On the structural test of 1.5-ton test WIG craft

This paper deals with the static structural test of 1.5-ton test WIG (Wing-In-Ground effect) craft designed and built to verify its aero/hydro-dynamic and structural design characteristics prior to the design of full scale 20-passenger WIG craft. The test WIG craft is scaled down from its full scale WIG craft by 1/2 and built using pre-preg carbon/epoxy composites for the majority of structural parts with metallic materials for fittings used at wings to hull connections. To initiate the test, the finite element (FE) models of the test WIG craft are developed and, based on their results, the details of applied load range, strain/deflection gauge types and locations, load application methods and data acquisition system specifications are determined. The test is performed with respect to main wings to hull connection part first and then vertical tail wing to hull connection part and, finally, horizontal tail wing to vertical tail wing/hull connection part. Both the test and FE model results are compared with respect to stresses and deflections and the comparison shows good correlation between them which implies satisfactory design and building of 1.5-ton test WIG craft.     

Numerical analysis of the influence of fixed hydrofoil installation position on seakeeping of the planing craft

This paper analyzes the hydrodynamic performance of a planing craft with a fixed hydrofoil in regular waves. Numerical simulations are carried out based on a RANS-VOF solver to study the hydrodynamic performance of the planing craft and the influence of the fixed hydrofoil on its seakeeping. To validate the numerical method, a series of hydrodynamic experiments of a bare planing craft without the hydrofoil were carried out, from which the seakeeping performance of the planing craft was recorded, the numerical method based on overset grid was compared with the experiment and verified reliable. Eight hydrofoil design cases were then studied, whereby, their seakeeping performance in regular wave conditions were predicted through the numerical method which has been verified reliable and compared with each other. Effects of hydrofoil parameters, such as angle of attack and installation height, on the seakeeping performance were investigated. Finally, the suitable installation parameters which can optimize the performance of hydrofoil and reduce the negative influence are verified. The influence of the speed on the effect of the hydrofoil and the flow field around the planing craft are also investigated.     

The response of a hydrofoil to wave orbital velocity fields

For study purposes, a simplified model of a hydrofoil craft is constructed with the assumption that it has:

• heave only motion;

• no surface proximity effect on the foil;

• no foil broaching.

It is then shown that a fully submerged hydrofoil, mounted at the bottom of rigid struts, can transmit large vertical force fluctuations to the hull, even in an idealized sinusoidal seaway because of the orbital velocity field in the water. But if the foil support struts are hinged, inclined aft and resiliently supported, so that the hydrofoil can swing about the strut's pivot in response to the changes in local water velocity, then the vertical accelerations transmitted to the hull are reduced. The more the strut is inclined to the vertical, the smaller are the accelerations transmitted to the hull. A hinged strut whose equilibrium angle (for 1 g) is 60° to the vertical can reduce the vertical accelerations by an order of magnitude. It also has two other practical advantages. The strut(s) and foil will ride up towards horizontal during the rare but inevitable impacts with large marine objects (such as whales or flotsam) and during groundings. And when they encounter a region of water moving rapidly downward (which can cause a conventional fully submerged hydrofoil to experience a violent hull impact on the water) they move in such a way as to maintain a roughly constant lift force on the hull, so that there is negligible hull motion in heave.     

Towing dynamics of a liferaft and fast rescue craft in a surface wave

The equations of motion for the coupled dynamics of a small liferaft and fast rescue craft in a surface wave are formulated in two dimensions using the methods of Kane and Levinson [1985. Dynamics: Theory and Applications. McGraw-Hill Inc., New York]. It is assumed that the motion normal to the wave surface is small and can be neglected, i.e. the bodies move along the propagating wave profile. The bodies are small so that wave diffraction and reflection are negligible. A Stokes second order wave is used and the wave forces are applied using Morison's equation for a body in accelerated flow. Wind loads are similarly modelled using drag coefficients. The equations are solved numerically using the Runge–Kutta routine “ode45” of MATLAB^®. The numerical model provides guidelines for predicting the tow loads and motions of small craft in severe sea states.     

