment systems (CCSs) have an advantage over other types of CCSs for FLNG facilities. They afford the maximum utilization of the ship's space, making use of the ship's support structure. However, due to cost efficiency and low-temperature preservation considerations, one drawback to the GTT membrane CCS is the capability of the thin insulation layer to bear liquid impact. Furthermore, one significant difference between the LNG CCS and FLNG CCS is that the FLNG system has no restriction for filling conditions (American Bureau of Shipping, 2010). Liquid tends to slosh in a partially filled tank. These slosh-generated loads have a considerable influence on the tank and support structure design. The slamming force has long been of interest to designers and researchers because it has resulted in structural local damage and liquefied natural gas leakage during LNG shipping (Abramson et al., 1974; Gavory and De Seze, 2009). Thus, it is essential to determine the slamming force caused by violent liquid movements in partially filled tanks in the design of CCSs for FLNG facilities. Both theoretical and numerical methods have limitations in predicting rapid overturning or the strongly nonlinear free surface with large amplitude slamming and complicated tank shapes, which is reported by many researchers (Abramson et al., 1974; Lee and Choi, 1999; Faltinsen and Timokha, 2009). The experimental approach is efficient and accurate in revealing the complicated physical phenomena during liquid sloshing. Monitoring the slamming force of CCS prototypes for in-service LNG carriers is expensive, difficult, and dangerous. Furthermore, it is difficult to eliminate uncertain factors that are not relevant to the slamming force in prototype experiments (Malenica et al., 2009). Hence, few prototype tests have been conducted, and there is little public information on such tests because of commercial conservation. The scaled model test is a practical approach for filling in the gaps between theoretical, numerical methods and prototype experiments. The slamming pressure obtained by the sloshing model test is processed to identify the most critical sloshing force on the containment system structure. Although many sloshing model studies have been conducted, the effect of scaling remains unclear (Faltinsen and Timokha, 2009).
- Asia (1.00)
- North America > United States > California (0.68)
- Research Report > New Finding (0.88)
- Research Report > Experimental Study (0.66)
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Oil & Gas > Midstream (1.00)
- Transportation > Freight & Logistics Services > Shipping > Tanker (0.34)
- Government > Regional Government > North America Government > United States Government (0.34)
After years of efforts, HydrOcean and Ecole Centrale Nantes, supported by GTT, succeeded in the development of an SPH software gathering all functionalities for relevant simulations of sloshing impacts on membrane containment systems for LNG carriers. Based on Riemann solvers, SPH-Flow deals with two compressible fluids (liquid and gas) that interact with the impacted structure through a complete coupling. The liquid, the gas and the structure are modeled by different kinds of dedicated particles allowing sharp interfaces. An efficient parallelization scheme enables performing calculations with a sufficiently high density of particles to capture adequately the sharp impact pressure pulses. The development of the bi-fluid version led, in a first stage, to unstable solutions in the gaseous phase for pressures below the ullage pressure. This difficulty was presented at ISOPE-2010 and has been overcome since. Simulations of a unidirectional breaking wave impacting a rigid wall after propagating along a flume are presented in this paper. The physical phenomena involved in the last stage of the impacts are scrutinized and compared with experimental results from the Sloshel project. A comparison between calculated results at full scale and at scale 1:6 is proposed. Conclusions about scaling in the context of wave impacts are given.
ABSTRACT: This paper presents the results of an experimental study of plunging wave action on a large horizontal cylinder in the splash zone. Impact pressures are found to range from localised impulsive pressures with time scales in the range of 0.001T to synchronous low-frequency pressure oscillations with oscillation time scales around 0.01T (T being the characteristic wave period). The highly impulsive cases, with peak pressures ranging from 4pC to 33pC, are associated with minimal entrapped air at impact. (ρ is the water density and C is the characteristic phase speed of the wave.) The highest peak pressure is obtained when the profile of the incident wave front is concentric to the cylinder boundary. Peak pressures as high as 5.4pC are obtained when a large pocket of air is entrapped by the plunging wave front at impact. The entrapped air also leads to synchronous pressure oscillations over a substantial region of the cylinder"s surface, covering an angular zone of about 105°. The corresponding peak impact force is about 8.6ρCR per unit length of the cylinder where R is the radius of the cylinder. Overall, the incident wave profile trajectory and the entrapped air are found to influence the impact load significantly. INTRODUCTION In a hostile sea environment, the safe and economic design of a marine structure depends significantly on the prediction of a representative design wave load. For structural members located at elevations between the mean sea level and the crest elevation, the highest loadings are those associated with wave impacts or slamming. This is especially so for horizontal structures with large dimensions compared to the incident wave height. Horizontal members such as the deck of an offshore structure are currently designed to be located well above the crest level to avoid slamming pressures.
Variability Of Plunging Wave Pressures On Vertical Cylinders
Chan, Eng-Soon (Department of Civil Engineering, National University of Singapore) | Tan, Boon-Cheng (Department of Civil Engineering, National University of Singapore) | Cheong, Hin-Fatt (Department of Civil Engineering, National University of Singapore)
ABSTRACT: The variability of pressures resulting from plunging wave impact on a smooth vertical cylinder is examined. Based on laboratory measurements, the fluctuations in the pressure characteristics are found to be associated with two main factors. One is the significant shift in the wave breaking location relative to the cylinder location, and the other is the randomness of the wave breaking kinematics and the trapped air dynamics. The variation of the peak pressure magnitudes associated with the latter is presented. Both the mean characteristics and the probability distributions are examined. INTRODUCTION In the design of offshore structures, one often seeks an estimate of the extreme wave loads. It has been long known that such a loading can result from waves breaking onto the structure. Research in recent years (Sawaragi and Nochino, 1984; Kjeldsen et al. 1986; Chan and Melville, 1987, 1988; Basco and Niedzwecki, 1989; and Tan et al., 1989) has in fact shown that wave impact forces can be more than two times higher than nonimpact forces from waves of comparable amplitudes. Moreover, the corresponding impact pressures "can be more than 10 times higher compared to nonimpact pressures. These impact loads are highly impulsive and transient in nature, and the physics of the impact process is complex. In a hostile ocean environment, particularly during stormy weather, the chances of encountering such wave impacts are very high. In particular, it has been shown that wave impact loads from plunging waves are much higher than those from spilling and non-breaking waves (Kjeldsen et al., 1986; Basco and Niedzwecki, 1989). Also, plunging wave impact occurs not at one critical structure location relative to the wave breaking location, but over a range of locations within the region of wave breaking (Chan and Melville, 1987, 1988; Tan et al., 1989).