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Offshore Technology

The Effects of Fish Cages on Ambient Currents

[+] Author and Article Information
Lars C. Gansel

Department of Marine Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norwaygansel@ntnu.no

Thomas A. McClimans

Marine Resources Technology, SINTEF Fisheries and Aquaculture, NO-7010 Trondheim, Norwaythomas.a.mcclimans@sintef.no

Dag Myrhaug

Department of Marine Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norwaydag.myrhaug@ntnu.no

J. Offshore Mech. Arct. Eng 134(1), 011303 (Oct 13, 2011) (5 pages) doi:10.1115/1.4003696 History: Received November 13, 2009; Revised November 29, 2010; Published October 13, 2011; Online October 13, 2011

Experiments were carried out to measure forces on and wake characteristics downstream from fish cages. Cylinders made from metal mesh with porosities of 0%, 30%, 60%, 75%, 82%, and 90% were tested in a towing tank. The drag force was measured with strain gauges, and the flow field downstream from the models was analyzed using particle image velocimetry. The Reynolds numbers ranged from 1000–20,000 based on the model diameter and 15–300 based on the diameter of the strings of the mesh as an independent obstacle. High porosities (here, 82% and 90%) lead to low water blockage and allow a substantial amount of water to flow through the model. The data indicate that the wake characteristics change toward the wake characteristics of a solid cylinder at a porosity just below 75%. The drag force is highly dependent on the porosity for high porosities of a cylinder.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Models with porosities of 0%, 30%, 60%, 75%, 82%, and 90%

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Figure 2

Scheme of the setup during PIV tests and force measurements

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Figure 3

Drag forces acting on the solid and porous cylinders; ◻: solid cylinder and ★, ○, and +: 75%, 82%, and 90% open cylinders, respectively

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Figure 4

Drag forces in percent of the drag force acting on the solid cylinder; ○: towing speed of 0.01 m/s and ★ and ◇: towing speeds of 0.05 m/s and 0.2 m/s, respectively

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Figure 5

Velocity defect (u-U∞) in the wake of solid and porous cylinders 1 (a), 1.5 (b), and 3 (c) diameters downstream from the centerline of the cylinders; ◻: solid cylinder and ★, ○, and +: 75%, 82%, and 90% open cylinders, respectively. The towing speed (U∞) was 0.2 m/s. Note the different scales on the y-axis.

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Figure 6

TKE (m2/s2) in the wake of a solid (a), a 75% open (b), a 82% open (c), and a 90% open (d) cylinder 1 diameter downstream from the centerline of the cylinders. The towing speed (U∞) was 0.2 m/s. Note the different scales on the y-axis.

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Figure 7

Vorticity (s−1) in the wake of a solid (a), a 75% open (b), a 82% open (c), and a 90% open (d) cylinder 1–3 diameters downstream from the centerline of the cylinders. The towing speed (U∞) was 0.2 m/s. Note the different scales.

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