Fluid-structure interplay in deforming nozzles under jet flow

Flexible pipes that deform under fluid flow are seen in both nature and industry, such as in oil pipelines and blood vessels. However, large, nonlinear deformations have posed challenges for theoretical models, making precise measurements and accurate predictions difficult. In this study, we experimentally investigate the time-varying large deformations of a flexible nozzle to understand the nonlinear fluid-structure interactions between the nozzle and a steady jet. Using a jet generator and a combination of Particle Image Velocimetry (PIV) and Digital Image Correlation (DIC), we track the interaction between the fluid and the nozzle. We systematically vary the nozzle's geometry by altering wall thickness, length, and outlet diameter, while keeping the inlet diameter constant. Instabilities emerge when the Reynolds number exceeds 4,000, leading to periodic deformations with both azimuthal and longitudinal oscillation modes as the nozzle's stiffness decreases. By measuring the kinetic energy of the jet at the outlet and tracking the nozzle's deformation, we analyze the energy exchange between the fluid and the structure. Our findings identify conditions where jet power is either amplified or attenuated, depending on the stiffness of the nozzle and flow conditions. These insights help explain flexible structures in biological systems like blood vessels and urinary tracts and may inform the design of bio-inspired propulsion systems.