Towards the Improvement of Autonomous Glider Navigational Accuracy Through the Use of Regional Ocean Models

Autonomous underwater gliders are robust and widely-used ocean sampling platforms that are characterized by their endurance, and are one of the best approaches to gather subsurface data at the appropriate spatial resolution to advance our knowledge of the ocean environment. Gliders generally do not employ sophisticated sensors for underwater localization, but instead dead-reckon between set waypoints. Thus, these vehicles are subject to large positional errors between prescribed and actual surfacing locations. Here, we investigate the implementation of a large-scale, regional ocean model into the trajectory design for autonomous gliders to improve their navigational accuracy. We compute the dead-reckoning error for our Slocum gliders, and compare this to the average positional error recorded from multiple deployments conducted over the past year. We then compare trajectory plans computed on-board the vehicle during recent deployments to our prediction-based trajectory plans for 140 surfacing occurrences.

Investigation of Wave Conditions in the Paranagua´ Estuarine Complex in the South of Brazil

This paper summarises results of investigations aiming at the improvement of the understanding about the wave conditions in the Paranagua´ Estuarine Complex (PEC) in the South of Brazil. The investigations were carried out in the framework of a joint-research project funded by the German Ministry of Education and Research and the Ministry of Science and Technology in Brazil. In this study a phase-averaged wave model was set-up and applied to the study area. The relevance of the main processes affecting wave generation and dissipation were investigated. Focus was given to the wave conditions in the vicinity of the harbour some 25km within the PEC. The fetch was found to have a major effect on wave generation in the study area. Significant wave heights up to about 0.6m resulted near the harbour during storms. The results obtained helped in advancing the development of the coupled process-based models for simulation of flow, waves and sediment transport in the PEC. It was found that although currents have a certain influence on the wave heights, the effect of the tidal variation resulted more significant. Moreover waves were found to affect the current velocities in shallow water areas and should therefore be included for enhancing the predictions of sediment transport rates particularly for more adverse wind conditions.

Hydrodynamic Analysis of Two Side-by-Side Moored Floating Bodies

The hydrodynamic interaction of two bodies floating in waves is studied. The two-body hydrodynamic coefficients of added mass, wave damping and exciting forces and moments are calculated using the irregular frequency free radiation-diffraction potential solution of the improved Green integral equation associated with the free surface Green function (Hong 1987) according to the conventional two-body analysis. It is well known that the conventional two-body potential solution with usual grid fineness largely overestimates the hydrodynamic coefficients at and near the resonance frequency of the free surface in the gap between two floating bodies moored side-by-side in close proximity (Huijsmans et al. 2001, Hong et al. 2005). The two-body diffraction problem has been solved by both the conventional two-body analysis without damped free surface condition and a boundary matching method with and without damped free surface condition. Numerical results of the wave exciting force coefficients of two identical caissons floating side by side obtained by the two methods have been presented and the discrepancies between them have been discussed. Particular attention is paid to the wave elevation in the gap at the resonance frequencies. Amplitudes and phases of the scattering wave elevations in the gap at the first three free surface resonance frequencies computed by the boundary matching method without damped free surface condition have been presented. It has also been shown that the unrealistic wave elevation due to the resonance of the free surface in the gap can be reduced by imposing the damped free surface condition upon the flow in the gap as used in the oscillating water column hydrodynamics (Hong et al. 2004).

Study on Hydrodynamic Performance of a Dish-Shaped Underwater Vehicle

A new kind of dish-shaped underwater vehicle was designed. The maneuverability of the dish-shaped underwater vehicle (UV) is predicted in this paper. Hydrodynamic coefficients of the vehicle were calculated in numerically. The numerical method applied is one of the tools available in the commercial computational fluid dynamics software FLUENT. The dynamic mesh system and post-processing system are adopted in the numerical method. By simulating numerically straight motion, inclined motion and planar motion mechanism (PMM) experiment, the hydrodynamic performance in different states were obtained. Based on the least square method, the hydrodynamic coefficients of maneuverability were obtained. The calculated results indicate that the numerical method is suitable.

Numerical Experiments Analysis of Wave-Induced Vertical Mixing’s Effects on Sea Surface Wind-Induced Momentum Transfer

A hydrodynamic sediment coupled model COHERENS-SED, which has been developed by the present authors through introducing wave-enhanced bottom stress, wave dependent surface drag coefficient, wave-induced surface mixing, SWAN, damping function of sediment on turbulence and sediment model to COHERENS, is modified to account for wave-induced vertical mixing. One equation k–ε turbulence model is taken into account in calculating vertical viscosity coefficient. COHERENS-SED consists of sediment model SED, current model COHERENS and wave generation model SWAN. The model can also calculate one-dimensional, two-dimensional and three-dimensional current separately. One-dimensional model and three-dimensional model are adoptted to study the wave-induced vertical mixing’s effects. The horizontal current velocity profiles given by the model, with same input conditions as what to get analytical results, are in nice agreement with analytical velocity profiles. Therefore the model can be reliable to identify wave-induced vertical mixing’s effects on horizontal velocity profiles and momentum transferring. Two group numerical experiments are built based on 130m water depth and 20m water depth for the one-dimensional model. Results show that higher wave height can generate larger vertical eddy viscosity and lower horizontal velocity generally. In order to find out such effects on fresh water flume momentum transfer towards down in vertical section of estuary, Yellow River delta is chosen to study the effects of wave-induced vertical diffusion on sediment vertical mixing and the Yellow River estuary vertical cross-section is chosen to study fresh water disperse range in vertical section. The results of fresh water shows that wave-induced vertical mixing increases the momentum of fresh water transferring ability towards down to seabed. So fresh water flume length is compressed obviously.

