D.A. Hudson

Climate change scenarios from a regional climate model: Estimating change in runoff in southern Africa

This paper describes an analysis of different ways of constructing climate change scenarios using output from three climate models. It focuses on using the HadRM3H regional climate model applied across southern Africa and a macroscale runoff model operating at a scale of 0.5 × 0.5° to simulate river runoff. HadRM3H has a spatial resolution of 0.44 × 0.44° and is driven by boundary conditions from HadAM3H, a global atmosphere general circulation model with a spatial resolution of 1.875 × 1.25°. This, in turn, used sea-surface boundary conditions from HadCM3, a coupled global ocean-atmosphere general circulation model that operates at a spatial resolution of 3.75 × 2.5°. Sixteen climate scenarios were constructed from the three models, representing different combinations of model scale, whether the climate model simulations were used directly or changes were applied to an observed baseline, and whether observed or simulated variations from year-to-year were used. The different ways of deriving climate scenarios from a single initial climate model experiment result in a range in change in average annual runoff at a location of at least 10%, and often more than 20%. There is a clear difference in the large-scale spatial pattern of change in runoff from HadCM3 to HadRM3H. Many of the climate features in HadRM3H are already present in HadAM3H simulations, as would be expected from the experimental design. This suggests that for studies over a large geographic domain, an intermediate-resolution global climate model can produce useful scenarios for impact assessments. HadRM3H overestimates rainfall across much of southern Africa and so results in too much runoff: This leads to smaller estimates of future change in runoff than arise when changes in climate are applied to an observed climate baseline. It is concluded that under these circumstances it is preferable to apply modeled changes in climate to observed data to construct climate scenarios rather than derive these directly from the regional climate model simulations. Incorporating increases in interannual variability as simulated by HadRM3H leads to little change in simulated annual mean runoff. However, it has a larger impact on the frequency distributions of runoff, with extreme flows predicted to increase more than mean flows and even to increase in areas where the mean flow decreases. This demonstrates the importance of considering not only changes in mean climate but also climate variability.

Impact of a free-falling wedge with water: synchronized visualization, pressure and acceleration measurements

A fixed 25deg deadrise angle wedge is allowed to fall from a range of heights into static water. A high-speed (up to 5000 frames s?1) camera is used to visualize the impact and subsequent formation of jet flows and droplets. Unsteady pressure measurements at six locations across the wedge surface are measured at 10 kHz. Two accelerometers (10g, 100g) are mounted above the apex of the wedge and measure the vertical acceleration. A purpose-built position gauge and analysis of the synchronized video allows the wedge motion to be captured. The synchronization of these data with the digital images of the impact makes it particularly suitable for the validation of computational fluid dynamics simulations as well as theoretical studies. A detailed experimental uncertainty analysis is presented. The repeatability of the test process is demonstrated and the measured pressures are comparable to previous studies. A 2.5 ms time delay is identified between the point of impact observed from the video and the onset of actual wedge deceleration. The clear definition of the free surface provides insight into jet formation, its evolution and eventual breakdown, further assisting with the development of numerical predictions

Comparison of Two Kite Force Models with Experiment

Kite propulsion has emerged as an attractive means to harness wind power in a way that yields environmental and finanical benefits. This paper compares results from two line tension models with experimentall recorded time histories for dynamic kite flight. New methodologies for investigating kite performance are established. The first zero mass model assumes that the kite and lines are weightless. The second lumped mass model considers the kites mass and thus makes use of the equations of motion. It is found that the two different models converge to the same result in the limit where the kite mass tends to zero. Kite mass is shown to affect performance only to a relatively small extent. The zero mass model has shown to compare favorably with experimental results for the prediction of performance during three-dimensional kite trajectories

A validation study on mathematical models of speed and frequency dependence in seakeeping

Abstract An extensive theoretical validation exercise is presented into the speed and frequency dependent solutions associated with surface piercing vessels travelling in waves. The basis of the study lies in the formulation of the Greens function satisfying the traditionally posed linearized boundary value problem of an oscillating ship with forward speed and the development/implementation of appropriate numerical schemes of study for solution. Two widely different mathematical models in formulation and numerical algorithm, denoted as methods A and B, are discussed and employed to predict hydrodynamic coefficients, wave loads and responses of a Series 60 form and an NPL monohull. Only a selection of results are shown, but great care was taken in the overall investigation to verify and validate intermediate steps within the separate calculation procedures as well as to compare final predicted values. The extensive qualitative and quantitative agreement of comparable results from methods A and B provides a measure of confidence that the presented findings are solutions to the posed seakeeping problem. The influence of speed and frequency dependence within the context of this study are discussed as well as a preliminary study into their influence in the occurrence of irregular frequencies in the numerical schemes.

Kayak blade–hull interactions: A body force approach for self-propelled simulations

A sprint kayak experiences an unsteady flow regime due to the local influence of the paddle. However, kayak designs are usually optimised for steady-state, naked hull resistance. To determine whether unsteady paddle effects need to be included in kayak design, the hydrodynamic interactions between a kayak paddle and a hull are assessed using computational fluid dynamics. A body force model of a drag-based paddle stroke is developed using a blade element approach and validated against experimental data. This allows the paddle-induced local velocities to be simulated without the need to fully resolve the detailed flow around a moving paddle geometry. The increase in computational cost, compared to the naked hull simulation, is 8%. A case study investigating the impact of different paddle techniques on the hydrodynamic forces acting on a self-propelled kayak is conducted. A 0.23% difference in self-propelled resistance was observed, while an estimated 0.5% additional increase can be attributed to paddle-induced draught increases. An estimate of small changes in resistance on race times indicates that reductions of even a fraction of a percent are worth pursuing, indicating that the developed methodology may provide a useful design tool in the future.

