A comprehensive assessment of turbulence at a tidal-stream energy site influenced by wind-generated ocean waves

Abstract

Velocity measurements collected by an upward-looking acoustic Doppler current profiler were used to provide the first study of ambient turbulence in Alderney Race. Turbulence metrics were estimated at middepth during peak flooding and ebbing tidal conditions. The dissipation rate ε and the integral lengthscale (L) were estimated using two independent methods: the spectral method and the structure function method. The spectral method provided ε and (L) estimates with standard deviations twice lower than that obtained from the structure function method. Removal of wave and Doppler noise-induced bias when estimating the dissipation rate was shown to be a crucial step in turbulence characterization. It allowed for a significant refining in (L) estimates derived from the spectral and structure function methods of 35 and 20 respectively. The integral lengthscale was found to be 2-3 times the local water depth. It is considered that these findings could be valuable for current turbine designers, helping them optimizing their designs as well as improving loading prediction through the lifetime of the machines. Highlights ► The first study of ambient turbulence in Alderney Race is provided. ► Two independent methods are used to quantify turbulence metrics. ► Removal of wave and Doppler noise-induced bias is a crucial step. ► The integral lengthscale was found to be 2-3 times the local water depth. ► Unique dataset of turbulence metrics computed at mid-depth are provided.

Wave-Turbulence Decomposition Methods Applied to Tidal Energy Site Assessment

“…These findings also reinforce the results of other studies that the estimation of turbulence parameters can be biased by the presence of surface waves. For instance, Thiébaut et al [43] has estimated turbulence dissipation rates and integral scales using ADCP data from Alderney Race, France. Their findings reveal estimated parameters were sensitive to wave-induced motion.…”

Underway velocity measurements in the Alderney Race: towards a three-dimensional representation of tidal motions

“…Thiébaut et al [42] analysed the velocity data recorded by a bottom-mounted ADCP deployed in Aldernney Race, 4 km west off the Cotentin Peninsular (latitude of Goury in figure 1), for assessing parameters of the turbulence in the tidal flow. It was argued that below the surface layer affected by wave motions, the velocity shear is probably the major parameter controlling 701 the evolution of turbulence in the water column.…”

The Alderney Race: general hydrodynamic and particular features

“…(Online version in colour.) information concerning current variability, wave effects and fluctuations that are described elsewhere [14,15] and in this theme issue [16]. In this study, we only used the average current through the water column at one station for comparison with two-dimensional hydrodynamic model results.…”

“…Measurements obtained during the HYD2M project and the development of coupled three-dimensional hydrodynamic models provide ways to account for all phenomena and improve our knowledge of most of the physical mechanisms involved. Recent studies have opened the way [15,40] and those presented in this theme issue have made significant progress. Differences between measurements and simulations show that many questions remain open concerning the turbulence and the kinetic energy transfer at different scales.…”