Flow-induced noise in merchant ship broadband source levels

Abstract

Source level spectra for 14 large merchant ships have been measured with hydrophones at about 400 m from the ships' tracks. The measured broadband levels tend to be fairly constant from 20 or 30 Hz down to 4 or 5 Hz, instead of decreasing. A possible cause of these high measured low-frequency levels is flow-induced noise radiated from the turbulent boundary layer on the ship's hull [E. A. Vecchio and C. A. Wiley, J. Acoust. Soc. Am. 53, 596–601(1975)]. This paper presents results of calculating the radiated flow-induced noise, as received at the experimental ranges. The smooth-plate expressions for boundary layer thickness and source spectral density have been modified to conform to typical hull roughness. The computed levels, above 5 Hz, are generally less than 10 dB from the measured received levels, suggesting that flow-induced noise is an important component of merchant ship radiated noise from about 5 to 20 Hz.