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• The first in-plane omnidirectional flutter piezoelectric wind energy harvester. • A design method of omnidirectional flutter wind energy harvesters. • Excellent omnidirectionalities of onset speed and electrical output power. • High output power in arbitrary wind direction and wide working wind speed range. Most wind energy harvesters that are based on wind-induced vibrations can only operate in a specific wind direction, which limits their application in environments with time-varying wind directions. Previously reported in-plane omnidirectional piezoelectric wind energy harvesters (PWEHs) based on vortex-induced vibrations can produce stable electrical output characteristics when the wind direction changes, but their output powers are relatively low and the working wind speed ranges are relatively narrow. To solve this issue, this work presents an in-plane omnidirectional flutter PWEH which is mainly composed of the bluff body of a hollow cylindrical shell and the inside elastic supporting structure of several piezoelectric composite beams. Finite element analysis shows that harvesters with both four and six supporting beams offer good stiffness and electromechanical conversion isotropies. Experiments demonstrate that both prototype devices are flutter PWEHs with rotation axes located on the rear of the geometrical center of the bluff body. The effect of wind speed on their directionality is small, and both prototypes show in-plane omnidirectional operation over a wide wind speed range, over which the electrical outputs remain relatively high. The prototype with six beams is preferred from the viewpoint of the wind direction adaptability. The onset wind speeds in different directions all range between 5.8 and 6.0 m/s, showing the good isotropy. The minimum and maximum powers are approximately 266.4 and 300.1 μW, respectively, at 10 m/s, and it shows in-plane omnidirectional energy harvesting performance in the wind speed range from 6.5 m/s up to more than 10 m/s. [ABSTRACT FROM AUTHOR]