This paper contributes to the investigations into the feasibility of improving the performance of a marine current turbine using a biomimetic concept inspired from the leading-edge tubercles on the flippers of humpback whales. An experimental test campaign was recently conducted in the Emerson Cavitation Tunnel at Newcastle University and details of this test campaign together with the findings are summarised in the paper A set of tidal turbines with different leading-edge profiles was manufactured and tested to evaluate the hydrodynamic performance. Various tests were conducted at different flow speed and different pitch angle settings of the turbine blades. The results showed that the models with the leading-edge tubercles had higher power coefficients at lower tip speed ratios (TSRs) and at lower pitch angle settings where the turbine blades were working under stall conditions. Therefore, the tubercles can reduce the turbines' cut-in speed to improve the starting performance. The biomimetic concept did not compromise the maximum power coefficient value of the turbine, being comparable to the device without the tubercles, but shifted the distribution of the coefficient over the range of the tip speed ratios tested
The study presents an energy performance improvement measure for an Autonomous Underwater Vehicle (AUV) carrying oceanographic equipment for collecting scientific data from the ocean. The required electric energy for the on-board equipment is harvested from tidal energy by using twin horizontal axis turbines which are integrated with thin-wall diffusers to enhance their energy capturing performance. The main focus and hence objective of the paper is the optimal design of the diffusers by using Reynolds Average Navier–Stokes Equations (RANSE) based Computational Fluid Dynamics (CFD) method and the validation of the design using physical model tests. A goal-driven optimisation procedure is used to achieve a higher power coefficient for the turbine while keeping the size and the drag of the diffuser as practically minimum as possible. Two main parameters of the optimisation are selected, the outlet diameter and the expansion section length of the diffusers, which are optimised for the highest flow acceleration ratio at the diffuser throat and for the minimum drag of the integrated diffuser and turbine system which is called as "Diffuser Augmented Tidal Turbine" (DATT) system. The numerical optimisation is validated by two sets of physical model tests conducted with a single turbine without diffuser and the same turbine integrated with the diffuser (DATT) in a cavitation tunnel and a circulating water channel. These tests demonstrated a performance enhancement for the turbine with the optimal diffuser by almost doubling the power coefficient of the turbine without the diffuser. However, the performance enhancement was dependent upon the pitch angle of the turbine
Although there are different strategies to control the operation of marine turbines, the so-called ‘stall-regulated strategy’ is one of the most widely used and mature control strategies. Since the stall phenomenon is closely related to flow separation around the turbine blades the treatment of this separation requires great care during the design and performance analysis of turbines when using computational fluid dynamics (CFD). This study investigates appropriate methodologies and approaches to simulate the hydrodynamic performance of horizontal marine turbines with a specific emphasis on the flow separation phenomena. The well-known viscous flow solver ANSYS-CFX was employed as the main CFD code to predict the power extraction coefficient of these turbines. The investigations were carried out by using both numerical and experimental methods applied on tidal stream turbine models tested in the Emerson Cavitation Tunnel of Newcastle University, UK and the circulating water channel of Harbin Institute of Technology, China. The measured power extraction coefficients generally agreed well with the numerically predicted ones except for one of the models with the lower pitch angle which displayed large discrepancies over the entire operating range. The detailed flow analyses from the CFD studies with this turbine and other model at higher pitch angles revealed that large-scale detached vortices developed downstream of the model with the lower pitch angle may have contributed to this large discrepancy. The study therefore draws attention to the importance of the combined use of the CFD and model test-based approaches in the design and performance analysis of marine turbines.
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