Multibeam sonar observations of hydrodynamic forcing functions and bubble persistence in a ship wake

Abstract

High frequency (240 kHz) upward‐looking multibeam echosounder data were collected from the wake of a twin‐screw 50‐m research vessel. Volumetric backscatter in each of the 101 1.5×1.5‐deg beams was recorded at a repetition rate of approximately 10 s as the ship passed over the moored sonar system. The multibeam sonar’s wide field of view and the low wake drift rate made it possible to image the wake over its entire lifetime of 30 min. The wake quickly separates into distinct port and starboard components. It is suggested that this is caused by vortices shed from the hull that propagate orthogonally to the ship’s direction, entraining bubbles and creating a convergence zone on the outboard side of the vortices as they oppose the buoyant rise of the bubbles. The speed at which the vortices separate is calculated from the data and used in a simple model describing the evolution of the bubbles in the wake. Results show that the hull shedded vortex model is plausible, and that the gas transfer rate from the bubbles must be approximately 25 times less than it would be for surfactant‐free bubbles in order to explain the bubble persistence observed in the data.

Department

Center for Coastal and Ocean Mapping

Publication Date

2004

Volume

116, Issue 4

Journal Title

Journal of the Acoustical Society of America

Pages

2650

Publisher

Acoustical Society of America

Digital Object Identifier (DOI)

10.1121/1.4785578

Document Type

Journal Article

Link to Full Text

Share

COinS