LEE- Gemi ve Deniz Teknolojisi Mühendisliği-Doktora
Bu koleksiyon için kalıcı URI
Gözat
Son Başvurular
1 - 5 / 6
-
ÖgeEffect of tip flow on vortex induced vibration of circular cylinders(Graduate School, 2022-04-07)As one of the complicated subject of flow-induced vibration (FIV), the physical background of Vortex-Induced Vibration (VIV) and its mathematical model can not be represented by a single theory. The solutions contain highly non-linear terms increasing the computational burden. Investigations on this area have not yet matured although it takes place in almost all fields of ocean engineering. VIV has destructive effects on deep-sea oil production and offshore industry equipment since the phenomenon is observed around the bluff bodies such as marine cables, moorings, risers and pipes. As the number of offshore structures around the world increased, the oil companies have identified new targets and started moving away from shallow waters to deep-waters. This is considered to increase the interest in VIV in the future. Moreover, recent researches have revealed that it is possible to benefit from this phenomenon. A recently invented device, VIVACE, succeeded to convert the energy in water currents into electric energy by fitting a power generator into conventional VIV models. Some other studies propose piezoelectric materials to derive energy from VIV mechanisms. Recent studies have also revealed that VIV may also be used for developing some gauges such as water measuring device. Taking these new developments into the account, the number of VIV researches are increasing rapidly in parallel with reachable higher computational technologies. Moreover, the reliability of numerical studies are improving thanks to the better approximations of flow and turbulence models. The nature of VIV phenomenon is highly non-linear. Mathematical models simplify the problem in many ways by leading to partly or entirely incompatible results between different studies, even if these studies are using the same non-dimensional parameters. At this point, three-dimensionality of the flow plays an important role in many of these studies. Depending on the mathematical model in numerical studies, or the lab setup in experimental ones; the oscillating body might be exposed to more 3D effects while in some others the flow might even be 2D in the entire wake. 3D effects are mostly dependent on the aspect ratio of the circular cylinder and end conditions (such as usage of end-plates or not). The flow partly escapes through the free-ends of the bluff body and creates a trailing vortex at the tips spoiling the shedding process, contributing to the oscillations of that body. The aspect ratio is inversely proportional to the three-dimensionality of the flow and its dominance on the VIV response. If the aspect ratio is sufficiently large, the escaping flow from the tips can even be neglected. Effect of tip flow on the VIV response are generally observed through the oscillation amplitude, the frequency response, and the phase difference between the oscillation of the cylinder and the vortex shedding. Due to the reasons explained above, the effect of tip flow should be taken into consideration in calculations. A 2D VIV approach typically neglects the finiteness of the aspect ratio and assumes that Karman vortex street covers the entire wake while it can be observed only around the mid-section of a three-dimensional VIV. Studies adopting a 3D model indicate that the vorticity type along a VIV cylinder changes from the mid-section to the tips of the cylinder due to cellular sheddings, cross-flow and tip-flow. Despite a boost in recent studies, numerical approaches to solve the VIV problem are still in progress since current methods are incapable of reflecting the experimental conditions sufficiently, requiring unaffordable computational power due to the complexity of spatial alteration of vortices in the wake. Therefore, researchers generally prefer relatively simpler 2D methods. Although these methods have the advantage of decreasing the computational cost; some characteristics of VIV phenomenon, observable in only 3D studies, such as cross-flows, cellular sheddings and tip flow (and tip vortices) can not be represented with a 2D flow assumption. An adoptable enhancement would be worth to pursue to implement into a 2D model, representing partial three-dimensional flow, so that these effects can be partly compensated to obtain more realistic results. A chapter of this thesis is devoted to this purpose by proposing a scaling factor to represent three-dimensional characteristics of the flow around circular cylinder in a 2D numerical model. The finite volume method is used to calculate force term at each time step acting on the oscillating cylinder. The lift force is scaled by a newly proposed term named as the "three-dimensionality factor". By using this factor in the equation of motion to reflect three-dimensionality, a reduction in oscillation amplitude is examined. This factor alters the lift force, the phase difference, and therefore the oscillation frequency. It also changes the synchronization range especially at the lower branch region. Eventually, the numerical method has been compared with some experimental data. The enhancement in the 2D numerical method is demonstrated and discussed. A suitable scaling factor is proposed for the chosen experimental cases. The experimental part of this thesis focuses on the effect of tip flow by changing systematically the aspect