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The efficiency of tidal-stream turbines in a large array depends on thebalance between negative effects of turbine-wake interactions and positiveeffects of bypass-flow acceleration due to local blockage, both of which arefunctions of the layout of turbines. Here we investigate the hydrodynamics ofturbines in an infinitely large array with aligned or staggered layouts for arange of streamwise and lateral turbine spacing. First, we present atheoretical analysis based on an extension of the Linear Momentum Actuator DiscTheory (LMADT) for perfectly aligned and staggered layouts, employing a hybridinviscid-viscous approach to account for the local blockage effect within eachturbine row and the viscous (turbulent) wake mixing behind each row in acoupled manner. We perform Large-Eddy Simulation (LES) of open-channel flow for28 layouts of tidal turbines using an Actuator-Line Method with periodicboundary conditions. Both theoretical and LES results show that the efficiencyof turbines (or the power of turbines for a given bulk velocity) in an alignedarray decreases as we reduce the streamwise spacing, whereas that in astaggered array remains high and may even increase due to the positive localblockage effect (causing the local flow velocity upstream of each turbine toexceed the bulk velocity) if the lateral turbine spacing is sufficiently small.The LES results further reveal that the amplitude of wake meandering tends todecrease as we reduce the lateral spacing, which leads to a lower wake recoveryrate in the near-wake region. These results will help to understand and improvethe efficiency of tidal turbines in future large arrays, even though theperformance of real tidal arrays may depend not only on turbine-to-turbineinteractions but also on macro-scale interactions between the array and naturaltidal currents, the latter of which are outside the scope of this study.

Performance and wake characteristics of tidal turbines in an infinitely large array