Forces on Nets With Bending Stiffness—An Experimental Study on the Effects of Flow Speed and Angle of Attack

Abstract

This study investigates the effects of changes in flow speed and angle of attack on drag and lift forces on nets with bending stiffness. Today most fish cage nets are made from nylon, but new cage materials are proposed in order to improve the stability of cages in currents and waves, to reduce biofouling, prevent escapes, and to secure fish from preda tor attacks. The use of some of these materials leads to nets with bending stiffness in at least one direction. However, not much is known about the performance of such nets in currents and waves. In this study, three different nets with bending stiffness were tested together with nylon nets. Net panels were subjected to different flow speeds at different angles between flow direction and net plane, and the forces on the nets were measured with a multi-axis force!torque sensor system. Based on the experiments, drag, and lift coefficients were determined for the different net materials and compared to existing theory with which they are in reasonably good agreement for the nets with low solidity. However, for nets with higher solidity the results are significantly lower than the drag and lift coefficients provided other authors. Also, the change of drag coefficient with changing flow speed and angle of attack was different for a monofilament and a multifilament net with similar solidity and aperture form and size. These differences may partly be due to differences in twine structures and net construction between the monofilament and multifilament net and between nets used by other authors and in the present study.

Normal and Tangential Drag Forces of Nylon Nets, Clean and with Fouling, in Fish Farming. An Experimental Study

“…Determining the drag coefficient on flat nets usually focuses on experimental work to measure the drag force F D perceived by the nets due to exposure to a flow [1][2][3][4][5][6][7]. This force is a function of the density of the fluid ρ, the projected normal area A, the velocity of approximation of the flow to the net V, and a drag coefficient C D .…”

“…Likewise, the nets are forced by the flow, which influences their wear and increases the possibility of fish escape, whose effects can be harmful and irremediable in the ecosystem. The high impact that this represents, in terms of economic costs and environmental risks, has motivated a large number of investigations of numerical modeling and experimental work, to characterize the hydrodynamic forces that are generated due to the interaction of the flow with the nets [1][2][3][4][5][6][7][8].…”

“…They are usually characterized in terms of the drag coefficient, which is a parameter that summarizes the characteristics of the nets and the flow-net interactions. From the perspective of experimental work, the problem is measuring the drag force for a given flow condition, a methodology that allows calculating the drag coefficient of the net [1][2][3][4][5][6][7][8].…”

“…On the other hand, during the operation of a cultivation center, the prolonged exposure of the nets to the marine waters causes seaweed and other types of organisms to adhere to their surfaces. These species are known as fouling organisms, and their presence causes major problems for the operations of the centers because they increase the exposed areas of the nets, which induces an increase in the drag force they resist [2,5,6].…”

“…Besides, depending on how the installation recreates the flow over the nets, the length of the tank can also play an important role. The reported facilities use recirculation channels to recreate the currents [5][6][7]11,15]. These simulate the flow by the use of centrifugal pumps or employ towing tanks [2][3][4]9,10,13,16,17], where the flow is recreated by moving the net along the tank using a carriage-type towing mechanism.…”