Abstract
In free-field operation, many aerodynamic systems are confronted with changing turbulent inflow conditions. Wind turbines are a prominent example. Here, the rotation of the rotor blades causes incoming wind gusts to result in a local change in the angle of incidence for the blade segments, which changes the effective angle of attack and can lead to dynamic non-linear effects like dynamic stall. Dynamic stall is known to produce a significant overshoot in the acting forces and thus an increase in loads acting on the wind turbine, leading to long-term fatigue. To gain a better understanding, it is necessary to perform wind tunnel experiments under realistic and reproducible inflow with defined conditions. In this study, a so-called 2D active grid is presented, which allows the generation of defined two-dimensional inflow conditions for wind tunnel experiments. The focus is on generating sinusoidal transversal and longitudinal gusts with high amplitudes and frequencies. Different grid configurations and sizes are tested to investigate differences in the generated flow fields. Transversal gusts imposed in this way can be used to study dynamic phenomena without having to move the object under investigation itself. Inertial effects during force measurements and a changing shadow casting due to moving airfoils in particle image velocimetry measurements are thus avoided. The additional possibility to generate defined longitudinal gusts allows to generate a broad range of reproducible inflow situations like yaw or tower shadow effects during experimental investigations.
Highlights
In nature, all kinds of objects are often exposed to rapidly changing inflow conditions
To test whether the later presented results are scalable to any setup, a smaller 2D active grid replica is manufactured in addition to the previous one
2D active grid and 2D AG the normal sized one. a Resulting transversal gust amplitudes φfor two different sized grids at various velocities and a fixed grid amplitude γ = 7.5◦ . b Normalized transversal gust amplitude φ∗ based on the results shown in a (b) amplitude γ
Summary
All kinds of objects are often exposed to rapidly changing inflow conditions. Studies have shown that the effects which occur are comparable (Gharali and Johnson 2012; Rival and Tropea 2010), the situation at hand is different and becomes even more complex on smaller time scales For this reason, experiments must account for this difference and use a varying inflow with a fixed rather than a pitching blade. A so-called 2D active grid is presented With this grid, significantly larger inflow variations can be generated in closed wind tunnel test sections than was previously possible. The different configurations can significantly affect the quality of the flow, producing flows capable to test the theories of Sears (pure transversal gusts, no fluctuations in the longitudinal component) and Atassi (a transversal gust and additional modulation of the longitudinal component) These results can be found in Wei et al (2019b).
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