A Review of the State-of-the Art in Marine Hydrodynamics

The purpose of the present paper is to summarise the present situation in the field of marine hydrodynamics. Since the William Froude time there has been considerable development in all fields of marine hydrodynamics, both experimental and particularly in theoretical methods and their numerical implementation. The role of computational methods is becoming more important because modern technology requires analysis of increasingly complex phenomena. The hydrodynamics institutes make efforts to expand their activities through integration of experimental and computational approach in order to be able to successfully answer the increased demands of the related industries

Hydrodynamic Optimization of a torpedo-shaped hull

Nowadays, it is not fully clear how the Ocean seabed can contribute to Earth ecosystems. However, several steps are being taken to completely understand Ocean’s seabed. Lately, many methods are being developed to explore the Oceans, although there is one method which fulfill the desired trade-off (between low operational costs and high quality data collection). This efficient method developed to explore the Ocean’s depth is known as submarine vehicles, and the most efficient of them, to explore and mapping, is certainly the Autonomous Underwater Vehicle (AUV). The increasing use of AUV’s is leading to a point in which its design parameters are crucial. Characteristics as high endurance, long operation time, high maneuverability and range are demanded at an early design stage; thus, it is essential to find an optimum hull shape design to improve these characteristics. This thesis presents the effect of hydrodynamic forces of axisymmetric underwater vehicles through the variation of the shape of a torpedo-shaped hull body. Furthermore, this thesis is intended to analyze, experimentally, the length-to-Diameter (D) ratios of nose (N) and tail (T), as well as its shapes, in order to find the optimum ratios and shape combinations for the minimization of Drag. The experimental tests were conducted in the towing tank of the University of Beira Interior (UBI). However, due to the Towing Tank dimensions, the development of a scaled model had to be made. A similarity between the scaled model and the full-scale prototype must be done to assume similar flow conditions. Several torpedo-shaped combinations were tested experimentally and further validated the numerical simulations. Moreover, parameters such as the pitch angles (or Angle of Attack (AoA)) [0 - 20°] and velocities [0.50 – 1 m/s] were investigated to understand their influence on the hydrodynamic Drag. The experimental setup is hereby fully described, showing the various procedures adopted until the data collection phase. A strain gauge system (load cell) was used to measure the Drag induced by the hull body. Experimental results demonstrate an optimum configuration for N/D = 0.8 (Elliptical shape) and T/D = 1.6 (Conical shape). From the experimental and numerical data, it could be seen that the Drag increases with the increase of velocity. Same occurrence happens for AoA, where Drag increases with higher AoA’s. Therefore, it can be concluded that the influence of AoA on Drag is higher for greater velocities. The experimental measurements have been used to validate results obtained from a Computational Fluid Dynamics (CFD) software that uses Reynolds Average Navier-Stokes (RANS) equations (ANSYSTM FLUENT). A mesh-independency study was made to investigate two turbulence models: Standard ?-e and ?-? SST models. Standard ?-e showed to be the most appropriate model to this study with a lower computational cost. Results between Experimental and Numerical methods showed a good agreement, considering the conditions mentioned.Hoje em dia, não é ainda completamente claro de que maneira o fundo dos oceanos podem contribuir para os Ecossistemas da Terra. Contudo, vários esforços estão a ser feito para compreender em profundidade os fundos marinhos dos Oceanos. Atualmente, o método mais eficiente, já desenvolvido, para explorar a profundeza dos oceanos é conhecido como veículos submarinos, e especificamente, o mais eficiente para pesquisa e exploração destes é conhecido como Veículo Autónomo Subaquático (AUV). O aumento do uso de AUV’s tem levado a um ponto em que os parâmetros de projeto são cruciais. Características como a resistência ao avanço, o alto tempo de operação, a grande manobrabilidade e o grande alcance são exigidos numa fase primária de projeto; desta forma, é fundamental encontrar uma forma ótima do corpo hidrodinâmico, ainda durante a fase de projeto, ambicionando melhorar as suas características. Esta dissertação apresenta o efeito das forças hidrodinâmicas de veículos subaquáticos axi- simétricos através da variação da forma de um corpo em forma de torpedo. Além disso, nesta dissertação pretende-se ainda analisar, experimentalmente, os rácios comprimento/diâmetro do nariz e da cauda do corpo, assim como as suas formas, para que seja possível os rácios e combinação ótimos do ponto de vista da minimização da resistência ao avanço. Os testes experimentais foram feitos num tanque de água da Universidade da Beira Interior (UBI). No entanto, devido às dimensões do tanque de água, o desenvolvimento de um modelo à escala foi a opção mais viável. Uma similaridade entre o modelo à escala e o protótipo foi feita para garantir as mesmas condições de escoamento entre ambos. Várias combinações foram testadas experimentalmente e seguidamente validadas por simulações numéricas. Adicionalmente, parâmetros como o ângulo de ataque (de 0 - 20°) e a velocidade (entre 0.50 – 1 m/s) foram alterados para perceber a sua influência na resistência hidrodinâmica. A preparação experimental é totalmente descrita, mostrando vários procedimentos adotados até à fase de recolha de dados. Um sistema de tensão/compressão (célula de carga) foi utilizado para medir a resistência induzido pelo corpo. Os resultados experimentais demonstraram uma configuração ótima que se situa nas proximidades de N/D = 0.8 (Forma Elítica) e T/D = 1.6 (Forma Cónica). Pode ser visto que a resistência aumenta com o aumento da velocidade. Da mesma forma para os ângulos de ataque, a resistência aumenta para ângulos de ataque maiores. Os dados experimentais foram usados para validar os resultados obtidos de um software CFD que usa as equações RANS. Um estudo de independência da malha foi feito para investigar dois modelos turbulentos: Modelos Standard ?-e e ?-? SST. O modelo turbulento Standard ?-e mostrou ser o mais apropriado para este estudo com um menor custo computacional. Os resultados entre os métodos experimentais e numéricos mostraram uma boa concordância, considerando as condições mencionadas

