In this study, the performance and wake profiles of an axial flow tidal stream turbine are numerically investigated using actuator line method. The simulation is conducted based on the open source library OpenFOAM. We first validate the simulation results and achieve good agreement with experimental results from previous studies. Then, the method is used to simulate two and three tidal stream turbines, which are placed axially in line. The results reveal the capability of actuator line method in simulating performance and wake of a tidal stream turbine. Moreover, the performance of downstream turbines is greatly influenced by upstream turbines' wake.
Tidal stream energy has been widely considered as one of the most important promising renewable and environmentally-friendly resources nowadays (Zheng et al., 2015; Mejia-Olivares et al., 2018). Compared with other kinds of sustainable energies, tidal stream energy is distinguished due to its richness and high predictability (Lamy and Azevedo, 2018). Among all devices that are employed to extract tidal stream energy from tidal current, horizontal-axis tidal stream turbines are the most preferable (Tedds et al., 2014). Besides, as these tidal turbines are normally configured into an array or a farm in order to make full use of potential sea sites (Myers and Bahaj, 2011), it is of significance to investigate performance of tidal stream turbines in an array. Wake properties of a turbine shall also be highly concerned due to the fact that the interaction of turbines inside an array has a major influence on their performance (Funke et al., 2014; Stallard et al., 2013).
A number of experimental researches have been conducted to investigate performance and wake properties of a tidal stream turbine in recent years. Bahaj et al. (2007) presented experimental results with a turbine of a range of tip speed ratio and pith settings. They also discussed influences of various conditions on the turbine's performance, including straight or yawed flow, tip immersion of the rotor and etc. Myers and Bahaj (2011) experimentally studied flow filed around a tidal array using actuator disks to present the turbine devices. They claimed an optimal lateral spacing between devices where, under certain conditions flow could be accelerated between a pair of rotor disks. Therefore, the performance of downstream turbines subjected to the accelerated flow would increase to some extent. Mycek et al. (2014a, 2014b) experimentally investigated performance of two axially aligned turbines with two ambient flow intensity rates considered. They results indicated that the higher ambient turbulence intensity rates reduced wake effects and thus allowed a better compromise between inter-device spacing and individual performance. Payne et al. (2018) focused on the variation with frequency of rotor thrust and torque loads using a scaled three-bladed tidal turbine in a recirculating flume. For frequencies above the rotational frequency, loads acting on the rotor blades were found to be highly affected by turbine operation phenomena. Martinez et al. (2018) experimentally investigated a horizontal axis turbine model subjected to combined oblique waves and current. They found that waves affected means and standard deviation of rotor power with off-axis waves associated with lower thrust loads than head-on waves.
