Tidal Current Conditions Research Articles

Nested numerical simulation of multi-scale flow characteristics around monopile foundation of offshore wind farm at Weihai coastal water, China

When constructing an offshore wind farm under tidal current conditions, flow field characteristics without structures are often used for design. However, the presence of structure will greatly affect the flow field around structures. Therefore, it is important to study the flow characteristics around the wind farm foundations under the tidal flow conditions to ensure safe construction and operation. Due to the spatial-temporal scale differences of different physical processes, it is difficult to apply the existing numerical models to accurately simulate the flow characteristics around the offshore monopile. Therefore, this study will link the large-scale TELEMAC with the small-scale OpenFOAM code to establish a multi-scale spatial nesting model to consider the temporal and spatial flow characteristics of real ocean environmental conditions. The nested model is validated using the measurements. The validated model is then applied to conduct some parametric study to evaluate the effect of the wind speed and direction and tidal flow on the flow characteristics around the monopile. Results reveal that the tidal flow velocity could be significantly suppressed by strong opposite wind and vice versa. Tidal flow separation occurring at the lee side of the pile generates the so-called von Karman vortex street downstream of the pile.

Use of an environmental proxy to determine turbulence regime surrounding a full-scale tidal turbine deployed within the Fromveur Strait, Brittany, France

Establishing a relationship between tidal current conditions and tidal turbine performance and loads is a critically important consideration for turbine reliability. Nonetheless, obtaining in-situ information is often challenging, and as a result both environmental and load data may be more sparse than desired. This study presents a method to make use of limited data sets by establishing a relationship between measurements of hydrodynamic variability and turbine power or blade strain variability, even when these measurements are not taken simultaneously. The method is tested on data from the deployment of a full-scale pilot tidal turbine: in situ velocity measurements and turbulence characteristics taken at times when the turbine was not installed were associated with power and strain measurements during the turbine’s deployment via a Delft3D proxy. The data show that the variability of active power correlates well with larger turbulence kinetic energy (TKE) when comparing similar populations via the proxy. Examination of blade strain variance against TKE shows a weaker correlation, with fat-tailed distributions and extremely high strain values prominent across all flow speeds. Acceleration or deceleration of the flow influenced the power variability of the turbine, with larger standard deviations recorded across accelerating flows. No significant difference was found when comparing blade strain variance in accelerating and decelerating flows. We conclude that the proxy method studied can establish a population-level relationship between non-simultaneous environmental and load data, but that the accuracy and precision of this relationship depends on the amount of data available: this method is therefore only suitable where there is a sufficiently rich dataset.

An Output Power Interval Control Strategy Based on Pseudo-Tip-Speed Ratio and Adaptive Genetic Algorithm for Variable-Pitch Tidal Stream Turbine

Power extraction has become a critical consideration in tidal stream turbine (TST) systems. In practice, the lumped disturbances under varying tidal current conditions may deteriorate the maximum power point tracking (MPPT) performance and cumulate fatigue damage over-rated power. Besides, the conventional pitch controllers are sensitive to parameter uncertainties of the nonlinear TST system. In this paper, a novel output power internal control strategy based on pseudo-tip-speed ratio and adaptive genetic algorithm (PTSR-AGA) is proposed to improve the anti-interference ability and reliability. The proposed control scheme consists of two parts. The first part proposes the PTSR method for MPPT to predict the TST’s operating point which contributes reducing the logical errors assigned to swell disturbances. The second part designed an AGA for the optimization of the pitch controller to conduct its angle delay. A reduced pitch control strategy is applied to the preprocessing of the pitch controller to reduce the mechanical wear over the rated power. The comparative simulation results validate the TST system can obtain a higher power efficiency of energy capture and a smoother power output with the proposed control strategies at full range of tidal current speed.

