Spray Cloud Formation Over Marine Vessels by a Water Breakup Model

Water breakup affects the variety of droplet sizes and velocities in a cloud of spray resulting from a sea wave striking a vessel bow. The Weber and Reynolds numbers of droplets are the main parameters for water breakup phenomena. “Stripping breakup” is a faster phenomenon than “bag breakup” and occurs at higher velocities and with larger diameters of droplets. A water breakup model employs droplet trajectories to develop a predictive model for the extent of spray cloud. The governing equations of breakup and trajectories of droplets are solved numerically. Stripping breakup is found as the major phenomenon in the process of the formation of wave-impact sea spray. Bag breakup acts as a complementary phenomenon to the stripping breakup. The extent of the spray as well as wet heights, for a Mediumsized Fishing Vessel (MFV), are obtained by numerical solutions. The results show that bag breakup occurs at higher heights. In addition, there is no breakup when droplets move over the deck.

Study of Self-Propelled Pufferfish Driven by Multiple Fins: A Comparison Between Rigid and Deformable Fins

In this work, we numerically studied the steady swimming of a pufferfish driven by the undulating motion of its dorsal, anal and caudal fins. The simulations are based on experimentally measured kinematics. To model the self-propelled fish swimming, a Computational Fluid Dynamics (CFD) tool was coupled with a Multi-Body-Dynamics (MBD) technique. It is widely accepted that deformable/flexible or undulating fins are better than rigid fins in terms of propulsion efficiency. To elucidate the underlying mechanism, we established an undulating fins model based on the kinematics of live fish, and conducted a simulation under the same operating conditions as rigid fins. The results presented here agree with this view by showing that the contribution of undulating fins to propulsion efficiency is significantly larger than that of rigid fins.

Impact Tests in an Air-Water Mixture

Experimental impact tests were performed using a shock machine and aerated water by means of an air-bubble generator. High speed shock test machine allows carrying out tests of impact on water (slamming). This machine permits to stabilise velocity with a maximal error equal to 10% during slamming tests. The air volume fraction in the bubble was measured by optical probe technique. The present work is aimed at quantifying the effects of the aeration on the hydrodynamic loads and pressures during the entry of a rigid body at constant speed in an air-water mixture. The impact tests were conducted with a rigid pyramid for an impact velocity equal to 15 m.s−1 and for two average void fractions, 0,46% and 0,84%. The reduction of the impact force and pressure due to aeration has been confirmed by these experiments.

Effective Power and Speed Loss of Underwater Vehicles in Close Proximity to Regular Waves

The present study is focused on performance issues of underwater vehicles near the free surface and gives insight into the analysis of a speed loss in regular deep water waves. Predictions of the speed loss are based on the evaluation of the total resistance and effective power in calm water and preselected regular wave fields w.r.t. the non-dimensional wave to body length ratio. It has been assumed that the water is sufficiently deep and that the vehicle is operating in a range of small to moderate Froude numbers by moving forward on a straight-line course with a defined encounter angle of incident regular waves. A modified version of the Doctors & Days [1] method as presented in Skejic and Jullumstrø [2] is used for the determination of the total resistance and consequently the effective power. In particular, the wave-making resistance is estimated by using different approaches covering simplified methods, i.e. Michell’s thin ship theory with the inclusion of viscosity effects Tuck [3] and Lazauskas [4] as well as boundary element methods, i.e. 3D Rankine source calculations according to Hess and Smith [5]. These methods are based on the linear potential fluid flow and are compared to fully viscous finite volume methods for selected geometries. The wave resistance models are verified and validated by published data of a prolate spheroid and one appropriate axisymmetric submarine model. Added resistance in regular deep water waves is obtained through evaluation of the surge mean second-order wave load. For this purpose, two different theoretical models based on potential flow theory are used: Loukakis and Sclavounos [6] and Salvesen et. al. [7]. The considered theories cover the whole range of important wavelengths for an underwater vehicle advancing in close proximity to the free surface. Comparisons between the outlined wave load theories and available theoretical and experimental data were carried out for a submerged submarine and a horizontal cylinder. Finally, the effective power and speed loss are discussed from a submarine operational point of view where the mentioned parameters directly influence mission requirements in a seaway. All presented results are carried out from the perspective of accuracy and efficiency within common engineering practice. By concluding current investigations in regular waves an outlook will be drawn to the application of advancing underwater vehicles in more realistic sea conditions.

Design Aspects of Breakwaters for Cold Climate Oil and Gas Terminals

This paper focuses on design aspects regarding breakwaters for cold climate terminals, in particular, the different types of berm breakwaters, their stability, armour mobility criteria and armour size. A methodology is analyzed in order to determine the mean weight of the heaviest armour class as a function of wave parameters such as the significant wave height and the mean wave period, both for non-reshaping and reshaping stable berm breakwaters. The influence of the wave period on the stone mass required will be of special importance in the discussion. This methodology will enable us to determine the required median armour weight for a specific wave parameter, being easily able to compare the feasibility of different concepts or availability of the required stone size at the location. As a case study, the breakwater selection for the Melkøya terminal in the Norwegian Barents Sea, is assessed through a comparison of the necessary armour unit masses for the different berm breakwaters. The armour mobility criteria currently established is reviewed and a recommendation for an updated criterion for the statically stable non-reshaped berm breakwater category is proposed.

