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 characteris-
ing these coherent structures are developed 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 length-
scales fall in the range 2.5 to 51 m. Focused study on a 30-minute 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 0.9 and 0.4 times the water
depth respectively, these features have a periodicity of ∼127 s. Methods us-
ing 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 is found to be a poor predictor water column turbulence in the presence of coherent struc-
tures.
Keywords: Hydrodynamics, Tidal stream turbines, Coherent Structures,
Tidal Power, Variance Method, Alternative Energy Site Assessment