Hydro-Oceanographic and Bathymetric Survey in Tanjung Merah as a Basis for Modelling Coastal Spatial Plans of Bitung City

The purpose of this study was to examine the parameters of Hydro-Oceanography (Tidal and Tidal Currents) and Bathymetry (Sea Depth) in Tanjung Merah, Bitung City. The method used in this research is a field survey method. The survey was carried out at the research location, precisely in Tanjung Merah, Matuari District, Bitung City. Equipment such as GPS Fish Finder, Floater Current Meter, Tidal Pole and Boats are supporting devices in obtaining accurate data on tides, tidal currents, and bathymetry in the waters of Tanjung Merah. The results showed that the tidal current conditions at the study site with several variations in the depth of the tidal current velocity ranged from 0.2 – 0.3 cm/sec with the dominant direction of the current towards the south. While the tides using tidal harmonic analysis for 15 days of observation, the Formzal Index is 0.93 which means that the tides in this location are mixed type with a single daily trend. Bathymetric conditions in Tanjung Merah identified the topography of the bottom of the waters is steep with depth variations between 2 -30 meters along the coast of Tanjung Merah while shallow areas are found at the mouth of the Tanjung Merah river.

Ship Maneuvering Prediction based Pivot Point Estimation

In the last years, the size and the number of ships has grown remarkably. At the same time, the size of harbors and ports remain constantly. Asa result, the ship maneuvering in harbors has become more problematic and harder to execute. This is the reason that many sailors affirm that it is an art than a science to execute some maneuverings. Ship maneuvers represented a complex vessel motions influenced most of the time by external environmental conditions in the navigation area (i.e. ocean and tidal current conditions, water depth, wind and waves). Navigator`s knowledge and experiences overcome to control the ship speed, course and heading in some close encounter situations. For example, combinations of different environmental conditions (draft variations in a passage from fresh to sea water with other ships near the same navigation area), create not only additional navigation difficulties, but also could compromise and threatens the navigation.

Risk Countermeasures for Jiangyin Waters of Yangtze River Based on Oilmap Model

The common calculation methods for vessel fuel oil leakage were summarized. By analysing the fuel carrying capacity of non-tanker vessels statistically and regressively, the relationship between fuel carrying capacity and gross tonnage was worked out. The possible leakage from vessel of 10,000 gross tonnage was estimated based on the result. By using a numerical simulation method, the trajectory and polluting range of the vessel oil spill was simulated in Jiangyin waters of Yangtze River downstream, and different wind direction and tidal current conditions were considered in the simulation scenarios. The prevention and emergency response measures were proposed according to the simulated result.

Shape design and numerical analysis on a 1 MW tidal current turbine for the south-western coast of Korea

The study concentrates on the shape design and numerical analysis of a 1 MW horizontal axis tidal current turbine (HATCT), which can be applied near the southwest regions of Korea. On the basis of actual tidal current conditions of south-western region of Korea, configuration design of 1 MW class turbine rotor blade is carried out by blade element momentum theory (BEMT). The hydrodynamic performance including the lift and drag forces, is conducted with the variation of the angle of attack using an open source code of X-Foil. The optimized blade geometry is used for Computational Fluid Dynamics (CFD) analysis with hexahedral numerical grids. This study focuses on developing a new hydrofoil and designing a blade with relatively shorter chord length in contrast to a typical TCT blade. Therefore, after a thorough study of two common hydrofoils, (S814 and DU-91-W2-250, which show good performance for rough conditions), a new hydrofoil, MNU26, is developed. The new hydrofoil has a 26% thickness that can be applied throughout the blade length, giving good structural strength. Power coefficient, pressure and velocity distributions are investigated according to Tip Speed Ratio by CFD analysis. As cavitation analysis is also an important part of the study, it is investigated for all the three hydrofoils. Due to the shorter chord length of the new turbine blade in contrast to a typical TCT blade design, a Fluid Structure Interaction (FSI) analysis is also done. Concrete conclusions have been made after comparing the three hydrofoils, considering their performance, efficiency, occurrence of cavitation and structural feasibility.

