Kapalı yanaşma yapılarında çift pervane jet akımı etkisinde oyulma mekanizmasının araştırılması
Investigation of the scour mechanism under the influence of twin propeller jet flow in closed berthing structures
Yazar: AMINA SULJEVIC
Danışman: DR. ÖĞR. ÜYESİ ERDAL KESGİN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kıyı Bilimleri ve Mühendisliği Ana Bilim Dalı / Kıyı Bilimleri ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Mühendislik Bilimleri = Engineering Sciences ; İnşaat Mühendisliği = Civil Engineering

Özet:
Gemi pervanelerinin oluşturduğu jet akımları, deniz tabanında ciddi manada oyulmalara ve birikmelere sebep olmaktadır. Günümüzde gemi trafiğinin de artmasıyla, bu oyulma ve birikmeler liman yapılarının temelinde çok ciddi sorunlar meydana getirmektedir. Bu çalışmada farklı tipteki kapalı yanaşma yapılarına yanaşan ve ayrılan gemilerin pervanelerinden çıkan çift pervane jet akımlardan dolayı oluşan oyulmalar deneysel olarak incelenmiştir.
Çalışmada 10 cm çapında pervaneler kullanılmıştır. Pervaneler arası mesafe 2Dp ve 3Dp seçilerek farklı durumlar test edilmiştir. Pervane merkezi ile taban arasındaki mesafe ise 10 cm ve 15 cm olacak şekilde ayarlanmıştır. İki farklı dönüş hızı 30 hertz ve 40 hertz olarak belirlenmiştir. Duvarlı durumların daha iyi anlaşılabilmesi için öncelikle duvarsız durum dikkate alınarak deneyler gerçekleştirilmiştir. Daha sonra deney sistemine pervanelere dik ve paralel olacak şekilde iki farklı duvar konfigürasyonu yerleştirilmiş ve farklı parametrelerin incelendiği birtakım deneyler gerçekleştirilmiştir. Pervanelere dik olarak yerleştirilen duvar pervanelerden 3Dp, 5Dp ve 7Dp uzaklıkta olacak şekilde üç farklı durumda incelenmiştir. Pervanelere paralel olarak yerleştirilen duvar ise pervaneden 2Dp ve 5Dp uzaklıkta olacak şekilde yerleştirilmiştir. Deneyler sonucunda dönüş hızı arttıkça maksimum oyulmanın da arttığı sonucuna varılmıştır. Pervaneler ile taban arasındaki mesafe arttıkça, maksimum oyulmanın azaldığı gözlemlenmiştir. Pervaneler arası mesafenin artması ile de maksimum oyulmanın azaldığı ve 2Dp ve 3Dp durumları için tek oyulma çukurunun oluştuğu gözlenmiştir.

Abstract:
The jet flows generated by ship propellers cause significant scouring and deposition of material on the seabed. As ship traffic increases, these issues have led to major problems with the foundations of port structures. This study experimentally investigates the scour caused by twin propeller jet flows from ships docking and departing in various types of closed berthing structures.
It is well known that when the propeller is at a distance to interact with the ground, areas of scour and deposition caused by the jet flow are observed at the base. This base profile, consisting of scour and deposition, can be expressed with three different regions. These are: the small scour hole, the primary scour hole and the upper region where the deposition takes place.
The majority of studies in the literature to date relate to the experimental and numerical investigation of scour around piled structures under the influence of single propeller flow. There are not enough studies investigating scour in front of quay walls, and there are also very few studies investigating the scour hole created by twin propeller ship types in front of the wall. In this study, the scour caused by twin propeller jet flows in front of quay walls was investigated experimentally using different configurations of closed berthing structures.
In order to investigate the effect of the twin propeller jet on scour and deposition, experiments were carried out under certain conditions and the aim was to show the differences. The study considered cases where the unconfined and confined effects of the twin propeller jet were considered.
It is known that after a certain period of time from the start of propeller rotation, the scour formations reach equilibrium conditions and the dimensions of the scour hole remain unchanged. Experiments were carried out to understand the time dependent evolution of scour. When the results of these experiments were examined, it was decided that the total duration of the experiment would be 120 minutes. Although the experiments lasted two hours, the rate of change of scour was high in the first few minutes of the experiment and decreased as the scour approached its maximum value.
