Attitude-adjustable aquaculture net cage hydrodynamic force numerical calculation method

Abstract

The invention discloses a method for calculating the hydrodynamic force value of a posture-adjustable culture net cage, and the method comprises the steps: defining a wave field where the net cage is located, and setting the wave coverage range, the seabed range and the water depth where the net cage is located; establishing a net cage frame finite element model; establishing a netting finite element model; establishing a net cage mooring system model; environment variables are defined, and ocean current waves are simulated; loading the attitude parameters of the net cage; solving the motion response and the structure stress of the net cage; and comparing and analyzing each attitude motion response and structure stress difference of the net cage. Wherein the step of establishing the net cage frame finite element model comprises the substeps of defining circular tube nodes and section attributes; and performing beam unit simulation, and calculating the wave force by using a Morison equation. And establishing a netting finite element model, namely simulating the netting by adopting an equivalent surface unit, defining compactness parameters, and calculating resistance and lift force by using a screen model.

Description

Technical Field

[0001] This disclosure relates to the field of marine engineering numerical simulation technology, specifically a method for numerical calculation of hydrodynamics of an attitude-adjustable aquaculture cage. Background Technology

[0002] Insufficient resistance to wind and waves and the problem of biological adhesion to the netting surface are two major technical challenges currently hindering the high-quality development of the marine cage aquaculture industry. To effectively address these issues, a type of attitude-adjustable aquaculture cage structure has been proposed in recent years. This type of structure achieves attitude control through the filling and draining of internal ballast tanks, thereby improving environmental adaptability. This type of cage structure typically has the following characteristics: the main body is composed of numerous slender rods, lacking large-scale floating bodies; the frame structure is covered with a closed, flexible netting, with densely interwoven mesh and wires, resulting in a complex yet highly flexible overall structure; and the cage's attitude is frequently adjusted, rather than remaining completely static.

[0003] However, existing numerical simulation methods for hydrodynamic characteristics generally have significant limitations when dealing with the aforementioned structural characteristics and attitude adjustment functions. One mainstream approach is based on high-precision simulation techniques using computational fluid dynamics (CFD), such as RANS or VOF models for solving the global flow field. Existing research has used software like OpenFOAM or Fluent to model the local fluid behavior of the cages, such as wave diffraction and seepage flow. While these methods offer high accuracy, they suffer from severe limitations in engineering applications: the model building process is cumbersome, mesh generation is time-consuming and requires high quality; computational resources are enormous; and in attitude adjustment scenarios, each change in structural state (such as rotation or submersion) necessitates mesh reconstruction, lacking reusability and failing to meet the simulation requirements for multiple operating conditions and rapid response. Another commonly used method is a simplified modeling approach combining potential flow theory and the Morison equation, which has been widely applied to the hydrodynamic response estimation of rigid or semi-flexible cage structures. For example, in the study of submersible cages, this method can effectively predict their motion response characteristics. However, for the attitude-adjustable cage involved in this study, due to its composition of numerous slender rods and lack of large floating structures, potential flow theory is prone to significant errors when dealing with such irregular structures. Furthermore, when processing the mesh portion, this method typically requires modeling the mesh and wires as independent rod units, resulting in an extremely large model size, significantly reducing simulation efficiency, and leading to complex parameter settings and poor convergence. More critically, most current methods can only predict the structure's motion response independently during the simulation process, while the internal forces of the structure must be calculated using a secondary model or post-processing, making it difficult to achieve integrated coupled calculation of structural forces and motion states.

[0004] In summary, existing hydrodynamic calculation methods suffer from problems such as cumbersome modeling processes, low simulation efficiency, poor adaptability to attitude changes, and insufficient coupling analysis capabilities when dealing with the dynamic adjustability, high flexibility, and structural complexity of attitude-adjustable marine cages. Summary of the Invention

[0005] One embodiment of this disclosure provides a method for numerically calculating the hydrodynamics of an adjustable aquaculture cage, comprising the following steps:

[0006] A1, define the wave field of the cage, and set the wave coverage area, seabed range and water depth of the cage;

[0007] A2, Establish the finite element model of the wire mesh frame;

[0008] A3, Establish the finite element model of the mesh garment;

[0009] A4, Establish a model of the cage mooring system;

[0010] A5, define environmental variables to simulate ocean currents and waves;

[0011] A6, Load cage attitude parameters;

[0012] A7, solve for the motion response and structural forces of the cage;

[0013] A8, comparative analysis of the motion response and structural stress differences of the cage at various postures.

