Lars Gansel

PIV on Inclined Cylinder Shaped Fish Cages in a Current and the Resulting Flow Field

The present experimental investigation focuses on the flow structure around short free end cylinders with an aspect ratio of L/D = 3 at a Reynolds number of Re = 5000. Cylinders with varying degrees of inclination and porosity were tested in a towing tank, acting as models for fish farm cages. According to studies of fish cages at commercial Norwegian fish farms, the inclination angle of the structure change up to 25 degrees from the vertical when exposed to a current. Cylinders with porosities of 0% and 75% were tested, the latter representing fish cage netting with fouling. To visualize the flow around the cylinders, 2- and 3-Component (2C and 3C) Particle Image Velocimetry (PIV) was used. The effects of inclination and porosity on the three-dimensional flow field will be described and discussed.Copyright

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

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.

Drag of Clean and Fouled Net Panels – Measurements and Parameterization of Fouling

Biofouling is a serious problem in marine aquaculture and it has a number of negative impacts including increased forces on aquaculture structures and reduced water exchange across nets. This in turn affects the behavior of fish cages in waves and currents and has an impact on the water volume and quality inside net pens. Even though these negative effects are acknowledged by the research community and governmental institutions, there is limited knowledge about fouling related effects on the flow past nets, and more detailed investigations distinguishing between different fouling types have been called for. This study evaluates the effect of hydroids, an important fouling organism in Norwegian aquaculture, on the forces acting on net panels. Drag forces on clean and fouled nets were measured in a flume tank, and net solidity including effect of fouling were determined using image analysis. The relationship between net solidity and drag was assessed, and it was found that a solidity increase due to hydroids caused less additional drag than a similar increase caused by change in clean net parameters. For solidities tested in this study, the difference in drag force increase could be as high as 43% between fouled and clean nets with same solidity. The relationship between solidity and drag force is well described by exponential functions for clean as well as for fouled nets. A method is proposed to parameterize the effect of fouling in terms of an increase in net solidity. This allows existing numerical methods developed for clean nets to be used to model the effects of biofouling on nets. Measurements with other types of fouling can be added to build a database on effects of the accumulation of different fouling organisms on aquaculture nets.

The Effects of Fish Cages on Ambient Currents

Experiments were carried out to measure forces on and wake characteristics downstream from fish cages. Cylinders made from metal mesh with porosities of 0%, 30%, 60%, 75%, 82%, and 90% were tested in a towing tank. The drag force was measured with strain gauges, and the flow field downstream from the models was analyzed using particle image velocimetry. The Reynolds numbers ranged from 1000–20,000 based on the model diameter and 15–300 based on the diameter of the strings of the mesh as an independent obstacle. High porosities (here, 82% and 90%) lead to low water blockage and allow a substantial amount of water to flow through the model. The data indicate that the wake characteristics change toward the wake characteristics of a solid cylinder at a porosity just below 75%. The drag force is highly dependent on the porosity for high porosities of a cylinder.

Forces on Nets With Bending Stiffness—An Experimental Study on the Effects of Flow Speed and Angle of Attack

This study investigates the effects of changes in flow speed and angle of attack on drag and lift forces on nets with bending stiffness. Today most fish cage nets are made from nylon, but new cage materials are proposed in order to improve the stability of cages in currents and waves, to reduce biofouling, prevent escapes, and to secure fish from predator attacks. The use of some of these materials leads to nets with bending stiffness in at least one direction. However, not much is known about the performance of such nets in currents and waves. In this study, three different nets with bending stiffness were tested together with nylon nets. Net panels were subjected to different flow speeds at different angles between flow direction and net plane, and the forces on the nets were measured with a multi-axis force/torque sensor system. Based on the experiments, drag, and lift coefficients were determined for the different net materials and compared to existing theory with which they are in reasonably good agreement for the nets with low solidity. However, for nets with higher solidity the results are significantly lower than the drag and lift coefficients provided other authors. Also, the change of drag coefficient with changing flow speed and angle of attack was different for a monofilament and a multifilament net with similar solidity and aperture form and size. These differences may partly be due to differences in twine structures and net construction between the monofilament and multifilament net and between nets used by other authors and in the present study.

