Broadband Acoustic Transmission Measurements in Surface Ship Wakes

摘要

Since the time of World War II the propagation of sound in ship wakes has been subject of considerable interest. A ship's wake is a mixture of turbulent seawater created by the motion of a surface ship's hull and air bubbles that are created by the breaking of a ship's bow and stern waves, and by the cavitation of the ship's propellers. The signature of a ship's wake will vary, depending primarily on hull design, speed through the water, and local oceanographic conditions. As the wake ages, it goes from a violent breakup and mixing of bubbles due to turbulent diffusion, to one where the turbulence decays and the bubbles begin to rise slowly toward the surface due to their buoyancy and changes in the buoyancy of the water mass. This time and frequency dependent bubbly mixture has dramatic effects on acoustic signals due to increases in absorption and refraction. As the wake ages, bubbles of different sizes rise at different rates, and the horizontal and vertical distributions of bubble densities give rise to changes in both sound speed and absorption. It is these generated bubble densities that create large acoustic resonance scattering cross sections that are responsible for the acoustic signature of a ship's wake. In this paper, we will describe a method of measuring the average across the wake excess absorption due to bubbles within the wake. Pulsed cw signals ranging from 30 kHz to 140 kHz, in 10 kHz increments, were transmitted across a surface ship wake. Using the average excess signal absorption at 12 frequencies and, and their resonance frequencies, estimates of bubble number densities as a function of wake age were obtained at 7.4-meter increments behind the wake generating ship. These measurements were continued for about 2 km behind the ship.