  Through experiments, we idealise a plant leaf as a flexible, thin,
rectangular plate clamped at the midpoint and positioned perpendicular to an
airflow. Flexibility of the structure is considered as an advantage at moderate
flow speed because it allows drag reduction by elastic reconfiguration, but it
can also be at the origin of several flow-induced vibration phenomena at higher
flow speeds. A wind tunnel campaign is conducted to identify the limitation to
elastic reconfiguration that dynamic instability imposes. Here we show by
increasing the flow speed that the flexibility permits a considerable drag
reduction by reconfiguration, compared to the rigid case. However, beyond the
stability limit, vibrations occur and limit the reconfiguration. This limit is
represented by two dimensionless numbers: the mass number, and the Cauchy
number. Our results reveal the existence of a critical Cauchy number below
which static reconfiguration with drag reduction is possible and above which a
dynamic instability with important fluctuating loads is present. The critical
dimensionless velocity is dependant on the mass number. Flexibility is related
to the critical reduced velocity, and allows defining an optimal flexibility
for the structure that leads to a drag reduction by reconfiguration while
avoiding dynamic instability. Furthermore, experiments show that our flexible
structure can exhibit two vibration modes: symmetric and anti-symmetric,
depending on its mass number. Because the system we consider is bluff yet
aligned with the flow, it is unclear whether the vibrations are due to a
flutter instability or vortex-induced vibration or a combination of both
phenomena.
