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 (Δx=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 (nondimensionalized) spectral power density of flow noise Φ(ω) ⟨U∞⟩/ (⟨δ∗⟩ (1/2ρ ⟨U∞⟩)2) is found to scale like (ω⟨δ∗⟩/⟨U∞⟩)−4.3 in a wide frequency range at higher towing speeds. Here, ω, ⟨δ∗⟩, and ⟨U∞⟩ 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 χ2-(Tsallis) superstatistics provides a reasonable representation of the probability distribution function of flow noise at higher towing speeds.