Motion response solving method for mooring aquaculture net cage under influence of irregular waves

Abstract

The invention discloses a method for solving motion response of a mooring aquaculture net cage under the influence of irregular waves, which relates to the technical field of ocean engineering structure motion attitude calculation, and comprises the following steps: acquiring a wave height time sequence of the irregular waves according to observed Jonswap spectrum parameters, and obtaining wavelets through fast Fourier transform; simulating a reproducible irregular wave load in the numerical simulation pool in a wavelet superposition mode; and based on the VOF model and the overlapped grids, adopting a local encryption grid in a key area of a flow field, and adopting a fluid-solid coupling method to simulate the attitude response of the floating aquaculture net cage structure model of the catenary anchor system under the action of irregular wave load in a numerical pool. Therefore, by adopting the method for solving the motion response of the mooring aquaculture net cage under the influence of the irregular waves, the uncertainty influence caused by generating random irregular waves does not need to be eliminated through long-time calculation, the calculation time is shortened, and identical irregular wave loads can be applied to different targets.

Description

Technical Field

[0001] The present invention relates to the technical field of motion posture calculation of marine engineering structures, and in particular to a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves. Background Art

[0002] Irregular waves are a common wave pattern in the real ocean. They are composed of the random superposition of a large number of simple harmonic waves of varying frequencies, wave heights, and periods, and are characterized by remarkable non-repetitiveness and unpredictability. Irregular wave loads can significantly alter the motion of anchored floating aquaculture cages. Therefore, the study of irregular waves is crucial for coastal engineering, energy development, and disaster prediction.

[0003] Domestic scholars have conducted extensive research on this issue. For irregular waves, push-plate wave generation and numerical simulation are commonly used. Due to the random nature of irregular waves, random phases are often used in numerical simulation software. However, this method introduces errors in controlled experiments, requiring lengthy simulation calculations to eliminate, significantly increasing computing power. Based on this, the present invention proposes a boundary wave generation method for simulating and generating reproducible irregular waves, providing a reference for controlled experiments evaluating the motion posture of catenary-anchored aquaculture cages under the influence of irregular waves. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for solving the motion response of anchored aquaculture cages under the influence of irregular waves, which can address the problem that random irregular waves generated by traditional methods cannot be reproduced, reduce the error in control experiments, and achieve stable emergence of numerical simulation of irregular waves with known wave height time series.

[0005] To achieve the above object, the present invention provides a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves, comprising: Step 1: According to the parameters of the JONSWAP spectrum corresponding to the sea conditions reached by the random irregular waves, the wave spectrum density of the irregular waves is calculated using the JONSWAP spectrum formula; The dispersion equation is solved by Newton's iteration method to obtain the wave number of the irregular wave; According to the wave spectrum density and wave number of irregular waves, the wave height of each component is calculated, and random phases are generated to iteratively obtain the wave height time series; Based on fast Fourier transform, the wave height time series is decomposed into wavelets and the parameter information of the wavelets is determined; Step 2: Establish a fluid domain geometry model for the area where the anchor structure is located as the background pool area. Within the background pool area, determine the wave surface fluid domain grid area based on the maximum wave height of the irregular wave. Subtract the target anchor structure through a subtraction operation to generate an overlapping fluid domain. The interface between the overlapping fluid domain and the background pool is used as the overlapping grid interface to establish flow field information exchange between the two areas. The fluid domain geometry model defines the continuous shape and physical properties of the computational domain. The length direction is the propagation direction of the irregular wave, and the height direction is the depth direction of the background pool. Step 3: Based on the VOF model, construct the mesh model of the background water pool and the catenary anchor structure, perform local densification on the mesh area of ​​the wave surface fluid domain in the background water pool, and set the mesh volume growth rate to slow; Define the irregular wave input boundary, wave breaking boundary, front and rear boundaries of the pool, and the pool bottom boundary as velocity inlet boundary conditions, and define the upper boundary of the pool as the pressure outlet boundary condition; define the irregular wave numerical pool depth, and define the interface between seawater and air as the irregular wave generation reference surface; Step 4: Construct the local coordinate system of the background pool and the catenary anchor structure coordinate system; Define the surface boundary of the target catenary anchor structure as the wall boundary condition; Define the fluid-structure interaction module to simulate the dynamic, bidirectional interaction between the fluid and the solid target; set the inertia moment of the target catenary model with respect to the catenary anchor structure coordinate system, as well as the mass of the target catenary anchor structure; According to the local coordinate system of the background pool and the coordinate system of the catenary anchoring structure, the relative positions of the two ends of the catenary are determined, and the relaxation length and stiffness of the catenary are set; Step 5: Add the wavelet parameters decomposed in step 1 to the wavelet of the superimposed wave by using the superimposed wave boundary wave generation method to generate a reproducible irregular wave load. Define the wave damping wave length as twice the wavelength of the wavelet with the largest wavelength, and set the velocity of the wave damping boundary, the front and rear boundaries of the pool, and the bottom boundary of the pool to 0 m / s. Step 6: Initialize the grid model and calculate the flow field of the grid model of the target catenary anchor structure based on the initial state of sway, heave and pitch of the target catenary anchor structure until the calculation end time is reached. , terminate the calculation.