ratios and the distance from the edges of the cylinders to walls of the circulation channel. The experiments are carried out at İstanbul Technical University (İTÜ) Ata Nutku Ship Model Testing Laboratory (ANSMT Lab) located in the Faculty of Naval Architecture and Ocean Engineering. Mass ratios of 1.93, 2.24, and 2.52 are considered and the resulting Reynolds number range is 1.6×104 – 8×104 corresponding to the sub-critical TrSL2 and TrSL3 (transition in shear layer) flow regimes. The nondimensional velocities (U^*) range from 3 to 13. Six different circular cylinders are used with different aspect ratios varying from 11.225 to 17.7875. The cylinder with the longest length extends to the walls of the circulation channel (as much as possible) and the length of each cylinder is shortened systematically while the diameter is kept constant at 0.08m. As the length of the cylinder is reduced, three-dimensionality of the flow increases and the flow escaping from the tips gets higher. This is accompanied by vortex disturbances which causes a loss on the lift force due to the decreasing Karman vortex street in the wake of the cylinder. Eventually, VIV response of the cylinder differentiates into a narrower synchronization range, lower oscillation amplitude and larger differences in phase angles. In the last section of the thesis, effect of aspect ratio and tip flow on VIV is investigated through hydrokinetic energy harnessing from the phenomenon. Three-dimensional effects, reducing the effective length of the cylinder, are discussed in terms of energy generation. Converted power and maximum system efficiencies are calculated from experiments conducted in the recirculation channel of the ANSMT Laboratory. It was found that the end-zones of the cylinder, which do not induce lift due to tip flow, are more dominant in lower aspect ratio cylinders. More power can be captured from TrSL3 flows due to higher shear-flow momentum while higher efficiency in power conversion is achieved in TrSL2.
-
ÖgeManeuvering of high speed displacement vessels in regular waves(Graduate School, 2023-10-16)This doctoral thesis presents a comprehensive exploration of maneuvering performance in waves, aiming to gain insight into the interactive behavior of waves and vessels and use this interaction to develop a mathematical model of maneuvering in waves. The research focuses on the adaptation of experimental methods, practical system-based methods, and computational analysis techniques to enhance the integration of solution methods. The central objective of this thesis is to systematically reveal the complicated characteristics of mean wave drift loads. Additionally, the thesis aims to demonstrate the influence of these loads on the assessment of maneuvering performance. The research seeks to bridge the gap between traditional investigation of maneuvering in calm water and growing demand for determining the minimum propulsion power needed to maintain a ship's maneuverability in adverse conditions. The study employs a systematic approach, combining various research techniques. It utilizes a system-based mathematical method along with laboratory experiments and simulation-based analyses to achieve its objectives. The methodology encompasses theoretical modeling, computational simulations, and practical measurements. The research reveals that integrating mean wave drift loads, namely surge wave drift force, sway wave drift force, and yaw wave drift moment, obtained with a practical experimental approach or computational methods, can significantly enhance the capacity of system-based models. The data reduction procedure enables to reach of zeroth-frequency values representing the mean wave drift loads. This thesis contributes a novel approach to research endeavors, experimental initiatives, and software development projects aimed at elucidating the concept of "maintaining maneuverability" in challenging sea conditions. The findings underscore the importance of adapting wave conditions to maneuvering applications developed with new approaches. The insights from this research hold wide-ranging implications for ocean engineering practitioners and researchers. The presented framework offers a holistic perspective that can guide maneuvering performance and navigational safety. The findings of this thesis will be also presented in the activity named "Assessment of Experiments and Prediction Methods for Naval Ships Maneuvering in Waves", the activity period of which is 2021-2023 and still continues. The activity is carried out within the scope of Applied Vehicle Technology (AVT) and is referred to by the code AVT-348. In conclusion, this doctoral thesis presents a practical experimental and numerical approach to achieve surge, sway, and yaw wave drift loads in regular waves which are then used to demonstrate the effect of wave-related loads on the maneuvering performance of surface vessels. The measured wave drift forces and moments are incorporated into the equations of motion for maneuvering to predict the effect of waves on the maneuvering performance of a vessel. A simplified mathematical model to show the behavior of the vessel in waves is represented by four nonlinear equations of motion for surge, sway, yaw, and roll. The turning circle, which is one of the standard maneuvering tests is simulated for ONR Tumblehome (ONRT) at different wave conditions in order to demonstrate the effect of wave drift forces on the trajectories and time histories of maneuvering parameters.
