Sloshing in a rotating liquid inside a closed sea cage for fish farming

Sloshing in a sea cage with a slowly rotating liquid is investigated. The cage is axisymmetric, and the liquid is subjected to a nearly uniform angular velocity about the vertical axis of the cage. Both experimental and theoretical investigations are presented. It is shown that rotation modifies the sloshing regimes of a non-rotating liquid by splitting the natural frequencies. Therefore, resonant sloshing regimes can be manipulated by varying the rotation rate of the liquid.acceptedVersio

Numerical study of closed rigid fish cages in waves and comparison with experimental data

Closed fish cages have gained increased interest in marine aquaculture. However, knowledge on the seakeeping behaviour of a floating closed cage and the influence of the contained water inside the cage are still limited. In this paper, a coupled numerical model is developed for the simulation of closed rigid cages in waves. Numerical studies are conducted both in the frequency domain and in the time domain, and compared with scaled physical experiments. Special attention has been drawn to the coupling effects of sloshing on cage response and the resulting mooring line forces. The comparative analyses show that sloshing of the contained water has large influence on the coupled surge and pitch motions of the cage. Sloshing is also found to have significant effect on the mean-drift forces in regular waves. In the tested/simulated irregular waves, the mooring forces are found to be dominated by the slow-drift motions, which indicates that the slowly-varying wave drift forces need to be considered in the design of the mooring system for a floating closed cage.publishedVersio

Geometry and Kinematics of Breaking Waves

The objective of this thesis is to experimentally study different breaking waves cases. This is done by measuring in detail the free surface geometry and the internal kinematics of the waves as they approach breaking. Three principal wave cases were chosen for the study: A plunging breaker, a spilling breaker, and an intermediate breaker. A major part of this work is the design, construction and building of a wave laboratory. The laboratory contains a glass wall waveflume which is 13.5m long, 1m deep and 0.6m wide, as well as equipment for measuring both the wave kinematics and geometry optically. The wave kinematics is measured using the Particle Image Velocimetry (PIV) method, while the wave profile geometry is measured using image analysis (space domain geometry), as well as standard wave gauges (time domain geometry). The analysis of both the wave kinematics and geometry is done using parameters describing quantitatively important features in the wave evolution. The surface geometry is described using the commonly known zero-downcross parameters, and in addition, new parameters are suggested and used in the study, The kinematics are described by a set of four parameters suggested for the first time in this work. These parameters are: Velocity at the surface, velocity at the still water line (z = 0), mean velocity direction, and local wave number. The purpose of these parameters is to give a better understanding of the space and time domain development of the kinematics, and they appear to be a reasonable compromise between simplicity and accuracy. The results presented here represents a thorough and detailed mapping of the breaking process. Much data is gathered and analysed, and throughout this thesis it is sought to present the data in the most intuitive way, so that other investigations may benefit from it

Review of cage and containment tank designs for offshore fish farming

Fish farm operators worldwide are planning to move offshore due to lack of available nearshore production sites in heavily utilized coastal zones, where there is increasing community opposition to coastal development and conflict with other usages such as shipping, fishing, tourism, conservation and recreation. Moreover, offshore sites provide more sea space and generally better water quality, which are needed to increase the production of healthy fish. This review paper begins with the definition of offshore for fish farming based on unified viewpoint and proceeds to highlight the challenges faced by going offshore. Next, the paper presents a review of designs of fish cages from conventional nearshore fish farms to next-generation offshore fish farms, which have to contend with a high energy environment. The fish cages may be divided into the open net cage system and the closed containment tank system. The open net cage system can be categorized further into 5 types. The advantages and disadvantages of the various fish cage designs will be discussed. Further, different types of cage designs are compared with the view to guide feasibility of offshore fish farming. Co-location with other synergetic industries is discussed as a possible example of future offshore fish farms

Sloshing in a rotating liquid inside a closed sea cage for fish farming

Sloshing in a sea cage with a slowly rotating liquid is investigated. The cage is axisymmetric, and the liquid is subjected to a nearly uniform angular velocity about the vertical axis of the cage. Both experimental and theoretical investigations are presented. It is shown that rotation modifies the sloshing regimes of a non-rotating liquid by splitting the natural frequencies. Therefore, resonant sloshing regimes can be manipulated by varying the rotation rate of the liquid

Experimental study of installation procedure and volume estimation of tarpaulin for chemical treatment of fish in floating cages

In the salmon industry chemical delousing is done by enclosing the cage in a tarpaulin, and then mixing the chemical agent in the enclosed water volume. Correct dosage is important to obtain the wanted effect, and in order to calculate the correct dose of the agent the volume of the enclosed water is estimated based on the geometry of the tarpaulin. The accuracy of this volume estimation is investigated by model experiments in a flume tank. Different tarpaulin shapes, installation procedures and current velocities were examined, and in addition to measuring the volume of the enclosed water, the drag force on the cage/tarpaulin was also measured. The accuracy of the volume estimation was found to be quite low (Root-mean-square (RMS) value of the deviation between estimated and measured volume for all runs was 21%). Certain combination of tarpaulin shape and installation procedure showed less deviation between estimated and volume, and it was found that increased current velocity was favourable with respect to this accuracy.publishedVersio

Numerical and Experimental Study on the Seakeeping Behavior of Floating Closed Rigid Fish Cages

The present paper addresses the seakeeping behavior of a rigid type of floating closed fish cages, with focus on effects of sloshing on the coupled motions and mooring loads. Closed cages have gained much attention recently as a strategy to avoid sea-lice infections in farming of Atlantic salmon. However, closed cages are novel structures and more knowledge is needed on the wave induced motions and coupling effects with sloshing for development of reliable closed cage structures to reduce risk for failure and possible escape of fish. In this paper, dedicated scaled model tests of closed cages in waves are presented and compared with numerical simulations using linear potential theory in frequency domain. The results shows that the influence of sloshing on the rigid body motion is significant. A nonlinear effect of sloshing was observed for a small region of excitation frequencies, where the surge amplitude increased with increasing wave steepness. Mean wave loads were also affected by sloshing. Hence, coupled motions with sloshing is important to consider in design of floating closed rigid fish cages and their mooring system.publishedVersionCopyright © 2018 by ASM

Sloshing in a rotating liquid inside a closed sea cage for fish farming

Sloshing in a sea cage with a slowly rotating liquid is investigated. The cage is axisymmetric, and the liquid is subjected to a nearly uniform angular velocity about the vertical axis of the cage. Both experimental and theoretical investigations are presented. It is shown that rotation modifies the sloshing regimes of a non-rotating liquid by splitting the natural frequencies. Therefore, resonant sloshing regimes can be manipulated by varying the rotation rate of the liquid

Hydrodynamic Loads on Net Panels With Different Solidities

Drag forces on nets represent the largest contribution to hydrodynamic loads on traditional fish farms, and will have a large impact on total loads on new designs utilizing netting as containment method. Precise methods for estimation of drag loads are needed. This paper gives new knowledge on hydrodynamic forces acting on aquaculture nets. It presents results from towing tests, including updated drag and lift coefficients for Raschel knitted netting materials used in nets for aquaculture, and quantify wake effect. The results include high solidity nets and high towing velocities. It was found that drag loads were close to proportional with the netting solidity for netting solidities ranging from 0.15 to 0.32. The wake effect is quantified through the average velocity reduction factor, which is given as a linear function of solidity. Much of previously published data are close to the data found through these tests. However, for high solidity nets, the deviation is significant. Therefore, previously published data and models may overestimate drag loads for high solidity nets