Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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The aquaculture industry in Norway Norway is today ... • the worlds biggest producer of Atlantic salmon • the second-largest exporter of seafood • exporting sea food for 51.8 G NOK (5.6 G EUR) yearly (2015) • producing 31 million meals of Norwegian seafood every single day!
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Norwegian Seafood Council (Seafood.no)
Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Aquaculture challenges The open net-cage fish farms, impacts the environment. Pressure is experienced to deal with ecological challenges related to: Alexandra Morton. Lewis Research Group http://www.math.ualberta.ca/~mlewis/SeaLice.htm
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Sea Lice
Michael L. Webe | SeaWeb Aquaculture Clearing House http://www.wri.org
Avisenagder.no, Arkiv/illustrasjonsfoto
Pollution
Escapes
Are Closed (Flexible) Fish Cages the answer?
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What are Closed Flexible Fish Cages ? The idea: Close of the system to get better control of the environment
Circular nett cage illustration from AKVA Group (akvagroup.com)
Traditional net cage system
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Closed Flexible Fish Cage
Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Meeting todays challenges with a CFFC Advantages: • Closer to conventional fish farms than rigid alternatives Technical Challenges • Large changes in behavior compared to conventional net system • No comparable systems in use • No existing model of the system
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Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Modelling of forces and deformations on the CFFC Hydro-elastic behavior: Structural response and forces interdependent
External Sealoads • Wave forces and motions • Current forces Internal bag forces • Internal water motion and forces Structure forces • Structure motion and deformation • Tension
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Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Focus on internal water motion: Sloshing in CFFC
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What is sloshing? In fluid dynamics, sloshing refers to the movement of liquid inside another object, which is, typically, also undergoing motion
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Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Modelling of sloshing in a 2D CFFC • • • • • •
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Models the internal motion 2D The system is highly coupled Potential theory Linear theory Must be solved numerically
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Case study on a rectangular sloshing tank with a flexible wall • • • •
Simplified system Existing literature Analytical solution Captures important physical phenomena • Validation case for numerical method
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Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Effect of sloshing on the membrane The effect of sloshing on the membrane 𝝏𝟐 𝒗(𝒛, 𝒕) 𝝏𝟐 𝒗 𝒛, 𝒕 𝝏𝝋 𝒎 − 𝑻 = −𝝆 𝝏𝒕𝟐 𝝏𝒛𝟐 𝝏𝒕 Assume modal representation of deformation on cable 𝑣 𝑧, 𝑡 = 𝜼𝒅𝒋 (t)sin(𝑗𝜋/𝐿(𝑧 + ℎ)) 𝑗
Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Effect of flexible wall on eigenfrequencies Eigenfrequencies of the coupled system: • dependent on the tension in the cable • lower than for a rigid tank • is highly coupled between modes • behaves as a rigid tank for 𝑇→∞
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Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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New knowledge • An analytical solution for validation of numerical code have been found • An added free surface stiffness have been found, previously not known of, which it exists an analytical expression for • A method for estimating frequency independent added mass numerically have been developed • A method for decoupling of structure fluid interaction have been found
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Centre for Autonomous Marine Operations and Systems - NTNU AMOS
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Further work
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Centre for Autonomous Marine Operations and Systems - NTNU AMOS
