6,096 research outputs found
Simulation of a free falling wedge into water using 2D CFD with applications in the prediction of high speed craft motions
No full textThe simulation of free surface flows with Computational Fluid Dynamics
No full textComputational fluid dynamics is a powerful and versatile tool for the analysis of flow problems encountered in themaritime environment. The University of Southampton Fluid-Structure Interactions research group use ANSYS CFX tomodel a wide variety of flow problems; to gain insight into flow physics, improve designs and increase the efficiencyand safety of marine vehicles. A series of three case studies from on-going research looks at: loads applied on liquefiednatural gas tanks due to sloshing, slamming pressures experienced by high speed craft as well as the influence ofpropellers on the resistance characteristics of autonomous underwater vehicles. The presence of the free surface,complex shapes and the unsteady nature of these applications make their simulation with computational fluid dynamicsparticularly challenging. The successful validation of the computational models has resulted in the development of aselection process for suitable multiphase models as well as cost-effective meshing strategies
Improvement to body impact predictions using CFD through analysis of an unsteady boundary layer.
No full textThe problem of a body impacting water and subsequent settling requires an unsteady boundary layer to be modelled. In order to ensure that this boundary layer is being modelled correctly,
the flow along an impulsively started flat plate moving parallel to the flow is investigated. The boundary layer on an impulsively started flat plate is well understood and is also reasonably simple to model using CFD. A commercial Naver-Stokes equations solver is used to carry out the simulation (ANSYS, 2008). The results are used to develop a methodology for generating a mesh capable of boundary layer resolution in this type of flow. A bow section impacting with water is then modelled, using the techniques developed for accurate prediction of the boundary layer
Predicting motions of high-speed rigid inflatable boats: improved wedge impact prediction
No full textModelling the performance of high speed planing craft is
challenging. The inherent unsteady free surface flows are
complex to model numerically, resulting in computationally expensive predictions. Non-linear equations of motion are required to predict the movement of the vessel. Previous work used a strip theory that predicts the forces of wedge impact using a potential flow method. This work aims to improve this numerical model by increasing the accuracy of predicting the forces acting on a wedge during impact. A viscous two dimensional computational fluid dynamic analysis is used to compute the wedge impacts, and the results are compared with the potential flow method used in the original motions prediction solver. Results of the computational fluid dynamics predictions are also compared with experimental data. Overall, good agreement is found with the experimental results and the computed wedge impact prediction improves upon the 2D potential theory results
A computational and experimental search for polymorphs of parabanic acid – a salutary tale leading to the crystal structure of oxo-ureido-acetic acid methyl ester
No full textA computational search to predict the crystal structure of parabanic acid produced the known P21/c crystal structure as the global minimum in the lattice energy. However, there are many hypothetical structures only 2–6 kJ mol?1 less stable than the known form, which are within the energy range of possible polymorphism and have reasonable mechanical properties and relative growth rates. The harmonic intermolecular frequencies and the attachment energy estimate of relative growth rates suggest that the known polymorph is thermodynamically and kinetically favoured, but the possibility of other polymorphs cannot be excluded. A simultaneous experimental search for new polymorphs found crystals with a new morphology and X-ray powder pattern when a solution of parabanic acid in methanol was left to evaporate. Eventually, the structure was shown by single crystal X-ray diffraction to be that of oxo-ureido-acetic acid methyl ester. Thus, under the conditions of recrystallisation from methanol, parabanic acid had undergone a previously unreported ring-opening reaction, and had not crystallised as a new polymorph as had seemed likely prior to single crystal characterisation. The combination of the experimental and theoretical studies indicates that new polymorphs of parabanic acid are unlikely to be found readil
A comparison of experimental measurements of high-speed RIB motions with non-linear strip theory
No full textAccurate prediction of the motions of high-speed craft is an essential element in understanding the response of crew to a particular design configuration. The aim of this work is to evaluate the capability of a numerical method for use in the context of a procedure for designing high-speed craft. A numerical model is used to predict the motions of a planing craft in regular waves. The model is based on non-linear strip theory, through calculation of the forces occurring on wedge impact. This numerical model and its limitations are well understood for lower planing speeds (up to a length-based Froude number of around 1.2). This paper describes the limitations and accuracy for higher-speed craft (Froude number around 3). At present there is an inadequate knowledge of the model performance at these speeds. Validation of the model is achieved using experimental data obtained from testing two scale models: A wave piercing rigid inflatable boat (RIB) and an Atlantic 21 RIB. The experiments are conducted in a range of regular wave frequencies for three wave heights. Results are promising, with good correlation between the heave motion of the numerical model and the measured experimental data. Based on these results, a number of potential enhancements to the existing numerical model are discussed
Predicting the motions of high speed RIBs: a comparison of non-linear strip theory with experiments
No full textAccurate prediction of the motions of high speed craft is an essential element in understanding the response of crew to a particular design configuration. The aim of this work is to evaluate the capability of a numerical method for use in the context of a procedure for designing high speed craft. A numerical model is used to predict the motions of a planing craft in both regular and irregular waves. The model is based on non-linear strip theory, through calculation of the forces occurring on wedge impact. This numerical model and its limitations are well understood for lower planing speeds (up to a length based Froude number of around 1.2). This paper investigates the limitations and accuracy for higher speed craft (Froude number around 3). At present there is an inadequate knowledge of the model performance at these speeds. Lower speed validation is carried out using results from published experiments although this data does not extend to the higher speeds. Validation of the model at higher speeds is achieved using experimental data attained from testing two scale models: A wave piercing rigid inflatable boat (RIB) and an Atlantic 21 RIB. The experiments are conducted in a range of regular wave frequencies for three wave height together with a realistic JONSWAP sea spectrum. Results are promising, with good correlation between the heave motion of the numerical model and the measured experimental data. Based on these results, a number of potential enhancements to the existing numerical model are discussed
Design metrics for evaluating the propulsive efficiency of future ships
No full textThere is an increasing need for the ship design process to take account of environmental issues such as the emission of greenhouse gases and the likely extension of a carbon dioxide charging mechanism to international shipping. These issues, together with the need for economic viability, provide further incentives to improve the efficiency of propulsion of ships. The main components of powering are firstly reviewed. Individual components and other power saving devices are identified which should contribute to improvements in the overall efficiency of propulsion. Suitable design metrics and procedures, taking into account economic and environmental factors, are recommended for the design of future ships
Evaluating the self-propulsion of a container ship in a seastate using computational fluid dynamics
No full textAn important area of ship design that requires the development of unsteady computational fluid dynamics is the ability to evaluate accurately the unsteady propulsive efficiency of a ship in waves. A reliable capability to do this would allow design selection of hull forms that have maximum propulsive efficiency across their required operating range of seastates. In this paper we consider the necessary steps in validating the assessment of wave and viscous hull resistance, the computational efficiency of representing the propulsion effects of a propeller and finally the influence of an incident wave on the overall propulsive forces. The Korean Container Ship, KCS, is chosen due to the availability of good quality experimental data and the relative magnitudes of the resistance components. Two different flow solvers are applied and a variety of meshing strategies. Overall, good predictions of the self-propelled ship condition are possible if an appropriate, flow feature adapted, mesh of sufficiently high density and quality is use
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