Measuring and Modeling Bubbles in Ship Wakes, and Their Effect on Acoustic Propagation

This paper presents measurements and model predictions of the density and size distribution of bubbles in the wakes of full scale ships, and the effects of the bubbles on acoustic propagation. The ship wake is primarily a hydrodynamic process, characterized by temporally and spatially varying flow conditions (i.e., local fluid velocity, pressure, turbulent kinetic energy and dissipation, etc.) coupled with the transport of large numbers of bubbles. We present results of ship wake modelling utilizing hydrodynamic and bubble transport equations in which we assume that the bubble density is such that, to a very good approximation, the bubbles have negligible effect on the momentum of the local (water) flow. This allows transport of the bubble field to be performed after the flow calculation, i.e., there is no feedback. Bubble transport includes the following processes: buoyancy, drag, turbulent dispersion, and response to pressure fluctuations. We present measurements of acoustical backscattering from full-scale ship wakes, which reveal the wake’s time-varying shape and dependence upon operating conditions (e.g., speed). The bubbles dominate the optical and acoustic characteristics of the wake, which means that remote probing of the wake provides direct information about bubble distribution and only by inference other properties such as fluid velocity. Finally, the effect of the bubbles on acoustic propagation is investigated using a full-field acoustic propagation model that responds to the range-dependent refractive and attenuative effects of the bubbles. Both sound speed and attenuation depend upon the bubble radius, pressure and acoustic frequency, and the radius depends upon pressure (i.e., depth). The nature of the wake bubble field leads to depths and frequencies at which sound is attracted toward or away from the wake.