Planform geometry effects of piezoelectric wind energy harvesting composite inverted flags

Energy harvesting devices based on the inverted flag configuration have recently attracted attention as a viable option to power small electronics and remote wireless sensors from wind excitation. Despite showing high potential, relatively little research has considered how the dynamics and power generation performance of these devices vary with planform geometry. This study considers composite inverted flags constructed using flexible polyvinylidene difluoride strips as active elements sandwiching a stainless-steel shim as elastic structural support, a configuration allowing for enhanced ability to tailor mechanical and geometrical properties (and hence responses) of the flag. We explore the effect of three key parameters (planform aspect ratio, mass ratio, second moment of area) on flag dynamics (amplitude, flapping frequency) and operational characteristics (power, velocity range). Twelve flags have been manufactured and tested under controlled wind excitation, selected to cover aspect ratios in the range of 0.9-7 and mass ratios in the range of 7-14. Our results expose a number of useful insights into how these devices perform. First, we show that for a given mass ratio, flapping amplitude, frequency, power, and power density are all inversely proportional to aspect ratio and proportional to second moment of planform area. Second, we show that second moment of planform area is a better indicator parameter to assess the flags performance, as compared to the aspect ratio. Third, we show that as mass ratio increases higher power densities and wider operational velocity ranges are allowed; however, this is on the expense of the flag being more sensitive to variations in operating conditions and the possibility of experiencing some unfavourable motion behaviour such as the 'one-sided' low-amplitude high-frequency low-power flapping mode.