Repeatable techniques for assessing changes in passive swimming resistance

Two different methods of measuring the passive resistance of swimmers are used to compare system accuracy and repeatability. Method I uses a submerged glide tow, and Method II, a novel, simpler approach, is based on measuring deceleration during a submerged push-off glide. The comparison of each method is made for specific changes in passive resistance. A set of three male and three female swimmers compare the use of drag shorts to make swimmer-specific increases in drag. In a second study, the effect of hair removal is quantified on a single male swimmer (Method I 9.7% reduction and Method II 9.4% reduction). For five repeat tests, a 1.8% difference in resistance can be resolved with 95% and 70% confidence levels for the passive tow and push-off glide experiments, respectively.

An investigation into the hydrodynamic analysis of vessels with a zero or forward speed

The occurrence of irregular frequencies is known to affect the prediction of hydrodynamic coefficients, whether a rigid body or hydroelasticity analysis is used. For conventional ship-like structures, these irregular frequencies lie outside the range of practical interest for rigid body motions. On the other hand, for a large ship-like offshore structure, such as a Floating Production, Storage and Offloading unit (FPSO), the irregular frequencies are in the range of practical interest. Furthermore, they can create difficulties for the analysis of vessels treated as flexible bodies and of multi-hulled vessels. In these cases, distinguishing between genuine physical effects (e.g. wave interaction between hulls) and that of irregular frequencies is important. The aim of this paper is twofold: first, elimination of irregular frequencies for the zero-forward-speed case and, second, improvement in predictions for the forward-speed case through a more accurate evaluation of the waterline integral term. Although the investigations are illustrated for rigid hulls, namely a rectangular box and a Series 60 hull, the methodology and its effects are equally valid for flexible hulls. For the zero-forward-speed case an extended boundary integral method (the lid method) is used, with the imaginary interior free surface placed approximately 0.1% below the mean free surface. It is confirmed that the lid boundary approach is an efficient and robust method for treating irregular frequencies for stationary rigid floating bodies experiencing low-frequency oscillations. For the forward-speed case a treatment of waterline integral terms is introduced, with reference to sources lying on the free surface. The applications highlight the importance of waterline integral terms which are shown to introduce oscillatory behaviour in the predicted hydrodynamic data. It is also shown that, for the forward-speed case, irregular frequencies do not occur; however, as the forward speed tends to smaller values, the hydrodynamic data exhibit oscillations which depend on the treatment of the waterline integral terms and the use of the direct potential or source distribution method. These methods are also suitable for hydroelasticity analysis with a forward speed.

A non-linear numerical method for the simulation of parametric roll resonance in regular waves

In this paper, a non-linear time-domain method is used for the prediction of parametric roll resonance in regular waves, assuming the ship to be a system with three degrees of freedom in heave, pitch and roll. Coupled heave and pitch motions are obtained using a three-dimensional frequency-domain potential flow method which also provides the requisite hydrodynamic data of the ship in roll i.e. the potential flow based added inertia and damping. Periodic changes in the underwater hull geometry due to heave, pitch and the wave profile are calculated as a function of the instantaneous breadth. This is carried out using a two-dimensional approach i.e. for sections along the ship and at each time step. This formulation leads to a mathematical model that represents the roll equation of motion with third order non-linearities in the parametric excitation terms. Non-linearities in the roll damping and restoring terms are also accounted for. This method has been applied to two different hull forms, a post-Panamax C11 class containership and a transom stern Trawler, both travelling in regular waves. Special attention is focused on the influence of different operational aspects on parametric roll. Obtained results demonstrate that this numerical method succeeds in producing results similar to those available in the literature, both numerical and experimental.Copyright

Very large floating structures

Very Large Floating Structure (VLFS) is a unique concept of ocean structures primary because of their unprecedented length, displacement cost and associated hydroelastic response. International Ship and Offshore Structures Congress (ISSC) had paid attention to the emerging novel technology and launched Special Task Committee to investigate the state of the art in the technology. This paper summarizes the activities of the committee. A brief overview of VLFS is given first for readers new to the subject. History, application and uniqueness with regard to engineering implication are presented. The Mobile Offshore Base (MOB) and Mega-Float, which are typical VLFS projects that have been investigated in detail and are aimed to be realized in the near future, are introduced. Uniqueness of VLFS, such as differences in behavior of VLFS from conventional ships and offshore structures, are described. The engineering challenges associated with behavior, design procedure, environment, and the structural analysis of VLFS are introduced. A comparative study of hydroelastic analysis tools that were independently developed for MOB and Mega-Float is made in terms of accuracy of global behavior. The effect of structural modeling on the accuracy of stress analysis is also discussed. VLFS entails innovative design methods and procedure. Development of design criteria and design procedures are described and application of reliability-based approaches are documented and discussed.

An improved matching method to solve the diffraction—radiation problem of a ship moving in waves

On the underlying assumption that the disturbance of the far-field velocity potential caused by the motion of a ship is small and may be linearized, an improved matching method is developed. Two arrays of fundamental singularities are placed inside the ship hull, which satisfy the linear free surface condition outside the truncated fluid domain and the far-field radiation condition. The choice of fundamental singularity depends on the problem under investigation (e.g. pulsating, translating or translating and pulsating source). The unknown strength of these singularities and the near-field velocity potential are determined in a coupled manner. It is shown from numerical examples that the present method provides a more efficient and accurate representation of waves in the far field than conventional matching methods.