Net Solids Research Articles

Numerical study on the flow characteristics of an integrated fish cage based on the monopile offshore wind turbine foundation

The integration of net cages and offshore wind turbines can effectively develop and utilize marine resources and is one of the important ways of intensive use of the sea. The numerical simulation of the hydrodynamics of an integrated fish cage based on the monopile offshore wind turbine foundation in currents is carried out. The realizable k-ε model is adopted and the porous media model is used to simulate the net. To verify the validity of the numerical model, the numerical result is compared with the data of the corresponding literature. The results show that the numerical method is effective. On this basis, the flow field distribution results of the integrated structure at different velocities are calculated. At the center of the downstream zone of the integrated structure, the time-average velocity is lower than 80 % of the initial velocity. The effects of the net solidity and the cage draught on the flow field characteristics of the integrated structure are analyzed. The results show that the increase of net solidity will significantly reduce the velocity inside the net cage and in the downstream zone of the cage. As the net solidity increases from 0.14 to 0.32, the average velocity attenuation inside the net cage increased by 13.9 %. The change of cage draught will affect the velocity distribution in the horizontal and vertical directions. When the cage draught increases, the velocity on both sides in the downstream zone of the integrated structure increases more than 5 %. It is expected that the results of this study can provide data support for the design of integrated structure of net cages and offshore wind turbines.

Improvement of waste sludge dewaterability by a novel co-conditioning approach with polyaluminum chloride-sludge based carbon-polyacrylamide

A novel sludge conditioning method incorporating sludge-based carbon (SBC) as a filter aid into the coagulation and flocculation co-conditioning process is proposed to strengthen sludge dewaterability, namely the co-conditioning of polyaluminum chloride (PAC)-SBC-polyacrylamide (PAM). The results show that the co-conditioning method can effectively reduce the specific resistance to filtration (SRF), moisture content (MC) of the filter cake and sludge compressibility coefficient (CC), meanwhile increase the net sludge solids yield (YN) of municipal sludge (MS) and textile dyeing sludge (TDS), resulting in a significant improvement in sludge dewatering performance for two types of sludge. The response of sludge dewatering performance to different conditioning agents and the variation characteristics of sludge EPS in co-conditioning process suggest that the co-conditioning mechanism involves a synergistic effect of coagulation, adsorption, and flocculation, which process includes the aggregation and disintegration of microbial cell flocs caused by PAC, the skeleton-building action and biopolymer adsorption by SBC, and sludge floc agglomeration facilitated by PAM. In addition, SBC also optimizes coagulation by electrostatic neutralization, while strengthening flocculation by bridging biopolymers and PAM. Moreover, co-conditioning exhibits a remarkable ability to eliminate dissolved organic matter from the filtrate, leading to an enhancement in the filtrate quality and a reduction in the downstream burden for sludge filtrate treatment. Consequently, the PAC-SBC-PAM co-conditioning emerges as a promising method for sludge pretreatment.

Hydrodynamic Analysis of a Multi-Pile-Supported Offshore Wind Turbine Integrated with an Aquaculture Cage

The integrated development of offshore wind power and marine aquaculture is becoming increasingly important. However, the impact mechanism of integrating a net cage on the dynamic characteristics of offshore wind turbines remains unclear. This paper presents a design scheme for a multi-pile-supported offshore wind turbine integrated with an aquaculture net cage and conducts a preliminary theoretical analysis of the influence of an additional net cage on the wind turbine. The analysis reveals that the primary effect is an increase in hydrodynamic loads on the wind turbine foundation, while the structural frequency of the wind turbine remains largely unaffected. Furthermore, computational fluid dynamics (CFD) numerical models, whose accuracy is verified by physical experiments, are utilized to compare the hydrodynamic characteristics of the offshore wind turbine foundation with and without the net cage, considering different net solidities. The simulations identify significant changes in the flow field surrounding the foundation due to the presence of the net cage, resulting in a considerable increase in the overall hydrodynamic load on the foundation. Moreover, the mutual interference between the netting and the foundation amplifies their respective hydrodynamic loads and concentrates these loads at the upstream section of the structure. The maximum increase in hydrodynamic load for a single pile reaches 6.32 times its original value, posing significant risks to the structure. Finally, a preliminary feasibility analysis of the scheme was conducted. The results presented in this article can serve as a theoretical basis for the design of such innovative structures.

