Flow past porous cylinders and effects of biofouling and fish behavior on the flow in and around Atlantic salmon net cages
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- Institutt for marin teknikk 
Norwegian salmon aquaculture has experienced a tremendous growth within the past decades and Norway is today the biggest salmon producer in the world. The growth of aquaculture production was enabled by technological advances, but with the development of larger fish farms and the general increase in fish production came new challenges of which some are not of technological and structural nature, but connected to biological and ecological factors. The research community as well as governmental bodies identified a number of areas that need to be addressed in order to assure sustainability and further sustainable growth of the industry. These areas include environmental effects of fish farms in terms of the release of nutrients and waste products, disease control and fish welfare, amongst others. All of these are connected to the flow past fish farms and water exchange between fish cages and the surrounding environment. Fish cages are porous, flexible structures and most Norwegian salmon is farmed in circular net pens. Unfortunately, there is little knowledge about the flow past such structures. Fish cages are subjected to change during their submergence not only due to their flexible nature, but also because biological factors like the accumulation of biofouling will influence the characteristics of structural elements of fish cages. It was previously shown in small scale tests that fish inside net cages can influence the water flow, but there is very little knowledge about the effect of up to several hundred tons of fish swimming inside large fish cages. The primary objectives of this thesis are to i) investigate the flow past porous cylinders to understand the effect of porosity on water blockage, flow patterns and wake spreading, ii) understand the role of fish for the flow past stocked fish cages and iii) investigate the effect of net solidity changes due to biofouling on the drag of aquaculture nets. Many factors influence the behavior of fish cages in the sea. Laboratory experiments were conducted to investigate the effect of solidity on the flow through porous cylinders. The drag on porous and solid cylinders in a uniform flow was measured and the flows through and around the cylinders were studied. The drag force was highly dependent on the solidity of cylinders, and the relationship between solidity and drag was solidity-dependent. Single strands of the netting of porous cylinders excited separate vortex streets in their wakes. At solidities around 0.25 a shift in the wake characteristics towards that of solid cylinders appeared, and at high solidities (between solidities of 0.4 and 0.7) the flow structure became more similar to that around solid cylinders. Three different flow regimes depending on the solidity were identified for the flow around and through porous cylinders. An inclination of porous cylinders had only a small effect on the flow, but an inclination strongly impacted the flow past solid cylinders. The accumulation of fouling organisms on fish cages poses a major problem for marine aquaculture. Biofouling can add to the weight of nets and equipment and it can change the hydrodynamic properties of fish cages. Biofouling also decreases the size of net apertures, thereby increasing the solidity of nets. It is known that biofouling can accumulate in substantial amounts within weeks or even days in peak growth periods. The drag forces on a number of clean and fouled nets were measured in a flume tank. The fouling was almost exclusively hydroids so that the effect of a single type of fouling could be investigated separately. The relationship between drag force on and solidity of clean nets was different from that of fouled nets. Thus, the same solidity increase due to an increase of clean net characteristics was different from the increase due to the accumulation of fouling organisms. The effect of fouling on nets on the drag force was independent from the initial clean net solidity of fouled nets. However, only two different net types with varying amounts of fouling were used for the tests. The results can be used as a database to compare the effect of solidity of clean and fouled nets on the drag force, which can also act as a proxy for the water blockage of nets. This type of information is important, as different types of fouling are likely to influence the flow across nets differently. Knowledge about the amount of fouling on aquaculture nets alone will therefore not allow a good estimate of the effect of fouling in terms of water blockage. It is necessary to have information about the type of fouling as well and its effect on the water flow in addition. This study proposes a method to build a database to compare different types of clean and fouled nets based on the example of hydroids. Such data allows the effect of biofouling to be implemented into numerical models developed for clean nets. Fish inside net cages have to be considered when investigating the flow past the cages. Previous studies showed fish motion to be able to introduce forces that could push water out of the cages even against weak currents. However, these studies were conducted on a very small scale and the situation might be different in much larger salmon fish cages. Measurements of the flow around single empty and stocked fish cages confirmed that fish can substantially alter the flow around the cages. Experiments visualizing the surface flow inside a stocked fish cage were conducted and the results from all tests were used to develop a simple qualitative model of the effect of fish motion inside net cages. Salmon tends to school inside cages, which leads to fish swimming at relatively high densities on a circular path along the net wall. That means that the fish need to accelerate towards the center of the cage, which leads to a radially outwards directed force on the water. Fish motion causes an outwards directed water flow at depths of high fish density. This results in a relatively low pressure in the center of the swimming fish, leading to water being sucked towards this area from adjacent water layers.