[0014] This disclosure proposes an efficient, flexible, and precise numerical calculation method for hydrodynamics of attitude-adjustable net cages. It can integrate multi-attitude response simulation and stress analysis of complex structures while ensuring computational efficiency, providing reliable numerical support for performance evaluation and engineering design of deep-sea aquaculture equipment. Attached Figure Description

[0015] The above and other objects, features, and advantages of this disclosure will become readily apparent from the following detailed description of exemplary embodiments, taken in conjunction with the accompanying drawings. Several embodiments of this disclosure are illustrated in the drawings by way of example and not limitation, in which:

[0016] Figure 1 A schematic diagram of an adjustable aquaculture cage structure according to one embodiment of the present disclosure.

[0017] Figure 2 A flowchart of a method for numerical calculation of hydrodynamics of an attitude-adjustable cage according to one embodiment of the present disclosure.

[0018] Figure 3 A schematic diagram of a net cage numerical model according to one embodiment of the present disclosure.

[0019] Figure 4 A schematic diagram comparing the peak values ​​of vertical displacement of a cage in semi-submerged and fully submerged attitudes according to one embodiment of this disclosure.

[0020] Figure 5 A schematic diagram comparing the peak values ​​of horizontal displacement of a cage in semi-submerged and fully submerged attitudes according to one embodiment of this disclosure.

[0021] Figure 6 A comparison diagram of the maximum mooring anchor chain tension on the wave-facing and wave-back sides under two different attitudes, according to one embodiment of this disclosure.

[0022] Figure 7 A schematic diagram of the axial force on the unit at the center pressure bar of the cage under two postures according to one of the embodiments of this disclosure.

[0023] Figure 8 A schematic diagram of the tension on the mesh of an adjustable mesh cage according to one embodiment of the present disclosure.

[0024] Figure 9 The effective tension cloud diagram of the cage under semi-submersible and fully submersible postures according to one embodiment of the present disclosure.

[0025] 1—Central pressure bar, 2—Circular tube, 3—Hub-shaped ring. Detailed Implementation

[0026] This disclosure relates to a posture-adjustable aquaculture cage structure. The cage comprises a net, a hub-shaped ring, a circular tube, and a central pressure rod. The net is bound to cover the surface of the cage. Both the central pressure rod and the hub-shaped ring have ballast chambers inside. The cage can achieve circumferential rotation, depth adjustment, and tilting by filling and defilling the ballast chambers. Filling the ballast chambers in the central pressure rod with water can raise and lower the cage. Filling the ballast chambers in the hub-shaped ring with water can generate an asymmetric torque to drive the cage to rotate for net drying.

[0027] The structure of this net cage differs from common gravity-type and truss-type net cages. It is horizontally arranged, resembling a flat-lying cylinder. The cage consists of a central pressure rod, two hub-shaped rings, and several connecting tubing. Internal ballast chambers are located inside the central pressure rod and the rings. The operation of these chambers allows the cage to float, sink, and tilt; the operation of the annular ballast chambers within the rings allows the cage to rotate. The combined functions of floating and rotating effectively remove netting. Specifically, the cage rises from a fully submerged state until one-fifth of the netting is exposed above the water. After several days of sun exposure, this portion of the netting is rotated back into the water, causing algae and shellfish attached to it to detach automatically. Furthermore, floating and tilting also help avoid typhoons and attract fish for easier harvesting. Therefore, the attitude adjustment of this net cage refers to its floating (fully submerged and semi-submerged), rotation, and tilting.

[0028] This disclosure proposes a rapid numerical calculation method for the hydrodynamic response of aquaculture cage structures with adjustable posture. This method integrates posture simulation based on equivalent mass adjustment, an equivalent modeling strategy using a dense netting screen model, and a structure-motion-load coupled solution framework. It overcomes key challenges of traditional methods, such as complex modeling, the need to reconstruct the model for posture changes, and the inability to integrate hydrodynamic and structural stress analysis. This method is suitable for performance evaluation of cage structures in multi-posture and complex environments.

[0029] According to one or more embodiments, a method for numerically calculating the hydrodynamics of an attitude-adjustable net cage includes the following steps:

[0030] S100 defines the wave field extent, setting the wave coverage area, seabed extent, and water depth to determine the marine environment boundary of the cage system, specifically including:

[0031] S101, Set the wave coverage area and the seabed (seafloor) area. Both areas are rectangular in shape, and define their length and width;

[0032] S102, Defines water depth.

[0033] S200, Establish the finite element model of the cage frame, specifically including:

[0034] S201, Establish frame nodes, define node coordinates and degrees of freedom. All nodes in the mesh cage frame are free nodes, with no restrictions on degrees of freedom;

[0035] S202 defines the cross-sectional properties of the gabion frame, including unit mass, cross-sectional area, element type, stiffness, load equations, and hydrodynamic coefficients. Beam elements are used to simulate the gabion frame. The Morison equations are used to calculate the wave loads on the frame members, incorporating wave forces... Decomposed into inertial forces and drag force Calculate in two parts separately:

[0036]

[0037]

[0038]

[0039] In the formula: The density of seawater; The projected area of ​​a bar per unit area; It is the drag coefficient; It is an additional quality coefficient; It is the inertial force coefficient. ; The volume per unit length of the component; and These are the velocity and acceleration components of the water particles, respectively. and These are the velocity and acceleration components of the component, respectively.

