Date of Award

5-28-2025

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

fluid structure interaction; harvester; piezoelectric; Particle Image Velocimetry; renewable energy; vortex induced vibrations

Supervisor

Vesselin Stoilov

Supervisor

Vesselina Roussinova

Rights

info:eu-repo/semantics/openAccess

Abstract

This research investigates a hydrokinetic energy harvesting system consisting of oscillating tethered spheres submerged in water flow. When flow encounters a tethered sphere, the vortex induced vibrations (VIV) develop and the sphere starts to oscillate. The hydrokinetic energy of the turbulent flow is thus transferred to the mechanical energy of sphere vibrations. The implications of VIVs may be destructive for hydraulic structures and pipelines and are of significant practical interest. Alternatively, in this study, VIVs are harnessed and further converted to electrical power by piezoelectric sensor attached to the sphere. While VIV of elastically mounted cylinders and rigid spheres have been studied extensively, little is known for the wake of a tethered sphere. The laboratory experiments are first undertaken to study the wake of a single tethered sphere using Particle Image Velocimetry (PIV) technique. The PIV experiments provide information for the wake mean velocity, deficit and wake growth under VIV. Simultaneously with velocity measurements the sphere position was tracked and output voltage from the piezoelectric sensor were recorded for the same duration of the experiment. Analysis of sphere centroid motion provide information for the amplitude of VIVs and the outputted voltage signal is used to estimate the energy conversion. The wake deficit and wake growth obtained from a single harvester was further applied to five array sizes and the concept of wake induced vibrations (WIV) and the effect each unit has on the neighboring ones was investigated. Several Particle Image Velocimetry (PIV) and array configuration image processing experiments were conducted along with the collection of voltage measurements. This was done to study the effect of the array arrangement on the energy output and flow structures. Results have shown that the edges of the wake lie at a specific slope. These angles were used to determine the placement of additional units. The arrays were evaluated in terms of dominant centroid and voltage frequencies, mechanical energy, power, and efficiency. It was determined that as the array size increases, less spheres experience a reduction in centroid dominant frequency, mechanical energy, power, and efficiency. This study could be utilized towards engineering applications, relating to fluid structure interactions, VIVs, and novel renewable energy studies.

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