Experimental investigations on hydrodynamic interactions between the cylinder and nets of a typical offshore aquacultural structure in steady current

A series of physical experiments were performed in steady current to investigate the hydrodynamic interactions between the cylinder and nets which constitute a main part of offshore aquacultural platforms. The hydrodynamic characteristics of only the cylinder, only nets and combined cylinder-net structures are measured and analyzed systematically under different current velocities, inflow angles and solidity ratios of nets. Based on experimental data, fitted formulas for hydrodynamic coefficients of a single resin net are proposed and compared to previously published empirical formulas. It is observed that the existence of the cylinder brings an increment up to 9.2% to drag coefficient of net panels whose solidity ratios are higher than 0.347, whereas this effect is negligible for nets with lower solidity ratios and perpendicular to the inflow. For nets inclined with 45°, the increment of the drag coefficient of net panels due to the existence of the cylinder is more significant (up to 22.9%) than that for perpendicularly placed nets. Furthermore, the existence of nets also leads to a noticeable increase in the drag coefficient of the cylinder, up to 40.18% for a relatively large net solidity ratio of 0.458, representative of biofouling condition. The increment increases with the rise of the solidity ratio of nets and it is larger for nets placed perpendicularly to the inflow than inclined. Effects of the cylinder and nets on lift coefficients of each other are a bit complicated, leading to an increase or reduction of lift coefficients depending on the inflow velocity, inflow angle and net solidity ratios. Finally, it is worth noting that the hydrodynamic interaction between the cylinder and nets deserves to be considered in current practice of hybrid methods by combining potential flow theory, Morison and screen models for aquacultural structures, especially for biofouling conditions.

Quantitative evaluation on the suitable flow region inside the shielding device of submersible aquaculture facility

The primary purpose of the aquaculture facility is to provide a comfortable living environment and achieve welfare aquaculture for cultured fish in the open sea. Nevertheless, it has been a challenge to accurately assess the suitable flow region of the cultured fish inside the aquaculture facility for the severe ocean environment, which is a pressing concern for aquaculture farmers. First, the maximum domed swimming speed (MDSS) is used as an indicator to assess the swimming ability of Sciaenops ocellatus in this study, which can be measured through a vertical circulation water channel. Subsequently, the porous media theory is adopted and applied to predict the flow field inside the shielding device of the submersible aquaculture facility based on the shear stress transport k-omega turbulence model using the computational fluid dynamics (CFD) approach. In addition, combining the swimming ability of Sciaenops ocellatus and current speed distribution inside the shielding device of a submersible aquaculture facility, the suitable flow region is quantitatively assessed. The results indicate that when middle height, the inclined angle and net solidity are 0.4 m, 3π/12 and 0.65, respectively, the current speed inside the shielding device attenuates below the maximum domed swimming speed of Sciaenops ocellatus - 0.76 m/s and meets the survival of Sciaenops ocellatus for the actual sea condition with an incident current speed of 1.5 m/s. In addition, a suitable flow region for Sciaenops ocellatus, which is 204.512 m3 and accounts for 17.76% of the total aquaculture volume, is formed and located inside the shielding device, close to the downstream sides, and gradually extends towards the middle of the shielding device. Based on the swimming ability of Sciaenops ocellatus, using numerical means to describe the suitable flow region in detail is to provide clear and macroscopic guidance for aquaculture farmers.

