Analysis of Turbulence Parameters for a Tidal Energy Site in a Wave-Current Environment

Tidal power converters deployed in areas where waves and tidal currents coexist face high levels of turbulences, which can lead to unstable power generation and increase the risks of turbine blade fatigue failure. In order to improve the design and safety operation of tidal turbines in a combined wave-current environment, it is necessary to understand and characterise the turbulence parameters such as turbulence intensity (TI)/turbulent kinetic energy (TKE) etc. It is also important to characterise how waves and currents separately affect these parameters. In this study, field data collected by an Acoustic Doppler Current Profiler (ADCP) at the Pentland Firth, Orkney Islands, Scotland, are analysed to evaluate the levels of TI produced separately by waves and tidal currents. The empirical mode decomposition (EMD) method has been utilised for this analysis. At first, using the EMD, the velocity components corresponding to waves and tidal current components are separated. In order to verify the separation methodology, a two step process is adopted. In step one, wave component velocities are converted to significant wave heights using linear wave transfer function. For the second step, the significant wave heights obtained from step one are compared with the same hindcast by a coupled wave-current numerical model. A very good match between wave heights is observed with a correlation coefficient of 0.8, thus validating the methodology followed. Then, the depth-varying vertical profiles of the turbulence intensity (TI) of the decomposed wave and current components are calculated. It is found that the streamwise wave-induced TI is about 5% at the height of 15m where the turbine hub is placed, while the tidal current-induced TI is 10% - 17%. The wave-current combined TI is 13%-20%. This study has demonstrated a methodology to successfully separate and quantify turbulence intensities produced individually by waves and tidal currents when they coexist at a site.