An experimental method for determining the frequency-dependent added mass and added mass moment of inertia for a floating body in heave and pitch motions

Most of the existing relevant materials have been obtained from experiments, in which evaluating the added mass at the resonant frequency corresponding to the peak of a frequency-response curve obtained from the “forced” vibration analysis is the most popular technique. In this paper, a simple experimental method was presented where the “free” vibration responses instead of the “forced” ones were used to determine the values of m_ah and I_ap. The main part of the experimental system is composed of a floating body (model) and a spring–shaft shaker. The “free” vibration of this main part was induced by imposing on it an initial displacement (and/or an initial velocity), and from the time histories of displacements information such as the “damped” natural frequencies, damping ratios, sectional added mass coefficients (C_V and C_P) were obtained. Since the displacements of the spring–shaft shaker are “translational” and those of the floating body due to pitch motions are “angular”, a technique for the transformation between the associated parameters of the two components of the main part was presented.     

Hydrodynamic performance improvement on small horizontal axis current turbine blade using different tube slots configurations

Vortex generators are used extensively as a passive flow control devices to delay or remove the boundary layer separation, which affects the hydrodynamic performance of the hydrofoil. In this paper, a new approach is introduced to overcome the boundary layer separation on the NACA S1210 hydrofoil. The outcome of tube slots combination in the S1210 hydrofoil on the boundary layer separation are numerically investigated. The performance is compared with respect to the force coefficients and glide ratio. The effects of tube slot inlet positions with different diameters on S1210 hydrofoil are presented here. The results show that the smaller diameter tube slots starting near the leading edge improves the hydrodynamics performance of the hydrofoil.     

Towards the modeling of the ditching of a ground-effect wing ship within the framework of the SPH method

Ditching often takes place for a ground-effect wing (WIG) ship. During the ditching, the extreme load developed by water impacts may cause damages to structures, posing a great threat to the safety of crew and passengers. In the paper, a weakly compressible smoothed particle hydrodynamics (SPH) model combined with enhanced numerical techniques has been adopted to tackle the ditching problems. In order to handle the motion of a rigid body in the three-dimensional ditching problems, the six degrees-of-freedom (6-DOF) equations of motion are incorporated into the SPH scheme. The accuracy of the SPH model is validated through two benchmarks, respectively, the two-dimensional wedge water entry and the three-dimensional stone-skipping. The former is aimed to validate the prediction of pressures during the free surface impact while the latter is a good case for testing the coupling motions of the rigid body. Furthermore, the ditching of the real scale WIG ship under different conditions is simulated with the established SPH model, through which some useful conclusions are drawn.     

Static and dynamic aspects of a capsize phenomenon

Capsizing of ships constitutes a primary group of casualties that leads to loss of life and money. Unfortunately, its mechanism has yet to be fully resolved due to underlying complex dynamics and parameters. Upon studying the causes in more detail, designing safer ships against capsizing may become a reality. In the present study, a relatively different approach called “reserve of stability” or “stability margins” which utilizes both statical and nonlinear dynamical aspects of stability is employed to analyze ship hydrodynamics. For this purpose a nonlinear roll model in beam waves has been implemented. In order to apply the theory, a capsized vessel is chosen to be analyzed in terms of stability. The necessary data about the vessel at the time of capsizing were collected from the published work found in the naval architecture literature. Suggestions are made based on the results of the analysis to improve ship stability qualities in a seaway.     

Longitudinal stability and dynamic motions of a small passenger WIG craft

The longitudinal stability characteristics of a Wing-In-Ground (WIG) effect craft are quite different from those of the conventional airplane due to the existence of force and moment derivatives with regard to height. These stability characteristics play an important role in designing a safe and efficient WIG due to its potential danger in sea surface proximity. The static and dynamic stability criteria are derived from the motion equations of WIG in the framework of small disturbance theory and discussed in this paper. The static and dynamic stability analyses of a 20-passenger WIG are conducted based on wind tunnel test data, and dynamic motion behaviors are investigated for changes in design parameters. Finally, the flying quality of the 20-passenger WIG is analyzed at cruising conditions according to the military regulations.     