Ship-Hull Shape Optimization with a T-spline based BEM-Isogeometric Solver

In this work, we present a ship-hull optimization process combining a T-spline based parametric ship-hull model and an Isogeometric Analysis (IGA) hydrodynamic solver for the calculation of ship wave resistance. The surface representation of the ship-hull instances comprise one cubic T-spline with extraordinary points, ensuring C2C2 continuity everywhere except for the vicinity of extraordinary points where G1G1 continuity is achieved. The employed solver for ship wave resistance is based on the Neumann–Kelvin formulation of the problem, where the resulting Boundary Integral Equation is numerically solved using a higher order collocated Boundary Element Method which adopts the IGA concept and the T-spline representation for the ship-hull surface. The hydrodynamic solver along with the ship parametric model are subsequently integrated within an appropriate optimization environment for local and global ship-hull optimizations against the criterion of minimum resistance

The Design and Development of a Recirculating Water Channel: A Critical Assessment

The thesis critically assesses the design, development and equipment of a recirculating water channel: the channel in the Department of Engineering at the University of Liverpool It is intended to provide a valuable reference manual for such facilities. The channel is a versatile item of equipment wliich can be used in four different configurations:- as a free surface water channel, including the ability to simulate shallow water and also to generate regular head waves; as a water tunnel; as a free surface cavitation channel or as a cavitation tunnel, with a minimum pressure of 1/30 atmosphere. Uie working section is 3.66m long, 1.4m wide and 0.84m deep with a maximum water speed of 5.7 m/s. The thesis includes: 1. A description of the water channel in its four configurations and earlier development of a jet injection system to overcome a velocity defect at the water surface. The flow velocity in tlie working section and methods of flow measurement are discussed together with new measurements of turbulence velocity and the time to reach equilibrium speed. The wavemaker has been shown to produce waves which are good approximations to a sinusoidal shape at low water speeds and can be maintained over a long period of time. At higher speeds it was found that the wave shape progressively degenerated; methods for improving the wave-maker are suggested. 2. The force measuring equipment is reviewed including a dynamometer for ship models and a miniature version for much smaller forces obtained on sea shells at low water speeds. 3. The principles of model testing are discussed together with a technique for measuring the actual wetted surface area in the case of fast ships. A new experimental and theoretical investigation into the effect of model size relative to the size of the working section is reported. 4. A final chapter reviews the design of the flume and all its equipment. Suggestions are made for further improvements suitable for a future water channel