Hybridization of Theory and Experiment in Optimizing Di-Hull Configuration With Respect to Wave Resistance

A fast method for optimizing the configuration of a di-hull system is to take advantage of the wave-cut signatures of each hull and evaluate the combined resistance of the hull system using analytical expressions that portray the interference effects of the hull-generated waves. This interference formula is available in Yeung et al. [1] and can be used in conjunction with the wave-cut signatures. The Longitudinal Wave-cut Method (LCM) is utilized to acquire the wave-making spectrum for each monohull. Then the di-hull interference wave resistance is deduced by substituting these experimentally-acquired information into analytical expressions for resistance computation. The pre-acquired wave-spectrum information can be stored and used for a combination of any component hulls, identical or not. This hybridization procedure of theory and experiments is tested and evaluated. Its merits and deficiencies are discussed.

Models and Methods for Efficiency Estimation of a Marine Electric Power Grid

Diesel electric propulsion has become industry standard for many marine applications. Typically, a significant part of the operations of vessels with diesel electric propulsion is done with low loads on the motors and generators. However, the efficiency of a drive train is typically only calculated for full load conditions. This underestimates the losses during low load conditions. This article presents modeling methods for the electric drive train, which can be used to estimate the efficiencies, also at low load. The models are established with limited parameter sets, as detailed information about of the components are seldom available. This article compares the estimated efficiency of the generator and motor with the given data from datasheets.

Impact of Climate Modes on Shoreline Evolution: Southwest Coast of India

Coastal geomorphology is a complex phenomenon which is governed by nearshore wave and tidal climate. Change in climate indices (like sea surface temperature, sea level, intensified cyclone activity, among others) and climate modes (like El Nino Southern Oscillation (ENSO), Southern Annular Mode (SAM), Indian Ocean Dipole (IOD)) affect the wave climate and modify many coastal processes thereby altering the geomorphology of shorelines. In countries like India where tropical and sub-tropical cyclones are common, the coastal geomorphology is under constant threat. Coasts are also vulnerable to anthropogenic factors like offshore structures, harbours, wave farms and other constructional activities along the shoreline. It is thus necessary to understand the evolution of coastlines under the changing climate scenario. The rapidly growing socio-economic development in south-west coast of India has generated the need to investigate the longshore sediment transport (LST) regime in this region under the influence of variable climate factors like the wave characteristics. The presence of numerous river deltas, estuaries and mud banks makes the situation worse especially during the south-west monsoon season (June-September). The investigation on the contemporary evolution of this coastline has not been undertaken and the knowledge of the climate factors that influence the shorelines of the southern tip of India are unknown. This study attempts to understand the temporal dynamics of the longshore sediment transport in this region.

Investigation of Free Surface Damping Models With Applications to Gap Resonance Problems

In this paper two methods for modelling the damping in a narrow gap are investigated. The first method is called the Pressure Damping Model. This method has been used in studies of wave energy devices. An attractive feature of this model is that the modified input is directly related to the energy dissipation in the gap, which means that if this dissipation is estimated the input to the model can be obtained directly. The idea of the method is to add a pressure input in the gap to suppress the resonant motion. A challenge with the method is that it contains a non-linear term. The second method is the Newtonian Cooling damping model. The method is based on introducing a dissipation term in the free surface boundary condition. This dissipation term contains a coefficient which is not directly related to the energy dissipation. Hence this method is not so easy to relate directly to the estimated energy dissipation. An advantage with this method is that it is linear and hence can be expected to be more robust. In the first part of the paper a 2-dimensional problem is investigated using both methods. In addition to the numerical performance and robustness, much focus is put on investigation of the energy balance in the solution, and we attempt to relate both models to the energy dissipation in the gap. In the second part the Newtonian cooling method is implemented in a 3-dimensional potential flow solver and it is shown that the method provides a robust way to handle the resonance problem. The method will give rise to a modified set of equations which are described. Two different problems are investigated with the 3D solver. First we look at a side-by-side problem, where the 3D results are also compared with 2D results. Finally, the moonpool problem is investigated by two different 3D solvers, a classical Green’s function based method and a Rankine solver. It is also shown how this damping model can be combined with a similar model on the internal waterplane to remove irregular frequencies.

Nonlinear and Dissipative Characteristics of a Combined Random-Cnoidal Wave Field

Images of the 2004 Indian Ocean tsunami at landfall shows a leading edge marked by short waves (“fission” waves). These waves appear to be cnoidal in shape and of a temporal and spatial scale in line with the longest swell present in the region, and may interact with the longer waves in the background random wave spectrum. As part of a comprehensive series of experiments, the Large Wave Flume at Oregon State University (USA) was used to generate and measure the properties of cnoidal, random, and combined cnoidal-random wave trains. Both the nonlinear energy transfer characteristics (via bispectral analysis) and dissipation characteristics (via a proxy dissipation function) are studied for all generated wave conditions. It is generally determined that the characteristics of the cnoidal wave dominate the combined cnoidal-random wave signals if the energy of the cnoidal wave is at least equal to that of the random wave.