Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Tidal Blade Performance Analysis Based on Cavitation Inception
Afrooz Kazemi Vanhari a, Edward Fagan a, Oscar De la Torre b, Finlay Wallace c, Jamie Goggins aa MaREI Centre, Ryan Institute & School of Engineering, National University of Ireland Galway,

Galway, Irelandb Department of Fluid Mechanics, Polytechnic University of Catalonia,

Catalonia, Spainc Orbital Marine Power Ltd,

Orkney, Scotland

performance of tidal turbines. First of all, it causes both thrust and torqueto reduce, hence efficiency is reduced (Lindau et al. 2005). Secondly, itcauses erosion of the turbine blade surface, which can cause significant
damage to the blade structure (Karimi and Martin 1986).

ABSTRACTThis paper assesses cavitation inception on an 8.5 m tidal turbine bladeby 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 areobserved in this blade: tip cavitation and cloud cavitation. To avoid thesetypes of cavitation, the degree of freedom of the pitch angle must belimited to below 8 degrees. It was also found that stall regulated control
is more suitable than the pitch-regulation in avoiding both types ofcavitation.

The possibility of cavitation inception on the turbine blades is supported
by experimental evidence and computational studies. Bahaj et al. (2007)
designed an 800 mm diameter marine current turbine (MCT) rotor basedon BEM theory, and both numerical simulations and a towing tank test
were performed to analyse the rotor performance. In addition, theoccurrence of cavitation in a tunnel was studied. While these experiments
are useful for classifying the type of cavitation and observing its onset,
tidal turbines should be designed to generally prevent the onset ofcavitation (GL 2015). Numerical simulation of the model is also used tostudy the cavitation phenomenon in the Bahaj turbine by Guo et al. (Guo,Zhou, and Wang 2015). Most of the tidal turbine rotor design algorithms
used are based on the Blade Element Momentum (BEM) method, which
is a combination of momentum and blade element theories. The firstprimary cavitation model was integrated into BEM by Batten et al.
(2008).

KEY WORDS: Tidal turbine blade; cavitation; stall regulated control;
pitch regulated control; pitch angle; rotor speed; AeroDyn.

INTRODUCTION
Tidal energy is a renewable source of energy where turbines are used togenerate electricity from the movement of tidal waters. Although the tidevaries in velocity and direction twice during the day as the tide rises andfalls, these changes are well understood and accurately predictable. Thispredictable nature of tidal energy is the main benefit of tidal energy overother renewables such as solar and wind energy (Watchorn and Trapp2000) 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 toregulate power above rated speed, variable pitch blades are pitched tofeather. 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 oneof the major differences between water- and air-based turbines (NichollsLee, Turnock, and Boyd 2008). Cavitation is the process where gas
vapour structures form in a liquid in which the pressure at a constantambient temperature decreases below a certain critical level (Knapp,Dailey, and Hammitt 1970). Cavitation is first mentioned by LeonhardEuler in 1754 in his work on the theory of turbines (Dumas 1839). It has
significant destructive effects on the hydrodynamic and structuralDepending on the blade geometry, hydrodynamic conditions and fluid
properties, a number of cavitation forms can develop on the turbine
blades (e.g. tip vortex, sheet, bubble, cloud etc.) (Wang, Atlar, andSampson 2007). Kaufmann et al. (2017) showed that a radial flow
component is developed at the blade tip, which causes a tip vortex. Dueto the presence of this tip vortex, cavitation occurred on the blade suction
surface and near the blade tip region. Wang et al. (2007) showed that thesheet cavitation can extend more than 20% of the blade chord from its
leading-edge of the suction side of the blades. However, the tail end of
the sheet cavitation displays some instability and bursts to form unstableand erosive cloud cavitation. Comparing cavitation types on the turbine
blades proved that the most damaging form of cavitation is cloudcavitation, which can be developed from sheet and bubble cavitation(Kim et al. 2014). Numerical modelling of cavitation inception onturbine blades by Guo et al (2015) indicated that cloud cavitation could
cover up to two-thirds of the blade.

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