1,721,151 research outputs found
CFD simulations of vertical ship motions in shallow water
Full text linkThe seakeeping behaviour of a vessel in shallow water differs significantly from its behaviour in deep water. In shallow water, a vessel’s motion responses to incident waves will be affected by hydrodynamic effects caused by the presence of a finite depth. Given that a vessel will sail in shallow water at various times during its service life, such as when entering harbours, it is important to have an understanding of the influence of shallow water on ship motions. In this study, using a commercial unsteady Reynolds-Averaged Navier-Stokes solver, a numerical study of ship motions in shallow water was carried out. Firstly, the characteristics of shallow water waves were investigated by conducting a series of simulations. Then, a full-scale large tanker model was used as a case study to predict its heave and pitch responses to head waves at various water depths, covering a range of wave frequencies at zero speed. The motion results obtained were validated against related experimental studies available in the literature, and were also compared to those from 3-D potential theory. The results were found to be in good agreement with the experimental data. Finally, it was shown that vertical motions were significantly affected by shallow water
Russell’s solitary wave in 21st century Scotland
No full textAt the end of the 19th century, Scotland's relatively new canals were abuzz with activity. Canal boats were transporting bulk cargo and people, stimulating a flourishing economy. Darrigol [1] recants the well-known fable of the discovery of the solitary wave by John Scott Russel when a horse pulling a boat was frightened into gallop. To everyone's surprise, the boat offered less resistance than at low speeds. Exploiting that discovery, businesses began offering high-speed boat services from Glasgow to Edinburgh along the Forth and Clyde Canal. This paper aims to investigate the occurrence of solitary waves on Scotland's canal
Hydrodynamic analysis of ship manoeuvrability in shallow water using high-fidelity URANS computations
Full text linkThe manoeuvring performance of a ship in shallow water is substantially different from its performance in deep water, attributed to shallow water effects caused by the presence of a finite water depth. Without a doubt a ship will navigate in areas of shallow water at various times during its operational life (such as when approaching harbours or ports), which underscores the importance of understanding the shallow water effects on ship manoeuvrability. In the present paper, the manoeuvrability of the KRISO Container Ship (KCS) model in different shallow water conditions was comprehensively analysed by means of the unsteady Reynolds-Averaged Navier-Stokes (URANS) computations coupled with the equations of rigid body motion with full six degrees of freedom (6DOF). A dynamic overset grid approach was implemented to allow the ship hull to move in 6DOF in a computational domain and to enable the rudder to be deflected according to a rudder controller for free-running manoeuvres. A series of manoeuvring simulations were performed in shallow waters with water depth to draft ratios varying between 1.2 and 4.0, and partially validated with the available experimental data from a free running test. The numerical results revealed that the ship advance, transfer, and tactical diameter mainly increased with the decrease in the ratio of water depth to draft, closely associated with the complicated interactions between the hull wake, boundary layer, propeller, vortex, and sea floor
A numerical assessment of the scale effects of a ship advancing through restricted waters
Full text linkRestricted waters present several challenges for ship builders and operators. The proximity of the seabed and river or canal banks cause viscous effects to be more pronounced than in unrestricted waters. These effects do not follow a linear scaling law, which is typically assumed in terms of sinkage and trim. Moreover, the resistance of the ship is increased in a complex fashion, which has largely eluded researchers. The present study will aim to elucidate scale effects in shallow water performance predictions. Particular attention is placed on the form factor, wave resistance, and frictional resistance. Scale effects are confirmed in the two former parameters. Justification for the obtained results is sought in terms of flow properties. Specifically, the flow velocity and boundary layer thickness are examined in detail. The selected case-study reflects recent experimental work on the KCS hull form in restricted waters
Operability assessment of high speed passenger ships based on human comfort criteria
No full textThe growing popularity of passenger cruise lines means continual challenges are faced concerning both a vessel׳s design and its operational ability. Vessel dimensions, service speeds and performance rates are rapidly increasing to keep pace with this expanding interest. It is essential that vessels demonstrate high performances, even in adverse sea and weather conditions, and ensure the comfort of passengers and the safety of cargo. A vessel׳s operability can be defined as the percentage of time in which the vessel is capable of performing her tasks securely. In order to calculate a vessel׳s operability index, many key parameters are required. These include the dynamic responses of the ship to regular waves, the wave climate of the sea around the ship׳s route, and the assigned missions of the vessel. This paper presents a procedure to calculate the operability index of a ship using seakeeping analyses. A discussion of the sensitivity of the results relative to three different employed seakeeping methods is then given. The effect of seasonality on a ship׳s estimated operability is also investigated using wave scatter diagrams. Finally, a high speed catamaran ferry is explored as a case study and its operability is assessed with regards to human comfort criteria
A posteriori error and uncertainty estimation in computational ship hydrodynamics
