Influence of mixed flows on ship hydrodynamics in dredged channels

Although there is a significant body of research devoted to the shallow water hydrodynamic aspects of ships, several unexamined topics remain. Among these is that of critical outer flow in a dredged channel and its influence on parameters of interest. While empirical methods can be used with ease to resolve this, they can provide results with reliability sufficient only for an early design stage. On the other hand, more sophisticated potential flow theories are either inapplicable or do not perform well at the critical limit. However, RANS (Reynolds Averaged Navier-Stokes) – based tools can accurately capture all underlying phenomena without relying on limiting assumptions. This paper presents an attempt at comparing some results obtained via a CFD-based RANS solver and the slender body theory for critical outer flow in a dredged channel

Numerical modelling of ship-generated solitary waves

A ship sailing at the wave speed in shallow water produces a complex wave pattern including a downstream disturbance in addition to a periodically generated upstream disturbance. The upstream component consists of solitary waves which are generated at the ship bow and emitted forward as soon as the local water depth is sufficiently modified enabling them to bypass the shallow water wave speed limit. Reynolds Averaged Navier-Stokes numerical simulations are carried out to explore this phenomenon in a fully non-linear and viscous virtual towing tank in cases where a ship sails at the waterway centreline as well as off-centreline conditions. Results indicate that friction attains no more than approximately 25% of the total resistance coefficient, justifying the use of inviscid methods by previous studies in the literature. Water depth may have a significant impact on the frequency and amplitude of ship-generated solitary waves, but the manner in which this occurs is highly sensitive to the width of the waterway

A short review of scale effects in ship hydrodynamics with emphasis on CFD applications

The 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 posteriori error and uncertainty estimation in computational ship hydrodynamics

The 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

Virtual replica of a towing tank experiment to determine the Kelvin half-angle of a ship in restricted water

The numerical simulation of ship flows has evolved into a highly practical approach in naval architecture. In typical virtual towing tanks, the principle of Galilean relativity is invoked to maintain the ship as fixed, while the surrounding water is prescribed to flow past it. This assumption may be identified, at least partly, as being responsible for the wide-scale adoption of computational solutions within practitioners' toolkits. However, it carries several assumptions, such as the levels of inlet turbulence and their effect on flow properties. This study presents an alternative virtual towing tank, where the ship is simulated to advance over a stationary fluid. To supplement the present work, the free surface disturbance is processed into Fourier space to determine the Kelvin half-angle for an example case. The results suggest that it is possible to construct a fully unsteady virtual towing tank using the overset method, without relying on Galilean relativity. Differences between theoretical and numerical predictions for the Kelvin half-angle are predominantly attributed to the assumptions used by the theoretical method. The methods presented in this work can potentially be used to validate free-surface flows, even when one does not have access to experimental wave elevation data

A geosim analysis of ship resistance decomposition and scale effects with the aid of CFD

Historically, the prediction ship resistance has received its fair share of attention by the scientific community. Yet, a robust scaling law still lacks, leaving testing facilities to rely on experience-based approaches and large datasets accumulated from years of operation. Academia’s concern regarding this has not led to an extrapolation procedure, capable of bearing scrutiny adequately. One way to circumvent what has become the bane of the study of ship resistance is to perform Reynolds averaged Navier-Stokes (RANS) simulations directly in full-scale. The rapid advent of such methods has meant that confidence levels in predictions achieved by RANS simulations are low. This paper explores and demonstrates scale effects on the constituent components of ship resistance by performing a geosim analysis using a Computational Fluid Dynamics approach. Emphasis is placed on challenging the assumptions imposed as part of the currently accepted ship resistance extrapolation procedure. Our results suggest that a high degree of uncertainty exists in the calculated full-scale resistance depending on the approach taken towards its evaluation. In particular, scale effects are demonstrated in wave resistance, while free surface effects are palpable in the frictional resistance

Numerical investigation of the behaviour and performance of ships advancing through restricted shallow waters

Upon entering shallow waters, ships experience a number of changes due to the hydrodynamic interaction between the hull and the seabed. Some of these changes are expressed in a pronounced increase in sinkage, trim and resistance. In this paper, a numerical study is performed on the Duisburg Test Case (DTC) container ship using Computational Fluid Dynamics (CFD), the Slender-Body theory and various empirical methods. A parametric comparison of the behaviour and performance estimation techniques in shallow waters for varying channel cross-sections and ship speeds is performed. The main objective of this research is to quantify the effect a step in the channel topography on ship sinkage, trim and resistance. Significant differences are shown in the computed parameters for the DTC advancing through dredged channels and conventional shallow water topographies. The different techniques employed show good agreement, especially in the low speed range

Experimental and numerical study of an obliquely towed ship model in confined waters

In this study, the forces and moments acting on the KCS ship model as a result of oblique towing at 10 and 20 degrees drift angles are evaluated experimentally and numerically via a commercial Reynolds averaged Navier-Stokes solver. For the purposes of this work, the KCS hull is modelled both experimentally and numerically at a scale factor of 1:75. The adopted case-studies feature both horizontal and vertical restrictions. Thus, the subject of this work is the oblique motion of a ship in a narrow canal with a depth of h/T=2.2. The relative impact of turbulence modelling is assessed by comparing the computed integral quantities via several eddy-viscosity closure strategies. These include significant variants of the k-ϵ and k-ω models as well as a widely used one-equation closure. Multiphase numerical simulations are performed at several of the experimentally investigated depth Froude numbers for each drift angle condition in order to fully capture the physics of the problem at hand. The present study aims to provide a quantitative evaluation of the performance of the adopted turbulence models and recommended the best closure strategy for the class of investigated problems

Prediction of the aerodynamic behaviour of a full-scale naval ship in head waves using Detached Eddy Simulation

The airwake behaviour around a ship provides useful information for the safe operation of helicopters on naval ships as well as in helicopter pilot training. This study investigates the impact of ship motions on the airwake behind the superstructure of a naval ship using Detached Eddy Simulation. A full-scale simplified frigate geometry is analysed stationary and in head waves at three different wavelengths under a uniform wind field and in the presence of an atmospheric boundary layer. The results reveal that an atmospheric boundary layer impacts significantly the airwake, as well as the vertical wave-induced motions of the ship, which reduce in amplitude by between 20.9% and 22.39% in heave, and up to approximately 38% in pitch. Moreover, the results show that the presence of an atmospheric boundary layer impacts the ship's heave and pitch motion periods. The flow field is also significantly altered depending on the ambient wavelength and period of motion, particularly in the case where an atmospheric boundary layer is modelled

Investigating the influence of sheared currents on ship hydrodynamics in confined water using Computational Fluid Dynamics

The field of ship hydrodynamics in confined water has received increased attention by the academic community in recent years. Nevertheless, a number of phenomena occurring in confined waters are yet to be examined using high fidelity Computational Fluid Dynamics (CFD) or experimentally. One particular case is the presence of sheared currents and their impact on the performance of a ship. Such currents can be generated in confined waters as a result of the natural flow of water in rivers or due to the action of tidal influences in long canals. Alternatively, due to the short fetch of many inland waterways, the action of wind may result in the production of a sheared current. This work aims to investigate these effects by making use of a commercially available Reynolds Averaged Navier-Stokes (RANS) solver. A number of current profiles are numerically modelled to determine their influence on ship performance and the manner in which ship waves interact with the background current. The present study will contribute to the understanding of restricted water effects by revealing the impact of shear currents on ship performance