Date of Award

Winter 2025

Project Type

Dissertation

Program or Major

Ocean Engineering

Degree Name

Doctor of Philosophy

First Advisor

Martin Wosnik

Second Advisor

Thomas C Lippmann

Third Advisor

Ivaylo Nedyalkov

Abstract

The oceans are a largely untapped resource with regard to their tidal, wave, and ocean current energy potential and could significantly contribute to clean energy objectives. Cross-flow turbines (CFTs) can be used to extract energy from tidal currents and provide electrical energy to the power grid. However, one of their main barriers to market entry is their levelized cost of energy (LCOE), which at present is still quite high. Three parameters that have not previously been studied together, and that could make CFTs more cost-competitive, include the use of less rigid and less expensive materials, a reduction in the number of required blade supports, and the use of additive manufacturing with its associated blade surface roughness. This work aimed to understand the implications of these parameters for CFTs and was divided into three objectives.

The first objective was to investigate turbine performance in relation to varying blade materials and unsupported blade span lengths correlated with overall turbine torque, thrust, and blade strain data. This involved conducting experiments with a 1-meter diameter vertical-axis CFT consisting of three NACA 0018 blades with two support struts in a turbine test bed within the University of New Hampshire (UNH) towing tank. The impact of blade flexibility on turbine performance was examined by varying the blade materials and support strut position. The blade materials tested were carbon, E-glass, and hollow E-glass fiber composites, in decreasing order of stiffness and cost. The lower strut was fixed in position while the upper strut was adjusted to five different positions, changing the length of the free blade end: $z/H = $ 0.14, 0.25, 0.5, 0.75, and 1.0, where z is the distance between the lower and upper struts and H is the blade span. Previous and ongoing studies have captured blade strain at discrete locations, but one unique aspect of this work was to embed high-resolution low-noise distributed fiber optic sensors. The sensors recorded hundreds of strain measurements in two of the three rotor blades along both sides of the blades. Turbine performance was be measured by varying Reynolds numbers and tip speed ratios. Each power curve utilized a towing speed that was sufficiently high for the performance to be independent of Reynolds number. E-glass blades and carbon blades performed similarly for the most rigid strut configurations of $z/H =$ 0.5 and 0.75. Higher strain was measured on the E-glass blades, and their performance was reduced for less rigid configurations compared to the carbon fiber blades. The performance of the highly deflective hollow E-glass blades was lower overall and became even more degraded for longer unsupported blade span less than $z/H =$ 0.5. The results provided insight into the use of various blade materials in cross-flow turbines and guidance on allowable free end length for each material type.

The second objective was to analyze blade strain and rotor torque to gain insight into cross-flow turbine rotor dynamics. Phase-averaged blade strain and rotor torque were examined to characterize variations in azimuthal position associated with maximum blade strain. Maximum blade strain occurred over a large azimuthal range of about $30^\circ$ to $80^\circ$ for tip speed ratios less than two, but shifted to a higher and tighter azimuthal range of about $70^\circ$ to $94^\circ$ for tip speed ratios greater than two. This indicated that the location where the leading edge vortex forms and sheds was shifted in azimuthal position. The peak in blade strain was closely associated with the peak in torque, while a downstream blade strain ripple was present that aligned with peaks in torque associated with the other two blade passes. At low tip speed ratios, there was high positive torque and high negative torque values due to a stronger dynamic stall at lower tip speed ratios. At higher tip speed ratios, there was a small reduction in positive torque, but a larger reduction in negative torque that resulted in a higher power coefficient. Differences between upstream and downstream loading, as well as blade twist and displacement were analyzed. The blade displacement and twist was largest for the least rigid configuration of $z/H =$ 0.25 for all three blade materials due to the distributed loading on the unsupported blade span. These results linked the azimuthal position of maximum blade strain and rotor torque to blade geometry, material selection, and strut support spacing, providing guidance to advance cross-flow turbine design and performance.

The third objective was to investigate the impact of blade surface roughness on performance from an additive manufacturing perspective. In particular, additive manufacturing can provide multiple benefits including lowering the cost of production through a reduction in material waste, a quick turnaround in comparison to conventional techniques, and the ability to print complex geometries. However, the resulting physical properties of the product, specifically surface roughness, can negatively impact cross-flow turbine performance, similar to marine biofouling. The impact of surface roughness on performance was analyzed by comparing two helical blade materials with varying surface roughness: electron beam melting (EBM) 3D-printed titanium and composite E-glass fiber. With these two blade material types, turbine performance was compared across a total of three surfaces with varied roughness, i) titanium blades with the as-printed rough surface, and ii) titanium blades after smoothing the blade surface, and iii) smooth E-glass fiber blades coated with epoxy. An entirely negative power curve was produced when stepping through a range of tip speed ratios for the rough titanium blades. After reducing surface roughness through sanding, the rotor with smoothed titanium blades produced a positive power curve. However, the maximum power coefficient was still significantly lower than the reference turbine with smooth E-glass blades. Blade surface roughness measurements show that turbine performance correlates with surface roughness viscous heights calculated from operating conditions, which can indicate the presence of a rough wall boundary layer. It is suggested that turbine blades must generally be hydraulically smooth to obtain their maximum power coefficients, except where strategically placed roughness could be beneficial under certain conditions. These results were in agreement with prior studies of roughness on CFTs.

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