Abstract
Experiments with a three-bladed, constant chord tidal turbine were undertaken to understand the influence of free surface proximity on blockage effects and near-wake flow field. The turbine was placed at various depths as rotational speeds were varied; thrust and torque data were acquired through a submerged sensor. Blockage effects were quantified in terms of changes in power coefficient and were found to be dependent on tip speed ratio and free surface to blade tip clearance. Flow acceleration near turbine rotation plane was attributed to blockage offered by the rotor, wake, and free surface deformation. In addition, particle image velocimetry was carried out in the turbine near-wake using time- and phase-averaged techniques to understand the mechanism responsible for the variation of power coefficient with rotational speed and free surface proximity. Slower wake propagation for higher rotational velocities and increased asymmetry in the wake with increasing free surface proximity was observed. Improved performance at high rotational speed was attributed to enhanced wake blockage, and performance enhancement with free surface proximity was due to the additional blockage effects caused by the free surface deformation. Proper orthogonal decomposition analysis revealed a downward moving wake for the turbine placed in near free surface proximity.
Highlights
Axial flow tidal turbines are installed near the free surface of a channel/river to take advantage of maximum volumetric flux available near the free surface [1]
Improved performance at high rotational speed was attributed to enhanced wake blockage, and performance enhancement with free surface proximity was due to the additional blockage effects caused by the free surface deformation
Though the solid blockage is constant for a turbine in its deployed channel area, the extent of wake blockage varies with flow speed and turbine rotation speed [3]
Summary
Axial flow tidal turbines are installed near the free surface of a channel/river to take advantage of maximum volumetric flux available near the free surface [1]. The majority of the reported experimental investigations available in the literature that quantify the effect of blockage on turbine performance are limited to turbines operating at a single depth of immersion; very little is known about the effect of free surface proximity on the turbine performance and the dynamics of the downstream wake. This work is the first detailed experimental study to explore the effects of free-surface proximity on the near-wake characteristics of a tidal turbine model. It adds to the data reported in a previous publication by the authors [3] that focused on the effects of free surface proximity on the performance characteristics the turbine model.
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