Marine Hydrokinetic Turbines (MHkT) are a new class of renewable energy devices that harvest the kinetic energy of the flowing water in rivers or tides. In these environments, the approach flow contains elevated levels of free-stream turbulence (FST) and large coherent structures, which affect the performance and structural loading of the turbine as well as the signature of the downstream wake. Very little is understood about these interactions and how they cross-couple to impact river morphology, flood conveyance, and sediment transport. The current work uses controlled laboratory experiments to investigate the effects of FST on both component and system level metrics of MHK turbines. Homogeneous, free-stream turbulence levels ranging from 1% to 10% were achieved by employing a Makita type active-grid turbulence generator that is placed at the entrance of the water channel test section and is equipped with motor controlled winglet shafts. For component level measurements, loads acting on an MHK turbine hydrofoil at angles of attack ranging from −40° to +40° were measured using a two-axis load cell. Turbulence was shown to influence the stall angle of the hydrofoil. Stall occurred at ∼17° in the laminar free-stream but was postponed to ∼25° in the turbulent free-stream. An increase in the lift was observed at all angles greater than 10° with the increase being more significant in the post-stall regime. The drag measured in the turbulent free-stream was higher at angles less than 20° and greater than 25 ° but was lower at points in between. Concluding from the obtained lift to drag ratios, FST was observed to decrease performance while operating at angles less than the stall angle, and increase performance in the post-stall regime. For the system level measurements, a comparison of performance characteristics that includes the mean and standard deviations of the power coefficient (CP), and thrust coefficient (CT) between the turbulent and the laminar free-stream cases was performed. The results show a ∼2 fold increase in standard deviation of CT; however, elevated levels of FST have a weak effect on mean performance characteristics.
Effect of Free-Stream Turbulence on the Loads Experienced by a Marine Hydrokinetic Turbine
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Lawrence, AM, Vinod, A, & Banerjee, A. "Effect of Free-Stream Turbulence on the Loads Experienced by a Marine Hydrokinetic Turbine." Proceedings of the ASME 2016 International Mechanical Engineering Congress and Exposition. Volume 7: Fluids Engineering. Phoenix, Arizona, USA. November 11–17, 2016. V007T09A091. ASME. https://doi.org/10.1115/IMECE2016-68395
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