An implicit Eulerian–Lagrangian model for flow-net interaction using immersed boundary method in OpenFOAM

In this paper, we propose an implicit Eulerian–Lagrangian model to resolve the fluid–structure interaction of submerged nets. The Reynolds-averaged Navier–Stokes equations are solved on an Eulerian grid using OpenFOAM and the geometry of the flexible net structures is tracked using Lagrangian points. The motion and deformation of the flexible net structures are advanced using an implicit scheme to avoid excessively small time intervals. The force acting on the net structures is calculated based on a mass–spring system. The screen model and the immersed boundary method are implemented to capture the interaction between the net structures and the surrounding fluid. We present a new drag coefficient formulation as based on a piecewise function for different net solidities to obtain a much improved drag force prediction. The numerical results are compared with a series of existing experiments showing excellent agreement and improved drag force predictions.

Numerical Simulation of Scalar Mixing and Transport through a Fishing Net Panel

With the rapid development of commercial aquaculture in recent decades, large numbers of submerged cages or pens are clustered in fish farms that are commonly located within inland lakes, reservoirs, and coastal embayments around the world. The submerged structures have significant influence on both the flow fields and mass transport processes in surrounding water bodies. While existing studies have concentrated mainly on the flow blockage effects produced by fish cages, the associated effect on near-field mass transport processes, important for pollution transport and dispersal, remains largely unclear. To address this knowledge gap, a CFD (computational fluid dynamics) model was established using OpenFOAM to investigate the wake characteristics and scalar transport processes through a fishing net panel, as representative of a key component of the fish cage or pen. In this model, the net panel was represented as porous media, and the finite volume method was applied to solve the governing flow equations with the standard k-ε model used for turbulence closure. Experimental data from previous studies were used to calibrate and validate the numerical model, which was applied to different scenarios over a range of net solidities and incoming flow velocities. Overall, the numerical model results demonstrated that porous media schematization could adequately reproduce the blocking effect from the net panel on the mean flow field, as well as the induced changes to scalar transport, with satisfactory accuracy. The flow velocity reduction across the net panel was found to strengthen with increasing net solidity and decreasing incoming velocity, while the scalar concentration decay tended to become enhanced when the incoming velocity was decreased. The lateral profile of the scalar concentration exhibited a self-similar Gaussian distribution with the spreading width of the plume reduced by increasing the incoming velocity. This lateral concentration distribution was minimally affected by the upstream scalar source location relative to the net panel, when adopting the current RANS and porous media modelling approach. The model results provide useful references for the assessment of the environmental impacts and carrying capacity of cage-based fish farming.

Numerical investigations on fluid characteristics around the bottom-fixed aquacultural farm

Offshore aquacultural farms are world widely booming in recent years. The bottom-fixed aquacultural farm gains more and more attention because of the large volume and great strength. This study numerically investigates the fluid velocity distribution characteristic around the bottom-fixed aquacultural farm, which is of great importance for fish welfare and pollutant diffusion. The numerical model is developed based on the porous media model for the nets and the rigid walls for the fixed frame. The k-omega turbulence model and the finite volume method are employed to solve the Navier-Stokes equations. The flow velocity distributions around and inside the farm in the varied current velocity are analyzed. The vertical and horizontal flow velocity distributions in various current directions with different net solidities are investigated in detail. Results demonstrated that the configurations of trusses, vertical and horizontal columns of the farm and the interaction with the nets result in complex velocity distributions inside and around the farm. The velocity attenuations at different depths are quite different in uniform and non-uniform currents. The velocity distributions in different attack angles of the flow provide valuable suggestions on the arrangement and layout of the multiple farms.

