Normal and Tangential Drag Forces of Nylon Nets, Clean and with Fouling, in Fish Farming. An Experimental Study (original) (raw)
Related papers
Drag of Clean and Fouled Net Panels – Measurements and Parameterization of Fouling
PLOS ONE, 2015
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. Fig 4. Example for the image analysis. (a) the original (RGB) image and (b) the segmented (binary) image as a result of the image analysis. The net solidity can be determined from the ratio of black (net and fouling) and white pixels (background).
Flow through and around fish farming nets
2008
BIBLIOGRAPHY vm 103 4.2 Normalized velocity measurements for the double point tow configuration 4.3 Porous media resistance coefficients for the higher solidity net. .. 108 4.4 Meshes tested and obtained drag force on the cage 4.5 Model settings tested and obtained drag force on the cage 114 ix 4.6 Turbulence quantities tested, and obtained drag force on the cage. 4.7 Porous resistance coefficients tested, and obtained drag force on the cage 4.8 Porous resistance coefficients tested, and obtained drag force on the porous media 4.9 Turbulence quantities tested, and obtained drag force 4.10 Porous coefficients for the biofouled net 4.11 Turbulence in full scale 4.12 Numerical results from simulations x LIST OF FIGURES 1-1 A typical tow tank setup 1-2 The net is modeled as a thin volume with added resistance 8 2-1 The different Re regimes for porous flow 22 2-2 Two coordinate systems that differ by a rotation 2-3 The distance through the porous media changes with angle of attack 2-4 Control volumes used to illustrate discretization of a scalar transport equation 2-5 Tetrahedral cell 2-6 Control volume used to calculate forces on a net panel in current. . 3-1 Overview of the setup for measuring forces on the net panel .... 3-2 Pictures of the setup for measuring forces on the net panel 3-3 Loadcell arrangement 3-4 Instrumentation for measuring carriage speed and current velocity. 3-5 The net used for the tests 3-6 Time series of drag and lift force and measured current 3-7 Drag and lift coefficients as a function of angle of attack 3-8 Current reduction behind the net panel 3-9 Velocity reduction across the net panel wake for different meshes. . 3-10 Velocity reduction across the net panel wake for different inlet turbulence 3-11 The variation of r n with angle of attack 3-12 Data from the fit using LSNE 3-13 Data from the fit using LAE 81 xi 3-14 Data from the fit using LANE 82 3-15 The variation of r with angle of attack 3-16 Data from the fit of the data from Rudi et al. (1988) using LANE. 85 3-17 Comparing CFD results to data 87 3-18 Sensitivity of analytical method to offset in porous resistance coefficients 89 3-19 Sensitivity of CFD method to offset in porous resistance coefficients 4-1 Overview of a gravity cage 4-2 Overview of the cage 4-3 Cage and tow setup 4-4 Overview of the tow tank 4-5 The current meter calibration setup 4-6 The tow setup used 4-7 The cage deployed in the tank 101 4-8 Porous resistance coefficients as a function of solidity 4-9 The geometry of the cage model 4-10 Mesh used for CFD of small cage 4-11 Velocity distribution around and inside the cage Ill 4-12 Pressure distribution at the surface level 4-13 Velocity reduction of different meshes 113 4-14 Velocity reduction of different model settings 4-15 Velocity reduction using different turbulence quantities 4-16 Velocity reduction using different porous resistance coefficients ... 4-17 Comparing nets of different thickness parallel to the flow 4-18 Velocity in scaled up cage 4-19 Overview of the full size cage and domain xii 4-20 The mesh for the full size cage and domain 130 4-21 Velocity in full size cage 132
The effect of mesh orientation on netting drag and its application to innovative prawn trawl design
Fisheries Research, 2015
