Towed body measurements of flow noise from a turbulent boundary layer under sea conditions

Results from an underwater experiment under sea conditions on flow noise beneath a flat-plate turbulent boundary layer are presented. The measurements were performed with a towed body at towing speeds U = 2.3, ... , 6.1 m/s and depths h = -150, ... , -100 m. Flow noise is measured with a linear array of equally spaced hydrophones (Delta x - 70 mm) that is orientated in streamwise direction and embedded within a laterally attached flat plate. In order to separate flow noise from ocean ambient noise and other acoustical noise sources wavenumber-frequency filtering is applied. The (non-dimensionalized) spectral power density of flow noise Phi(omega) < U-infinity >/(<delta*> (1/2 rho < U-infinity >)(2)) is found to scale liked (omega <delta*>/< U infinity >)(-4.3) in a wide frequency range at higher towing speeds. Here, omega, <delta*>, and < U-infinity > denote frequency, boundary layer displacement thickness, and potential flow velocity in the array region, respectively. Potential flow velocity is estimated from numerical simulations around a symmetrical, two-dimensional body with a semi-elliptical nose. Evidence is given that a chi(2)-(Tsallis) superstatistics provides a reasonable representation of the probability distribution function of flow noise at higher towing speeds. (C) 2014 Acoustical Society of America.