[0006] Furthermore, the parameters of the JONSWAP spectrum include significant wave height, spectrum peak period, spectrum peak enhancement factor and average wave period.

[0007] Furthermore, the spectral density of the ocean wave spectrum is expressed as follows: ; Where, is the wave spectrum energy density, For the sake of righteousness, is the peak period, is the frequency, is the peak enhancement factor, is the peak width parameter, is the scale parameter.

[0008] Furthermore, the dispersion equation is expressed as: ; Where, is the wave circular frequency, is the acceleration due to gravity, is the wave number, For water depth.

[0009] Furthermore, the fluid domain geometric model in the area where the anchoring structure is located is set as a cuboid.

[0010] Furthermore, in the local coordinate system of the background pool, the origin of the coordinate system is the interface between air and seawater. Direction is the direction of wave propagation, The direction is the depth of the pool. Direction is the width direction of the pool; in the catenary anchor structure coordinate system, the origin of the coordinate system is the center of mass of the anchor structure. The direction is the length direction, The direction is the height direction, The direction is the width direction.

[0011] Furthermore, the steps of initializing the grid model include: Step 1: Determine the wavefront position based on the position vector field function, use the irregular wave theoretical model as the linear superposition wave model, and set the initial time t=0 of the waveform equation based on the wavelet parameter information determined in step 1. The waveform of the generated superposition wave is consistent with the wave height time series. Step 2: Initialize the mesh models of the water pool and the target catenary anchor structure based on the VOF method, activate the mesh model interface between the background water pool and the target catenary anchor structure, and then exchange data through interpolation; Step 3: Initialize the catenary so that the force at time t=0 is 0; Step 4: Based on the overlapping grid theory, the fluid domain grid model is processed by digging holes to identify and eliminate invalid solid areas; the information interpolation between the background water pool and the target catenary anchor structure grid model is defined to be second-order accuracy.

[0012] Therefore, the present invention adopts the above-mentioned method for solving the motion response of anchored aquaculture cages under the influence of irregular waves, which has the following technical effects: (1) The method of the present invention uses local grid encryption and wavelet superposition to recreate the wave boundary, generating irregular waves with a known wave height time series. The control of the generated wave front is more accurate than that of traditional wave generation methods. It has the characteristics of fast and simple wave front generation, easy data collection, and high success rate. (2) The present invention adopts overlapping grids, which can more accurately describe the details of the catenary anchored aquaculture cage model, reduce the risk of divergence of calculation results caused by traditional deformable grid methods, and realize the posture motion assessment of irregular waves on deep-sea structures; (3) The present invention obtains the sub-waves constituting the wave height time series by performing fast Fourier transform on the wave height time series, thereby realizing the stable emergence of the numerical simulation of irregular waves of known wave height time series, effectively solving the problem that random irregular waves generated by traditional methods are not reproducible, and reducing the error of control tests.