On the design of a longitudinal motion control system of a fully-submerged hydrofoil craft based on the optimal preview servo system

The current control system of a fully-submerged hydrofoil craft based on optimal feedback control has several problems: it has no good performance of contouring waves and it cannot reduce the effect of wave disturbance. In this paper, we apply the optimal preview servo system to the longitudinal control system of a fully-submerged hydrofoil craft in order to settle the problems of current control system. The control system based on optimal preview servo system is composed of feedback controller and feedforward controller. Also in order to design the control system, the necessary prediction range of future information, the weight function in performance index, the future reference input and wave disturbance are determined. Finally, the validity of the proposed control system in this paper is confirmed by simulation. The simulation results show that the control system has good performance of contouring waves in following seas and is effective to settle the problems of current control system of a fully-submerged hydrofoil craft.     

The BCI criterion for the initiation of breaking process in Boussinesq-type equations wave models

The Breaking Celerity Index (BCI) is proposed as a new wave breaking criterion for Boussinesq-type equations wave propagation models (BTE).The BCI effectiveness in determining the breaking initiation location has been verified against data from different experimental investigations conducted with incident regular and irregular waves propagating along uniform slope [Utku, M. (1999). “The Relative Trough Froude Number. A New Criteria for Wave Breaking”. Ph.D. Dissertation, Dept. of Civil and Enviromental Engineering, Old Dominion University, Norfolk, VA; Gonsalves Veloso dos Reis, M.T.L. (1992). “Characteristics of waves in the surf zone”. MS Thesis, Department of Civil Engineering, University of Liverpool., Liverpool; Lara, J.L., Losada, I.J., and Liu, P.L.-F. (2006). “Breaking waves over a mild gravel slope: experimental and numerical analysis”. Journal of Geophysical Research, VOL 111, C11019] and barred beaches [Tomasicchio, G.R., and Sancho, F. (2002). “On wave induced undertow at a barred beach”. Proceedings of 28th International Conference on Coastal Engineering, ASCE, New York, 557–569]. The considered experiments were carried out in small-scale and large-scale facilities. In addition, one set of data has been obtained by the use of the COBRAS model based upon the Reynolds Averaged Navier Stokes (RANS) equations [Liu, P.L.-F., Lin, P., Hsu, T., Chang, K., Losada, I.J., Vidal, C., and Sakakiyama, T. (2000). “A Reynolds averaged Navier–Stokes equation model for nonlinear water wave and structure interactions”. Proceedings of Coastal Structures ‘99, Balkema, Rotterdam, 169–174; Losada, I.J., Lara, J.L., and Liu, P.L.-F. (2005). “Numerical simulation based on a RANS model of wave groups on an impermeable slope”. Proceedings of Fifth International Symposium WAVES 2005, Madrid].Numerical simulations have been performed with the 1D-FUNWAVE model [Kirby, J.T., Wei, G., Chen, Q., Kennedy, A.B., and Dalrymple, R.A. (1998). “FUNWAVE 1.0 Fully Nonlinear Boussinesq Wave Model Documentation and User's Manual”. Research Report No CACR-98-06, Center for Applied Coastal Research, University of Delaware, Newark]. With regard to the adopted experimental conditions, the breaking location has been calculated for different trigger mechanisms [Zelt, J.A. (1991). “The run-up of nonbreaking and breaking solitary waves”. Coastal Engineering, 15, 205–246; Kennedy, A.B., Chen, Q., Kirby, J.T., and Dalrymple, R.A. (2000). “Boussinesq modeling of wave transformation, breaking and run-up. I: 1D”. Journal of Waterway, Port, Coastal and Ocean Engineering, 126, 39–47; Utku, M., and Basco, D.R. (2002). “A new criteria for wave breaking based on the Relative Trough Froude Number”. Proceedings of 28th International Conference on Coastal Engineering, ASCE, New York, 258–268] including the proposed BCI.The calculations have shown that BCI gives a better agreement with the physical data with respect to the other trigger criteria, both for spilling and plunging breaking events, with a not negligible reduction of the calculation time.     

