Optimization and experimental validation of bio-inspired flexible nozzles for thrust enhancement in pulsed laminar jet propulsion

Nozzles are commonly used in jet engines, fuel-injection systems, and underwater propulsion to control fluid flow and enhance thrust generation.  Flexible nozzles have been shown to be promising components for enhancing underwater propulsion by passively adapting to flow conditions and amplifying thrust generation.  This work focuses on validating optimized rigid and flexible nozzle designs through a pulsed laminar jet experimental setup that enables direct thrust measurement and flow characterization.  A custom dynamometer was developed to perform controlled pulse jet experiments, incorporating a flow stabilizer, two porous screens, and a honeycomb flow straightener to ensure stability and uniformity of the flow. A contraction section is placed at the end of the nozzle to minimize turbulence and create flow conditions analogous to a pulse jet.  Nozzles with varying flexibility and geometry were fabricated using additive manufacturing to measure thrust and power under identical input conditions and validate the computational predictions. The rigid-flexible connection was precisely fabricated to ensure a secure seal and stable structural coupling between the nozzle and mounting assembly.  Flow fields were captured using Particle Image Velocimetry (PIV) to visualize jet formation, wall deformation, and vortex dynamics. These measurements validated computational predictions that controlled deformation of flexible nozzles, amplifying jet velocity and hydrodynamic impulse compared to rigid counterparts. The integration of thrust, power, and PIV data establishes a robust experimental foundation for optimizing bio-inspired flexible propulsion systems for underwater vehicles.