Abstract
CFD modelling of tidal turbines in arrays is described and assessed against experimental studies of turbines operating either at constant speed or constant torque. Rotor blades are represented by rotating actuator lines, whilst supports are represented by partially-blocked-out cells. For a single turbine the model successfully reproduces towing-tank measurements of thrust and power coefficients across a range of tip-speed ratios. For two turbines staggered streamwise, it is demonstrated that loads may be reduced or augmented, according as the downstream turbine is in the wake or bypass flow of the upstream turbine. When the downstream turbine is partially in the wake, individual blades are subject to large cyclic load fluctuations. Array performance is evaluated by comparison with experimental data, modelling up to 12 turbines in up to three staggered rows. The speed of each turbine is continuously adjusted in response to flow-induced torque. Distribution of thrust coefficients within the array is well reproduced, but there is greater discrepancy in angular speed. With actuator representation of blades, the choice of turbulence model has little effect on load coefficients for an isolated turbine or row of turbines, but a significant effect on the wake, and hence on downstream turbines in an array.
Highlights
Tidal energy represents a promising, yet largely underexploited, source of renewable energy, offering high energy density and predictable generating periods
Public opposition to the expense and unknown environmental consequences of large barrages have led to consideration of tidal-stream devices—predominantly axial-flow turbines—which aim to extract the kinetic energy of a tidal current rather than the potential energy built up by impounding water
In numerous sites around the world, narrow straits lead to tidal currents in excess of 2.5 m s−1, where tidal-stream energy is expected to become commercially viable (Black and Veatch 2005)
Summary
Tidal energy represents a promising, yet largely underexploited, source of renewable energy, offering high energy density and predictable generating periods. To model interactions between multiple turbines (and between turbines and waves) we have turned to the practice used in the wind-energy industry (Sørensen and Shen 2002; Sarlak et al 2016) of replacing a geometry-resolved turbine (Fig. 1a) by an “actuator” model—essentially a bodyforce distribution that provides as closely as possible the same reactive forces as a real turbine This may be done at a coarse axially symmetric level with an actuator disk (Fig. 1b), where momentum (and, sometimes, angular momentum) body-force density is distributed over a swept volume (either uniformly, or as a function of radius) to match total thrust and torque (Schluntz and Willden 2015a; Olczak et al 2016), or, with extra computational expense but better near-wake agreement, resolution of fluctuating loads and no a priori assumption about total loads, by representing the blades as rotating actuator lines, or, more strictly, lines of discrete points (Fig. 1c) (Sørensen and Shen 2002; Schluntz and Willden 2015b; Porté-Agel et al 2011; Baba-Ahmadi and Dong 2017).
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