  Vibration-based flow energy harvesting enables robust, in-situ energy
extraction for low-power applications, such as distributed sensor networks.
Fluid-structure instabilities dictate a harvester's viability since the
structural response to the flow determines its power output. Previous work on a
flextensional-based flow energy harvester demonstrated that an elastic member
within a converging-diverging channel is susceptible to the aeroelastic
flutter. This work explores the mechanism driving flutter through experiments
and simulations. A model is then developed based on channel flow-rate
modulation and considering the effects of both normal and spanwise flow
confinement on the instability. Linear stability analysis of the model
replicates flutter onset, critical frequency, and mode shapes observed in
experiments. The model suggests that flow modulation through the channel throat
is the principal mechanism for the fluid-induced vibration. The generalized
model presented can serve as the foundation of design parameter exploration for
energy harvesters, perhaps leading to more powerful devices in the future, but
also to other similar flow geometries where the flutter instability arises in
an elastic member within a narrow flow passage.