Coupling sludge-based biochar and electrolysis for conditioning and dewatering of sewage sludge: Effect of char properties

The addition of sludge-based biochar during electrochemical pretreatment of sewage sludge, as an efficient hybrid technology, is potentially to be applied in sludge deep-dewatering. The chars functioned as conductors, catalysts and skeleton particles could enhance the sludge dewaterability and increase the calorific value of the dewatered sludge cake. However, the effect of synthesis conditions on the char properties and further on the dewatering performance is still unknown. Herein, the sludge-based particle electrodes (SPEs) under three main synthesis conditions, including liquid-solid ratio, pyrolysis temperature and time, were prepared. The sludge-based biochars (i.e., SPE-400, SPE-600, and SPE-800 pyrolyzed under 400, 600 and 800 °C, respectively) were characterized and utilized as three-dimensional electrodes during sludge electrolysis. The increased pyrolysis temperature (within 400–800 °C) resulted in the enrichment of metallic ions and increment of specific surface area and pore volume of SPE, which led to the increased catalysis and adsorption sites for viscous proteins (PNs). Particularly, the pores of SPE-800 provided more drainage channels as skeleton builders. Compared with raw sludge, the capillary suction time (CST) and the specific resistance of filtration (SRF) of the treated sludge with 3D-SPE-800 were reduced by 58.12% and 81.01%, respectively, but the net sludge solids yield (YN) was increased by 87.05%. The highest decrease of hydrophilic α-Helix content in PNs (from 9.93% to 7.30%) was observed when using SPE-800 as particle electrode, revealing the crucial role of char characteristics on protein reduction and subsequent dewatering enhancement. The synergistic effects of electrolysis and sludge-based biochar provided a new insight for a closed-loop pretreatment of sewage sludge in the wastewater treatment plant.

Numerical Investigation of the Hydrodynamic Behavior of Trash-Blocking Nets for Water Intake Engineering of Nuclear Power Plant

In order to ensure the safety of the cooling water source of coastal nuclear power plants (NPP), trash-blocking nets (TBNs) are usually installed at the entrance of the penstock to prevent marine sewage and organisms flowing into the front pool of the pump house of the nuclear power plant. The safety evaluation of these trash-blocking nets is of paramount importance for the stable operation of a nuclear power plant. However, there is no reliable analysis method for improving the design of trash-blocking nets and mooring systems. In this study, a numerical model of in-current trash-blocking nets based on the lumped mass method was developed to calculate the tension force on the trash-blocking nets and mooring system. A comparison with the experimental data indicates that the present numerical model is appropriate for calculating the in-current hydrodynamic loads on the trash-blocking nets. In addition, the effects of the width of trash-blocking nets, hanging ratio, water depth, and net solidity are discussed in detail, and the damage process of trash-blocking nets was also investigated. The results indicate that the maximum tension force on the trash-blocking net linearly increases with the increasing width of trash-blocking nets, and it is greatly decreased with the increase in the horizontal hanging ratio of trash-blocking nets. It can be increased by 200% when the net solidity is increased from 0.16 to 0.6. Two damage modes for mooring lines can be observed, which are determined by the strength of mooring lines.

Experimental study of flow and mass transport in the near-wake region of a rigid planar metal net panel

Aquaculture cages constructed from rigid material have become increasingly popular in recent years due in part to the continuous expansion of the aquaculture industry offshore, where cages need to withstand much harsher ocean conditions. Most previous studies of hydrodynamic interactions with aquaculture cages have explored the impacts of more traditional flexible cage net panels on wake flow fields and mass transport properties. In the current experimental study, Particle Image Velocimetry and Planar Laser Induced Fluorescence techniques were used to measure flow and concentration fields in the near wake region of a steady current through a rigid metal net panel to analyze both the flow patterns and mass transport processes. The results indicated that, similar to a flexible polyethylene net panel, the time-average velocities and turbulence characteristics in the near wake region were highly inhomogeneous and anisotropic, with the turbulent intensity following a short-range, power-law decay downstream of the panel. In direct comparison with a flexible net panel of similar net solidity, the rigid net panel was found to slightly enhance the lateral spreading of the plume, with cross-sectional profiles of the mean concentration and concentration fluctuation shown to exhibit self-similar Gaussian distributions. The rigid net panel also resulted in a lower flow blockage, as measured by a velocity reduction coefficient, with the corresponding turbulent intensity also reduced compared to the flexible panel due to an overall reduction in the drag force. In addition, the rigid panel tended to create larger wake eddies than the flexible panel, leading to enhanced scalar mixing and, hence, faster rates of concentration decay in the near-wake region. These experimental findings have potential implications for cage-based aquaculture operations, where rigid fish cages are comparatively more likely to generate favorable environments for fish growth, as well as helping to reduce negative impacts on the surrounding aquatic environment.

