Bubble Number Densities In the Wake of a Propeller and a Pump Jet Ship

Document Type

Conference Proceeding

Publication Date

9-23-2013

Department

Marine Science

School

Ocean Science and Engineering

Abstract

Bubbly wakes generated by a surface ship are generally the result of cavitation generated by its propulsion system, and air being entrapped along the water line as the ship moves through the water. As this turbulent wake decays, bubbles of different sizes coalesce, break up, and rise at different rates. This results in changing horizontal and vertical bubble distributions that are a function of ship's speed, wake depth, and age. The changing vertical and horizontal distributions have a time and frequency dependent effect on acoustic signals that creates an excess attenuation. A series of acoustic attenuation measurements were taken across the wake of a propeller driven ship using NRL's acoustic wake characterization system. These measurements were taken over a broad range of acoustic frequencies (30 kHz to 140 kHz), ship speeds (18 and 22 knots), wake depths (3 m to 6 m), and wake age (4.5 to 9.7 minutes). The acoustic attenuations across the wake due to varying bubble-size densities were determined experimentally. Using these measured average acoustic attenuations across the wake and the resonant bubble approximation, estimates of the average bubble number densities in the wake of the propeller driven ship were derived. These bubble number densities are compared to those obtained for the wake of a similar sized pump jet ship, (Stanic et al JOE v34, Jan 2009). Results show that the bubble number densities for the propeller driven ship (18 knots) and the pump jet ship (15 knots) at a wake depth of 3 m were very similar. These bubble number densities were also seen to decrease with wake depth and age. As the propeller ship's speed increased, the bubble number densities for the propeller driven ship were lower than those for the pump jet ship at a speed of 15 knots. These results also showed that in general the wake of the propeller driven ship was not as intense and not as persistent as the wake of the pump jet ship at wake depths greater than 3 m and at all ship speeds. © 2013 MTS.

Publication Title

OCEANS 2013 MTS/IEEE - San Diego: An Ocean in Common

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