[0040] S203, Define the wire mesh frame line segment type, apply the defined cross-section attributes to the line segment type, and set the line segment length and the number of units to be divided;

[0041] S204, Create the wire mesh frame segments, and select the defined segment type and the nodes at the beginning and end of the segments.

[0042] S300, establish the finite element model of the mesh garment, specifically including:

[0043] S301, Establish a mesh node;

[0044] S302, Define section properties

[0045] The netting is configured with a unit mass, equivalent unit area, density of 0.2, element type, and hydrodynamic coefficient. Rod elements are selected for the netting. Hydrodynamic loads are calculated using a screen model, decomposing the hydrodynamic forces on the netting into resistance forces. and lift .

[0046]

[0047]

[0048] In the formula: and Let A and B be the drag and lift coefficients of the mesh, respectively, which can be expressed as:

[0049]

[0050]

[0051] In the formula: For density; Mesh fabric angle of attack. Regarding density. Calculate using the following formula, where The diameter of the network cable. The side length of the mesh.

[0052]

[0053] In the formula: For density; Mesh fabric angle of attack. Regarding density. Calculate using the following formula, where The diameter of the network cable. The side length of the mesh.

[0054] S303 defines the mesh segment type, applying the defined section properties to the segment type. It sets the mesh segment length and the number of units to be divided.

[0055] S304. Create mesh segments, select the defined mesh segment type and the nodes at the beginning and end of the segments, and repeat the steps to generate a complete mesh.

[0056] S400, establishing mooring anchor chains, specifically including:

[0057] S401. Establishing the anchor point node: Establishing a mooring anchor chain first requires defining the anchor point. The vertical coordinates of the anchor point should align with the water depth, and its degrees of freedom must be completely restricted. The other end of the anchor chain is connected to the net cage, or, depending on the designed mooring system, to a buoy.

[0058] S402 defines the anchor chain section properties, setting unit mass, cross-sectional area, axial stiffness, element type, and hydrodynamic loads. The hydrodynamic load equation for the anchor chain is the Morison equation. The gabion mooring anchor chain is considered as a rod element, which can be used to calculate axial tensile deformation, but does not undergo bending or torsional deformation. Its axial tension is:

[0059]

[0060] In the formula: For the axial force of the unit; This is the length after stretching and deformation; This is the initial length; It is the elastic modulus; It represents the cross-sectional area.

[0061] S403 defines the anchor chain segment type, applies the defined anchor chain section properties to the segment type, and sets the length of the mooring anchor chain and the number of units to be divided.

[0062] S404, Create anchor chain segments, select the defined anchor chain segment type and the nodes at the beginning and end of the segment, and generate the mooring anchor chain.

[0063] S500 defines ocean current and wave parameters, specifically including:

[0064] S501 defines the ocean current parameters, using a uniform flow model. Simulation is performed by defining the flow direction and velocity. The ocean current load can be calculated using the following formula:

[0065]

[0066] in: These are the design values ​​for the loads of the wire mesh frame system; It is the projected area of ​​a component per unit length in the direction perpendicular to the ocean current; It is the design flow rate.

[0067] S502, define wave parameters:

[0068] When simulating waves, linear wave theory or stochastic wave theory is usually used to simplify the waves. Input parameters include wave height, wave period, and wave angle.

[0069] S600, attitude adjustment settings, specifically including:

[0070] S601, Determine the structural components corresponding to the ballast tanks. Identify the structural components corresponding to the locations of the ballast tanks used for attitude adjustment in the cage, such as the central pressure bar (for lifting and lowering), the hub-shaped ring (for rotation or tilting), etc., and clarify the node and element relationships of these components in the finite element model.

[0071] S602 defines an equivalent mass variable parameter. The unit mass of the cross-section of the structural component corresponding to the ballast tank is set as an adjustable variable parameter. This parameter is used to equivalently simulate the impact of mass changes during the filling and emptying of the ballast tank on the overall center of gravity and attitude of the cage.

[0072] S603 sets equivalent mass adjustment values ​​for different attitudes. For the rising / falling attitude: calculate the equivalent mass increase of the central pressure bar section based on the designed water filling ratio of the central pressure bar ballast tank. For the rotating attitude: set differentiated equivalent mass increase values ​​on the components corresponding to the asymmetrical ballast area of ​​the hub-shaped ring, causing the overall center of gravity of the cage to shift and generate a rotational torque. For the tilting attitude: divide the central pressure bar into several segments, applying different section properties to each segment. Add equivalent mass at both ends of the central pressure bar, simulating the tilting angle through center of gravity offset.

