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Vibrant bodies of swirling flow: On the limits of mechanical power transformation
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Vibrerande kroppar av virvlande vatten : Energiomvandlingens gränser hos mekaniska system (Swedish)
Abstract [en]

This thesis pinpoints the intertwined nature of inertia. A vibrant body of a swirling flow is neither just a flow nor a body. It is both. The overall aim is to foresee and visualize vibrant motion and unsteady flow phenomena that follow with the release of power, addressed in the following two research problems: i) How are unsteady flow phenomena during the start-up sequence of a Kaplan turbine manifested as a vibratory twisting and heaving motion of its rotating system? ii) How does inertia come into play in the vibrant motion of a cylindrical interface adjacent to a solid core and a swirling flow confined by a concentric fixed cylinder? Dynamic measurements of axial and torsional strain on the middle shaft of a Kaplan-unit are represented by their energy intensity as a function of the orthogonal coordinates time and frequency. High energy intensities during adverse flow conditions are identified associated with coherent structures and their decay of turbulent flow in the draft tube and the swirl chamber above the runner. Previously non-identified self-sustained vibrations are detected during the loading of the generator, proposed to be induced by interaction of a resonator in the electrical system with an eigenmode of the hydraulic circuit. A strategy is developed to identify the dynamic behaviour of the rigid body mode of a swirling flow in a fixed cylindrical container with a solid core. The rigid body mode defines the fluid motion that follows by a motion of the rigid interface of the solid core to the swirling fluid. Interplay between fictitious forces yields rotating waves in the swirling flow and a vibrant motion of the solid core. The equation of motion of the rigid interface is analogous to a point mass subjected to external motion-induced forces. The vibratory motion of the rigid interface follows by a departure from a merged homogenous state, in which the solid core and fluid are of equal density and the fluid rotates as a solid body. As the rigid interface moves with a constant velocity, the constant vorticity of the fluid induces a fictitious force that opposes the fictitious Coriolis force induced by the solid body. If the angular velocity of the fluid is not constant, these forces do not cancel out each other, which implies that a swirling flow can withhold a solid core denser than the fluid in a concentric position.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2013.
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Research subject
Fluid Mechanics
URN: urn:nbn:se:ltu:diva-25835Local ID: b5c86814-2259-4434-b5c9-ece601183d8aISBN: 978-91-7439-714-7ISBN: 978-91-7439-715-4 (PDF)OAI: diva2:998992
Godkänd; 2013; 20130823 (idajoh); Tillkännagivande disputation 2013-09-06 Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Ida Jansson Ämne: Strömningslära/Fluid Mechanics Avhandling: Vibrant Bodies of Swirling Flow: On the Limits of Mechanical Power Transformation Opponent: Professor Don-Hyun Kim, Dept of Aerospace and System Engineering, Gyeongsang National University, Jinju, Sydkorea Ordförande: Professor Staffan Lundström, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Fredag den 27 september 2013, kl 09.00 Plats: E246, Luleå tekniska universitetAvailable from: 2016-09-30 Created: 2016-09-30Bibliographically approved

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