Numerical Modeling of Flexible Net Panels under Steady Flow Using a Coupled Fluid–Structure Partitioned Scheme

Fluid–structure interactions of flexible net panels are complex and lack sufficient exploration. To examine the flow characteristics of flexible net panels with large deformation, we propose a partitioned coupling scheme in this paper. The coupled fluid–structure equations are solved separately under finite volume and finite element frameworks. The interface traction from the fluid solver is considered as a Neumann boundary condition for the solid domain, and the interface velocity is applied as a Dirichlet boundary condition for the fluid problem. Then, the forces can be transferred along the interface via Dirichlet-to-Neumann mapping. The results show that both the drag coefficient and the velocity reduction increase alongside the net solidity ratio (Sn), but they decrease as the Reynolds number/attack angle increases. A comparative study of drag coefficients is made between the present numerical simulations and the analytical predictions. This paper also examines the velocity distribution and vortex formation of flexible net panels. A single vortex forms in the shear layers and the wake when Sn = 0.16, and a pair of vortices mostly forms in the wake when Sn = 0.33. The vertical net twines predominantly affect the formation of the vortex behind the net, leading to delayed vortex shedding. The flow exhibits wake interactions due to the interference between the net twines in the high-solidity net panel. No such interference occurs in the low-solidity net panel, but the altered shear layers could cause severe velocity fluctuations in the near field.

An integrated investigation on anaerobic membrane-based thickening of fecal sludge and the role of extracellular polymeric substances (EPS) in solid-liquid separation

Pre-thickening or primary treatment of fecal sludge (FS) is a major bottleneck in designing fecal sludge treatment plants. This research demonstrates a practical analysis to improve the thickening efficiency of FS using woven fiber microfiltration membrane. A laboratory-scale anaerobic membrane-based thickening tank (MBTT) was investigated. Firstly, the system was operated with unconditioned FS at a flux range of 1–3 L/m2h. Secondly, the system was operated at an optimized flux of 2 L/m2h with conditioners, chitosan, and charcoal dust, at their optimized dosages reported in previous studies. It was observed that the solids accumulation in MBTT was linear and was specific to net solids accumulation. Feed FS was thickened to around 15% of total solids (TS) in 9–14 days. The overall solids accumulation rate was higher with conditioned FS. The less EPS accumulation and higher dewaterability in conditioned FS reduced the brushing frequency of the membrane and consequently, the average filtration duration per cycle was increased. Strong correlations of dewatering time with floc size, EPS, and electrical conductivity, indicated that higher EPS in FS tends to form flocs which can increase the dewatering rate in unstabilized FS. In permeate, the average TS and CODt removal observed were 72–78% and 87–91%, respectively.