[0013] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a fluid domain geometric model in an embodiment of a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves; Figure 2 It is a schematic diagram of the structure of overlapping grids in an embodiment of a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves; Figure 3 This is a schematic diagram of overlapping boundary grids divided based on overlapping grid theory in an embodiment of a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves; Figure 4 In an embodiment of a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves, the irregular waves are generated and compared with a known wave height time series; Figure 5 The present invention is an embodiment of a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves, and is a velocity vector diagram of a flow field in which the anchored structure is subjected to an irregular wave load; Figure 6 The diagram is a planar three-degree-of-freedom variation diagram of a catenary anchored structure in an embodiment of a method for solving the motion response of an anchored aquaculture cage under the influence of irregular waves, wherein (a) is longitudinal sway, (b) is pitch, and (c) is heave. DETAILED DESCRIPTION

[0015] The present invention can be explained in more detail by the following examples. The purpose of disclosing the present invention is to protect all changes and improvements within the scope of the present invention. The present invention is not limited to the following examples.

[0016] like Figure 1 As shown, the present invention provides a method for solving the motion response of anchored aquaculture cages under the influence of irregular waves, including generating the required wave height time series based on the VOF model and the overlapping grid method through the known JONSWAP spectrum parameters and performing fast Fourier transform (FFT) to obtain wavelets; in the numerical simulation pool, reproducible irregular wave loads are simulated by superposition of wavelets, specifically by The turbulence model generates irregular waves in a tank containing gas and liquid, with the same wave height time series as the calculated one. The model then examines the effect of these irregular waves on the attitude of the catenary-anchored structure. Furthermore, by using locally refined meshes in key areas of the flow field, a fluid-structure interaction method is employed to simulate the attitude response of a floating aquaculture cage structure anchored by a catenary in a numerical tank under the action of irregular wave loads. Compared to traditional wave generation methods involving push-plate and pull-plate treads, this method improves the accuracy of the generated wave surface and achieves stable reproducibility of irregular waves. This method eliminates the need for lengthy computations to eliminate the uncertainties associated with random irregular wave generation, shortens computation time, and allows the application of identical irregular wave loads to different targets. This approach provides a reference for attitude control of floating aquaculture cage structures anchored by catenary.

[0017] In this embodiment, the JONSWAP spectrum parameters used are the significant wave height =3.5m, spectral peak period =10.5s, peak enhancement factor =2.2, average wave period =8.7s; the catenary anchoring structure used is a rectangular parallelepiped, weighing 2050kg, with a length of 2m, a width of 1m, and a height of 2m. The initial center of gravity is at the same height as the water surface, and the diagonal components of its moment of inertia are [854.17, 1366.67, 854.17] kg-m 2 , the catenary is defined as a preloaded state, the preload force is 100N, the stiffness is 5000.0N / m, and the mass per unit length is 0.005kg / m.

[0018] Based on the above parameters, the present invention provides a method for solving the motion response of anchored aquaculture cages under the influence of irregular waves. The specific implementation process includes: Step 1: First, according to the parameters of the JONSWAP spectrum corresponding to the sea conditions reached by the random irregular waves, the wave spectrum density of the irregular waves is calculated using the JONSWAP spectrum formula.

[0019] The expression of the wave spectrum density is as follows: ; Where, is the wave spectrum energy density, For the sake of righteousness, is the peak period, is the frequency, is the peak enhancement factor, is the peak width parameter, is the scale parameter. hour, =0.07; when hour, =0.09. , here =0.27.

[0020] Secondly, the dispersion equation is solved by Newton iteration method , get the wave number of the irregular wave .in, is the wave circle frequency; is the acceleration due to gravity, which is 9.81 m / s 2 ; is the water depth, which is 30m.

[0021] Next, according to the spectral density of the irregular wave spectrum and wave number , generating random phases , calculate the wave height of each component , is the frequency interval, which is set to 0.0011. The wave height time series is obtained by iteration .in, The initial spatial position of the wave generation, usually x=0; , is the number of frequency components. In this embodiment =450; the time series is , is the sampling frequency. In this embodiment, =200Hz.

[0022] Then, the wave height time series is decomposed into sub-waves based on fast Fourier transform.