-
ÖgePhysical investigation of 2d free falling wedge(Graduate School, 2023-06-23)Water entry is an important phenomenon for sea-going vessel in terms of planing and slamming. Pressure distribution on the solid surface, at the moment of impact on water is a critical parameter during the initial design and optimization of a vessel, especially on planing vessels, trawlers or free-fall lifeboats and alike. The aim of this study is to create a simple yet accurate method to aid ship designers on the initial process & optimization of the ship design. Porpoising motion of planing hulls or releasing the free-fall lifeboat from the ship during the emergency situation are perfect two examples of this specific design problem. During this specific water impact event, none of the vessels shall suffer any kind of damage which results in either structural failure on the said vessel or injuries/death of persons onboard. It is fair to say that this specific design factor has its importance to this date. Starting from this point of view, for the easiness of the calculations a wedge shape is taken into account for both planing vessels and trawlers & fishing vessels. This wedge shape can be considered as similar to planing hulls, small craft such as trawlers or free-fall lifeboats. Based on Wagner's famous method for calculating pressure distribution and pile-up of water on plates, this study proposes a new method using Schwarz-Christoffel conformal mapping to calculate pressure distribution along the surface as well as pile-up of water and force history. At the beginning, the definition between mapping plane and real plane is presented. By doing this, pile-up coefficients are calculated. Based on the Wagner's flat plate theory, the pile-up coefficients are plotted from 0 degrees of deadrise to 90 degrees deadrise, where 0 degrees deadrise is a flat plate. The results are further compares against Mei's results [17], which are not only improving the Wagner's fixed pile-up coefficient but also found by similar conformal mapping method. Upon calculating the pile-up coefficient, this coefficient incorporated into existing empirical methods for individual usage. Then, conformal mapping calculations are continued to plot pressure distribution along the wedge. The pressure distribution values are compared against Yettou's Experiment [15], Oien's Experiment [54], Sun's Results [36] and Dong's Experiment [20]. These experiments consist of respectively 25 degrees of deadrise, 30 degrees of deadrise, 10 degrees of deadrise and 45 degrees of deadrise. The results are very well matching. Upon completion of comparison, the force history is calculated. The force history is compared against Sun's results, which are consisting of four different impact speeds. It is found some overestimation on the force history, but in general results are satisfactory. As an additional verification, comparison against real ship sections was also examined. This proposed method can be expressed on monotonic sections. Based on this method of application, alternative sections were proposed against real ship sections and comparison for slamming pressure is conducted. Results were discussed accordingly. Lastly, a comparison against CFD application is done. The results are shown as very well matching. By doing this, presented method in this study was compared against drop test experiments, real ship sections and CFD application all together. It is accuracy proved in all aspects. In conclusion, the aim is considered to be achieved by presenting pressure distribution and force history results by making different comparisons. This study provides an easy method to calculate this critical design factor. It is noted that the method is very accurate for initial impact loads and can be used in initial design stage to understand the loads and pressure along the ship section.
-
ÖgeNumerical investigation of maneuvering performance of monohull and multihull marine vessels(Graduate School, 2022-04-22)This Ph.D. study is based on the numerical analysis of the manoeuvring performance of two monohull and two multihull ship forms. To achieve this, the CFD-based system simulation method based on Abkowitz's model has been employed and new closed-form solution techniques have been proposed. The first monohull form is a displacement type surface combatant, which is widely used in the literature and has a significant number of comparison data. It is known by the name DTMB5415, which is the model number in the David Taylor Model Basin (DTMB) experimental laboratory where it was produced. Of the two known configurations of this surface combatant, the one with discontinuous bilge keel geometry and named DTMB5415M has been preferred with full appendages. The second monohull is the R/V Athena, which has the form of a planing hull and is also known in the literature as Model5365. For multihull ships, Delft Catamaran 372 (DC372) and surface combat trimaran have been selected. The CFD-based system simulation method first dictates the development of a mathematical model for ship manoeuvres. Depending on the application purpose, the mathematical model can include environmental factors, rudder forces, propeller forces, hull hydrodynamics and other external effects, if there is any. Hull hydrodynamic characteristics are commonly modelled by special parameters representing the ship hull form. These parameters are the core of the manoeuvring mathematical model and they determine how the ship form will react to any effect entering or leaving the system. These parameters are called hydrodynamic derivatives or manoeuvring coefficients in the literature and can be calculated either experimentally or numerically. Abkowitz's manoeuvring mathematical model has been adopted and the hydrodynamic derivatives have been calculated separately for all selected ship forms by analyzing the planar motion mechanism (PMM) tests with the help of CFD in the computer environment.
