Characteristics of the turbulence in the flow at a tidal stream power site

TLDR

The study aims to improve confidence in the tidal energy industry by characterizing high‑frequency flow components to enable more realistic performance and loading predictions. The authors analyzed velocity time histories recorded 5 m above the seabed in a tidal stream with a mean speed of 2.5 m s⁻¹. During spring‑tide cycles, streamwise turbulence intensity was 12–13 %, with a 1 : 0.75 : 0.56 intensity ratio, an f⁻²⁄³ inertial subrange, local isotropy above 0.5 Hz, and integral time and length scales of about 6 s and 11–14 m, all larger than transverse and vertical components.

Abstract

This paper analyses a set of velocity time histories which were obtained at a fixed point in the bottom boundary layer of a tidal stream, 5 m from the seabed, and where the mean flow reached 2.5 m s −1 . Considering two complete tidal cycles near spring tide, the streamwise turbulence intensity during non-slack flow was found to be approximately 12–13%, varying slightly between flood and ebb tides. The ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities is typically 1 : 0.75 : 0.56, respectively. Velocity autospectra computed near maximum flood tidal flow conditions exhibit an f −2/3 inertial subrange and conform reasonably well to atmospheric turbulence spectral models. Local isotropy is observed between the streamwise and transverse spectra at reduced frequencies of f >0.5. The streamwise integral time scales and length scales of turbulence at maximum flow are approximately 6 s and 11–14 m, respectively, and exhibit a relatively large degree of scatter. They are also typically much greater in magnitude than the transverse and vertical components. The findings are intended to increase the levels of confidence within the tidal energy industry of the characteristics of the higher frequency components of the onset flow, and subsequently lead to more realistic performance and loading predictions.