[0023] Step 2. Establish a geometric model of the fluid domain in the area where the anchor structure is located, and define the continuous shape and physical properties of the calculation domain. The geometric model of the fluid domain is a cuboid with lengths of 200m, 2m, and 60m in length, width, and depth, respectively. The length direction is the propagation direction of the irregular wave, and the height direction is the water depth direction of the test pool, with a water depth of 30m. Set the area within 5m above and below the gas-liquid interface as the wave surface fluid domain grid area. This is determined based on the maximum wave height of the irregular wave, and is used for mesh encryption in this area when subsequently generating the grid model, reducing the risk of calculation divergence and errors, and increasing the accuracy of generating irregular waves; based on the irregular waves and anchor structures, divide the overlapping grid boundaries, and generate overlapping fluid domains through subtraction operations, that is, the follow-up fluid domain around the target structure. This area can move after being affected by the irregular wave load, such as Figure 2 shown.

[0024] The fluid domain geometry model is assigned to the background pool as a stationary wave-generating area. The overlapping fluid domain is assigned to the anchor structure area, and the overlapping mesh interface between the anchor structure area and the background pool is generated to establish flow field information exchange between the two areas.

[0025] Step 3: Generate a mesh model of the background water pool and the fluid domain surrounding the target structure (i.e., the overlapping fluid domain). Define the interface between the overlapping fluid domain and the background water pool as the overlapping mesh boundary condition. Based on the cell size of the overlapping region boundary, perform local encryption on the mesh area of ​​the wave surface fluid domain. At the same time, set the mesh volume growth rate to slow, such as Figure 3 shown.

[0026] The irregular wave input boundary, wave breaking boundary, front and rear boundaries of the pool, and the bottom boundary of the pool are defined as velocity inlet boundary conditions, and the upper boundary of the pool is defined as the pressure outlet boundary condition. Compared with the traditional boundary condition setting method of numerical simulation pools, this method can completely eliminate the boundary reflection effect and simplify the control logic of the wave field.

[0027] Define the depth of the irregular wave numerical pool and define the interface between seawater and air as the reference surface for irregular wave generation. Specifically, based on the VOF (Volume of Fluid) model, add the Euler polynomials of liquid (water) and define the concentration as 997.561 kg / m 3 , add the Euler polynomial gas (air) and define the concentration as 1.18415 kg / m 3 , define the inner solitary wave numerical pool, set the position of liquid and gas along the depth direction of the pool, and define the interface between liquid and gas as the reference surface for generating irregular waves.

[0028] Step 4: Construct the local coordinate system of the background pool and the coordinate system of the catenary anchor structure. In the local coordinate system of the background pool, the origin of the coordinate system is located at the interface between air and seawater. In this embodiment, it is set at the midpoint of the interface between the liquid and gas at the wave-making boundary. Direction is the direction of wave propagation, The direction is the depth of the pool. Direction is the width direction of the pool. In the catenary anchor structure coordinate system, the origin of the coordinate system is the center of mass of the anchor structure. The direction is the length direction, The direction is the height direction, The direction is the width direction.

[0029] The surface boundary of the catenary anchor structure is defined as a wall boundary condition, and a surface control is defined to prohibit the generation of prismatic layer meshes at the interface between the overlapping fluid domain and the background pool, so that the prismatic layer meshes are only attached to the surface of the buoy.

[0030] Define the fluid-structure interaction module to simulate the dynamic, bidirectional interaction between the fluid and the solid target. Set the diagonal components of the moment of inertia of the target catenary model with respect to the catenary anchor structure coordinate system to [854.17, 1366.67, 854.17] kg. m 2 , and the target catenary model mass is 2050 kg.

[0031] According to the local coordinate system of the background pool and the coordinate system of the catenary anchoring structure, the relative positions of the two ends of the catenary are determined, and the catenary is set to a pre-tightened state with a pre-tightening force of 100N, a stiffness of 5000.0N / m, and a mass per unit length of 0.005kg / m.