Prawn fisheries around the world comprise fuel intensive enterprises currently stressed financially by rising diesel costs. An avenue for relieving the situation is to improve the energy efficiency of trawling by raising the productivity of fishing per litre of fuel consumed. This paper presents work to develop a new prawn trawl design that leads to reduced trawl system drag. The trawl has a 'double-tongue' format, which refers to extensions forward of the upper and lower panels to form two additional towing points for the trawl. For this design concept, named 'W' trawl, drag generated in the trawl is largely directed to the centreline tongues and transferred forward to the trawler through a connected sled and towing wire. The associated reduction of dragtransfer to the wings makes the trawl substantially easier to spread and results in smaller otter boards being required and subsequently reduced overall drag of the trawl system. The study determined the effect on frameline tensions of implementing T0 (diamond) and T45 (square) mesh in the main body and side sections of trawl models of conventional and 'W' configuration, with the aim to establish an optimal combination of mesh orientation for the principle parts of the 'W' trawl. The objective was to achieve minimum netting drag and beneficial strain transfer within the trawl such that maximum trawling performance (catch per unit of fuel) might be obtained in the field. T45 mesh in the side sections of the trawl was found to exhibit a progressively lower drag compared to T0 mesh as the flow speed increased, but the extent of drag reduction was not of practical significance. The 'W' trawl showed a capacity of redirecting 59% of the total netting drag to the centre line tongues when T45 netting was implemented in the body section, and only 40% when T0 orientation was used. However, the introduction of bracing ropes (at E = 0.71) along the upper and lower centrelines of the T0 version of the "W" trawl improved the drag transfer to the tongues from 40% to 50% of the total drag. Overall, the most practical and economic configuration of the model 'W' designs tested produced an estimated drag reduction of 8.3% ± 0.6%, compared to the conventional trawl. It is expected that drag saving benefits in practice will be more substantial as the tested trawl models were not completely representative of practical commercial gear in that they had minimum twine area to make the experiment most sensitive to the drag-effect of mesh orientation.
Journal of Marine Science and Engineering, 2020
In Japan, the marine aquaculture net cage has an important role in farming pacific bluefin tuna farming in oceans, and the design of the net cage needs to ensure robustness against hostile oceanic conditions. Accordingly, this study focuses on the drag forces and the cage volume of the net cage, and on their variations induced by different design parameters (netting solidity ratio, netting height, and bottom weight). A series of parametric studies on drag force and deformation of the net cage was conducted using a numerical simulation model. Accordingly, the contribution of each parameter to the drag and volume was analyzed using a generalized additive model. The results indicate that the bottom weight had the highest contribution to the holding ratio of the cage volume, whereas the netting height had the highest contribution to the drag coefficient of the net cage. Finally, a fast prediction model was created by a backpropagation (BP) neural network model and was examined for the a...
Effects of drag coefficient of netting for dynamic similarity on model testing of trawl nets
Fisheries Science, 2001
For design or improvement of fishing nets, a model test based on Tauti's law or Froude's law is well executed. However, because these modeling rules are based on the proportion of the drag of the net to the square of water velocity, the converted drag of the net from the result of model testing is considerably large compared with the observed value. In this study, Tauti's law and Froude's law were corrected considering the drag coefficient of the net to the Reynolds number based on the twine diameter used experimentally. Three different scale models: 1/12, 1/20 and 1/50 of a midwater trawl net of 61 m in total length were made according to the modified Tauti's law and Froude's law, and the full-scale and model experiment were carried out. Consequently, the difference between the converted drags of full-scale by three scale models and measured values was within 10%. Each of these modified modeling rules is effective for the model test of trawl nets.