Shape design and CFD analysis on a 1MW-class horizontal axis tidal current turbine blade

This study aims to develop a 1MW-class horizontal axis tidal current turbine rotor blade which can be applied near the southwest island regions of South Korea. On the basis of actual tidal current conditions of southern region of Korea, configuration design of 1MW class turbine rotor blade is carried out by BEMT (Blade element momentum theory). The hydrodynamic performance including the lift and drag forces, is conducted with the variation of the angle of attack using an open source code of X-Foil. The purpose of the study is to study the shape of the hydrofoil used and how it affects the performance of the turbine. After a thorough study of many airfoils, a new hydrofoil is developed using the S814 and DU-91-W2- 250 airfoils, which show good performance for rough conditions. A combination of the upper and lower surface of the two hydrofoils is tested. Three dimensional models were developed and the optimized blade geometry is used for CFD (Computational Fluid Dynamics) analysis with hexahedral numerical grids. Power coefficient, pressure coefficient and velocity distributions are investigated according to Tip Speed Ratio by CFD analysis.

Tidal effects on ecosystem variability in the Chesapeake Bay from MODIS-Aqua

Eight-year ocean color observations between 2002 and 2009 from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the satellite Aqua are used to quantitatively assess the tidal effects on variability of ocean's optical, biological, and biogeochemical properties in the Chesapeake Bay. We demonstrate that tidal variation in the lower Chesapeake Bay is one of the important ocean processes that drive significant changes of the ocean's optical and biogeochemical properties. Normalized water-leaving radiance spectra (nLw(λ)), water diffuse attenuation coefficient at the wavelength of 490nm (Kd(490)), and total suspended sediment (TSS) concentration show significant tidal variations in the lower Chesapeake Bay. Indeed, in the lower Bay region, Kd(490) increases from 0.9m−1 under low tidal current conditions at 0.10–0.15m/s to 1.35m−1 under high tidal current conditions at 0.45–0.50m/s, while chlorophyll-a concentration in the lower Bay region decreases slightly with an increase of the tidal current. Tide-driven variability in Kd(490) and TSS in the lower Bay is similar to and slightly larger than the seasonal variability in the region. On the other hand, different from the variability in the lower Bay, the tidal effects on the ecosystem are small and negligible in the middle and a large part of upper Chesapeake Bay regions.

장죽수로 조류발전건설시 작업특성에 따른 산업잠수 작업한계

The Korea has significant tidal current energy resources, but it is so hard to work underwater for tidal turbine installation. Therefore commercial diving work is very important for tidal current generator. Also, Jangjuk channel is vary famous as proper area to generate tidal current energy. Nevertheless, no one is studied about characteristics of commercial diving works with installation of tidal current generator. The purpose of this study is to introduce commercial diving with work types and investigate critical limits of diving working under the conditions, which are working only to minutes at slack tide during the neap tide. As the results, work types are five as like mooring installation, OMAS(Offshore Maintenance Access System), support structure installation, cable and turbine installation. Here, the original construction period is expected about 4 months, but the construction take 18 months to complete. The cause of extends construction period is insufficiency of researching tidal current conditions at the site and ignorance of slack tide which need to secure diving working time. Total diving working times are 110th during 18 months, the highest percentage of diving times is turbine installation about 43.6 %, and cable, mooring installation and support structure construction are 27.3 %, 15.5 %, 13.6 %, respectively. On the basis of this study, estimation of times of commercial diving is possible with work types of tidal current power, and has a significance as basic data to determining construction period.

COMPARISON OF BATHYMETRIC FEATURES DETECTED BY ERS2 SAR AND LANDSAT TM DATA OVER SAN MATÍAS GULF, ARGENTINA

It is well known that Synthetic Aperture Radar (SAR) is capable of imaging bathymetric features by detecting changes in sea surface roughness associated with bathymetry-induced surface current modulations. The high dynamic range of LANDSAT Thematic Mapper (TM) and Enhanced TM+ (ETM+) sensors allow them to observe oceanic surfaces affected by solar specular reflection, or sun glint, without being saturated. This ability allows the imaging of sea surface roughness patterns in the sun glint affected area that are associated with surface current changes such as those produced by bathymetric features. ERS SAR and LANDSAT TM/ETM+ images obtained over the San Matias Gulf, Argentina during similar tidal current conditions are compared and the surface signature generated by the interaction of the tidal current with the bottom topography analyzed. Each type of sensor presents advantages and limitations to the observation of bottom features due to the way each one detects sea surface roughness. Still, when sun glint is present, TM/ETM+ and SAR show similar bathymetric patterns under the same tidal conditions. While both sensors provide consistent information over significant bathymetry features, increased differences in the detection is observed over weaker features