Prediction of Wave Crest and Height Distributions and Wave Groups Using a Numerical Wave Tank

A major goal in current model test practice is the correct modeling of the environmental conditions, as they denote the starting point for all further hydrodynamic analyses. As a standard, wave power spectra are calibrated prior to the actual model tests whereas the corresponding wave group spectra follow from the arbitrarily chosen wave seeds and are not being predicted in advance. Wave crest and height distributions can be determined from the measured wave time traces at different reference locations in the basin but they are not calibrated purposely either. In this paper, a numerical wave tank based on a boundary element method is used to predict wave time traces measured in the wave basin. Resulting wave crest and height distributions are compared with theoretical distribution functions and wave measurements in MARIN’s Offshore Basin. Some thoughts on a possible application to the generation of “deterministic wave seeds” conclude the paper.

Evaluation of Seismic Displacement of Quay Walls for the Passive Case Under Earthquake and Tsunami

Prediction of the seismic rotational displacements of retaining wall under passive condition is an important aspect of design in earthquake prone region. In this paper, a rotating block method is developed to calculate the rotational displacements of quay walls based on rigid foundations under seismic loading and tsunami for the passive earth pressure condition. The proposed method considers the combined effect of the seismic forces, hydrostatic and hydrodynamic forces and tsunami force acting on the quay wall. Variations of different parameters involved in the analysis suggest sensitiveness of the rotational displacement and provides a better guideline for design.

Research and Design of Ballast System for a Science Icebreaker

With the process of industry productions, the polar region which is the last maiden land for human being on the earth is confronted to the environment pollution increasingly. In accordance with the development of green shipbuilding technologies in the world, the ballast system of a science icebreaker should be researched and designed particularly because the International Maritime Organization (IMO) has developed international legislation, the International Convention for the Control and Management of ships’ ballast water and sediments, to regulate discharges of ballast water and reduce the risk of introducing non-native species from ships’ ballast water. Not only the scientific research icebreaker is ensured to work normally in the polar region, but also the weak ocean environment is protected.

Average Flow Inside and Around Fish Cages With and Without Fouling in a Uniform Flow

The average flow field inside and around the bottom of porous cylinders in a uniform flow is explored using Particle Image Velocimetry (PIV). Tests were conducted on six cylinders with porosities of 0%, 30%, 60%, 75%, 82% and 90% in a flume tank where the flow field inside and around the models is time averaged over 180 seconds. The models had a height-to-diameter ratio of 3 and were made from metal mesh. The Reynolds numbers ranged from 5,000 to 20,000 based on the diameter of the models and from 75 to 300 based on the diameter of individual strands of the mesh, which corresponds to the Reynolds numbers occurring at salmon fish cage netting used along the Norwegian coast. The porosities of 82%, 75% and 60% correspond to those of a fish cage netting in Norwegian Salmon farming with no, light and heavy biofouling, respectively. The results from this study are discussed with respect to the instantaneous flow field in and around the same cylinders at identical Reynolds numbers. The focus is on the effect of porosity on the ventilation inside the cages and the vertical transports within the near wake. It is shown that heavy fouling of aquacultural nettings can lead to internal circulation inside fish cages and therefore has the potential to reduce the ventilation of the net pens dramatically. The description of the time-averaged flow field inside and around porous cylinders can be used as benchmarks to validate and adjust numerical models of the flow past porous cylinders. The results from this study can be valuable also for the fish farming industry, as bio-fouling and the reduced porosity of fish cages can be monitored and controlled directly by fish farmers.

Investigation of Liquid Loading in Gas Well: Simultaneous Measurements of Drop-Size, Film Thickness and Pressure Drop

The mechanism of atomization of part of the liquid film to form drops in annular two-phase flow is not entirely understood. It has been observed that drop creation only occurs when there are large disturbance waves present on the film interface. Woodmansee and Hanratty [1] observed that ripples on these waves were a precursor to drops. Though it has been reported that drops occur in bursts by Azzopardi [2], all previous drop size or concentration measurements have always been time integrated to simplify data analysis. Dynamic time averaged drop-size measurements are reported for the first time for annular two-phase flow. Experiments were carried out on a 19mm internal diameter vertical pipe with air and water as fluids. Spraytec, a laser diffraction-based, drop size measurement instrument, was used in the data acquisition. Simultaneous time-resolved measurements were made of: film thickness using conductance probes employing a pair of flush mounted rings as electrodes; and pressure gradient. The gas superficial velocity was 13–43 m/s at liquid superficial velocities of 0.05 and 0.15 m/s. Additional tests were carried out with the gas velocity at 14 m/s for liquid superficial velocities of 0.03–0.18 m/s. Though structures are not clearly visible in the signals acquired, they have been analyzed in amplitude and frequency space to yield Probability Density Function (PDF) and to identify the dominant frequency. Cross-correlation between two film thickness probes provides the wave velocities. Based on the signal analysis, links between film thickness, drop concentration and pressure drop have been identified.