Technical report | Bubble Cloud Generation by an Airgun: Laboratory Experiments and Modelling

ABSTRACT

Laboratory experiments in support of the development and validation of a model of bubble cloud resulting from an underwater explosion are described in this report. The underwater explosion was emulated by a small airgun. The elements of the model are presented in detail. Special attention is paid to the model improvements with respect to accounting for bubble interaction in the rising bubble cloud through water entrainment. Model results are compared to the experimental measurements and high‑fidelity numerical simulations.

Executive Summary

The Defence Science and Technology Group collaborates with the United States Naval Undersea Warfare Center (NUWC) in the development of various bubbly wake models for the Weapons Analysis Facility (WAF), and its Australian version, the Torpedo Analysis Facility (TAF). There is a requirement under the WAF/TAF model development program to develop a model of a bubble cloud generated by an underwater explosion (UNDEX). The ultimate purpose of such a model is to simulate the response of the bubble cloud to an active sonar pulse. For this, the size distribution of the bubbles resulting from the disintegration of the initial explosion bubble needs to be known. The bubble size distribution in the UNDEX remnant bubble cloud is not static and changes with time mainly due to the rise of bubbles to the surface, but also bubble coalescence and break-up. The problem of the rising bubble cloud is mathematically simpler to model than the problem of the explosion bubble disintegration into smaller bubbles, but is still too complex to solve analytically. The complexity of the multiphase flow in the rising bubble cloud is caused by the presence of bubbles of many different sizes and their interaction with each other, mainly through the water entrained by the bubble motion.

A previously developed model of the remnant bubble cloud of an underwater explosion did not take into account the motion of entrained water and its influence on the bubble rise. In this research the model has been improved by taking into account interaction between bubbles through water entrainment. In the new model the bubbles in the cloud are divided into two fractions of large and small bubbles, and a simplified model of water entrainment by the large bubble fraction has been developed. The dynamics of the rising small bubbles is calculated on the assumption that their velocity is constant and is the sum of the terminal velocity in still water and that of the entrained water. The time history of the bubble size and spatial distribution in the cloud can then be easily computed. The calculation of acoustic properties of the bubble cloud is straightforward after that.

To validate the new model, an experiment was conducted in the acoustic tank of the Underwater Acoustic Scattering Laboratory. The underwater explosion was emulated by a small airgun. The model of the explosion bubble dynamics was modified to the parameters of the airgun. The modified model is demonstrating a fair agreement with measured time history of the oscillating bubble radius. The simplified model of the water entrainment was validated by comparison of the water velocity at the axis of the bubble cloud with the corresponding measurements using an acoustic Doppler velocimeter. Finally, a comparison was made between the simulated and measured acoustic transmission through the bubble cloud. Although a perfect agreement was not achieved, a significant improvement compared to the previous model was demonstrated.