Numerical study on the flow environment for a novel design of net cage with a shielding device

As aquaculture moves from inshore to the deep sea, developing a comfortable swimming environment for the farmed fish to cope with the strong current in the open sea becomes vitally imperative. This study designs a novel net cage with a shielding device to provide a sheltered area for the farmed fish at extreme ambient current speeds. Considering the time-consuming and substantial economic loss caused by the destruction of the net cage in situ measurements, a numerical method based on computational fluid dynamics (CFD) modeling is employed to evaluate the flow field of the net cage. Three characterized parameters, namely the height of the middle plane nets, the inclined angle of plane nets, and the net solidity, are used to estimate their influences on the flow field of a net cage and the drag force loaded on it. The results highlight that the height variation of the middle-plane nets has little effect on the flow field and the drag force. Nevertheless, the increased inclined angle of plane nets and net solidity leads to a dramatic fluctuation regarding the current velocity distribution and the drag force. In particular, the increase of the inclined angle for the plane nets causes a noticeable increase in the cage volume. With this understanding, further aquaculture net cages can potentially be designed to better withstand the ocean's rigors and maintain the aquaculture industry's sustainability.

An improved Morison hydrodynamics model for knotless nets based on CFD and metamodelling methods

The hydrodynamic loadings on the knotless nets are studied using Improved Delayed Detached Eddy Simulations (IDDES) with Kriging metamodelling, then an improved Morison model for predicting hydrodynamic loadings on net twines is derived for the first time. The high-fidelity IDDES turbulence model is capable of simulating the separated flows from cylindrical objects. The 6 × 6 portions of smooth and fabric nets with roughness features are considered in flow models. The accuracy of the numerical method is validated through a typical cruciform case in fluid, wherein the pressure fields and turbulent intensity are compared with physical measurements. Initially, a comparative study of flowing over a circular cylinder, cruciform, and net panel is conducted, illustrating the necessity of the net twines utilisation to analyse the flow fields and hydrodynamic loadings accurately. Based on the review of existed Morison model for net structures, the design point cases varying the normal incoming velocities, diameters and lengths are simulated numerically, then Kriging metamodels of hydrodynamic loading coefficients are trained. It is addressed that the over-estimation of Reynolds number effects, the overlook of net solidity and tangential forces on the twine, or the constant-adoption in the previous models contribute to the deviations of hydrodynamic modelling. The polynomial-type, as well as power-type formulae of normal and tangential force coefficients, are fitted non-linearly using the metamodelling results. Finally, the applications of newly-proposed Morison models embedded in finite element modellings are presented to emphasise its improving abilities. The hydrodynamic loadings with new formulae are validated and compared with existed models through net panels, fish cage and trawl net cases.

Modelling the flow around and wake behind net panels using large eddy simulations

The fluid flow through a fixed net panel is investigated using large eddy simulations. Computational meshing strategies suitable for smooth and fabric twines are proposed. Numerical validations are conducted by comparing with measurements of the flow around a circular cylinder and hydrodynamics of net panels. It is shown that the hydrodynamic simulation of a small portion of the net panel is sufficient to replicate the full-scale net. Hence, the effect of the incoming velocity, the diameter and the length of the twines on surrounding turbulence fields are studied. Unique flow patterns are revealed in contrast to existing side-by-side cylinder simulations. Distributions of turbulent kinetic energies in the wake region behind intersections are crucial. It is further shown that the net solidity has a more considerable impact on the flow patterns in the wake than the Reynolds number. Further, the velocity reduction through the net panel is determined by weighting the relative velocity over the net panel area in the far-field. The analysis reveals less velocity reduction in comparison to existing velocity reduction models based on theoretical derivations and good agreement with physical measurements. The comparison of smooth and fabric nets indicates additional shielding effects of the twisted configurations on the velocity reduction which is not addressed in previous studies.