The total length of the experimental setup is 6 m, the height is 1 m and the width is 1.5 m. In the experimental system, the water level was set at 70 cm and filled with sand to a depth of 30 cm. In this experimental setup, the effective parameters in the case of scour under propeller jet flow were tested under different conditions. Following the completion of the channel in which the experiments will be carried out, the propeller system to be used in the experimental system was designed. This system design includes a dual propeller motor system that has been developed specifically for this study and has not been done before. The motor has been mounted on the top of the Plexiglass box system and the propellers have been designed to work with a belt system. The propellers can be easily moved up and down using an adjustable system (control box) and the distance from the centre of the propeller to the ground (G) can be adjusted. There is also a similar adjustment system for moving the propellers to the left and right, and the distance between the propeller axes (ap) can be adjusted in this way. Propellers with a diameter of 10 cm were used in the experiments. The Wageningen propellers were designed in a 3D drawing programme and produced on a 3D printer using PLA material. The twin propeller system used in the study was set to rotate clockwise and in the same direction. Sieve analysis was carried out at the Civil Engineering Geotechnical Laboratory of Istanbul Technical University to determine the granulometric curves of the material. The grain size of the material used in the study was determined by sieve analysis to be 1.2 mm. A limnimeter was used to take measurements at the end of each experiment. When the scour profile formations reached equilibrium, measurements were taken at 1 cm intervals.
Different scenarios were tested by setting the distance between the propellers to 2Dp and 3Dp. The distance between the propeller center and the base was adjusted to 10 cm and 15 cm. Two rotational speeds, 30 hertz and 40 hertz, were examined. The operating speed of the propeller (in rpm) was controlled by the frequency regulator and set to 30 hertz and 40 hertz, giving propeller speeds of 355 rpm and 470 rpm. Initially, to understand the impact of walls, experiments were conducted without walls. Subsequently, two different wall configurations were placed perpendicular and parallel to the propellers in the experimental setup, and a series of tests were performed to investigate various parameters. In the study, the walls were placed at a distance of Xw and Yw, perpendicular and parallel to the axis of the propeller. The wall positioned perpendicular to the propellers was examined at distances of 3Dp, 5Dp, and 7Dp, while the wall placed parallel to the propellers was set at distances of 2Dp and 5Dp.
Dimensional analysis and scaling are required to fully understand the scour mechanism in closed berthing structures under the influence of propeller jet flow. After applying dimensional analysis, many dimensionless parameters were found to estimate the maximum scour depth.Taking into account the maximum scour depths and dimensionless parameters from the no wall, vertical wall and parallel wall experiments, different equations were obtained for each case. In the present study, the prototype ship is considered as a Ro-Ro ship.
It was assumed that the scour holes formed from the unconfined case experiments were similar to the single propeller system, and that a small scour hole was formed just below the twin propeller system, primary scour hole downstream of the small scour hole and an accumulation zone downstream of the primary scour hole. As a result of the experiments, it was concluded that the maximum scour increases with increasing speed. At the same time, it was found that this had a major effect on the geometry of the scour hole between the propellers. In addition, some experiments were carried out in cases where the distance between the propeller axes was 4Dp and 5Dp and it was observed that twin scour holes were formed. As it was assumed that the distance between the propeller axes caused the formation of twin holes after 3Dp, this study focused on the cases where the distance between the propeller axes was 2Dp and 3Dp. It was observed that as the distance between the propellers and the base increased, maximum erosion decreased. Additionally, as the distance between the propellers increased, maximum erosion decreased, with a single scour hole forming for the 2Dp and 3Dp scenarios. As the distance between the propeller axles was known to cause the formation of twin hole after 3Dp, this study focused on the cases where the distance between the propeller axles was 2Dp and 3Dp. In the experiments carried out when the distance between the propeller and the wall (Xw) is small, i.e. when the vertical wall is closer to the propeller, it was found that the maximum scour depth deepens significantly. It was found that when the vertical wall was 3Dp and 5Dp from the propeller, the maximum scour generally occurred at the bottom of the wall, but when it was 7Dp away, the wall had little effect on scour.
In addition, when the results are examined, it can be seen that the effect of the parallel wall on scour is almost negligible when it is 5Dp away from the propeller.