[0073] S604 establishes a dynamic mapping relationship between attitude and mass. It constructs the correlation logic between attitude changes and equivalent mass parameters, enabling dynamic numerical simulation of attitude changes such as cage lifting depth, rotation angle, and tilt degree by directly modifying preset equivalent mass variable values, without requiring model or mesh reconstruction.

[0074] S700, establish the nonlinear finite element dynamic equilibrium equations, construct a set of equations including the structural mass matrix, hydrodynamic mass matrix, damping matrix, stiffness matrix and external force vector, and use the Newmark-β method for time integration solution;

[0075] S800, setting hydrodynamic calculation parameters, specifically including:

[0076] S801, Static Calculation

[0077] Select volume forces such as gravity, buoyancy, and ocean current forces in the load application order. Set the maximum number of iterations and the number of steps to complete the static calculation. Calculate the initial position of the net cage under the action of gravity, buoyancy, and ocean current forces.

[0078] S802, Dynamic Calculation

[0079] In the dynamic calculation parameters, set the number of periods / calculation duration and time step. Select the number of periods if using a regular wave, and the calculation duration if using a random wave. Select the calculation results you want to save, such as displacement, force, etc. Calculate the hydrodynamic response of the net cage under time-varying wave-current coupling.

[0080] The S900 outputs hydrodynamic response results, acquiring dynamic response parameters such as nodal displacement, component internal force, anchor chain tension, and netting tension of the cage under different postures and environmental loads.

[0081] According to one or more embodiments, a rapid numerical calculation method for the hydrodynamic response of an attitude-adjustable aquaculture cage structure is achieved through the following steps:

[0082] (1) Define the range of the wave field

[0083] The wave field range mainly includes the wave field coverage area, the seabed extent, and the water depth. The wave field coverage area relates to the wave initiation and termination positions in the dynamic analysis of the cage; the seabed extent affects the setting of mooring anchor points; and the water depth affects the anchor chain length, etc. When defining the wave field coverage area and the seabed extent, they should completely cover the cage system. The water depth is set according to requirements.

[0084] (2) Establish the finite element model of the cage frame

[0085] The main frame of the gabion is composed of circular tubes of varying lengths. The gabion frame is modeled using the finite element method. Each circular tube component consists of two nodes and one element. Nodes define coordinate positions and degrees of freedom, while the element, combined with cross-sectional properties (unit mass, stiffness, area, hydrodynamic parameters, etc.), constructs a physical model to simulate the structure's inertial response and external loads. Unit mass, cross-sectional area, and stiffness are calculated and set based on actual materials. Cross-sectional types include rod elements and beam elements; beam elements are generally used when simulating the gabion frame. The load expression is the Morison equation, which incorporates wave forces... Decomposed into inertial forces and drag force Calculate in two parts separately:

[0086]

[0087]

[0088]

[0089] In the formula: The density of seawater; The projected area of ​​a bar per unit area; It is the drag coefficient; It is an additional quality coefficient; It is the inertial force coefficient. ; The volume per unit length of the component; and These are the velocity and acceleration components of the water particles, respectively. and These are the velocity and acceleration components of the component, respectively.

[0090] (3) Establish the finite element model of the mesh garment

[0091] A complete mesh is modeled using a set of line segments, each representing a mesh panel. These segments are defined by rod sections with equivalent properties. The definitions include unit mass and volume, axial stiffness, mesh density, and velocity attenuation coefficient. Hydrodynamic loads on the mesh are calculated using a screen model, decomposing the hydrodynamic forces into drag forces. and lift .

[0092]

[0093]

[0094] In the formula: and Let A and B be the drag and lift coefficients of the mesh, respectively, which can be expressed as:

[0095]

[0096]

[0097] In the formula: For density; Mesh fabric angle of attack. Regarding density. Calculate using the following formula, where The diameter of the network cable. The side length of the mesh.

[0098]

[0099] (4) Establish a mooring system

[0100] Establishing a mooring anchor chain first requires defining the anchor point. The vertical coordinate of the anchor point must be consistent with the water depth, and its degrees of freedom must be restricted. Additionally, the unit mass, cross-sectional area, and axial stiffness of the anchor chain also need to be defined. The cage mooring anchor chain is treated as a rod element, which can be used to calculate axial tensile deformation, but does not undergo bending or torsional deformation. Its axial tension is:

[0101]

[0102] In the formula: This refers to the axial force of the unit. This is the length after stretching and deformation; This is the initial length; It is the elastic modulus; It represents the cross-sectional area.