Optimal design of two-dimensional wings in ground effect using multi-objective genetic algorithm

The shape optimization of the 2-dimensional wing in ground effect (WIG) has been performed by the integration of CFD (computational fluid dynamics) and MOGA (multi-objective genetic algorithm). Because of the trade-off between the aerodynamic forces and the height stability, it is difficult to satisfy the design requirements of efficiency and stability at the same time. In this study, the lift coefficient, the lift-drag ratio and the static height stability are chosen as the objective functions to obtain the optimal wing profiles of a WIG craft. An NACA0015 airfoil is used for the baseline model; the aerodynamic characteristics of the base model are compared with that of the optimal solutions. The profile of the airfoil is constructed by four Bezier curves with fourteen control points resulting in the eighteen coordinates, which are adopted as the design variables. The optimal solutions of the multi-objective optimization are not unique but a set of the non-dominated optima: the Pareto frontiers or a Pareto set. As the results of the multi-objective optimization, the forty Pareto optima, which include high-lift, high-efficiency, and more stable airfoils on the edge of the 3-dimensional objective space, are obtained at thirty evolutions of the generation.     

The discrete methods for free vibration analyses of an immersed beam carrying an eccentric tip mass with rotary inertia

In this paper, a beam without contact with water is called the “dry” beam and the one in contact with water is called the “wet” beam. For a partially (or completely) immersed uniform beam carrying an eccentric tip mass possessing rotary inertia, the conventional analytical (closed-form) solution is achieved by considering the inertial forces and moments of the tip mass and rotary inertia as the boundary conditions at the tip end of the beam. However, it has been found that the approximate solution for the last problem may be achieved by two techniques: Method 1 and Method 2. In Method 1, the basic concept is the same as the conventional analytical method; but in Method 2, the tip end of the beam is considered as a free end, while the inertial forces and moments induced by the tip mass and rotary inertia are considered as the external loads applied at the tip end of the beam. The main differences between the formulation of Method 1 and that of Method 2 are: In Method 1, the “normal” shapes of the “dry” beam are functions of the frequency-dependent boundary conditions but the external loads at the tip end are equal to zero; On the contrary, in Method 2, the “normal” mode shapes of the “dry” beam are determined based on the zero boundary conditions at the tip end of the beam but the external loads at the tip end due to the inertial effects of the tip mass and rotary inertia must be taken into consideration for the free vibration analysis of the “wet” beam. Numerical results reveal that the approximate solution obtained from Method 2 are very close to that from Method 1 if the tip mass moment of inertia is negligible. Besides, the two approximate solutions are also very close to the associated analytical (closed-form) solution or the finite element solution. In general, it is hoped that there exist several methods for tackling the same problem so that one may have more choices to incorporate with the specified cases. It is believed that the two approximate methods presented in this paper will be significant from this point of view.     

Total (practical) stability of ships

In this paper the “total (practical) stability” concept is introduced to nonlinear forced rolling motion of a ship. This is achieved by employing “boundedness” and “Lyapunov's function” approach. In this respect two new theorems are proved and conditions and domain of Practical Stability are evaluated. The Paper also contains a critical review of the present status of international intact ship stability regulations. A qualitative discussion of oscillatory rolling motion and the capsizing phenomena is presented.     

Assessment of water exchange between a discharge region and the open sea – A comparison of different methodological concepts

Two different methods of estimating the water exchange through the Baltic coastal region of Laxemar have been used, consisting of particle trajectories and passive tracers. Water is traced from and to a small discharge region near the coast. The discharge material in this region is treated as zero-dimensional particles or tracers with neutral buoyancy. The real discharge material could be a leakage of radio-nuclides through the sea floor from an underground repository of nuclear waste.Water exchange rates between the discharge region and the model domain are estimated using both forward and backward trajectories as well as passive tracers. The Lagrangian trajectories can account for the time evolution of the water exchange while the tracers give one average age per model grid box. Water exchange times such as residence time, age and transient times have been calculated with trajectories but only the average age (AvA) for tracers. The trajectory calculations provide a more detailed time evolution than the tracers.On the other hand the tracers are integrated “on-line” simultaneously in the sea circulation model with the same time step while the Lagrangian trajectories are integrated “off-line” from the stored model velocities with its inherent temporal resolution, presently 1 h. The sub-grid turbulence is parameterised as the Laplacian diffusion for the passive tracers and with an extra stochastic velocity for trajectories. The importance of the parameterised sub-grid turbulence for the trajectories is estimated to give an extra diffusion of the same order as the Laplacian diffusion by comparing the Lagrangian dispersions with and without parameterisation. The results of the different methods are similar but depend on the chosen diffusivity coefficient with a slightly higher correlation between trajectories and tracers when integrated with a lower diffusivity coefficient.     

