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 nonslack 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.

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An improved characterisation of the hydrodynamic blade loads due to onset turbulence is essential in order to mitigate premature failures, reduce excessive levels of conservativeness and ultimately ensure the commercial viability of tidal turbines. The literature focussing on the turbulence in fast flowing tidal streams and of the unsteady loads that are subsequently imparted to rotors has previously been very limited. However, increased activity in the tidal energy community has led to new investigations and insights which are reported in this paper. It has been found that through the use of acoustic Doppler-based sensors, the streamwise turbulence intensities generally tend to a value of approximately 6–8% at the mid-depth of proposed tidal energy sites. Evidence that the anisotropic structure and scales of the turbulence are more consistent with open-channel-based models than atmospheric-based correlations has also been found. Rapid distortion theory has been applied to estimate that the standard deviation of the streamwise turbulent velocity fluctuations in the onset free-stream flow may be amplified significantly by 15% due to the presence of a turbine. The turbulent fluctuations have also been predicted to remain well correlated over the outer span of the blades at the rotational frequency of the rotor. Recent model-scale experiments have enabled the unsteady hydrodynamic loading to be isolated from the steady-flow loading. For cases where the boundary layer remains primarily attached across the blades, this has enabled linear transfer functions to be developed and applied to model the response to a multi-frequency forcing. It has also been found that phenomena consistent with delayed separation and dynamic stall can result in a blade root bending moment that exceeds the steady value by 25%, and this needs to be taken into account in design to reduce the probability of failure

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An improved characterisation of the unsteady hydrodynamic loads on tidal turbine blades is necessary to enable more reliable predictions of their fatigue life and to avoid premature failures. To this end, this paper presents a set of blade-root bending moment responses for a scale-model tidal turbine subjected to an unsteady planar forcing in a towing tank. In cases where the boundary layer was believed to be attached to the outer sections of the blade, the out-of-plane bending moment amplitude for unsteady flow was up to 15% greater than the corresponding load measured in steady flow and exhibited a phase-lead of up to 4.5°. Both these observations are qualitatively consistent with the effects of dynamic inflow and non-circulatory forcing. The bending moment responses for a forcing time history that comprised multiple frequencies, as well as for a discrete half-sinusoidal perturbation, were able to be reconstructed reasonably well using the responses obtained from single-frequency oscillatory flows. This suggests that blade designers could utilise relatively low fidelity techniques and conduct potentially fewer experimental tests to acquire the fatigue load spectrum