[0103] (5) Define environment variables, specifically including:

[0104] 1) Ocean current simulation

[0105] During their service life, aquaculture cages primarily withstand ocean currents and wave loads. Ocean currents are generally simulated as uniform flows by defining the flow direction and velocity. The ocean current load can be calculated using the following formula:

[0106]

[0107] in: These are the design values ​​for the loads of the floating frame system; It is the projected area of ​​a component per unit length in the direction perpendicular to the ocean current; It is the design flow rate.

[0108] 2) Wave Simulation

[0109] In wave simulation, linear wave theory or stochastic wave theory is typically used to simplify the waves. The surface conditions in linear wave theory are linearized. Through linearization, a sinusoidal wave applicable to a wide range of applications can be obtained. Based on linear wave theory, combined with boundary conditions and the assumptions of fluid incompressibility and irrotation, the velocity potential function can be derived:

[0110]

[0111] In the formula: x is the horizontal coordinate of the calculation point. It is the amplitude; It is the frequency; Here, k is time, d is the number of waves, z is the water depth, and z is the vertical coordinate of the calculation point.

[0112] Random wave theory assumes that wave motion is the superposition of an infinite number of cosine waves with different amplitudes, frequencies, initial phases, and directions. Wave height. It can be represented as:

[0113]

[0114] In the formula, , , , , These are the amplitude, wave number, angular frequency, propagation direction angle, and initial phase of the q-th component wave, respectively.

[0115] Random waves are typically described using non-random frequency and direction spectra to depict the distribution of wave energy relative to wave frequency, propagation direction, etc. The frequency spectrum is usually described using... This indicates that its physical meaning is the energy density of the wave amplitude, as shown in the following formula:

[0116]

[0117] Commonly used spectra include the Pierson-Moskowitz spectrum (PM spectrum) and the JONSWAP spectrum. The PM spectrum is suitable for fully developed ocean waves. Its specific form is as follows:

[0118]

[0119] In the formula, = It is the angular frequency of the spectral peak period; The period of the spectral peak; For the sake of righteousness, the waves rise high.

[0120] The JONSWAP wave spectrum is a wave spectrum developed from the PM wave spectrum. It takes into account the influence of wave development state and is generally applicable to underdeveloped waves under finite wind distance. Its specific form is as follows:

[0121]

[0122] In the formula, It is the peak elevation factor; the observed value range is 1.5-6; It is the spectral bandwidth coefficient; It is a function of the causeless wind zone.

[0123] (6) Nonlinear finite element dynamic equilibrium equations

[0124] The nonlinear finite element dynamic equilibrium equations for a cage system consisting of netting and mooring chains are as follows:

[0125]

[0126] in, The structural mass matrix; The hydrodynamic mass matrix; Here is the structural damping matrix; The hydrodynamic damping matrix; Here is the structural stiffness matrix; This is the vector of external forces; Let be the displacement vector of the structure. The equation is solved using the Newmark-β numerical integration method.

[0127] (7) Define variables

[0128] By setting the cross-sectional properties of the core components in the cage structure that are directly related to the attitude adjustment function as dynamically adjustable variables, we can achieve accurate simulation of the mass changes caused by the filling and draining of the ballast tank, and provide a flexible and controllable parameter basis for subsequent multi-attitude hydrodynamic response analysis.

[0129] (8) Setting of hydrodynamic response calculation parameters for the cage, specifically including:

[0130] 1) Static calculation

[0131] Static calculations primarily provide the initial position for dynamic calculations, aiding in their convergence, and do not involve wave forces. The order of load application is selected by choosing volume forces such as gravity, buoyancy, and ocean current forces. The maximum number of iterations and the number of iterations are set to complete the static calculation. The initial position of the net cage under the influence of gravity, buoyancy, and ocean current forces is then calculated.

[0132] 2) Dynamic calculation

[0133] In the dynamic calculation parameters, set the number of periods / calculation duration and time step. Select the number of periods if using a regular wave, and the calculation duration if using a random wave. Select the calculation results you want to save, such as displacement, force, etc. Calculate the hydrodynamic response of the net cage under time-varying wave-current coupling.

[0134] To make the technical solutions of this disclosure clearer, based on one or more embodiments, and in conjunction with an adjustable-position aquaculture cage, the technical solutions of this disclosure are clearly and completely described. Adjustable-position aquaculture cage ( Figure 1The cage consists of a hub-shaped ring, a circular tube, and a central pressure bar. Netting is tied and covers the surface of the cage. Ballast chambers are located inside both the central pressure bar and the hub-shaped ring. The cage can rotate circumferentially, adjust its depth, and tilt by filling and emptying the ballast chambers. Filling the ballast chambers inside the central pressure bar with water raises and lowers the cage; filling the ballast chambers inside the hub-shaped ring generates an asymmetrical torque that drives the cage to rotate for net drying. Figure 1 Combination Figure 3 As can be seen, the attitude-adjustable aquaculture cage used for hydrodynamic numerical calculations comprises a net, a hub-shaped ring, circular tubes, and a central pressure rod. The net is bound to and covers the surface of the cage, which is cylindrical in shape. The upper and lower circumferential surfaces of the cylindrical cage are formed by the hub-shaped ring, and the cylindrical cage walls are composed of multiple longitudinally arranged parallel circular tubes. A central pressure rod is located at the axis of the cylindrical cage. Ballast chambers are located inside both the central pressure rod and the hub-shaped ring. The circumferential rotation, water depth, and tilt angle of the cage are adjusted by controlling the filling and emptying of these ballast chambers. When water is added to or removed from the ballast chambers inside the central pressure rod, the submersion depth of the cage is controlled; when water is added to or removed from the ballast chambers of the hub-shaped ring, an asymmetrical torque is generated in the cage, driving it to rotate and dry the nets.