Effects of soft marine fouling on wave-induced forces in floating aquaculture cages: Physical model testing under regular waves

Previous experiments on the fouling/current and fouling/wave interactions were mostly limited to two dimensional (2D) net panels or isolated twines. The current paper deals with the wave-induced forces/moments in three dimensional (3D) physical models of a gravity net-cage. The colonial hydroid Ectopleura larynx, a common fouling organism in the aquaculture industry, was artificially modelled and considered in the current experiments. A clean cage and cages with different fouling lengths were tested under deep-water regular wave trains. The time series of the wave-induced forces/moments in the surge, sway, roll and pitch degrees of freedom were recorded and analysed. The experimental results showed that the surge force amplitude increased up to 72% by the fouling, but the sway force amplitude decreased up to 26%. The asymmetry of the surge and sway force waveforms increased by the fouling, respectively. The effects of the fouling on the force/moment response amplitude operators (RAO) were also studied. Pixel histogram analysis was performed to define equivalent net solidities of the fouled nets. In general, the tests under regular waves were found to underestimate the soft fouling effects on the wave-induced forces/moments, as compared to those carried out under irregular waves.

Hydrodynamic assessment of a semi-submersible aquaculture platform in uniform fluid environment

Predicting the distribution of flow velocity inside a semi-submersible aquaculture platform and taking measures to optimize the flow field are critical to provide a dependable inhabit for farmed fish. This paper examines the hydrodynamics of a semi-submersible fish farm under current using the porous media method combined with a rigid wall based on the shear stress transport k-omega turbulence model. The results show that the presence of nets expands the low flow area and provides a better inhibit for farmed fish in the severe currents. The increase of net solidity enhances the damping effect on the flow, and it also simultaneously increases the current loads acting on the net. The attenuation caused by different net solidities produces a 66.10 % deficit of the flow velocity in maximum inside aquaculture platform. A maximum increase of 36.82 % for the drag force acting on the aquaculture platform is obtained. Furthermore, the distribution of drag force acting on the aquaculture platform in current is also assessed. Although the drag force of the frame-only is approximately twice compared to the net-only case, the improvement in the strength of the net structure under the action of flow should not be ignored.

Experimental and numerical study of a semi-submersible offshore fish farm under waves

The interaction between the semi-submersible offshore fish farm (SOFF) and waves was investigated by numerical simulations and dedicated model tests. The SOFF generally consists of three main components, such as large pontoon, frame system, and net system. And the hydrodynamic forces cannot be calculated by the same method due to the big difference in the main dimensions of them. Therefore, a hybrid numerical method of direct time domain and indirect time domain for the SOFF was proposed in this paper, and the calculation program was originally developed. In the present numerical method, the three dimensional potential theory was used to calculate the hydrodynamic loads on the large pontoon through indirect time domain, and the Morison equation and screen model were used to calculate the hydrodynamic loads on the frame system and net system, respectively. Then the total hydrodynamic loads on the SOFF at each time step can be obtained, and the motion responses of the SOFF were calculated in the time domain. The model tests were carried out in the wave basin, and the heave and pitch motions with wave periods were obtained. The numerical results were verified by the experimental results. A good agreement exists between the numerical results and experimental results. Then the effects of some key parameters on the motion responses were investigated by the present numerical model, such as draft, wave height and net solidity ratio. The results show that the draught of SOFF has an obvious influence on the heave and pitch motion. In addition, heave and pitch response amplitudes are positively correlated with wave amplitude. And the change of net solidity ratio has a notable effect on the heave motion and pitch motion.

Experimental analysis of hydrodynamic forces on net panel in extreme waves

As fish farms move from nearshore region to open sea, understanding the hydrodynamic force on net structures in extreme waves plays a significant role in the design of offshore aquaculture structures. In this study, a series of experiments on different types of net panels in extreme waves were conducted in a wave flume. The dispersive focusing method with the constant steepness spectrum was adopted to generate extreme waves and the effects of the net solidity, the net material and the knot pattern of net panels were experimentally analyzed. The experimental results indicated that the force on the net panel in extreme waves was heavily dependent on the net solidity and the KC number, and it was slightly dependent on the net material. A concept of quasi-static forces was adopted to establish the wave force model to predict the forces on net panels in extreme waves, and then, a quadratic function describing the dependence of the drag coefficient on the solidity and the KC number was obtained. The results indicated that the wave force model could simulate the loading process of the submerged net panels in extreme waves very well.