[0032] Step 5: Add the wavelet parameters obtained by step 1 to the wavelet of the superposition wave by using the wavelet boundary generation method to generate a reproducible irregular wave load. Specifically, the parameters of the first fifty wavelets with the largest amplitudes obtained by step 1, including phase, amplitude, and period, are added one by one to the wavelet of the superposition wave, as shown in the following example: Figure 4 shown.

[0033] Activate the damping wave at the wave-breaking boundary to eliminate the non-physical reflection of irregular waves at the boundary of the numerical simulation pool, ensuring the accuracy of the results; define the length of the wave-breaking damping wave as twice the wavelength of the sub-wave with the largest wavelength, which is 121m here, and set the speed of the wave-breaking boundary, the front and rear boundaries of the pool, and the bottom boundary of the pool to 0m / s.

[0034] Step 6. First, initialize the grid model as follows: (1) According to the position vector field function, the liquid area range is set to determine the location of the sea surface (i.e., the wave surface position). The linear superposition wave model is used as the irregular wave theoretical model. The waveform of the irregular wave generated by the superposition wave boundary wave generation method is previewed so that the recorded wave height sequence is consistent with the wave height time series obtained in step 1. This avoids the discovery of inconsistent waveforms after the calculation starts, which wastes computing resources. In this process, the initial time t = 0 of the waveform equation is set, the irregular wave waveform is transmitted forward over time, and the wave height sequence is recorded by the wave height meter.

[0035] (2) Based on the VOF method, the grid models of the background water pool and the target catenary anchor structure are initialized so that the interface between the grid models of the background water pool and the target catenary anchor structure is activated to achieve data exchange.

[0036] (4) Initialize the catenary so that the force on it is 0 at time t = 0.

[0037] (5) Based on the overlapping grid theory, the fluid domain mesh model is processed by digging holes to identify and eliminate invalid solid areas, ensuring that the solver only performs calculations in the fluid area, avoiding redundant calculations in invalid areas and reducing the amount of calculation. In addition, the information between the mesh models defining the background water pool and the target catenary anchor structure is interpolated with second-order accuracy, improving the accuracy of data transmission and reducing numerical dissipation and dispersion errors.

[0038] Then, based on the initial state of the target catenary anchor structure's sway, heave, and pitch, create reports, monitors, and plots of x-direction movement, z-direction movement, and y-axis rotation at different times. Set the calculation time step to 0.025s, and perform flow field calculations on the mesh model of the target catenary anchor model until the calculation end time is reached. , terminate the calculation, and then the velocity vector change diagram of the flow field during the entire simulation process and the plane three-degree-of-freedom motion simulation results of the target under the influence of the generated irregular waves can be obtained, such as Figure 5 and Figure 6 shown.