[0135] This disclosure discloses a method for calculating the hydrodynamic values ​​of an adjustable-attitude cage, the calculation method being as follows: Figure 2 It includes the following steps:

[0136] S10, Define the range of the wave field.

[0137] The wave field and seabed area are both set to 200×200 m, and the water depth is set to 30 m.

[0138] S20. Establish the finite element model of the cage frame, specifically including:

[0139] (1) Establish nodes

[0140] The cage frame structure consists of a central pressure member, a hub-shaped ring, and circular tubes. The central pressure member is 17.13 m long, with the origin located at its center. The length direction of the pressure member is defined as the y-axis, meaning the coordinates of its left and right endpoints are (0, -8.565, 0). The pressure ring is a circular structure, divided into 16 equal segments in the model. Its xoz plane node coordinates can be calculated using the following formula:

[0141]

[0142] Where R is the radius of the cage; and n ranges from 1 to 16.

[0143] (2) Establish section properties

[0144] The structural parameters of the fish cage are shown in Table 1. Based on these parameters, cross-sectional properties such as unit mass, cross-sectional area, and stiffness were calculated and set, using beam elements for simulation. The hydrodynamic loads were calculated according to the Morison equations.

[0145] (18)

[0146] (19)

[0147] (20)

[0148] In the formula: The density of seawater; The projected area of ​​a bar per unit area; It is the drag coefficient; It is an additional quality coefficient; It is the inertial force coefficient. ; The volume per unit length of the component; and These are the velocity and acceleration components of the water particles, respectively. and These are the velocity and acceleration components of the component, respectively.

[0149] (3) Establish the cage frame

[0150] Assign the defined cross-section to the line segment type and set the length. Select the line segment type, select two nodes, and repeat to generate the mesh cage frame.

[0151] Table 1 Main structural parameters of the wire mesh cage

[0152]

[0153] S30. Establish the finite element model of the mesh garment, specifically including:

[0154] (1) Establish nodes

[0155] The mesh covering of the adjustable cage is applied to the surface of the cage frame, and its node coordinates are consistent with the coordinates of the cage frame.

[0156] (2) Define section properties

[0157] The mesh garment has a unit mass of 1.5 kg / m and an equivalent unit area of ​​0.0005 m². 2 With a density of 0.2, rod elements were used for simulation, and the hydrodynamic loads were calculated using a screen model, decomposing the hydrodynamic forces on the netting into resistance. and lift .

[0158]

[0159]

[0160] In the formula: and Let A and B be the drag and lift coefficients of the mesh, respectively, which can be expressed as:

[0161]

[0162]

[0163] In the formula: For density; Mesh fabric angle of attack. Regarding density. Calculate using the following formula, where The diameter of the network cable. The side length of the mesh.

[0164]

[0165] (3) Establish a netting

[0166] Assign the defined cross-section to the line segment type and set its length. Select the line segment type, select two nodes, and repeat to generate the mesh.

[0167] S40. Establish a mooring system.

[0168] The mooring system is constructed using the same method as the cage frame. Four-point mooring is employed, with the coordinates of the four anchor points being (67, 74, -30), (-67, 74, -30), (67, -74, -30), and (-67, -74, -30). The mooring chain has a unit mass of 15 kg / m and an equivalent cross-sectional area of ​​0.007854 m². 2 The rod element model was used for simulation, and the Morison equation was used to calculate its hydrodynamic loads. The mooring length was 100m. The established numerical model of the cage is as follows: Figure 3 .

[0169] S50. Define environment variables.

[0170] The flow velocity was set to 0.1 m / s. Regular waves were used, and detailed operating conditions are shown in Table 2.

[0171] Table 2 Environmental Conditions Table

[0172]

[0173] S60, Implementation of cage attitude adjustment.

