  We report a comprehensive study of the wake of a porous disc, the design of
which has been modified to incorporate a swirling motion at an inexpensive
cost. The swirl intensity is passively controlled by varying the internal disc
geometry, i.e. the pitch angle of the blades. A swirl number is introduced to
characterise the competition between the linear (drag) and the azimuthal
(swirl) momentums on the wake recovery. Assuming that swirl dominates the near
wake and non-equilibrium turbulence theory applies, new scaling laws of the
mean wake properties are derived. To assess these theoretical predictions, an
in-depth analysis of the aerodynamics of these original porous discs has been
conducted experimentally. It is found that at the early stage of wake recovery,
the swirling motion induces a low-pressure core, which controls the mean
velocity deficit properties. The measurements collected in the swirling wake of
the porous discs support the new scaling laws proposed in this work. Finally,
it is shown that, as far as swirl is injected in the wake, the characteristics
of the mean velocity deficit profiles match very well those of both lab-scale
and real-scale wind turbine data extracted from the literature. Overall, our
results emphasise that by setting the initial conditions of the wake recovery,
swirl is a key ingredient to be taken into account in order to faithfully
replicate the mean wake of wind turbines.