Transient effects in wave-current boundary layer flow

Previous studies of combined wave and current bottom boundary layer flow have concentrated on the final converged state of the flow following the addition of waves to a current. While this final state is of primary interest to modellers and engineers, it pre-supposes that such a state is actually attained in reality, and this may not always be the case. In addition, it overlooks the interesting and complicated transient effects which occur as a wave-current flow evolves from one state to another. The present study concentrates attention on the transient effects predicted by a “one-equation” turbulence closure model. Results of case studies are presented in which waves are superimposed co-linearly on a current (“forward problem”), and are then removed from the converged wave-current flow (“backward problem”). Two formulations of the “forward” and “backward” problems are discussed. In the first the steady component of the pressure gradient driving the mean flow is held constant throughout, and in the second the steady component of the mass flux is held constant. In each case the detailed evolution of the profiles of mean velocity, turbulent energy, mixing length, eddy viscosity and shear stress are discussed. More generally, the question of the convergence timescale of a combined wave-current flow is considered, and a convergence criterion is proposed.     

On extracting current profiles from vertically integrated numerical models

A transformation method is presented by which current profiles (of tidal or wind-induced origin) can be extracted at any horizontal position and moment in time from a vertically integrated, two-dimensional, hydrodynamic numerical model. An arbitrary vertical variation of eddy viscosity can be included in the method, which can incorporate a no-slip bottom boundary condition. The technique assumes that the sea is homogeneous.The method is used to improve the representation of bottom stress within the two-dimensional model, whereby the bottom stress is no longer related simply to the depth-mean current as in the “conventional” two-dimensional, vertically integrated model.Idealized calculations for a range of eddy viscosity profiles, show that elevations, current profiles, and time series of current extracted from this “enhanced” two-dimensional numerical model are in good agreement with currents obtained from a full three-dimensional model.     

Alternative placement technique for antifer blocks used on breakwaters

In this study, a placement technique named as “alternative placement technique” was developed for antifer blocks and the results of its application to a breakwater model were presented. This placement technique was compared with the existing techniques such as the “regular placement technique”, the “irregular placement technique” and the “sloped wall placement technique” by experiments. The comparison was carried out considering armor layer stability, prototype placement, clarity of the placement technique’s definition, armor layer cost, and wave runup. As a result of this investigation the “alternative placement technique” was found to be superior to the other existing placement techniques.     

Inventory of nonvolatile fatty acids and hydrocarbons in the oceans

A survey of the distribution of nonvolatile fatty acids and hydrocarbons in the oceans is given. The results represent more a “feel” from the literature rather than a mathematical analysis and are given in Table III. Fatty acid concentration appears to show a greater variation, presumably because they are more prone to biological influences than the hydrocarbons.     

A new view of near-shore dynamics based on observations from HF Radar

Since 1984 the OSCR HF Radar system has been used in over 50 deployments to measure near-shore surface currents for both scientific and engineering applications. The enhanced scope, resolution and accuracy of these measurements have yielded new insights into the tidal, wind and density driven dynamics of the near-shore zone.Tidal current ellipses obtained from these radar measurements have been shown to be in good aggrement with values calculated by numerical models both for the predominant constituents and also for higher harmonics. Coherent patterns of wind-forced currents ahve been determined with strong evidence of a “slab-like” surface response. In one deployment, with offshore winds blowing over relatively deep water, this “slab” rotated clockwise at near-inertial frequency. Strong (up to 20cm s⁻¹), persistent surface residual currents are commonly observed, these are almost certainly generated by (small) horizontal density gradients. These observed surface residuals provide ideal data for rigorous testing of 3-D numerical models.With a threatened rise in sea level, HF Radar is well-suited for observing the expected changes in the dynamics of near-shore regions. Continuing development of these radar systems offers exciting prospects of remote sensing of both surface waves and currents. Future applications may extend beyond the near-shore region to measurements along the shelf-edge, in oceanic gyres and for “beach-processes”.