  Wake redirection is a promising approach designed to mitigate turbine-wake
interactions which have a negative impact on the performance and lifetime of
wind farms. It has recently been found that substantial power gains can be
obtained by tilting the rotors of spanwise-periodic wind-turbine arrays. Rotor
tilt is associated to the generation of coherent streamwise vortices which
deflect wakes towards the ground and, by exploiting the vertical wind shear,
replace them with higher-momentum fluid (high-speed streaks). The objective of
this work is to evaluate power gains that can be obtained by tilting rotors in
spanwise-periodic wind-turbine arrays immersed in the atmospheric boundary
layer and, in particular, to analyze the influence of the rotor size on power
gains in the case where the turbines emerge from the atmospheric surface layer.
We show that, for the case of wind-aligned arrays, large power gains can be
obtained for positive tilt angles of the order of 30{\deg}. Power gains are
substantially enhanced by operating tilted-rotor turbines at thrust
coefficients higher than in the reference configuration. These power gains
initially increase with the rotor size reaching a maximum for rotor diameters
of the order of 3.6 boundary layer momentum thicknesses (for the considered
cases) and decrease for larger sizes. Maximum power gains are obtained for
wind-turbine spanwise spacings which are very similar to those of large-scale
and very large scale streaky motions which are naturally amplified in turbulent
boundary layers. These results are all congruent with the findings of previous
investigations of passive control of canonical boundary layers for
drag-reduction applications where high-speed streaks replaced wakes of
spanwise-periodic rows of wall-mounted roughness elements.
