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Title: A fluid dynamic study of free-surface proximity and inertia effects on tidal turbines
Author: Whelan, Joanna Isabelle
ISNI:       0000 0004 2685 5461
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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A combination of theoretical, computational and experimental work has been conducted to investigate both free-surface proximity effects and inertia effects on tidal turbines. The free-surface proximity effects include transverse blockage in linear arrays of rotors. The inertia (added mass) effects investigated are such as a turbine may be subjected to due to passing waves. Free-surface proximity effects on the flow field around tidal-stream turbines have been modelled using actuator disc theory. Theoretical results are presented for a blocked configuration of tidal-stream turbines. As a first step to simulate the flow field a porous disc and a strip were tested in a water flume and measurements of axial thrust were taken using a cantilevered strain gauge arrangement. These results were then complemented by more detailed measurements on a rig which was designed and built for taking steady measurements of both power and thrust on a horizontal-axis rotor. The measurements of power were obtained using a friction brake (torque) and an infra-red tachometer (rotation rate). Measurements were carried out in both a water flume and a wind tunnel representing highly blocked and effectively unblocked cases, respectively. The results of these investigations into free-surface proximity have been incorporated into a blade element momentum code as used in the industry and agreement is found to be good for a range of turbine operating states. Where agreement weakens alternative corrections are suggested. A second rig was designed and built for investigation of inertia effects on horizontal- axis rotors. This rig employed an electromagnetic brake to provide a constant torque and was subjected to planar oscillations in a water flume. Instantaneous measurements of axial thrust and rotation rate were taken using a strain gauge arrangement and a shaft encoder respectively, in order to assess the magnitude of the out-of- plane (axial) and in-plane (rotational) added inertia. The experiments conducted spanned a range of Keulegan-Carpenter numbers and tip speed ratios applicable to a full-scale turbine. These experimental results were compared to theoretical results generated using semi-empirical theories and unsteady aerofoil theory. The component of the force in phase with acceleration is shown to be small compared with the force in phase with the velocity in both the out-of-plane and in-plane direction. However the added inertia is shown to be significant compared to the structural inertia. Two-dimensional unsteady aerofoil theory, applied on a sectional basis, provides a prediction for the added mass. The axial added mass is predicted to be of greater magnitude than the structural added mass, whilst the rotational added mass is predicted to be comparable to the rotational structural inertia. Unsteady aerofoil theory also provides an insight into the added mass and wake effects which overlap for a rotor operating in unsteady flow.
Supervisor: Graham, Mike Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available