This paper assesses cavitation inception on an 8.5 m tidal turbine blade by varying design parameters such as pitch angle, rotor speed, and tidal inflow speed. The research method uses JavaFoil to calculate hydrofoil data and AeroDyn to investigate cavitation and performance characteristics of the blade. It was found that two types of cavitation are observed in this blade: tip cavitation and cloud cavitation. To avoid these types of cavitation, the degree of freedom of the pitch angle must be limited to below 8 degrees. It was also found that stall regulated control is more suitable than the pitch-regulation in avoiding both types of cavitation.
Tidal energy is a renewable source of energy where turbines are used to generate electricity from the movement of tidal waters. Although the tide varies in velocity and direction twice during the day as the tide rises and falls, these changes are well understood and accurately predictable. This predictable nature of tidal energy is the main benefit of tidal energy over other renewables such as solar and wind energy (Watchorn and Trapp 2000) where intermittency must be accounted for. Horizontal Axis Tidal Turbine (HATT) technology is inherited from the wind turbine industry with two control modes of variable or fixed pitch blades. In order to regulate power above rated speed, variable pitch blades are pitched to feather. Fixed pitch designs, however, rely on the stall characteristic of the rotor blades (Whitby and Ugalde-Loo 2014). There are other issues that do not affect wind turbines. For example, the risk of cavitation is one of the major differences between water- and air-based turbines (Nicholls-Lee, Turnock, and Boyd 2008). Cavitation is the process where gas vapour structures form in a liquid in which the pressure at a constant ambient temperature decreases below a certain critical level (Knapp, Dailey, and Hammitt 1970). Cavitation is first mentioned by Leonhard Euler in 1754 in his work on the theory of turbines (Dumas 1839). It has significant destructive effects on the hydrodynamic and structural performance of tidal turbines. First of all, it causes both thrust and torque to reduce, hence efficiency is reduced (Lindau et al. 2005). Secondly, it causes erosion of the turbine blade surface, which can cause significant damage to the blade structure (Karimi and Martin 1986).
