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


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.


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Arredondo-Galeana A (2021) Unsteady load mitigation through a passive trailing-edge flap in Journal of Fluids and Structures

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Arredondo-Galeana A (2022) A Low Cost Oscillating Membrane for Underwater Applications at Low Reynolds Numbers in Journal of Marine Science and Engineering

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Muir RE (2017) The leading-edge vortex of swift wing-shaped delta wings. in Royal Society open science

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Ning D. (2015) Numerical investigation of nonlinear wave interaction with a submerged object in MARINE 2015 - Computational Methods in Marine Engineering VI

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Pisetta G (2022) Morphing blades for tidal turbines: A theoretical study in Renewable Energy

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Thomas Scarlett G (2020) Unsteady hydrodynamics of tidal turbine blades in Renewable Energy

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 100% 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; Scarlett et al., Renewable Energy, submitted).

2. The load fluctuations are maximum when the tidal stream velocity is maximum. Decreasing the tidal stream velocity, the absolute amplitude of the load fluctuations decreases but the relative amplitude compared to the mean load increases.

3. Waves and low-frequency turbulent eddies cause the most significant load fluctuations on a tidal turbine blade (Scarlett and Viola, OTE 2016; Scarlett et al., Renewable Energy, submitted).

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).

5. It is unlikely that the pitch control system will be capable to match frequencies much higher than one per revolution. Therefore, in low current speed and steep waves, the turbine will experience high-frequency 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).

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 leads 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 nonlinearity 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, and that leads to large oscillations in the power output. 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
Description Centre for Advanced Materials for Renewable Energy Generation
Amount £2,037,439 (GBP)
Funding ID EP/P007805/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 12/2016 
End 11/2020
Description DTP - University of Edinburgh
Amount £3,235,922 (GBP)
Funding ID EP/M508032/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 08/2015 
End 09/2019
Description Morphing-Blades: New-Concept Turbine Blades for Unsteady Load Mitigation
Amount £909,851 (GBP)
Funding ID EP/V009443/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2021 
End 06/2024
Description United Kingdom Centre for Marine Energy Research
Amount £1,517,202 (GBP)
Funding ID EP/P008682/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 12/2016 
End 05/2019
Description Unsteady hydrodynamics of tidal turbines
Amount £67,721 (GBP)
Funding ID 1667056 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2015 
End 12/2018
Description Open kick off meeting 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact We held a kick-off meeting of the project in the form of a public side event to the European Wave and Tidal Energy Conference, in September 2021 at Plymouth.
Year(s) Of Engagement Activity 2021
URL https://ewtec.org/wp-content/uploads/2021/08/EWTEC-Summary-Schedule_1-combined.pdf