Utilization of Decomposition Techniques for Analyzing and Characterizing Flows

Abstract: This thesis presents the utilization of two different decomposition techniques, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), for enhanced understanding of flow structures and their stability. The advantages of these techniques are shown for a range of flow situations, most of which are turbulent. It is shown that by these methods additional insight into complex flow situations can be gained. Such insight has been found to be needed for the flow in straight and 90 degree curved pipes. The so-called swirl switching phenomenon is investigated, which is a large scale oscillation of the flow after the bend. This phenomenon is classified into a low frequency and a high frequency switching, each with its own mechanism of formation. It is shown that while the low frequency switching stems from very-large-scale motions created in the upstream pipe, the high frequency switching results from the bend itself, making it an inherent property of the system. The second set of studies consider swirling flow in combustor-related geometries, using both high and low swirl levels. These investigations show highly energetic unsteady structures in the strongly vortical regions. The spatial symmetry of these flow modes reflect the level of confinement. While the vortices that are weakly confined show unsteady modes reflecting their displacement, the strongly confined vortices show low-order multipole deformations. For the low swirl burner, which is the only reacting flow considered, the flame is stabilized without the presence of vortex breakdown. To be able to investigate how the flame is anchored above the burner, an extended version of DMD (EDMD) is introduced, which helps to couple the flow with the flame. Using this method, a mechanism contributing to the flame stabilization is isolated. The third and final set of studies involve flow around cylinders and beams. These objects are flexible and respond to the forces that the flow exerts on them. For the flow around cylinders, which are connected to a spring system, the natural frequency of the spring-cylinder system and the frequency from the von Karman vortex shedding are the two a priori known frequencies of the system. Three different flow regimes are considered, one where the two frequencies are similar, giving resonance, and two cases where one frequency is far above/below the other. For flow around a single cylinder, an unexpected high energy low frequency mode is found off-resonance, which is argued to contribute greatly to the chaotic behaviour for the case with the loose spring. For a multiple cylinder array, while the strong low frequency mode found for the single cylinder case has been suppressed, an unexpected synchronization is seen. Considering the flow around a stiff and a flexible beam, a strong beat frequency is found for the lift force. While the beating is seen to be regular for the flexible beam, it appears intermittently for the stiff beam. The flow behaviour giving rise to this forcing is elucidated using the POD and DMD analyses.