Ian Milne

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

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.

The structure of turbulence in a rapid tidal flow

The structure of turbulence in a rapid tidal flow is characterized through new observations of fundamental statistical properties at a site in the UK which has a simple geometry and sedate surface wave action. The mean flow at the Sound of Islay exceeded 2.5 m s−1 and the turbulent boundary layer occupied the majority of the water column, with an approximately logarithmic mean velocity profile identifiable close to the seabed. The anisotropic ratios, spectral scales and higher-order statistics of the turbulence generally agree well with values reported for two-dimensional open channels in the laboratory and other tidal channels, therefore providing further support for the application of universal models. The results of the study can assist in developing numerical models of turbulence in rapid tidal flows such as those proposed for tidal energy generation.

A preliminary analysis of the effect of the onset flow structure on tidal turbine blade loads

A significant challenge designers of tidal stream turbines face is ensuring that the device can survive the harsh operating environment. Understanding the effect of the onset flow structure on the loading spectrum of the blades will lead to improved blade designs. The results of a preliminary parametric analysis suggest that the mean flow speed, longitudinal turbulence intensity, wave state and depth of hub relative to the channel depth are dominant drivers of the blade loads for horizontal-axis tidal turbines. They should therefore be accurately modeled in both fatigue and extreme load analyses.