Ship-hull shape optimization with a T-spline based BEM-isogeometric solver

In this work, we present a ship-hull optimization process combining a T-spline based parametric ship-hull model and an Isogeometric Analysis (IGA) hydrodynamic solver for the calculation of ship wave resistance. The surface representation of the ship-hull instances comprise one cubic T-spline with extraordinary points, ensuring C2 continuity everywhere except for the vicinity of extraordinary points where G1 continuity is achieved. The employed solver for ship wave resistance is based on the Neumann-Kelvin formulation of the problem, where the resulting Boundary Integral Equation is numerically solved using a higher order collocated Boundary Element Method which adopts the IGA concept and the T-spline representation for the ship-hull surface. The hydrodynamic solver along with the ship parametric model are subsequently integrated within an appropriate optimization environment for local and global ship-hull optimizations against the criterion of minimum resistance

Performance quantification of tidal turbines subjected to dynamic loading

The behaviour of Tidal Stream Turbines (TST) in the dynamic flow field caused by waves and rotor misalignment to the incoming flow (yaw) is currently poorly understood. The dynamic loading applied to the turbine could drive the structural design of the power capture and support subsystems, device size and its proximity to the water surface and sea bed. In addition, the strongly bi directional nature of the flow encountered at many tidal energy sites may lead to devices omitting yaw drives; accepting the additional dynamic loading associated with rotor misalignment and reduced power production in return for a reduction in capital cost. For such a design strategy it is imperative to quantify potential unsteady rotor loads so that the TST device design accommodates the inflow conditions and avoids an unacceptable increase in maintenance action or, more seriously, suffers sudden structural failure. The experiments presented as part of this work were conducted using a 1:20th scale 3-bladed horizontal axis TST at a large towing tank facility. The turbine had the capability to measure rotor thrust and torque, blade root strain, azimuthal position and speed. The maximum outof- plane bending moment was found to be as much as 9.5 times the in-plane bending moment, within the range of experiments conducted. A maximum loading range of 175% of the median out-of-plane bending moment and 100% of the median in-plane bending moment was observed for a turbine test case with zero yaw, scaled wave height of 2m and intrinsic wave period of 12.8s. A Blade Element Momentum (BEM) numerical model has been developed and modified to account for wave motion and yawed flow effects. This model includes a new dynamic inflow correction which is shown to be in close agreement with the measured experimental loads. The gravitational component was significant to the experimental in-plane blade bending moment and was included in the BEM model. Steady yaw loading on an individual blade was found to be negligible in comparison to wave loading (for the range of experiments conducted), but becomes important for the turbine rotor as a whole, reducing power capture and rotor thrust

Advanced Techniques for Design and Manufacturing in Marine Engineering

Modern engineering design processes are driven by the extensive use of numerical simulations; naval architecture and ocean engineering are no exception. Computational power has been improved over the last few decades; therefore, the integration of different tools such as CAD, FEM, CFD, and CAM has enabled complex modeling and manufacturing problems to be solved in a more feasible way. Classical naval design methodology can take advantage of this integration, giving rise to more robust designs in terms of shape, structural and hydrodynamic performances, and the manufacturing process.This Special Issue invites researchers and engineers from both academia and the industry to publish the latest progress in design and manufacturing techniques in marine engineering and to debate the current issues and future perspectives in this research area. Suitable topics for this issue include, but are not limited to, the following:CAD-based approaches for designing the hull and appendages of sailing and engine-powered boats and comparisons with traditional techniques;Finite element method applications to predict the structural performance of the whole boat or of a portion of it, with particular attention to the modeling of the material used;Embedded measurement systems for structural health monitoring;Determination of hydrodynamic efficiency using experimental, numerical, or semi-empiric methods for displacement and planning hulls;Topology optimization techniques to overcome traditional scantling criteria based on international standards;Applications of additive manufacturing to derive innovative shapes for internal reinforcements or sandwich hull structures

IIHR Currents Winter 2017-18

https://ir.uiowa.edu/iihrcurrents/1003/thumbnail.jp