Journal of Ocean University of China, 2017
Knotless polyethylene (PE) netting has been widely used in aquaculture cages and fishing gears, especially in Japan. In this study, the hydrodynamic coefficient of six knotless PE netting panels with different solidity ratios were assessed in a flume tank under various attack angles of netting from 0˚ (parallel to flow) to 90˚ (perpendicular to flow) and current speeds from 40 cm s −1 to 130 cm s −1. It was found that the drag coefficient was related to Reynolds number, solidity ratio and attack angle of netting. The solidity ratio was positively related with drag coefficient for netting panel perpendicular to flow, whereas when setting the netting panel parallel to the flow the opposite result was obtained. For netting panels placed at an angle to the flow, the lift coefficient reached the maximum at an attack angle of 50˚ and then decreased as the attack angle further increased. The solidity ratio had a dual influence on drag coefficient of inclined netting panels. Compared to result in the literature, the normal drag coefficient of knotless PE netting measured in this study is larger than that of nylon netting or Dyneema netting.
Drag characterisation of prawn-trawl bodies
Ocean Engineering, 2016
The drag of a prawn-trawl body is characterised by five design and three operational variables. The design variables comprise headline length, steepness of trawl side cut, width-to-depth mesh ratio of the trawl mouth (gape), vertical wingend mesh count, and netting solidity; all of which effectively determine the planar twine-area of the trawl. The operational variables include towing velocity, horizontal spread, and vertical opening (headline height)-these determine the extent that the netting is exposed to relative water movement. The individual drag effects of the above variables (except for headline length, gape, and netting solidity) were systematically examined in a flume tank with prawn-trawl models built with low-stiffness full-scale netting; and the existing literature was consulted on the drag effects of gape, while drag was assumed to be proportional to twine diameter, mesh size-1 and headline length 2. The developed equations in non-dimensional forms provide the basis for a drag-prediction model for a prawn trawl of any size, construction and operating conditions. Comparisons with previously published prediction equations showed considerable disagreement in some aspects, and suggest that using stiff, full-scale netting in past model experiments have produced significant modelto-full-scale prediction errors owing to the poor equivalence of twine bending-stiffness-to-netting-tension ratio.
An Investigation Into The Drag Increase on Roughen Surface due to Marine Fouling Growth
IPTEK: The Journal for Technology and Science, 2017
Marine biofouling attached to underwater ship hull has caused problems for many years to ship operators. It has been reported to increase the total drag thus can reduce the speed of ship and disturb the overall operation of marine vehicles. Changes laminar to turbulent flow becomes fast when passing through rough ships surface. The current study models the growth of marine fouling for one year periods basis on general cargo vessel. The methods have been used in this studied was model scale 1 : 53. The use of smooth model is also included in order to analyses the difference between the two conditions. Two models of the hull roughness are regular and irregular roughness. The modeling of roughness using sand with a specific size and have been scaled. The results show that an increase in skin friction drag of about 41% per year for biofouling spread not evenly at wetted surface area (WSA). Keywordstowing tank, biofouling, sand, drag. AbstrakBiofouling yang menempel pada lambung kapal merupakan permasalahan yang muncul ketika kapal mulai beroperasi. Penempelan ini akan menimbulkan kekasaran dan menambah tebal pada permukaan kapal yang tercelupdalam air laut. Perubahan aliran laminar ke turbulen menjadi cepat ketika melewati permukaan kapal yang kasar. Penelitian ini memodelkan pertumbuhan biofouling pada periode satu tahun untuk kapal general cargo. Metode yang digunakan dalam penelitian ini adalah pengujian model fisik dengan skala 1 : 53. Penggunaan smooth model juga harus disertakan untuk analisis perbedaan antara dua kondisi kekasaran lambung model. Dua kondisi kekasaran pada lambung model yaitu regular roughness dan irregular roughness. Pemodelan kekasaran akibat biofouling ini menggunakan pasir dengan ukuran tertentu dan sudah diskalakan. Hasil dari pengujian menunjukkan bahwa pertumbuhan biofouling selama kapal beroperasi dalam 1 tahun menimbulkan peningkatan hambatan gesek sebesar 41% untuk kapal dengan biofuling tidak merata pada luas permukaan basah. Kata Kuncitowing tank, biofouling, sand, drag.