[0039] Therefore, the present invention adopts the above-mentioned method for solving the motion response of anchored aquaculture cages under the influence of irregular waves, which can stably reproduce irregular waves with known wave height time series, improve the accuracy and controllability of irregular wave generation, and realize numerical simulation of the effect of ocean irregular wave loads on anchored aquaculture cage structures.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for solving the motion response of anchored aquaculture cages under the influence of irregular waves, characterized by: include: Step 1: According to the parameters of the JONSWAP spectrum corresponding to the sea conditions reached by the random irregular waves, the wave spectrum density of the irregular waves is calculated using the JONSWAP spectrum formula; The dispersion equation is solved by Newton's iteration method to obtain the wave number of the irregular wave; According to the wave spectrum density and wave number of irregular waves, the wave height of each component is calculated, and random phases are generated to iteratively obtain the wave height time series; Based on fast Fourier transform, the wave height time series is decomposed into wavelets and the parameter information of the wavelets is determined; Step 2: Establish a fluid domain geometry model for the area where the anchor structure is located as the background pool area. Within the background pool area, determine the wave surface fluid domain grid area based on the maximum wave height of the irregular wave. Subtract the target anchor structure through a subtraction operation to generate an overlapping fluid domain. The interface between the overlapping fluid domain and the background pool is used as the overlapping grid interface to establish flow field information exchange between the two areas. The fluid domain geometry model defines the continuous shape and physical properties of the computational domain. The length direction is the propagation direction of the irregular wave, and the height direction is the depth direction of the background pool. Step 3: Based on the VOF model, construct the mesh model of the background water pool and the catenary anchor structure, perform local densification on the mesh area of ​​the wave surface fluid domain in the background water pool, and set the mesh volume growth rate to slow; Define the irregular wave input boundary, wave breaking boundary, front and rear boundaries of the pool, and the pool bottom boundary as velocity inlet boundary conditions, and define the upper boundary of the pool as the pressure outlet boundary condition; define the irregular wave numerical pool depth, and define the interface between seawater and air as the irregular wave generation reference surface; Step 4: Construct the local coordinate system of the background pool and the catenary anchor structure coordinate system; Define the surface boundary of the target catenary anchor structure as the wall boundary condition; Define the fluid-structure interaction module to simulate the dynamic, bidirectional interaction between the fluid and the solid target; set the inertia moment of the target catenary model with respect to the catenary anchor structure coordinate system, as well as the mass of the target catenary anchor structure; According to the local coordinate system of the background pool and the coordinate system of the catenary anchoring structure, the relative positions of the two ends of the catenary are determined, and the relaxation length and stiffness of the catenary are set; Step 5: Add the wavelet parameters decomposed in step 1 to the wavelet of the superimposed wave by using the superimposed wave boundary wave generation method to generate a reproducible irregular wave load. Define the wave damping wave length as twice the wavelength of the wavelet with the largest wavelength, and set the velocity of the wave damping boundary, the front and rear boundaries of the pool, and the bottom boundary of the pool to 0 m / s. Step 6: Initialize the grid model and calculate the flow field of the grid model of the target catenary anchor structure based on the initial state of sway, heave and pitch of the target catenary anchor structure until the calculation end time is reached. , terminate the calculation.

2. The method for solving the motion response of anchored aquaculture cages under the influence of irregular waves according to claim 1 is characterized in that: The parameters of the JONSWAP spectrum include significant wave height, spectrum peak period, spectrum peak enhancement factor and average wave period.

3. The method for solving the motion response of anchored aquaculture cages under the influence of irregular waves according to claim 1 is characterized in that: The spectral density of ocean waves is expressed as follows: ； Where, is the wave spectrum energy density, For the sake of righteousness, is the peak period, is the frequency, is the peak enhancement factor, is the peak width parameter, is the scale parameter.

4. The method for solving the motion response of anchored aquaculture cages under the influence of irregular waves according to claim 1 is characterized in that: The dispersion equation is expressed as: ； Where, is the wave circular frequency, is the acceleration due to gravity, is the wave number, For water depth.

5. The method for solving the motion response of anchored aquaculture cages under the influence of irregular waves according to claim 1 is characterized in that: The fluid domain geometry model in the area where the anchor structure is located is set as a cuboid.

6. The method for solving the motion response of anchored aquaculture cages under the influence of irregular waves according to claim 1 is characterized in that: In the local coordinate system of the background pool, the origin of the coordinate system is the interface between air and seawater. Direction is the direction of wave propagation, The direction is the depth of the pool. Direction is the width direction of the pool; in the catenary anchor structure coordinate system, the origin of the coordinate system is the center of mass of the anchor structure. The direction is the length direction, The direction is the height direction, The direction is the width direction.

7. The method for solving the motion response of anchored aquaculture cages under the influence of irregular waves according to claim 1 is characterized in that: The steps of mesh model initialization include: Step 1: Determine the wavefront position based on the position vector field function, use the irregular wave theoretical model as the linear superposition wave model, and set the initial time t=0 of the waveform equation based on the wavelet parameter information determined in step 1. The waveform of the generated superposition wave is consistent with the wave height time series. Step 2: Initialize the mesh models of the water pool and the target catenary anchor structure based on the VOF method, activate the mesh model interface between the background water pool and the target catenary anchor structure, and then exchange data through interpolation; Step 3: Initialize the catenary so that the force at time t=0 is 0; Step 4: Based on the overlapping grid theory, the fluid domain grid model is processed by digging holes to identify and eliminate invalid solid areas; the information interpolation between the background water pool and the target catenary anchor structure grid model is defined to be second-order accuracy.

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