Deformation of Nets With Bending Stiffness Normal to Uniform Currents

The tremendous growth of the fish farming industry in Norway over the past decades was supported by new designs and materials for fish farms, enabling bigger fish cages to be positioned in more exposed sea areas. Today, the nets of most fish cages in Norway are made from nylon. Nylon nets are lightweight, relatively easy to handle and at the low cost end of proposed net materials. However, nylon nets also have some unfortunate characteristics like low abrasion resistance and limited tensile strength. Thus, new net materials are proposed to better prevent escapes, protect fish from predator attacks, improve the stability of fish cages and reduce bio-fouling. Some of these materials are stiff in at least one direction and there still is a lack of knowledge about the behavior of nets with bending stiffness in currents and waves. The aim of this study was to determine how nets with bending stiffness deform in different currents and how the deformation influences the drag on the nets and to compare the results with predictions from a numerical model. Three types of net (PET, copper and steel) were clamped to a solid steel bar on the top side, but were otherwise unrestricted. Reflective markers were mounted on the nets and an optical tracking system was used to determine the position of the markers during the tests, thus allowing the determination of the deformation of the net panels. The forces on the net panels were measured with a multi-axis force/torque sensor system. The nets were subjected to several flow speeds between 0.1 and 0.9 m/s. It is shown that bending stiffness and density of nets affect net deformation, as both parameters impact the balance between drag and gravitational forces on the nets. Net deformation leads to a decrease of the projected net area. As the rate of deformation with current speed varies greatly between different net types, the discrepancy between measured drag and drag values normalized to the projected net area at different current speeds follows different relationships for different nets. A numerical model, FhSim was able to predict net deformation of nets with bending stiffness well and it is shown that FhSim could not only account for the effect of bending stiffness on net deformation, but also that the model captures the structural dynamics of nets with bending stiffness in a current.Copyright

Average Flow Inside and Around Fish Cages With and Without Fouling in a Uniform Flow

The average flow field inside and around the bottom of porous cylinders in a uniform flow is explored using Particle Image Velocimetry (PIV). Tests were conducted on six cylinders with porosities of 0%, 30%, 60%, 75%, 82% and 90% in a flume tank where the flow field inside and around the models is time averaged over 180 seconds. The models had a height-to-diameter ratio of 3 and were made from metal mesh. The Reynolds numbers ranged from 5,000 to 20,000 based on the diameter of the models and from 75 to 300 based on the diameter of individual strands of the mesh, which corresponds to the Reynolds numbers occurring at salmon fish cage netting used along the Norwegian coast. The porosities of 82%, 75% and 60% correspond to those of a fish cage netting in Norwegian Salmon farming with no, light and heavy biofouling, respectively. The results from this study are discussed with respect to the instantaneous flow field in and around the same cylinders at identical Reynolds numbers. The focus is on the effect of porosity on the ventilation inside the cages and the vertical transports within the near wake. It is shown that heavy fouling of aquacultural nettings can lead to internal circulation inside fish cages and therefore has the potential to reduce the ventilation of the net pens dramatically. The description of the time-averaged flow field inside and around porous cylinders can be used as benchmarks to validate and adjust numerical models of the flow past porous cylinders. The results from this study can be valuable also for the fish farming industry, as bio-fouling and the reduced porosity of fish cages can be monitored and controlled directly by fish farmers.© 2010 ASME

Forces on Nets With Bending Stiffness: An Experimental Study on the Effects of Flow Speed and Angle of Attack

This study investigates the effects of changes in flow speed and angle of attack on drag and lift forces on nets with bending stiffness. Today most fish cage nets are made from nylon, but new cage materials are proposed in order to improve the stability of cages in currents and waves, to reduce biofouling, prevent escapes, and to secure fish from predator attacks. The use of some of these materials leads to nets with bending stiffness in at least one direction. However, not much is known about the performance of such nets in currents and waves. In this study three different nets with bending stiffness were tested together with nylon nets. Net panels were subjected to different flow speeds at different angles between flow direction and net plane, and the forces on the nets were measured with a multi-axis force/torque sensor system. Based on the experiments, drag and lift coefficients were determined for the different net materials and compared to existing theory [1,2]. The results are in reasonably good agreement with the existing theory for the nets with low solidity, however, for nets with higher solidity the results are significantly lower than the drag and lift coefficients provided by Aarsnes [1] and Loland [2].Copyright

Natural infection induced immune response against salmonid alphavirus in farmed salmon

