Fluid-structure interactions of net cages - full-scale pushing tests in the field

Abstract

This paper presents field tests on a full-scale cage, with and without fish, being pushed by a boat in Masfjorden at various speeds. The purpose was to imitate the exposure of net cages to different currents. The tests involved measuring cage deformations, fish behaviors, and the corresponding flow upstream, downstream, and inside the cage. The study found that the experimental setup used can achieve predictable and stable upstream flow for a full-scale net cage. Based on pressure tag data, the volume reductions of the cage, both with and without fish, were estimated at different speeds. Both cases show a similar trend of cage volume reduction with respect to flow speeds as the previous studies. Moreover, the presence of fish had limited influence on the net volume change. The reduction in speed inside and downstream from the cage was within the range reported in previous literature. Notably, when the cage becomes significantly deformed, it not only reduces flow speed but also alters flow directions, as evidenced by the high variability of flow direction inside the empty cage, particularly at high speeds. The measured flow speed inside the stocked cage also exhibited high variability, but the pattern of variation differed significantly from that of the empty cage, indicating the influence of fish. These findings suggest that traditional flow speed models might oversimplify the flow field in and around fish cages, especially in studies concerning the dispersion of particles, pathogens, and dissolved matter in and out of fish cages.