Deformation of Nets With Bending Stiffness Normal to Uniform Currents

Abstract

The tremendous growth of the fish farming industry in Norway over the past decades was supported by new designs and materials for fish farms, enabling bigger fish cages to be positioned in more exposed sea areas. Today, the nets of most fish cages in Norway are made from nylon. Nylon nets are lightweight, relatively easy to handle and at the low cost end of proposed net materials. However, nylon nets also have some unfortunate characteristics like low abrasion resistance and limited tensile strength. Thus, new net materials are proposed to better prevent escapes, protect fish from predator attacks, improve the stability of fish cages and reduce bio-fouling. Some of these materials are stiff in at least one direction and there still is a lack of knowledge about the behavior of nets with bending stiffness in currents and waves. The aim of this study was to determine how nets with bending stiffness deform in different currents and how the deformation influences the drag on the nets and to compare the results with predictions from a numerical model. Three types of net (PET, copper and steel) were clamped to a solid steel bar on the top side, but were otherwise unrestricted. Reflective markers were mounted on the nets and an optical tracking system was used to determine the position of the markers during the tests, thus allowing the determination of the deformation of the net panels. The forces on the net panels were measured with a multi-axis force/torque sensor system. The nets were subjected to several flow speeds between 0.1 and 0.9 m/s. It is shown that bending stiffness and density of nets affect net deformation, as both parameters impact the balance between drag and gravitational forces on the nets. Net deformation leads to a decrease of the projected net area. As the rate of deformation with current speed varies greatly between different net types, the discrepancy between measured drag and drag values normalized to the projected net area at different current speeds follows different relationships for different nets. A numerical model, FhSim was able to predict net deformation of nets with bending stiffness well and it is shown that FhSim could not only account for the effect of bending stiffness on net deformation, but also that the model captures the structural dynamics of nets with bending stiffness in a current.