Unsteady hydrodynamics of tidal turbines

The blades of horizontal-axis tidal turbines experience continuous variations of the angle of attack and flow speed. This is due to the high turbulent kinetic energy in the tidal stream, wave-induced current, vertical shear layer, tower shadow, yaw misalignment, blade oscillations and wakes of upstream turbines. This results in heavier and more expensive structures with shorter fatigue life, which is a key issue for the tidal energy sectors. The current reliability of axial tidal turbine blades has been estimated in one failure every two years, while the expected lifetime of a turbine should be 20 years. For comparison, on wind turbines, the blade failure rate is only one in every ten years.
The first aim of this project is to provide a better understanding of the flow experienced by the blades to enhance the performance and endurance of tidal turbines. This include:
• a review of tidal stream field measurements and its assessment with a view to tidal stream turbine design;
• the development of low order models capable to reproduce the key features of the unsteady flow experienced by the blade;
• the enhancement of inflow modelling techniques for state of the art computational fluid dynamic simulations (Reynolds-averaged Navier-Stokes and Large Eddy Simulations);
• validation the numerical models with physical experiments.
Flow fluctuations result in continuous variations of the fluid dynamic forces, which show a hysteresis loop with the instantaneous angle of attach and peaks beyond the quasi static values. For small angle of attack variations, potential flow theory is capable to predict the force’s hysteresis loops. On the contrary, for large angle of attack variations, dynamic stall occurs: the Kutta-Joukowsky condition at the trailing edge is broken down and non-linear effects dominate. This results in high load peaks, loss of mean performances and noise. A comprehensive theory for an accurate prediction of the fluid dynamics forces is not yet available.
The second aim of the project is to provide a better understanding of the large-amplitude, non-periodic three-dimensional dynamic stall of tidal turbines, and to develop low-order models which will enable the design of mitigating technologies.

The project is funded through 2 DTP scholarships.