The characterisation of the hydrodynamic loads on tidal turbines due to turbulence

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

Rotor Loading Characteristics of a Full-Scale Tidal Turbine

“…The variation in the spatial characteristics of turbulence suggests that the turbine's blades face anisotropic turbulent flow conditions during its operation. The time-dependent heterogeneous distribution of I x between ebb and flood tides was also found at other sites such as the Sound of Islay (UK) where the turbulence intensities can vary between 0.11 and 0.13 during flood and ebb tides respectively, or at the East river (US), which is slightly more turbulent attaining values of I x equal to 0.13 (flood) and 0.18 (ebb) [22]. Further analysis of the spatial variability of the velocities across the water column is performed in Figure 9 with the Power Spectral Density (PSD) distribution of 10 min bin velocities during peak flood and ebb tides at hub height (H hub ) and at bottom and top tips, i.e., z = H hub ± D/2.…”

Characterisation of Tidal Flows at the European Marine Energy Centre in the Absence of Ocean Waves

“…These include information varying across annual and seasonal time scales to fluctuations in velocity at time scales of seconds and below, with different scales of motion understood to have varying effects on energy extraction devices [10]. Similarly, knowledge of the spatial variation of flow parameters is required across a wide range, from orders of tens of blade diameters for, e.g., power curve production [11] to variations of metres and below for investigations into device loading and blade fatigue [12,13] and quality of power [14].…”

Currents, Waves and Turbulence Measurement: A View from Multiple Industrial-Academic Projects in Tidal Stream Energy

“…In Nova Scotia, Canada, Sustainable Marine Energy are operating a floating platform hosting 4 x 250 kW turbines, focussing on community scale systems. 1 Current, wave and turbulence metrics -and the interaction between these processes -are used to predict available resource, to determine power quality fluctuations and to assess fatigue and extreme loading of TECs [2]. Required both for individual TECs and interactions between arrays of TECs, incorporation of these metrics into engineering tools informs design.…”

“…Real time control of the TEC, seeking to maximise power output while minimising loads, depends on a feedback loop between measured and/or expected current speed and operation of the TEC [20]. • Turbulence: Generated by shear instabilities, turbulence is part of the velocity field in an area and has multiple scales from vortices the width of a tidal channel to microscale eddies, millimeters in size [2]. Understanding is required of the level (turbulence intensity) and nature (lengthscale) of both ambient turbulence (i.e.…”

“…large scale eddy structures) to blade lengths and smaller (e.g. microscale eddies) and at temporal scales ranging from years (solar/lunar cycle) to seconds (turbulent gusts) [2]. As illustrated in Fig.…”