  Wind turbines operate under very dynamic conditions in the atmospheric
boundary layer. Rapid changes of the angle of the incoming flow are caused by
so called extreme conditions, either directly by changes of the wind direction
or indirectly by gust induced changes of the angle for a rotating blade of the
wind turbine. This dynamic changes lead frequently to the aerodynamic
phenomenon of dynamic stall, which is known to generate a massive lift
overshoot and therefore an increase of loads acting on the wind turbine.
  In this study an experimental approach to modulate the inflow in a wind
tunnel in a two-dimensional manner will be presented. For a complete
characterization of the possibilities of the so-called 2D active grid, it is
investigated with different modifications of the used shafts. In addition to
the hardware alterations, the influence of the dynamic motion of the grid is
also extensively investigated. The benefit of a 2D active grid upstream of the
airfoil is the possibility to induce dynamic phenomena without the need to
dynamically pitch the airfoil itself. This allows for simultaneous measurements
of the flow field around airfoils with temporally high resolving particle image
velocimetry (PIV) and load cells. Disturbing effects, such as those caused by
the inertia of a moving airfoil, can be avoided. In addition to this dynamic
changes in the angle of attack, it will be shown that the 2D active grid design
enables to generate fast velocity fluctuations to mimic longitudinal inflow
variations. These two features can be combined, resulting in angle of attack
and velocity changes on small time scales with any phase shift between them to
imitate different inflow situations such as yaw effects.
