  Extracting the optimal amount of power from an array of tidal turbines
requires an intricate understanding of tidal dynamics and the effects of
turbine placement on the local and regional scale flow. Numerical models have
contributed significantly towards this understanding, and more recently,
adjoint-based modelling has been employed to optimise the positioning of the
turbines in an array in an automated way and improve on simple, regular
man-made configurations. Adjoint-based optimisation of high-resolution and
ideally 3D transient models is generally a very computationally expensive
problem. As a result, existing work on the adjoint optimisation of tidal
turbine placement has been mostly limited to steady-state simulations in which
very high, non-physical values of the background viscosity are required to
ensure that a steady-state solution exists. However, such compromises may
affect the reliability of the modelled turbines, their wakes and interactions,
and thus bring into question the validity of the computed optimal turbine
positions. This work considers a suite of idealised simulations of flow past
tidal turbine arrays in a 2D channel. It compares four regular array
configurations, detailed by Divett et al. (2013), with the configuration found
through adjoint optimisation in a steady-state, high-viscosity setup. The
optimised configuration produces considerably more power. The same
configurations are then used to produce a suite of transient simulations that
do not use constant high-viscosity, and instead use large eddy simulation (LES)
to parameterise the resulting turbulent structures. It is shown that the LES
simulations produce less power than that predicted by the constant
high-viscosity runs. Nevertheless, they still follow the same trends in the
power curve throughout time, with optimised layouts continuing to perform
significantly better than simplified configurations.
