How to design a 2D active grid for dynamic inflow modulation

Abstract

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.