Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 29-31). Also available via the Internet.

MASTER OF SCIENCE (2024) (School of Computational Science and Engineering) McMaster University Hamilton, Ontario, Canada TITLE: Vortex Analysis – Clustering and Temporal Tracking of Vortices AUTHOR: Yucheng Feng M.Eng. (Electrical Engineering) Xi’an Jiaotong University, Xi'an, Shaanxi, China B.Eng. (Electrical Engineering) Shandong University, Jinan, Shandong, China SUPERVISOR: Dr. Li Xi NUMBER OF PAGES: xix, 75 / The vortex is a fundamental concept in fluid dynamics, and analyzing it is crucial for explaining and predicting the behavior of fluids in practical applications. In this thesis, two techniques that can lead to a deeper understanding of vortices will be proposed and verified by applying them to Newtonian turbulence and polymer-added flow. The first technique is vortex clustering. By doing dimension reduction and clustering simultaneously, the performance of vortex clustering is notably improved since the hidden features that are immersed in the original input features but can efficiently distinguish different types of vortices can now be extracted objectively. Then, the reliability of the clustering technique is verified in various Newtonian flows. The second technique is vortex tracking based on vortex axis lines, which can efficiently provide complete evolving routines of each vortex over time. With this tracking method, temporal information of vortices, such as their detailed evolving routines and temporal drift positions, can be fully observed and recorded for a future study. The mechanisms and details of this tracking method will first be illustrated and verified using Newtonian flow. Finally, since these two techniques for vortex analysis are solely developed for Newtonian turbulence, a polymer-added flow, where a small amount of polymer can notably modify the behaviour of vortices in Newtonian turbulence, is introduced to check to which level these two techniques are still reliable. Moreover, these two techniques can be compatibly embedded into existing vortex analyzing tools. By doing this, the interested types of vortices can be found and isolated from others, and their specific features and routines can thus be thoroughly studied. / Thesis / Master of Science (MSc) / In turbulence research, efficient clustering and tracking of vortices are appealing. Hence, the fundamental motivation of this research is to investigate vortex clustering techniques and vortex tracking techniques to analyze vortices in turbulent flows automatically and objectively. With the proposed vortex clustering technique, the hidden features immersed in input data space that can efficiently distinguish different types of vortices can be extracted objectively to classify vortices into various groups. With the proposed vortex tracking technique, the temporal behaviours of vortices, such as their detailed developing routines, can be fully tracked, and recorded in a simple but efficient way. With these two techniques, our understanding of the differences between various types of vortices, the ways vortices evolve under different conditions, etc., can be further improved. Besides, embedding these two techniques in existing vortex analyzing tools makes them more powerful.

published_or_final_version / Mechanical Engineering / Master / Master of Science in Engineering

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Vortical structures are regarded as the dominant organized patterns in wall turbulence. They play a key role in physical phenomena of practical importance such as energy and momentum transport, combustion, mixing, and noise and drag production. Considerable investigations have been performed in drag and noise phenomena studies, with a main purpose of controlling and reducing them. Various techniques to control the drag reduction have been studied for over last five decades; however, the detailed understanding of the drag reduction mechanism is still lacking. Vortices play an important role in turbulence structure. Nevertheless, the identification of vortices is still unclear, not even a universal definition of a vortex is accepted. In the present study, several vortex feature extraction schemes are implemented. The methods are applied to analyze instantaneous two-dimensional velocity fields obtained by particle tracking Velocimetry (PTV) measurements of a turbulent channel flow with and without microbubble injection within the boundary layer. Microbubble injection is one of the drag reduction techniques, first studied in early 1970s, that has undergone extensive research in past years, and the generated information has aided into drag reduction understanding. As a general rule, vortex extraction methods can be either a simple visualization scheme or more sophisticated identification tools. The Reynolds decomposition and its variants are suitable due to their capacity to mark vortices advecting at different velocities. In the case of identification techniques, which yield a scalar field calculated from either the velocity vector field or the velocity gradient tensor, both the modified swirling strength Λci or the λ2 criteria were found to be well suited for vortex identification.

We study the interaction between superconductors and ferromagnets in two systems: a ferromagnet-superconductor bilayer, and a thin superconducting film with a periodic array of magnetic dots upon it, with spontaneous vortices appearing in the systems. We show that the superconducting phase transition is of the first order in a ferromagnet-superconductor bilayer and of the second order in the superconducting film with a periodic array of magnetic dots upon it. The shift of the transition temperature, (delta)Tc, due to the presence of a ferromagnetic layer may be positive or negative in the ferromagnet-superconductor bilayer and is always negative in the superconducting film with a periodic array of magnetic dots upon it. The dependence of (delta)Tc on geometrical factors and the external magneticfield is found. The theory is extended to multilayer structures. Next, we study the anisotropy dependence of the critical current in a thin superconducting film with a periodic array of magnetic dots with magnetization perpendicular to the film with spontaneous vortices and antivortices. The phase diagrams for the appearance of spontaneous vortices and antivortices are given for the square arrays of circular and square F dots respectively when the direction of the magnetization is parallel to the superconducting film.

Thesis (Ph. D)--Ohio State University, 2003. / Title from first page of PDF file. Document formatted into pages; contains xiii, 132 p.: ill. Includes abstract and vita. Advisor: Gregory R. Baker, Dept. of Mathematics. Includes bibliographical references (p. 131-132).

Thesis--M. Sc., University of Hong Kong, 1982.

Thesis (Ph. D.)--Lehigh University, 2003. / Includes vita. Includes bibliographical references (leaves 121-126).