The Physics of Flow Noise

The nearfield flow noise that is generated in the boundary layer of a vehicle in water turns out to be practically, independent of the kind of vehicle that generates the turbulence. Recorded levels depend on the size and shape of the recording hydrophone, even at very low frequencies when the linear dimensions of the hydrophone are small as compared to the convection wavelength of the turbulence. At high frequencies, because of their finite size, all hydrophones become nearfield insensitive and record the radiated noise. No significant difference was observed in the radiated-noise curve measured at a distance of about 100 ft from a rising buoyant unit, not even at frequencies as low as 150 Hz, whether the unit was of solid wood or had a thin metal shell. The radiation field depends on the second time derivative of the turbulent velocity fluctuations and, consequently, on the unsteadiness and intermittancy of the turbulence in the boundary layer. For a rotating cylinder, the measured radiation field levels approach those predicted by the recently improved Lighthill formula. For a ship, they are higher; for a buoyant unit, which is a small and a very unstable vehicle, the radiated noise becomes particularly great.