A flexible PVDF-based galloping flow sensor

Effective monitoring of low flow velocities in small rivers and irrigation channels is hindered by the power requirements and maintenance costs of existing technologies. This study proposes a novel flexible piezoelectric polymer flow sensor utilizing galloping vibration to detect flow velocity in the low range (≤ 0.1 m/s). The sensor features a flexible cantilever structure composed of a silicone rubber beam embedded with a polyvinylidene fluoride (PVDF) film and a tip pillar. Unlike conventional devices based on flow-induced vibration, the use of low-stiffness materials enables the induction of self-excited vibration even under weak fluid forces. Computational fluid dynamics (CFD) analysis has been conducted to optimize the tip shape; a D-shaped semicylinder is selected over a cylinder and a square prism because the geometry maximizes the lift force per unit mass, ensuring efficient energy conversion. To predict sensor behavior, a coupled mechanical-fluid-electrical model was developed. Specifically, the model accounts for the static deflection angle caused by fluid drag. Water channel experiments demonstrated that sensors with beam thicknesses under 4 mm successfully generated stable periodic outputs at 0.1 m/s, a regime previously difficult for galloping-based devices. Conversely, thicker beam which has a thickness of 8 mm achieved higher outputs at higher velocities but failed to actuate at low speeds. Furthermore, the study showed a vibration suppression phenomenon in flexible beams at high flow velocities due to excessive static deflection, which was accurately reproduced by the analytical model. These findings establish structural stiffness as the critical design parameter for optimizing the operable velocity range of flow sensors.