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

Abstract

The efficiency of tidal stream turbines in a large array depends on the balance between negative effects of turbine-wake interactions and positive effects of bypass-flow acceleration due to local blockage, both of which are functions of the layout of turbines. In this study we investigate the hydrodynamics of turbines in an infinitely large array with aligned or staggered layouts for a range of streamwise and lateral turbine spacing. First, we present a theoretical analysis based on an extension of the linear momentum actuator disc theory for perfectly aligned and staggered layouts, employing a hybrid inviscid-viscous approach to account for the local blockage effect within each row of turbines and the viscous (turbulent) wake mixing behind each row in a coupled manner. We then perform large-eddy simulation (LES) of open-channel flow for 28 layouts of tidal turbines using an actuator line method with doubly periodic boundary conditions. Both theoretical and LES results show that the efficiency of turbines (or the power of turbines for a given bulk velocity) in an aligned array decreases as we reduce the streamwise turbine spacing, whereas that in a staggered array remains high and may even increase due to the positive local blockage effect (causing the local flow velocity upstream of each turbine to exceed the bulk velocity) if the lateral turbine spacing is sufficiently small. The LES results further reveal that the amplitude of wake meandering tends to decrease as we reduce the lateral turbine spacing, which leads to a lower wake recovery rate in the near-wake region. These results will help to understand and improve the efficiency of tidal turbines in future large arrays, even though the performance of real tidal arrays may depend not only on turbine-to-turbine interactions within the array but also on macro-scale interactions between the array and natural tidal currents, the latter of which are outside the scope of this study.

Two-scale interaction of wake and blockage effects in large wind farms

“…Interestingly, some of the layouts seem to produce slightly higher power than predicted by the two-scale momentum theory. These layouts have a slightly higher than 0.75 (the value for an isolated turbine) and this seems to be due to locally accelerated flow caused by the local blockage effect (Nishino & Draper 2015; Ouro & Nishino 2021). These results suggest that both the array density and turbine–wake interactions are important for the performance for large finite-sized wind farms.…”

“…Conversely, local blockage effects (Nishino & Draper 2015) may increase because: (1) it creates locally accelerated flows which may allow the upstream velocity of most turbines to be higher than (see e.g. Ouro & Nishino 2021); and (2) it changes the mechanism of the flow around each turbine, allowing for a higher (for a given ) than that predicted by the classical actuator disc theory. However, such a positive effect of local blockage can be exploited only when the layout is carefully optimised for a specific wind direction.…”

Combining shallow-water and analytical wake models for tidal array micro-siting

“…An alternative approach for the local and global hydrodynamics may be undertaken using higher fidelity models such as those that utilise three-dimensional Reynoldsaveraged Navier-Stokes (RANS) (Abolghasemi et al 2016;Deskos et al 2017) or large-eddy simulation (LES) methods (Churchfield et al 2013;Ouro and Nishino 2021) which inherently allow for greater insight and accuracy in the nearwake region by allowing both horizontal and vertical wake dispersion through scale-resolving simulations. Such simulations emphasise how wake avoidance is not only critical for maximum exploitation of the channel potential, but also in reducing turbulence onto downstream turbines which may compromise the devices' lifetime due to fatigue (Thiébaut et al 2020).…”

Power density capacity of tidal stream turbine arrays with horizontal and vertical axis turbines

“…bines operate fully in the wake of upstream devices or in the bypass flow going through the closest upstream pair of turbines (Stansby and Stallard 2016;Ouro and Nishino 2021). During a tidal cycle, if the direction of the incident flow is exactly reversed from ebb to flood any array operating as aligned or staggered configuration will still operate with such layout during the other half of the cycle.…”

“…These provide an upper bound estimate of the energy that can be harnessed but fail to capture wake effects, which are detrimental to the energy yield of secondary rows (Stansby and Ouro 2022). Both the spacing between turbines and their layout as an staggered or aligned array determines its efficiency with the latter providing an increase in turbine performance due to the high-momentum bypass flow developed between two turbines of the same row and which impinges the next turbine in the following row (Draper and Nishino 2014;Nishino and Willden 2013;Ouro and Nishino 2021).…”