Fabrication of bio-inspired nozzle for underwater vehicle.

Nozzles are commonly used in jet engines, fuel-injection systems, and underwater propulsion to control fluid flow and enhance thrust generation. Recently, flexible nozzles have shown the potential to enhance propulsion performance for small-scale underwater vehicles by increasing hydrodynamic impulse and jet entrainment. This study aims to identify the optimal configuration for maximizing thrust and jet velocity by measuring the thrust force of the autonomous underwater vehicle (AUV) and fabricating various nozzle geometries. The AUV was fixed on a linear guide placed in a water tank, and the axial thrust generated was measured using an axial load cell and a voltage amplifier. Flexible nozzles were 3D-printed by varying material stiffness and nozzle geometry. Particle Image Velocimetry (PIV) then measured jet velocity from the AUV using a 2.5W continuous green laser and a high-speed camera. Results show that high-stiffness flexible nozzles produce similar thrust to rigid nozzles, while low-stiffness nozzles produce periodically undulating thrust due to the nozzle, which ultimately enhances axial thrust generation. Additionally, Digital Image Correlation (DIC) revealed a close relation between thrust frequency and nozzle deformation. These insights into enhanced thrust generation could have significant applications within naval and military technologies.