Flow-induced noise on underwater pressure-vector acoustic sensors

Flow-induced self noise on moored and drifting hydrophones when the sensors are subjected to oceanic currents such as due to wave motion and changing tides. Flow-induced self noise on underwater acoustic particle velocity (or acceleration) sensors, and on intensity probes that measure particle velocity and pressure simultaneously is the subject of this paper. The sensors considered are configured as neutrally-buoyant spheres that encase an accelerometer or geophone. These velocity sensors are very sensitive to the turbulent fluctuations in the free stream, or those, which are generated by the flow over the sensor body itself. The flow-induced self noise is thus determined by the level of unsteady forces created by the turbulence and unsteady flow. Experiments are conducted by towing a spherical velocity sensor from 0.5 to 2.0 knots in a quiet basin of water that is 9 m long. A model is developed for the flow-induced unsteady drag and side force of the sphere. This is based on integrating the local wall pressure fluctuations over the surface of the sphere. Wind tunnel experiments are performed to obtain the necessary statistics of these pressure fluctuations. The semi-empirical model is then validated using the towed sensor unsteady velocity spectra, which are easily converted to unsteady drag or side force spectra by Newton´s Second Law. Predictions of the flow noise response of neutrally-buoyant spherical velocity sensors of various sizes are then performed. It is concluded that at the same flow velocity, a large inertial sensor is less responsive than a small one to the turbulent flow over its surface and in the near wake.