Acoustic near field pressures from subsonic high-speed projectiles

Preliminary experiments performed to measure the radiated noise from subsonic high-speed underwater projectiles indicated the presence of a characteristic disturbance in the mean pressure field during the approach of the projectile to the location of the sensing hydrophone in the test tank. A numerical model based on the potential flow field surrounding the projectile was developed to explain this behavior. A series of 10 projectile shots were fired at speeds ranging from 700–900 m/s to investigate this phenomenon. The underwater range in the tank was instrumented with 5 PCB narrow span pressure transducers. The transducer pressure ranges and locations in the tank were selected using the potential flow model predictions. Results from these experiments are presented. The mean pressure field disturbances show steep cross range gradients. Passage of the projectile at a fixed location in the tank was determined by observing self-similar features of the pressure field at different transducer locations. Projectile velocity estimates were determined from time delays between transducers and are compared with break screen data. The cross range gradients and velocity measurement techniques will be used to develop an open range system to track underwater projectiles. [Work supported by ONR.]