Abstract
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.
Highlights
If tidal stream energy is to be competitive with other forms of energy generation, tidal turbines must be economical to manufacture and operate reliably over their design life of at least 20 years [1,2]
Such test data are necessary to quantify the significance of the unsteady hydrodynamic loading contributions, verify numerical models and develop design guidelines to account for unsteadiness
The primary aim of this paper is to present and discuss recent advances that have been made on the characterisation of turbulence at tidal energy sites and the hydrodynamic loading on tidal turbines
Summary
If tidal stream energy is to be competitive with other forms of energy generation, tidal turbines must be economical to manufacture and operate reliably over their design life of at least 20 years [1,2]. While numerous studies have characterised the hydrodynamic loads on tidal turbines for steady flow (see, for instance, [12,13]), studies for unsteady flow conditions have been limited Such test data are necessary to quantify the significance of the unsteady hydrodynamic loading contributions, verify numerical models and develop design guidelines to account for unsteadiness. The lack of test data on unsteady loading is arguably attributed to the substantial cost of using large, high quality facilities which are required to account for the effects of scaling and to reduce the effects of blockage This calls for appropriate methodologies to be developed to enable the underlying hydrodynamic phenomena and their complex interactions to be quantified. Obtaining measurements of the out-of-plane (thrust-wise) hydrodynamic loading is of utmost importance as it is this component which typically governs the total structural loading on a horizontal-axis tidal turbine [14,15]
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