Date

1-1-2020

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Arindam . Banerjee

Abstract

Tidal turbines are designed to be deployed in flows that are turbulent and spatially non-uniform in nature due to wave-current interactions, bottom boundary layer effects, and secondary flows resulting from bathymetry features. Accounting for the effects of such turbulent, sheared flow conditions are important from a design standpoint as it would aid in developing robust and cost-efficient turbine designs that would bring down the Levelized Cost of Energy. The presented research work provides detailed measurements of the dynamic near-wake of a tidal turbine model (1:20 scale) in experimentally generated turbulent flow environments similar to marine energy deployment sites. An active grid type turbulence generator developed during this dissertation research was used to generate the different turbulent inflow conditions in our free-surface water tunnel facility.

In this work, we report the first laboratory-scale experimental study that decouples the effects of inflow turbulence intensity (Ti) and integral length scale (L). To the knowledge of the authors, the work is also the first of its kind to explore the effects of sheared-turbulent inflows on a tidal turbine model. Five inflows were tested in the experiments, three homogenous cases; a 2.2% Ti that corresponds to a quasi-laminar flow, an elevated Ti case with a Ti, L of 12.6 % and 0.4D respectively (D is the diameter of the turbine model) and an elevated Ti-LD case with Ti =13.9% and L=D, and two non-homogeneous cases; a low-shear case with 15% Ti at the bottom of the rotor to 4% Ti at the top, and high shear case with 25% Ti at the bottom of the rotor to 6% Ti at the top. Based on the obtained measurements, Ti was observed to have a significant impact on the load fluctuations acting on the rotor; the maximum standard deviation of rotor torque measured in elevated Ti was found to be 4.5 times the corresponding value in the quasi-laminar flow. The increase in torque fluctuations observed in both the sheared inflow cases were similar to each other and were 40% higher than the quasi-laminar case. Energy recovery was also found to be considerably quicker in elevated Ti; at X/D=4, the percentage of inflow energy recovered was 37% and was twice the corresponding value in the quasi-laminar flow. The tendency of high ambient turbulence to disrupt the rotational character of the wake was observed by estimating the swirl number; in comparison to the quasi-laminar flow case, the drop-in swirl number in the elevated Ti ranged between 12% at X/D=0.5 to 71% at X/D=4. All rotor related periodicities were found to be eliminated in elevated Ti by X/D=4.

The integral length scales in the near wake of the turbine were evaluated; the integral scales calculated downstream of the rotor in elevated Ti were considerably larger (> 2 times) than the scales calculated in the quasi-laminar flow. An increase in inflow integral length scale was observed to result in larger wake structures, wake Ti and anisotropy; however, no noticeable influence was found on the rate of wake velocity recovery. Unlike the near symmetric wake profiles observed in the quasi-laminar and elevated Ti cases, inflow shear was observed to incite considerable levels of asymmetry in the near-wake. Turbulence intensity, Reynolds stresses, and power spectral density estimated in the near-wake of the rotor were found to be affected by the inflow shear at locations as close as 0.5D from the rotor. However, the effect of shear on the wake velocity deficit and integral length scales was more gradual and evident only at locations further downstream (≥ 2D). Wake swirl characteristics in the sheared inflows were found to be influenced by the depth-averaged turbulence intensity of the inflow. The detailed experimental datasets presented would be valuable to modelers as it can be used to develop, validate, and verify numerical Models that analyze performance and wake predictions of tidal turbines in open-water deployment.

Available for download on Saturday, January 29, 2022

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