Measured turbulence in ship wakes

Abstract

Large surface ships moving at high speed generate extremely turbulent velocity fields containing high concentrations of air bubbles that are distributed in a nonuniform manner. The bubbles are acoustically active, making sonar a useful tool for imaging the ship wake and measuring the spatial and temporal variations of the bubbles. We have employed a 250-kHz multibeam sonar, integrated into an autonomous vehicle (AUV), to image the wakes of two large surface ships from the underside. The AUVs traveled at high speed so that the spatial variation of wake of approximately the same age could be assessed, and ran under different age sections of the wake in order to observe temporal variation. Using an assumed bubble size spectrum for the wake, bubble density has been estimated from the backscattered signals. Variations in bubble density with depth, cross-wake, and along-wake dimension, and with time, are observed. Spatial wave number spectra are used to quantify the scales over which bubbles are distributed, which may be related to the scales of turbulent energy distribution. Comparing measurements of different age wakes indicate how the distribution over spatial scales evolves with time. The multibeam sonar appears to provide quantitative information about the distribution of bubbles, and possibly turbulence, in the ship wake. [Work supported by ONR.]