Research Papers: Ocean Space Utilization

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

[+] Author and Article Information
L. C. Gansel

SINTEF Fisheries and Aquaculture,
Trondheim N-7010, Norway
e-mail: Lars.Gansel@sintef.no

Ø. Jensen

SINTEF Fisheries and Aquaculture,
Trondheim N-7010, Norway
e-mail: Osten.Jensen@sintef.no

E. Lien

SINTEF Fisheries and Aquaculture,
Trondheim N-7010, Norway
e-mail: Egil.Lien@sintef.no

P. C. Endresen

SINTEF Fisheries and Aquaculture,
Trondheim N-7010, Norway
e-mail: Per.Christian.Endresen@sintef.no

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 27, 2012; final manuscript received May 26, 2014; published online July 24, 2014. Assoc. Editor: Hideyuki Suzuki.

J. Offshore Mech. Arct. Eng 136(4), 041201 (Jul 24, 2014) (8 pages) Paper No: OMAE-12-1078; doi: 10.1115/1.4027954 History: Received July 27, 2012; Revised May 26, 2014

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 predator 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.

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Fig. 1

Outlines of all nets. Back lighted photos of the nets were reduced to two colors using Gimp. The images show the nets in the orientation during the tests. Image (a) steel net, (b) copper net, (c) monofilament plastic net, (d) Nylon026, and (e) Nylon015.

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Fig. 2

Setup of the experiments. The experiments were conducted in the North Sea Center flume tank in Hirtshals, Denmark. The net panels were mounted in a rigid metal frame as shown in the perspective view on the right. In the drawings to the left, the velocity meter is indicated by the light circle 3000 mm upstream and 800 mm off center from the middle of the metal frame. The circles in the image to the right indicate the positions of reflective markers used to control the three-dimensional orientation of the frame using a Qualisys motion tracking system.

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Fig. 3

Drag and lift coefficient CD and CL as a function of the angle of attack. (a) steel net, (b) monofilament net, (c) Nylon015, (d) copper net, and (e) Nylon026. The net characteristics and the Reranges are summarized in Table 1.

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Fig. 4

Drag and lift coefficient CD and CL as a function of the flow speed. (a) Steel net, (b) monofilament net, (c) Nylon015, (d) copper net, and (e) Nylon026. The net characteristics and the Reranges are summarized in Table 1.

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Fig. 5

Drag coefficient as a function of solidity at Φ = 90 deg

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Fig. 6

Drag coefficient as a function of solidity at Φ = 45 deg

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Fig. 7

Lift coefficient as a function of solidity at Φ = 45 deg



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