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A preliminary offshore wind energy potential study for bay of İzmir: Quantifying the airflow distortion on local ferryboats for adjustment of wind data by 3D CFD analyses

Güngör, Şahin

Yüksek Lisans | 2017 | İzmir Katip Çelebi Üniversitesi Fen Bilimleri Enstitüsü

Humankind have an interest to obtain marine meteorological data for decades, therefore, constant and mobile meteorological stations have been used for the correct measurements. These meteorological data include wind speed and direction, sea surface and air temperature and cloud cover. Ship-mounted anemometers have been used for meteorological observations, obtaining the wind speed data and climate change analysis. Wind data are especially gathered and reported by Voluntary Observing Ships (VOS). World Meteorological Organization (WMO) created the VOS program to ensure reporting of the wind data from ships regularly. Ships participat . . .ed this program are cargo or tanker ships which are in different shapes and sizes. Anemometers are usually sited on a mast above the bridge of ships where the effects of flow distortion may be severe. Therefore, determining the wind speed bias around anemometers is so important for the reliability of data. Despite the wide range of usage for gathering wind data, only a few studies have taken the air flow distortion into account caused by the ship’s structure. In those studies, cargo ships or tankers have generally been used for wind data distortion-modelling in computational fluid dynamics (CFD) analysis. The aim of our study is quantifying the airflow distortion over local catamaran ships in Izmir Bay by 3D CFD analysis. 3D model of the catamaran ships is imported to Ansys CFX program and the air flow distortion caused by ship’s structure is analysed for different cases. The ship geometry has been modelled in detail to quantify the best results and the flow domain is made up of three bodies; one of them is a cylindrical core where the ship geometry is also in the centre of this layer. This layer’s radius is 1 ship lengths and height is 2 ship heights. This layer was arranged with detailed mesh sizes which were minimum 0.005 H, where the H was the height of the bridge above the waterline. Second part of flow domain is a ring shaped layer whose radius is 5 ship lengths and height is 2 ship heights. First part of the domain is in the centre of the second domain and they together form a disk like structure. Last part is also a cylindrical part which stands above the first and second parts. Third part’s radius is 5 ship lengths and height is 28.4 meters. These three flow domains form a model which has a radius of 5 ship lengths and a height of approximately 5 ship heights. Different mesh sizes were studied to quantify the air flow distortion in the flow domain correctly. The mesh sizes have been decreased at the positions closer the ship hull and increased away from the ship hull where the flow didn’t vary a great deal. Other air flow distortion studies in the literature used rectangular prism domains. In this study, the flow domain is sliced 8 equal parts. The cylindrical domain has advantages for correct results because the mesh model is fixed for every xviii analysis and wind directions can be changed simply with cylindrical domain’s 45° pieces. When the wind is impacted directly from the ship's bow, wind speed biases are approximately 5% around the anemometer site. Free stream velocity is accelerated up to 10% for 45° clockwise air flow that is similar with 315° wind direction. Accelerated flow regions are close to the anemometer position. The most important reason of the accelerated flow regions is the negatively inclined surface which is positioned in front of the master cabin of the ship. When the wind is impacted directly from beam (90° and 270°) of the catamaran, wind speed biases are between 17-20%. For the case that the air flow is affected from 135° and 225° clockwise, the flow accelerated between 6-8% . Decelerated flow regions are intensely behind the ship’s mast structure. When the wind is directly impacted from astern of the ship (180°), the mast behaves as an obstacle behind the anemometer. Because of this reason, the average wind speed values are approximately 30% lower than 10.8. Catamaran ship model has a closed part at the ship’s bow because of the platform which using for embarking and disembarking of the passengers. If the catamaran ship model was drawn symmetrically, the wind speed bias pairs for 45 and 315°, 90 and 270 °, 135 and 225° would be same. CFD analysis outputs were compared with information in the literature by means of wind data bias around the ships. Results of this study can be used for correcting the data collected from ship’s anemometer and to obtain the accurate offshore wind data to determine the offshore wind energy potential in Izmir Bay Daha fazlası Daha az

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