[0174] The adjustable net cage has four postures: semi-submerged, fully submerged, rotating, and tilted. The net cage is in a semi-submerged, rotating, or tilted posture when needed for drying nets and attracting fish, and fully submerged during normal aquaculture or when needing to avoid wind and waves. These four posture adjustments are achieved by adjusting the water volume in the ballast tank. In numerical simulations, this can be achieved by adding equivalent mass to the corresponding cross-section at the location of the ballast tank. For example, this embodiment compares semi-submerged and fully submerged postures. The unit mass of the cross-section of the central pressure bar of the adjustable net cage is set as a variable. A fully submerged net cage requires water to be injected into 40% of the working area inside the central pressure bar, with an equivalent mass of 5400 kg, meaning an increase of 320 kg in the unit mass of the cross-section. Simply adding 320 kg to this variable is sufficient to simulate the fully submerged posture. Similarly, to simulate the hydrodynamic response of the net cage rotating under wave and current, the unit mass of the cross-section can be increased in the corresponding parts of the two rings, causing the center of gravity of the net cage to shift and generating an asymmetrical moment to simulate rotation. The tilted posture of the net cage can also be simulated using this method. Schematic diagrams of the semi-submerged and fully submerged postures of the net cage are shown in [the diagram]. Figure 1 It has already been given in the text.

[0175] S70, Hydrodynamic Response Calculation Parameter Settings for Adjustable Net Cage

[0176] (1) Static calculation

[0177] We selected volume force and ocean current force as the applied loads, and set the maximum number of iterations and the number of steps to 20.

[0178] (2) Dynamic calculation

[0179] The wave was simulated using a regular wave pattern, with a period of 10 and a time step of 1500.

[0180] S80. Calculation results, specifically including:

[0181] (1) Motion response of the cage in semi-submerged and fully submerged states

[0182] Figure 4 This figure compares the peak vertical displacement of the net cage in semi-submerged and fully submerged states. It shows that at all three wave heights, the peak vertical displacement of the net cage in the fully submerged state is significantly smaller than that in the semi-submerged state. This indicates that submersion effectively reduces the vertical displacement of the net cage. By adding different equivalent masses, the influence of different submersion depths on the motion response of the net cage can be simulated. Figure 5This comparison shows the peak values ​​of horizontal displacement of the gabion cage in semi-submerged and fully submerged states. In this embodiment, the horizontal direction refers to the wave propagation direction. It can be seen that at wave heights of 0.5m and 0.7m, the peak values ​​of horizontal displacement of the gabion cage in the fully submerged state are much smaller than those in the semi-submerged state. However, at a wave height of 1m, the difference is not significant. This is because while submersion of the gabion cage has a wave-avoidance effect, it also loosens the anchor chain to some extent, weakening the anchor chain's displacement-limiting effect and leading to an increase in displacement. This method can comprehensively simulate and analyze the complex relationship between mooring anchor chain length, anchor chain tension, submersion depth, and the hydrodynamic response of the gabion cage under wave-current coupling. Figure 6 This is a comparison chart of the maximum anchor chain tension on the wave-facing and wave-back sides of the mooring line under two different attitudes. The chart shows that the anchor chain tension on the wave-facing side is greater than that on the wave-back side, and diving significantly reduces the maximum anchor chain tension.

[0183] (2) Stress on the central pressure bar and netting in semi-submerged and fully submerged states

[0184] Figure 7 These are the axial forces acting on the central compression member of the cage under two different orientations. In the fully submerged state, the force on the central compression member is significantly reduced, which helps improve the safety of the cage in wind and waves. Furthermore, the calculation results also include shear force and torque in different directions for each member. Figure 8 This refers to the tension on the mesh of the attitude-adjustable cage section. This method uses a screen model to calculate the hydrodynamic load on the mesh. A complete mesh section is simulated by a set of line segments, each representing a mesh panel. It can be used to quickly solve for mesh tension and to consider the hydrodynamic response of the cage. The figure shows that the axial force on part of the mesh is also significantly reduced in the fully submersible state. Figure 9 This is a contour map of the effective tension of the cage under both semi-submersible and fully submersible postures. This method achieves integrated coupled analysis of the cage's motion and structural forces.

[0185] The beneficial technical effects of this disclosure include:

[0186] (1) Efficient and simplified modeling process: This method does not require the construction of three-dimensional solids or complex meshes. Finite element modeling can be completed by defining structural nodes and cross-sectional parameters, which significantly reduces the threshold of engineering calculation and modeling time.

[0187] (2) Flexible and efficient attitude simulation: By adjusting the equivalent mass of the local structure, physical modeling of attitude changes caused by ballast water can be achieved without rebuilding the mesh or re-dividing the computational domain. It supports dynamic simulation under various attitudes such as semi-submersible, fully submersible, tilted, and rotating.

[0188] (3) Innovative and efficient mesh modeling: The screen model is used to convert the entire mesh into surface units, which significantly reduces the model's degree of freedom and computational load, avoiding the computational resource occupation problem caused by the complex mesh structure in traditional methods.