Pancreas disease (PD) is one of the most serious infectious diseases of salmon caused by salmonid alphavirus (SAV), and can be transmitted over distance in marine environments. Vaccination has already proven to be an important management tool for prevention and control of infectious diseases in profitable aquaculture. Currently, the inactivated whole-virus vaccine against PD is commercially available in Norway, however, its effectiveness in the field remains limited studied; lifetime protection has not been proven yet. In recent years, significant advances in molecular biology have led to the emergence of subunit vaccines. E2 protein is an envelope protein of SAV in Norway and supposed to be the carrier of neutralizing epitopes. Thus, E2 protein represents a potential candidate target for subunit vaccine-based immunotherapy against SAV. The aim of this study is to determine the natural infection induced humoral response against E2 protein in farmed salmon. Atlantic salmon reared in sea cage culture at the north western part of Norway was used in this study. Blood samples were collected monthly from March 2015 to Feb 2016. Diagnosis of SAV infection was based on the transcription-polymerase chain reaction PCR results and symptom on fishes. The serological reactivity was determined by an enzyme-linked immunosorbent assay (ELISA). A recombinant E2 protein was used as target antigen, and a recombinant ubiquitin protein was used as negative control in this study. A western blot (WB) assay was applied for the verification of specific binding. The serologic results demonstrated that no E2 specific antibody was identified in the plasma collected from March to August 2015. During August and September 2015, there was an outbreak of PD in the sea cage. In the heart tissues collected in August, the infection of SAV2 was documented by PCR assay. A significant elevated percentage of peripheral neutrophils were demonstrated during this period, which declined to the normal level afterwards. In plasma from salmon collected in September, one-month post outbreak SAV infection, a significant increase in serum antibodies against E2 protein was detected. Two months after infection, the detected antibody frequency remains 7 out of 10 samples, and the level of antibodies became relatively stable until February 2016 before slaughtering. The frequency of Antibody against E2 protein was nearly equal to the detected SAV2 infection rate. The specific bindings of salmon antibodies to the E2 protein were confirmed by the WB assay. These results demonstrate a natural infection of SAV2 induced humoral responses against E2 protein, which indicates that E2 protein can be a promising target for vaccine and diagnosis design. Furthermore, the antibodies from SAV2 infected salmon could react with SAV3, which indicates the potential cross protection of SAV3 whole virus vaccine. Further study is required to investigate into the actual protective effect of the humoral responses and to determine the targeted epitopes on E2 protein, and their correlation with fish survival. This will provide valuable information for the treatment of SAV caused salmon pancreas disease in the future.

Drag on Nets Fouled With Blue Mussel (Mytilus Edulis) and Sugar Kelp (Saccharina Latissima) and Parameterization of Fouling

Biofouling is a serious problem in marine finfish aquaculture with a number of negative impacts. Marine growth obstructs net openings, thereby reducing water exchange through the net and affecting fish welfare and health, as well as the spreading of dissolved nutrients, particles and pathogens. Furthermore, additional water blockage leads to increased hydrodynamic forces on fish cages, which potentially threaten the structural integrity of the fish farm. However, detailed knowledge about the effects of biofouling on the flow past, and the resulting forces on fish cages, is limited and systematic investigations of the effects of different types of fouling have been called for. This study investigates the effects of different amounts and sizes of two important fouling organisms in Norwegian aquaculture, blue mussel (Mytilus edulis) and kelp (Saccharina latissima) on the drag on net panels. Drag forces on a number of clean and fouled nets were measured in a flume tank at a flow speed of 0.1 m/s. Net solidity was calculated from images acquired of all nets in the current. The relationship between net solidity and drag was then found for clean nets and for each type of fouling, and biofouling was parameterized by comparing clean and fouled net results: for a given fouled net, a clean net can be found that experiences the same drag. The latter can then be used in numerical models to estimate the effect of fouling on net drag. That means existing models can be used to model the drag effect of fouling. This study found a solidity increase due to mussel and kelp fouling to affect drag roughly at the same rate as an increase in clean net solidity at a flow speed of 0.1 ms and within the tested fouling size range for two net types. Therefore, existing models, describing the relationship between net solidity and drag, can be used directly or with minor alterations (especially at high solidities) to estimate effects of additional mussel and kelp fouling on drag. In contrast, wet weight seems to be unsuitable as a measure to estimate drag on nets fouled with seaweed or mussels. It should be noted that these findings are only valid under similar conditions, and that other fouling types and sizes, as well as test parameters and tank size can affect the relationship between solidity and drag. INTRODUCTION Marine biofouling, the undesirable accumulation of organisms on submerged surfaces, has a number of adverse effects in marine aquaculture, including reduced water exchange across nets and increased net drag [1-4]. Shortly after submergence of nets fouling organisms settle on the surface, and, given suitable conditions, both mobile and sessile biofouling can then build up very fast. Several studies report a wet weight increase of biofouling organisms on aquaculture nets within the range of kilograms per m within few weeks (e.g. [5-7]) and one m of net can hold up to several 10 kg of biofouling wet weight [5]. The accumulation of fouling organisms is generally associated with the occlusion of net apertures, and several studies found rates of net aperture occlusion in accordance with rapid wet weight increase. For example, [8] found a net aperture occlusion of about 20 % within only two weeks in the sea in Clift sound, Shetland, and [9] report mesh occlusion on several nets in Tasmania of up to about 50 % and 80 % within about one and two weeks, respectively. The occlusion of net apertures results in additional water blockage, and several studies highlight the impact of biofouling for net drag on different scales. For example, [10] and [11] investigated the drag of clean and fouled nets in a flow and found that the drag coefficient of clean nets can increase about tenfold on heavily fouled nets. [12] measured fouling related