Abstract
Computational fluid dynamics is used to study the impact of the support structure of a tidal turbine on performance and the downstream wake characteristics. A high-fidelity computational model of a dual rotor, contra-rotating tidal turbine in a large channel domain is presented, with turbulence modelled using large eddy simulation. Actuator lines represent the turbine blades, permitting the analysis of transient flow features and turbine diagnostics. The following four cases are considered: the flow in an unexploited, empty channel; flow in a channel containing the rotors; flow in a channel containing the support structure; and flow in a channel with both rotors and support structure. The results indicate that the support structure contributes significantly to the behaviour of the turbine and to turbulence levels downstream, even when the rotors are upstream. This implies that inclusion of the turbine structure, or some parametrisation thereof, is a prerequisite for the realistic prediction of turbine performance and reliability, particularly for array layouts where wake effects become significant.
Highlights
The commercial exploitation of tidal energy on a large scale requires the deployment of arrays of full-scale tidal turbines
These were calculated at different locations along the centreline of the channel; the distance downstream is plotted in units of D, the rotor diameter of the tidal turbine to be modelled (16 m), with the origin at 250 m downstream of inflow boundary
This was done for two reasons: firstly, as the cylinder is in vertically-sheared flow, the Reynolds number can be expected to vary widely from the top to bottom, and secondly, increased turbulence near the seabed would cause large pressure fluctuations not associated with vortex detachment, giving a noisier signal
Summary
The commercial exploitation of tidal energy on a large scale requires the deployment of arrays of full-scale tidal turbines. Given individual turbines of rated power of 1–2 MW, such arrays would have to consist of 50–100 turbines to approach the operating capacities of modern offshore wind farms. Individual turbines within a farm array will be affected by the wake of any turbines located upstream, and the large-scale environmental flow impact of the farm as a whole must be understood; modelling tidal arrays becomes a true multiscale problem. The application of computational fluid dynamics (CFD) can shed light in both areas, but this is extremely challenging from a computational perspective. Wake effects in wind farms have been the subject of many studies. Models range from early empirical linear wake superposition approaches such as the Park model [1], through to Reynolds-averaged Navier–Stokes (RANS) CFD actuator disc models, large eddy simulation (LES)
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