[0189] (4) Achieve coupled analysis of motion and force: This method unifies the motion response and structural force of the cage in a set of dynamic equations, realizing the one-time output of parameters such as dynamic displacement, tension, and axial force under load, breaking through the limitation of traditional methods that require secondary analysis.

[0190] (5) The calculation results are accurate and reliable: The analysis results in the examples show that the response results of the cage under different postures and wave current conditions, such as motion displacement, anchor chain tension, and netting tension, have good stability and engineering credibility.

[0191] (6) Wide range of applications: This method is applicable to various structural types, material forms and layout schemes of adjustable cage systems. It has good versatility and scalability, which helps to form a standardized hydrodynamic analysis technology system and support the design and operation and maintenance management of intelligent aquaculture equipment in deep sea.

[0192] It should be understood that in the embodiments of this disclosure, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0193] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this disclosure, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this disclosure. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0194] The above description is merely a specific embodiment of this disclosure, but the scope of protection of this disclosure is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this disclosure, and these modifications or substitutions should all be covered within the scope of protection of this disclosure. Therefore, the scope of protection of this disclosure should be determined by the scope of the claims.

Claims

1. A numerical calculation method for the hydrodynamics of an adjustable aquaculture cage, characterized in that, Includes the following steps: A1 defines the wave field where the cage is located, and sets the wave coverage area, seabed area and water depth of the cage. A2, Establish the finite element model of the wire mesh frame; A3, Establish the finite element model of the mesh garment; A4, Establish a model of the cage mooring system; A5, define environmental variables to simulate ocean currents and waves; A6, Load cage attitude parameters; A7, solve for the motion response and structural forces of the cage; A8, comparative analysis of the motion response and structural stress differences of the cage at various postures.

2. The method according to claim 1, characterized in that, In step A2, a finite element model of the cage frame is established, including defining the circular tube nodes and cross-sectional properties; beam element simulation is performed, and wave forces are calculated using the Morison equation.

3. The method according to claim 1, characterized in that, In step A3, a finite element model of the mesh is established, including simulating the mesh using equivalent surface elements, defining density parameters, and using a screen model to calculate drag and lift.

4. The method according to claim 1, characterized in that, In step A4, a model of the cage mooring system is established, including defining anchor point coordinates and constraints, simulating the anchor chain using rod elements, and setting the axial stiffness.

5. The method according to claim 1, characterized in that, In step A5, environmental variables are defined, and ocean currents and waves are simulated, including ocean current simulations that calculate flow direction, velocity, and load, as well as wave simulations based on the principle of selecting linear / random waves.

6. The method according to claim 1, characterized in that, In step A6, the attitude parameters of the cage are loaded, including adjusting the equivalent mass of the ballast tank to achieve attitude setting.

7. The method according to claim 2, characterized in that, Step A2 further includes defining the wire mesh frame line segment type, applying the defined cross-sectional attributes to the line segment type, setting the line segment length and the number of units to be divided; then creating the wire mesh frame line segment, selecting the defined line segment type and the nodes at the beginning and end of the line segment.

8. The method according to claim 3, characterized in that, In step A3, a mesh node is created, cross-sectional properties are defined, and mesh segment types are defined. The defined cross-sectional properties are applied to the segment types. The length of the mesh segment and the number of units to be divided are set, mesh segments are created, the defined mesh segment types and the nodes at the beginning and end of the segments are selected, and a complete mesh is generated.

9. The method according to claim 4, characterized in that, In step A4, the anchor chain cross-sectional properties are defined, including setting the unit mass, cross-sectional area, axial stiffness, element type, and hydrodynamic load. The anchor chain segment type is defined, and the defined anchor chain cross-sectional properties are applied to the segment type. The length of the mooring anchor chain and the number of elements to be divided are set. An anchor chain segment is created by selecting the defined anchor chain segment type and the nodes at the beginning and end of the segment, and the mooring anchor chain is generated.

10. The method according to claim 6, characterized in that, In step A6, the cage attitude adjustment settings include: A601, determine the structural components corresponding to the ballast tanks, determine the structural components corresponding to the location of the ballast tanks used for attitude adjustment in the cage, and clarify the node and element relationships of the structural components in the finite element model; A602 defines an equivalent mass variable parameter, which sets the unit mass of the cross-section of the structural component corresponding to the ballast tank as an adjustable variable parameter. This parameter is used to equivalently simulate the influence of mass change on the overall center of gravity and attitude of the cage during the filling and emptying of the ballast tank. A603, set the equivalent mass adjustment value for different postures; A604 establishes a dynamic mapping relationship between attitude and mass, constructs the correlation logic between attitude changes and equivalent mass parameters, and realizes dynamic numerical simulation of the attitude changes of cage lifting depth, rotation angle, and tilt degree by directly modifying the preset equivalent mass variable values.

Images