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Cross-flow turbines, also known as vertical-axis turbines, have numerous features that make them attractive for wind and marine renewable energy. To maximize power output, the turbine blade kinematics may be controlled during the course of the blade revolution, thus optimizing the unsteady fluid dynamic forces. Dynamically pitching the blades, similar to blade control in a helicopter, is an established method. However, this technique adds undesirable mechanical complexity to the turbine, increasing cost and reducing durability. Here we introduce a novel alternative requiring no additional moving parts: we optimize the turbine rotation rate as a function of blade position resulting in motion (including changes in the effective angle of attack) that is precisely timed to exploit unsteady fluid effects. We demonstrate experimentally that this approach results in a 79% increase in power output over industry standard control methods. Analysis of the fluid forcing and blade kinematics show that maximal power is achieved through alignment of fluid force and rotation rate extrema. In addition, the optimized controller excites a well-timed dynamic stall vortex, as is found in many examples of biological propulsion. This control strategy allows a structurally robust turbine operating at relatively low angular velocity to achieve high efficiency and could enable a new generation of environmentally-benign turbines for wind and water current power generation.

One calendar year of Automatic Identification System (AIS) ship-traffic data was paired with hydrophone recordings to assess ambient noise in northern Admiralty Inlet, Puget Sound, WA (USA) and to quantify the contribution of vessel traffic. The study region included inland waters of the Salish Sea within a 20 km radius of the hydrophone deployment site. Spectra and hourly, daily, and monthly ambient noise statistics for unweighted broadband (0.02-30 kHz) and marine mammal, or M-weighted, sound pressure levels showed variability driven largely by vessel traffic. Over the calendar year, 1363 unique AIS transmitting vessels were recorded, with at least one AIS transmitting vessel present in the study area 90% of the time. A vessel noise budget was calculated for all vessels equipped with AIS transponders. Cargo ships were the largest contributor to the vessel noise budget, followed by tugs and passenger vessels. A simple model to predict received levels at the site based on an incoherent summation of noise from different vessels resulted in a cumulative probability density function of broadband sound pressure levels that shows good agreement with 85% of the temporal data.

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Whitt et al. Future of Autonomous Ocean Observations reductions. Cost reductions could enable order-of-magnitude increases in platform operations and increase sampling resolution for a given level of investment. Energy harvesting technologies should be integral to the system design, for sensors, platforms, vehicles, and docking stations. Connections are needed between the marine energy and ocean observing communities to coordinate among funding sources, researchers, and end users. Regional teams should work with global organizations such as IOC/GOOS in governance development. International networks such as emerging glider operations (EGO) should also provide a forum for addressing governance. Networks of multiple vehicles can improve operational efficiencies and transform operational patterns. There is a need to develop operational architectures at regional and global scales to provide a backbone for active networking of autonomous platforms.

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In laboratory experiments involving wind or water turbines, it is often desirable to correct measured performance for the effects of model blockage. However, there has been limited experimental validation of the analytical blockage corrections presented in the literature. Therefore, the objective of this study is to evaluate corrections against experimental data and recommend one or more for future use. For this investigation, we tested a crossflow turbine and an axial-flow turbine under conditions of varying blockage with other non-dimensional parameters, such as the free-stream Reynolds and Froude numbers, held approximately constant. We used the resulting experimental data to assess the effectiveness of multiple analytical blockage corrections for both turbine types. Of the corrections evaluated, two are recommended. However, as these methods are based on axial momentum theory, we observe that corrections are more effective for thrust than power. We also find that increasing blockage changes the local Reynolds number, which can affect turbine performance but is not reflected in axial momentum theory.

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