Characterization of vertical and longitudinal gusts generated by twin pitching airfoils

An experimental investigation is performed to characterize vertical and longitudinal gusts generated by a gust generator device in a low-speed water tunnel. The device is composed of two pitching airfoils with adjustable motion modes, namely, in-phase and out-of-phase. Phase-locked two-dimensional particle image velocimetry is used to measure the velocity field downstream of the airfoils. When the two airfoils pitch in phase, periodic vertical gusts dominate the flow field, while periodic longitudinal gusts dominate when the two airfoils pitch out of phase. A waveform similarity coefficient is proposed to quantify the spatial uniformity of the gust fields, which measures how similar gust profiles are at different spatial locations. For the two types of gusts, various airfoil spacings, pitching amplitudes, and reduced frequencies are considered to examine the effects of parameters on the downstream gust characteristics. The results show that a smaller airfoil spacing produces a larger gust amplitude; however, the region with a better uniformity is reduced. As the pitching amplitude increases, the gust amplitude increases linearly, and the uniformity of the measurement region improves. The slope between the gust amplitude and pitching amplitude increases with a reduced frequency, but further larger reduced frequencies could result in poor gust uniformity. Excluding the cases with poor uniformity, this device delivers a vertical gust amplitude up to 6.7% of the free-stream velocity with reduced frequencies of k = 0.2-0.6 and a longitudinal gust amplitude of 10% of the free-stream velocity with reduced frequencies of k = 0.2-0.8.