Turbulence and coherent structure characterisation in a tidally energetic channel

Understanding the temporal and spatial characteristics of turbulent coherent structures is of interest to the emergent sector of marine renewable energy for power generation from tidal stream turbines as loading due to these vortex structures has resulted in costly device failure. Here methods for characterising these coherent structures are developed in the Menai Straits, Anglesey, using an off-the-shelf broadband acoustic Doppler current profiler (ADCP) vertical beam with the metrics fast Fourier transforms and a wavelet element model. Results indicate lengthscales fall in the range 2.5–51 m. Focused study on a 30-min window finds the 5 most powerful features have a median lengthscale of 13.2 m and the strongest signal lies at ∼6.8 m, which scale to 0.86 and 0.44 times the water depth respectively, these features have a periodicity of ∼105 s. Methods using variance across ADCP beams are common for turbulence characterisation within the tidal energy sector, with turbulence intensity being appropriated from the wind energy sector. However, turbulence intensity when using an ADCP is found to be a poor predictor of water column turbulence in the presence of coherent structures.

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