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
