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Polar and Arctic Science and Technology

Flow Around the Free Bottom of Fish Cages in a Uniform Flow With and Without Fouling

[+] Author and Article Information
Lars C. Gansel

Department of Marine Technology, Norwegian University of Science and Technology, N0-7491 Trondheim, Norwaylars.gansel@ntnu.no

Thomas A. McClimans

Marine Resources Technology, SINTEF Fisheries and Aquaculture, N0-7010 Trondheim, Norwaythomas.a.mcclimans@sintef.no

Dag Myrhaug

Department of Marine Technology, Norwegian University of Science and Technology, N0-7491 Trondheim, Norwaydag.myrhaug@ntnu.no

J. Offshore Mech. Arct. Eng 134(1), 011501 (Oct 13, 2011) (8 pages) doi:10.1115/1.4003695 History: Received November 13, 2009; Revised July 23, 2010; Published October 13, 2011; Online October 13, 2011

This paper explores the flow around fish cages in a uniform flow with the focus on the flow patterns close to the bottom of the models. Towing tests were conducted with six straight cylinders with the porosities 0%, 30%, 60%, 75%, 82%, and 90%, two cylinders with an inclination of 12.5 deg, and the porosities 0% and 75% and two cylinders with an inclination of 25 deg and the porosities 0% and 75%. The models all had a height-to-diameter ratio of 3 and were made from metal mesh. The Reynolds number was 5000 based on the diameter of the models and 15 based on the diameter of individual strings of the mesh for all tests. Particle image velocimetry, a nonintrusive optical technique, was used to analyze the flow around the models in the plane of symmetry through the center of the cylinders. The porosities of 82%, 75%, and 60% correspond to those of a clean fish cage netting in Norwegian Salmon farming with no fouling, light fouling, and heavy fouling, respectively. The inclinations of 12.5 deg and 25 deg reflect the inclination of the net of a commercial fish cage in a slow and a fast current, respectively. The Reynolds number of the strings was within the range of Reynolds numbers occurring on fish cages along the Norwegian coast. The results from this study are discussed with respect to the flow around and through the same models at identical Reynolds numbers. It is shown that the inclination of the net cage and fouling of the netting have major effects on the flow pattern around fish cages. The flow around and through net cages defines the water exchange within fish cages and the distribution patterns of particles and nutrients released from a net-pen. The information provided in this study can be valuable for the fish farming industry, as the decrease of the porosity due to fouling, as well as the deformation of the netting of fish cages, can be controlled by fish farmers.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Scheme of the setup during PIV tests

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Figure 3

Dependence of the velocity defect (u) on the porosity 0.75 diameters upstream of porous cylinders

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Figure 4

W-velocity in a fixed reference system 0.75 diameters upstream from the straight models (a) and 1.5 (b) and 2.5 (c) diameters downstream from the straight models. ● solid, straight cylinder, ○, +, ◇, *, and ☆ 30%, 60%, 75%, 82%, and 90% porous, straight cylinders, respectively.

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Figure 8

Sketch of the flow around the end of a short solid cylinder

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Figure 6

W-velocity in a fixed reference system 0.75 diameters upstream from the inclined models (a) and 1.5 (b) and 2.5 (c) diameters downstream from the inclined models. ● solid, 25 deg inclined cylinder, ○ solid, 12.5 deg inclined cylinder. ◇ 75% porous, 25 deg inclined cylinder and ☆ 75% porous, 12.5 deg inclined cylinder.------- tow speed.

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Figure 5

U-velocity in a fixed reference system 0.75 diameters upstream from the inclined models (a) and 1.5 (b) and 2.5 (c) diameters downstream from the inclined models. ● solid, 25 deg. inclined cylinder, ○ solid, 12.5 deg inclined cylinder, ◇ 75% porous, 25 deg inclined cylinder and ☆ 75% porous, 12.5 deg inclined cylinder.------- tow speed.

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Figure 2

U-velocity in a fixed reference system 0.75 diameters upstream from the straight models (a) and 1.5 (b) and 2.5 (c) diameters downstream from the straight models. ● solid, straight cylinder, ○, +, ◇, *, and ☆ 30%, 60%, 75%, 82%, and 90% porous, straight cylinders, respectively. ------- tow speed.

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Figure 7

Vorticity downstream from a 90% porous cylinder (marked white). The flow/tow speed was 0.05 m/s and the legend shows the vorticity.

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