-
ÖgeSualtı patlamalarının saha ölçümleri ve sayısal modellemelerle incelenerek civardaki deniz araçlarında hasar tahminleri yapılması(Lisansüstü Eğitim Enstitüsü, 2022-04-22)Bu çalışmanın ilk kısmında denizde kontrollü bir sualtı patlamasının patlama basıncı ölçümleri yapılmıştır. Yirmi beş kg-TNT eşdeğeri bir patlayıcı infilak ettirilmiş ve patlama basınçları, sualtı ortamında rezonanssız yüksek voltaj çıkışında çalışan sekiz farklı yüksek performanslı basınç sensörü ile kaydedilmiştir. Kaydedilen tepe basınç değerleri, literatürde kabul edilen ampirik sualtı patlaması (UNDEX) basınç formülü ile kıyaslanmıştır. Söz konusu formülün sabitleri, ölçülen verilere en iyi şekilde uyması için iki farklı biçimde en küçük kareler yöntemi kullanılarak yeniden belirlenmiştir. Yeni belirlenen sabitlerin literatürde kabul görmüş formülün katsayıları ile karşılaştırıldığında nispeten küçük farklar tespit edilmiştir. Savaş gemilerinin stabilitesi, sistem ve alt sistemlerinin düzgün çalışmasını doğrudan tehdit eden sualtı patlamaları, hasar tahmini açısından ele alınmalıdır. Bu nedenle bu çalışmanın ikinci kısmında, LS-DYNA'nın Arbitrary Lagrangian-Eulerian (ALE) metodu/sayısal tekniği, gemi benzeri bir yapının yapısal bileşenleri üzerindeki şok etkilerini analiz etmek için kullanılmaktadır. Hesaplanan maksimum kalıcı deformasyonlar, doğrusal bir orantılılık faktörü elde etmek için karşılık gelen Omurga Şok Faktörü (OSF-Keel Shock Factor (KSF)) değerleriyle eşleştirilmektedir. Formülün tatmin edici tahminleri, şok dalgalarına maruz kalan gemi benzeri yapılara verilen hasarın büyüklüğünün bir ölçüsü olarak KSF'nin kullanılmasının oldukça kabul edilebilir olduğunu göstermektedir. Bu kasamda; bir gemi kirişini temsil eden barç benzeri bir yapı üzerindeki 3 boyutlu bir sualtı patlamasının etkileri, ALE yaklaşımı ile LS-DYNA yazılımı kullanılarak sayısal simülasyonlar yoluyla araştırılmıştır. KSF'deki değişimlerin etkilerini araştırmak için üç farklı simülasyon gerçekleştirilmiştir. KSF ≈ 1 için tasarlanan ikinci simülasyon, genel kullanıma açık olmasa da saha ölçümlerinin mevcut olduğu duruma benzer bir durumu temsil etmektedir. Hesaplanan maksimum kalıcı deformasyonların ölçülenlere yakınlığı, sayısal simülasyonların güvenilirliğinin bir göstergesidir. Ayrıca, simülasyon verilerinin kullanılmasıyla, burada ele alınan yapı için maksimum deformasyonlar ve karşılık gelen KSF değerleri arasında doğrusal bir ilişki kurulmuştur. Kalıcı deformasyonlar için iyi bir tahmin değeri veren bu basit ilişki, farklı yapısal özelliklere sahip gemi benzeri formlar için daha gelişmiş ve daha genel formüllerin öncüsü olarak görülebilir. Son olarak, hesaplanan başlangıç hızları (kick of velocity) göz önüne alındığında platformda, KSF ≈ 1 olduğunda ciddi hasarlar, KSF ≥ 1 olduğunda ise ölümcül hasarlar oluşacağı değerlendirilmektedir