The use of Landsat imagery to map fluvial sediment discharge into coastal waters

The sediment discharge of the river Yare, via Breydon Water estuary, into the North Sea has been studied. Sixty samples were taken, the average of each sample in the band TM1 was used to estimate the concentrations of suspended solids. Sixty samples were used to extract information from the reflective bands TM1–TM4. The spatial distribution of suspended solids as well as the spatial and vertical extent of different turbidity levels were then detected, estimated and mapped. Based on this information the water circulation pattern at the mouth of river Yare was identified and mapped. The influence of wind velocity and direction, as well as the tidal current conditions prior to and during the image time, on turbidity and water circulation patterns were examined. The tidal influence was found to be the most significant factor in shaping and guiding water circulation near the mouth of river Yare.

A STUDY OF ECOLOGICAL RESEARCH IN THE GEOGRAPHY OF FISHERIES: THE CASE OF THE USE PATTERNS OF THE FISHING GROUNDS FOR HAND-LINE FISHING (IPPON-ZURI) IN MUKUNA, THE OCHI ISLANDS

This article attempts to examine the time and space use patterns of the fishing grounds for hand-line fishing in Mukuna, a village on the 4chi Islands of Ehime Prefecture from the ecological point of view. Using the results of a field survey, applicability of ecological researches on the fishing ground use the geography of fisheries is also investigated. Data were collected by direct observation of fishing activities on the sea, as well as by interviewing the fishermen. The results of the field survey are summarized as follows: 1. It is nagashi-ipponzuri or the drifting hand-line fishing technique that is adopted by M ukuna fishermen. Motorboats operated by one fisherman are put to use for hand-line fishing. 2. The hand-line fishing takes place all year round. The fishing season is, however, divided into two periods in accordance with the seasonality of the catch: from April to January the fishing of madai (Chrysophrys major) and suzuki (Lateolabrax Japonicus) is practiced, while the fishing of kasago (Sebastiscus marmoratus) and mebaru (Sebastes inermis) is complementarily carried out in February and March. 3. Several sorts of bait are employed according to the seasonal variation in availability, 4. There are about thirty boats for hand-line fishing in Mukuna. This number is seasonally reduced according to shifts in the local climate. Merely ten are in use when the water temperature reaches its peak in the hottest months of August and September and when the water becomes cold and rough in winter with the northwest monsoon. 5. Mukuna waters provide twenty-two fishing spots, which can be classified into three categories in terms of the tidal current conditions. a) fishing spots used at ebb current (ten) b) fishing spots used at flood current (ten) c) fishing spots used at both ebb and flood current (two) 6. Considering the tidal current conditions, a fisherman decides the type of fishing spot which is suitable and then chooses by turns some of the fishing spots within the same category. 7. The hand-line fishing is conducted from early morning to evening. 8. It is not possible to do hand-line fishing when the tidal flow is fast. Fishermen cannot avoid stopping their work at sea until the tidal current conditions turn favourable. Some fishermen go back to the land to take a rest when the tidal current conditions are bad. 9. Mukuna fishermen can perceive clearly enough where they are at sea through yamasate, a location-finding technique by visual triangulation which is based on the observation of counter movements of two or more mountains or islands seen from the sea. An application of yamatate enables the fishermen to grasp the speed and direction of the currents. When their location at sea is once confirmed, the fishermen understand such attributes of the fishing ground as range, water depth, morphology of the sea bed, and so on, because they are well equipped with a large amount of empirical knowledge of each fishing spot. The author analyzed the use of the fishing ground in Mukuna from the viewpoints of the time-use pattern and the space-use pattern. In terms of the time-use pattern, data on daily, m onthly, seasonal, and annual cycles must be collected according to the fishing activity: In terms of the space-use pattern, three-dimensional space.……the surface, the water, and the sea bottom …… has' to be grasped as a whole. The author is intended to carry on this ecological study on the fishing ground use by accumulating more data on the time and space use of the fishing grounds and to integrate the data for a comprehensive ecological analysis of the fishing villages using these fishing, grounds.