Flow control to mitigate fatigue load through the use of flexible tidal turbine blades

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Engineering

Abstract

The project is a feasibility study on the use of flexible blades to increase the durability and survivability of tidal turbines. The highly turbulent flow experienced by tidal turbines leads to continuous load variations limiting the fatigue endurance. It is proposed to use flexible blades in order to control the flow field around the blades and allow constant stresses on the turbine's blades and shaft. Under the effect of a variation of the onset flow velocity at a certain location along the blade span, the flexible blade will change the local angle of attack and the foil shape. The new flow field around the blade section will be as such to minimise the load change on the blade section. The reduced, if not completely avoided, load variations will enhance the fatigue endurance of the blade and the shaft.

Planned Impact

The project could provide proof of concept that flexible blades can allow low-amplitude stress variations in turbulent flow conditions. The potential industrial impact would be significant because it would solve one of the key challenges of the emerging tidal energy industry. Being a short feasibility study, the results will be used as preliminary data for a wider study, which will be proposed as a research proposal to the next joint MoST/EPSRC call on marine energy.

The project will produce new high-quality flow measurements of a tidal turbine blade in highly turbulent flow conditions. These data will form an experimental benchmark for the validation of numerical codes, which aim to predict the flow field and/or forces on 3D wings in turbulent flow conditions, including applications in aeronautics, turbomachinery, and ocean engineering.

A novel potential flow code able to model the vorticity of turbulent flow will be developed, contributing to the fundamental research on turbulent flow and how to numerically model turbulence.

The project will lead to new collaborations between three institutes with a strong research activity on marine energy: the University of Edinburgh, the Ocean University of China and the Dalian University of Technology. Three researchers from the UoE (the PI, and two PGRs) will collaborate with the two researchers from OUC and two researchers from DUT, leading to an international team of seven researchers. Several opportunities of knowledge exchange, including more than 45 days of secondments in total, will enable long-lasting collaborations and synergies between these researchers.

Within the UoE team, one PGR with significant experience in PIV flow measurements will be employed at 20% of the time in order to provide guidance, advice and support to the project team on PIV flow measurements, thus this project will be opportunity for the research team to gain specialist expertise in PIV flow measurements.
 
Description The hydrodynamics of a tidal turbine in unsteady conditions were investigated numerically and experimentally. The following conclusions were drawn:

1. The unsteady flow fluctuations experienced by a tidal turbine blade may result in dynamic loads more than 20% higher than for steady conditions, i.e. if the turbine operated in a uniform tidal stream with a constant velocity (Tully and Viola, ISROMAC 2016).

2. The load fluctuations are maximum when the tidal stream velocity is maximum. Decreasing the tidal stream velocity, the amplitude of the angle of attack variations increases but the load fluctuations decrease.

3. Low frequency turbulent eddies and long period waves cause the most significant load fluctuations on a tidal turbine blade (Scarlett and Viola, OTE 2016).

4. If frequencies lower than two per revolution were filtered by, for instance, a pitch control system, then load fluctuations would decrease by one order of magnitude (Scarlett and Viola, OTE 2016). Further research is ongoing to determine the effect of yaw misalignment, which was neglected when the above conclusion was drawn.

5. It is unlikely that the pitch control system will be capable to match frequencies much higher than two per revolution. Therefore, in low current speed and steep waves, the turbine will experience high-frequency large angle of attack variations due to turbulence, wave and tower shadow (Scarlett and Viola, OTE 2016). These will result in small load fluctuations, because of the low tidal speed, but in periodic trailing edge separation - non dynamic stall - associated with loss of efficiency (Tully and Viola, ISROMAC 2016).

6 In these conditions, Theodorsen's method, which is widely used in industry as a predictive model for unsteady loads, may predict load fluctuations more than double that which was measured (Viola et al., OTE 2016). This highlights the need for advanced predictive tools that takes into account of non-linear effects.

7. A flexible trailing edge can be used to mitigate load fluctuations by more than 30% and to increase efficiency by more than 25%, preventing large trailing edge separation (Viola et al., OTE 2016; Tully and Viola, ISROMAC 2016).

8. The presence of a submerged foil lead to a modification of the surface waves. We found that the fundamental wave amplitude decreases and second and third free harmonic amplitudes increase by reducing the submergence or increasing the characterised length (Ning et al., MARINE 2015). There is a critical incident wave amplitude at which second free harmonic amplitude reaches the maximum. Also, a smaller incident wave amplitude can lead to a stronger wave non linearity in the shallower water region. Water depth shows less influence on fundamental wave amplitude and has no influence on second and third free harmonic amplitudes when water depth increases over a threshold.
Exploitation Route Our findings suggest that further research is needed in order to prevent large load fluctuations on tidal turbine blades that could result in fatigue and dynamic failures. In particular:

1. Advanced non-linear design tools, capable to predict the blade's dynamic loading, must be developed in order to enable the design of pitch control systems.

2. The use of flexible blades to decrease load fluctuations and improve the turbine efficiency must be further explored.
Sectors Aerospace, Defence and Marine,Energy

URL http://www.research.ed.ac.uk/portal/iviola