Characterizing Turbulent Events at a Tidal Energy Site from ADCP Data

In recent years, there has been increasing interest in using tides as a clean, renewable energy source, but in order to move from the design stage to production, a more comprehensive characterization of turbulence is needed to predict loads on a turbine in a tidal environment. The commonly used turbulence intensity metric does not describe turbulent length or time scales, nor does it give any directional information. Using acoustic doppler current profiler (ADCP) data from Admiralty Inlet, WA, several additional statistics are calculated to more fully characterize the turbulence that a turbine would experience in a tidal flow. These metrics include coherent turbulent kinetic energy, time-frequency (wavelet) properties, measures of anisotropy, and structure functions. Coherent turbulent kinetic energy (CTKE) highlights intermittent turbulent events and wavelet analysis of the CTKE can be used to predict loading events on the turbine. Anisotropy invariant analysis allows the anisotropy to be quantified in terms of one, two, or three (isotropic) component turbulence. Structure functions are typically used to predict spectral slopes or dissipation rates that are then used as inputs to synthetic turbulence generators for simulations of tidal turbines, but a more in-depth analysis can differentiate between isotropic, low-turbulence events and their counterparts which cause unwanted loads on a turbine. Not only do these results provide useful information about large, anisotropic eddies that affect tidal energy production, but they exemplify the wide range of possible quantities available from the simple velocity component observations of an ADCP. These quantities can be used to generate realistic inflow and boundary conditions for simulations of tidal turbines, and can also be used to validate results from the simulations.