Wavelet applications for turbulence characterisation of real tidal flows measured with an ADCP

Understanding turbulence is crucial to the design of tidal energy converters as it influences loads, fatigue life and power production. Acoustic Doppler Current Profilers (ADCPs) are normally employed in measurement campaigns to analyse turbulence at a particular location. Coherent turbulent structures are known to lead to the highest turbulence-induced stresses and are typically analysed by Fourier transform. However, this method can only yield parameters which are averaged over the signal length or the analysing window, and hence may not fully represent real flow turbulence, which is often non-stationary. This study applies wavelet time-frequency analysis to ADCP data from an active tidal test site to examine the turbulence spectrum and coherence characteristics of the flow, comparing these to Fourier methods. Results show that the average quantities from wavelet analysis agree favourably with Fourier analysis. Moreover, wavelet analysis yields instantaneous spectra and coherence, which show that at this site, high energy bursts occur at frequencies known to be significant for TEC loading and have turbulence intensities up to 80% higher than the average. We show that these intermittent, coherent bursts are obscured by the averages associated with Fourier analysis, highlighting the need for methods such as wavelets to understand interactions between fluid flow and TEC.

Automated summary

Understanding turbulence is crucial for tidal energy converter design. Acoustic Doppler Current Profilers (ADCPs) are typically used to analyze turbulence, but Fourier analysis may not fully represent the non-stationary flow turbulence. This study applies wavelet time-frequency analysis to ADCP data and compares it with Fourier methods. Results show that wavelet analysis provides better representation of the turbulence spectrum and coherence characteristics, highlighting the need for such methods to understand fluid flow and tidal energy converters.