Full text linkThe increasing relevance of simulation-based design has created a need to accurately estimate and bind numerical errors. This is particularly relevant to full-scale computational ship hydrodynamics, where measurements are difficult and expensive, simultaneously requiring a high degree of predictive accuracy even in early design stages. However, the field of ship hydrodynamics has yet to fully exploit the enhanced capabilities and potential benefits numerical verification methods have to offer. The present study presents a detailed application of numerical verification procedures in CFD as applied to local parameters, such as free surface elevation and skin friction. This is done in order to pinpoint specific locations in the computational domain responsible for heightened levels of error and uncertainty. Relationships between different parameters are demonstrated and discussed based on a set of full-scale simulations of the KCS advancing through a canal using CFD
A short review of scale effects in ship hydrodynamics with emphasis on CFD applications
Full text linkThe increased availability of computational resources has transformed the prediction of engineering quantities of interest at the design stage. For ship hydrodynamics, this means analysts are now able to predict the power requirements of a vessel at model-scale with good accuracy, routinely. As ever more intricate analysis methods and tools are developed, it has become apparent that modelling all physical phenomena at full-scale remains unattainable both presently, and in the near future. The difficulty in accounting for the full-scale performance frequently limits analysis to model-scale, causing scale effects. Scale effects arise due to the discrepancy in force ratios a model and the prototype will experience. One main consequence of the presence of scale effects is the difficulty in demonstrating the efficacy of new technologies, such as novel energy saving devices. The naval architecture community is therefore not ready to shed many of the historic assumptions made in the design of vessels. A prime example of this is the hydrodynamic modelling of a ship’s full-scale power requirements. Performing solely numerical simulations to obtain such data is considered risky, and is typically accompanied by model-scale experimentation and/or simulations. This work will focus on scale effects encountered when modelling the towed resistance of a ship at model and full-scale. The reasons scale effects are in many cases tolerated, and the problems they may cause are also reviewed. The only remedy to circumventing the presence of scale effects is to work in full-scale at the design stage, but there are a number of problems in doing so. These issues are also explored in this work, with special emphasis on the bottlenecks in adopting full-scale Computational Fluid Dynamics (CFD) numerical simulations as the only prediction tool used in the design process
A high-fidelity CFD-based model for the prediction of ship manoeuvrability in currents
Full text linkThe manoeuvring behaviour of a vessel in currents differs remarkably from its behaviour in water without a current, stemming from hydrodynamic effects caused by the presence of the current. Given that vessels operating in open seas and coastal waters are mostly exposed to ocean currents, it is important to have an understanding of the influence of currents on ship manoeuvrability. In the present paper, by means of an unsteady Reynolds-Averaged Naiver-Stokes solver, a numerical study of ship manoeuvrability in different currents was performed. Firstly, a model-scale container ship (the KRISO Container Ship) was used to develop the Computational Fluid Dynamics (CFD) model capable of performing a self-propelled free manoeuvre. Then, a validation study was carried out to assess the validity of the CFD model by comparison with the available experimental results from a free-running test. Following this, a series of manoeuvring simulations (i.e., standard turning manoeuvres) in deep waters with current speed to ship speed ratios varying between −0.552 and −0.138 / +0.138 and +0.552 were conducted using the present CFD model. The numerical results demonstrated that the inclusion of the current has a remarkable influence on the turning performance of the ship, leading to significant changes in the ship trajectory and its turning parameters when compared to the inherent ship manoeuvrability in deep water without a current
A numerical investigation of the squat and resistance of ships advancing through a canal using CFD
No full textAs a ship approaches shallow water, a number of changes arise owing to the hydrodynamic interaction between the bottom of the ship’s hull and the seafloor. The flow velocity between the bottom of the hull and the seafloor increases, which leads to an increase in sinkage, trim and resistance. As the ship travels forward, squat of the ship may occur, stemming from this increase in sinkage and trim. Knowledge of a ship’s squat is necessary when navigating vessels through shallow water regions, such as rivers, channels and harbours. Accurate prediction of a ship’s squat is therefore essential, to minimize the risk of grounding for ships. Similarly, predicting a ship’s resistance in shallow water is equally important, to be able to calculate its power requirements. The key objective of this study was to perform fully nonlinear unsteady RANS simulations to predict the squat and resistance of a model-scale Duisburg Test Case container ship advancing in a canal. The analyses were carried out in different ship drafts at various speeds, utilizing a commercial CFD software package. The squat results obtained by CFD were then compared with available experimental data
Scale effects and full-scale ship hydrodynamics : a review
Full text linkHistorically, the field of naval architecture has relied on a combination of model testing and scaling laws, known as extrapolation procedures, to predict full-scale power requirements. Numerous problems with extrapolation procedures were identified almost as soon as they were proposed, but since there were no alternative scaling laws their use persisted. This review article explores the cause of these uncertainties, the attempts to circumvent or correct them, and the current efforts to reduce and even eliminate the need for extrapolation of ship resistance through the use of full-scale Computational Fluid Dynamics. We find that while there are a number of developments and accomplishments in achieving robust and reliable full-scale numerical simulation, the research community is not yet ready to replace experimentation and extrapolation. The principal bottlenecks are the availability of open full-scale data, including ship geometries, and computational power to predict full-scale flows with the necessary accuracy
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