Research Papers: Ocean Space Utilization

Drag Forces on, and Deformation of, Closed Flexible Bags

[+] Author and Article Information
Pål Lader

SINTEF Fisheries and Aquaculture,
Trondheim 7465, Norway
e-mail: pal.lader@sintef.no

David W. Fredriksson

United States Naval Academy,
Annapolis, MD 21402
e-mail: fredriks@usna.edu

Zsolt Volent

SINTEF Fisheries and Aquaculture,
Trondheim 7465, Norway
e-mail: zsolt.volent@sintef.no

Jud DeCew

Department of Mechanical Engineering,
University of New Hampshire,
Durham, NH 03824
e-mail: jdecew@gmail.com

Trond Rosten

SINTEF Fisheries and Aquaculture,
Trondheim 7465, Norway
e-mail: trond.rosten@sintef.no

Ida M. Strand

Department of Marine Technology,
Norwegian University of Science
and Technology,
Trondheim 7491, Norway
e-mail: ida.strand@ntnu.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 November 24, 2014; final manuscript received May 7, 2015; published online May 21, 2015. Assoc. Editor: Colin Leung.

J. Offshore Mech. Arct. Eng 137(4), 041202 (Aug 01, 2015) (8 pages) Paper No: OMAE-14-1145; doi: 10.1115/1.4030629 History: Received November 24, 2014; Revised May 07, 2015; Online May 21, 2015

The use of closed flexible bags is among the suggestions considered as a potential way to expand the salmon production in Norway. Few ocean structures exist with large, heavily compliant submerged components, and there is presently limited existing knowledge about how aquaculture systems with flexible closed cages will respond to external sea loads. The flexibility and deformation of the bag are coupled to the hydrodynamic forces, and the forces and deformation will be dependent on the filling level of the bag. In order to get a better understanding of the drag forces on, and deformation of, such bags, experiments were conducted with a series of closed flexible bags. The bags were towed in a towing tank in order to simulate uniform current. Four different geometries were investigated, cylindrical, cubical, conical, and pyramidal, and the filling levels were varied between 70% and 120%. The main findings from the experiments were that the drag force was highly dependent on the filling level, and that the drag force increases with decreasing filling level. Comparing the drag force on a deflated bag with an inflated one showed an increase of up to 2.5 times.

Copyright © 2015 by ASME
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Fig. 1

Overview over the experimental matrix of different bag geometries, filling levels, and uniform current velocities. Drag force and bag deformation are recorded for each combination.

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Fig. 2

Overview of the setup on the towing carriage

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Fig. 3

The towing tank at the United States Naval Academy

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Fig. 4

The model attachment and the cameras

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Fig. 5

The principle for fixing the bag to the stiff frame

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Fig. 6

The square (top) and the circular (bottom) frame

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Fig. 7

Model bag geometries with dimensions

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Fig. 8

Filling levels for the different models. The images are taken from the side.

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Fig. 9

The positioning of the three cameras

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Fig. 10

Drag force on the different bag models, orientations, and filling levels as a function of velocity. On the abscissa axis, both towing velocity and full scale corresponding current velocity are given. On the ordinate axis, both drag force on the model, corresponding full scale drag force, and nondimensionalized drag force are given. The nondimensional drag force is normalized by the mass displacement of the undeformed volume (V100%ρg).

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Fig. 11

Geometry of the bags for different filling levels at towing velocity 5.8 cm/s

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Fig. 12

Geometry of the bags at the lowest filling level (70% and 80%) for different towing velocities

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Fig. 13

Principal deformation (shown for towing velocity 5.7 cm/s and deflated bags (70%/80%). On the right column is a schematic description of the shape of the bag front.

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Fig. 14

The drag coefficient on a cup section for flow toward the convex (Cd = 1.1) or the concave side (Cd = 2.3). Re = 2 × 104. Data found in Blevins [11]. Copyright 1984 by Van Nostrand Reinhold.




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