Østen Jensen

Transition from progressive to global buckling of aluminium extrusions – a numerical study

Abstract Numerical simulations using LS-DYNA were carried out in order to study the transition between progressive and global buckling of axially loaded aluminium extrusions in alloy AA6060 temper T6. A numerical model was validated against experimental tests and good agreement was found between the progressive buckling pattern in the numerical simulations and experimental tests. The numerical simulations were capable of giving a relatively accurate prediction of the collapse mode found in the experimental tests. The critical global slenderness is defined as the global slenderness, or length to width ratio (L/b), where direct global buckling or a transition from progressive to global buckling occurs. The validated numerical model was used in a parametric study where the influence on the critical global slenderness from impact velocity and material properties has been studied. The means to stabilize the response and increase the critical global slenderness has been investigated. It was found that the introduction of a heat affected zone at the impacted end increased the critical buckling length.

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.

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

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

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

Temporary-Creep and Postcreep Properties of Aquaculture Netting Materials With UHMWPE Fibers

This paper presents test results on temporary-creep properties, recovery of strain postcreep and postcreep tensile properties of a Raschel knitted netting material with a combination of ultrahigh molecular weight polyethylene (UHMWPE) and polyester fibers. Specimens of the material were subjected to uniaxial loading over a period of 30 mins, at a constant creep target load of 10–90% of average tensile strength. The specimens were wet and tested in room temperature. The netting structure experienced creep strain with mean values in the range of 1.3–4.5%, increasing with increased creep target load. In addition, the netting experienced 2% creep strain during on-loading. The creep strains were elastic, while large proportions of the elongation accumulated during on-loading (structural strain of 8.8–27.8%) were long lasting and possibly permanent. Tensile tests showed that for the highest creep target load, strength, and elongation at break increased by 17%.

Temporary-Creep and Post-Creep Properties of Aquaculture Netting Materials With UHMWPE Fibres

This paper presents test results on temporary-creep properties, recovery of strain post creep and post-creep tensile properties of a Raschel knitted netting material with a combination of UHMWPE and Polyester fibres. Specimens of the material were subjected to uniaxial loading over a period of 30 minutes, at a constant creep target load of 10–90 % of average tensile strength. The specimens were wet and tested in room temperature.The netting structure experienced significant creep strain, with mean values in the range of 1.3–4.5 %, increasing with increased creep target load. Large proportions of the elongation accumulated during on-loading and creep were long lasting and possibly permanent. Tensile tests showed that for the highest creep target load, strength and elongation at break increased by 17 %.The UHMWPE-netting experienced larger creep strains than PA6-netting for relatively large creep target loads (60–90 % of the average breaking strength), while creep strains were smaller for low loads. PA6-netting had a larger and faster recovery of strain post creep than the UHMWPE-netting, and the length and force at break were not significantly affected by the creep load history.Copyright