LEE- Gemi ve Deniz Teknolojisi Mühendisliği-Doktora
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Yazar "Kınacı, Ömer Kemal" ile LEE- Gemi ve Deniz Teknolojisi Mühendisliği-Doktora'a göz atma
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ÖgeEffect of tip flow on vortex induced vibration of circular cylinders(Graduate School, 2022-04-07) Duranay, Aytekin ; Kınacı, Ömer Kemal ; 508152105 ; Shipbuilding and Ocean EngineeringAs 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.
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ÖgeManeuvering of high speed displacement vessels in regular waves(Graduate School, 2023-10-16) Sarıgül Öztürk, Deniz ; Kınacı, Ömer Kemal ; 508192103 ; Shipbuilding and Ocean EngineeringThis 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.