A method for solving the motion response of anchored aquaculture cages under the influence of irregular waves

Abstract

The application discloses a kind of solving methods of irregular wave influence under anchoring aquaculture net cage motion response, it is related to marine engineering structure motion posture calculation technical field, including according to observed Jonswap spectrum parameter, obtain irregular wave height time series, and get wavelet by fast fourier transform, in numerical simulation pool by the way of wavelet superposition simulates reproducible irregular wave load;Based on VOF model and overlapping grid, by local encryption grid to the key area of flow field, and using fluid-solid coupling method in numerical tank simulates the posture response of catenary anchoring floating aquaculture net cage structure model under the action of irregular wave load. Therefore, by using the above-mentioned solving method of irregular wave influence under anchoring aquaculture net cage motion response, long-time calculation is not needed to eliminate the uncertainty influence brought by random irregular wave generated, the calculation time is shortened and the same irregular wave load can be applied to different targets.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ocean engineering structure motion posture calculation, in particular to a method for solving the motion response of an anchor-cultured net cage under the influence of irregular waves. BACKGROUND

[0002] Irregular waves are a common wave form in the real ocean environment, and are formed by the random superposition of a large number of simple harmonic waves of different frequencies, wave heights and periods, and have significant non-repeatability and unpredictability. Irregular wave loads can cause significant changes in the motion posture of an anchored sea surface floating aquaculture net cage structure. Therefore, the study of irregular waves is of key significance to coastal engineering, energy development and disaster prediction.

[0003] In view of the above problems, domestic scholars have carried out a large amount of research. For irregular waves, the push plate wave making method and numerical simulation method are usually used. Due to the randomness of irregular waves, a random phase is often selected in numerical simulation software, but this method brings errors in the control experiment and needs a long time of simulation calculation to eliminate, greatly increasing the cost of computing power. Based on this, the present application proposes a boundary wave making method for simulating and generating reproducible irregular waves, which provides a reference for the control experiment of the motion posture evaluation of a catenary anchor-cultured net cage under the influence of irregular waves. SUMMARY

[0004] The purpose of the present application is to provide a method for solving the motion response of an anchor-cultured net cage under the influence of irregular waves, which can solve the problem of non-reproducibility of random irregular waves generated by traditional methods, reduce errors in control experiments, and realize stable emergence of irregular waves with a known wave height time sequence.

[0005] To achieve the above purpose, the present application provides a method for solving the motion response of an anchor-cultured net cage under the influence of irregular waves, comprising:

[0006] Step one, according to the parameters of the JONSWAP spectrum corresponding to the sea state reached by the random irregular wave, the sea wave spectrum spectral density of the irregular wave is calculated by the JONSWAP spectrum formula;

[0007] The dispersion equation is solved by the Newton iteration method to obtain the wave number of the irregular wave;

[0008] According to the sea wave spectrum spectral density and the wave number of the irregular wave, the component wave heights are calculated, and random phases are generated to obtain the wave height time sequence by iteration;

[0009] Based on the fast Fourier transform, the wave height time sequence is decomposed into subwaves, and the parameter information of the subwaves is determined;

[0010] Step two, establish the fluid domain geometry model of the mooring structure area as the background pool area; in the background pool area, determine the wave surface fluid domain grid area according to the maximum wave height of irregular waves, and generate the overlapping fluid domain by subtracting the target mooring structure; the interface between the overlapping fluid domain and the background pool is the overlapping grid interface, and the flow field information exchange of the two areas is established;

[0011] Wherein, the fluid domain geometry model defines the continuous shape and physical properties of the calculation domain, the length direction is the propagation direction of irregular waves, and the height direction is the water depth direction of the background pool;

[0012] Step three, based on the VOF model, construct the grid model of the background pool and the catenary anchoring structure, and perform local encryption processing on the wave surface fluid domain grid area in the background pool, and set the grid volume growth rate to be slow;

[0013] Define the irregular wave incoming boundary, wave absorbing boundary, pool front and back boundary, pool bottom boundary as the velocity inlet boundary condition, define the pool upper boundary as the pressure outlet boundary condition; define the numerical pool depth of irregular waves, and define the air-water interface as the irregular wave generation reference surface;

[0014] Step four, construct the local coordinate system of the background pool and the catenary anchoring structure coordinate system;

[0015] Define the surface boundary of the target catenary anchoring structure as a wall boundary condition;

[0016] Define the fluid-structure coupling module to simulate the dynamic and bidirectional interaction between the fluid and the solid target; set the inertia moment of the target catenary model about the catenary anchoring structure coordinate system, and the mass of the target catenary anchoring structure;

[0017] According to the local coordinate system of the background pool and the catenary anchoring structure coordinate system, determine the relative position of the two ends of the catenary, and set the slack length and stiffness of the catenary;

[0018] Step five, add the sub-wave parameters obtained by decomposing step one to the sub-wave of the superposition wave, generate a reproducible irregular wave load, and define the wave absorbing damping wave length as twice the wavelength of the sub-wave with the maximum wavelength, and set the velocity of the wave absorbing boundary, pool front and back boundary, and pool bottom boundary to 0 m / s;

[0019] Step six, initialize the grid model, and take the initial state of the target catenary anchoring structure in the sway, heave and pitch as the benchmark, and perform flow field calculation on the grid model of the target catenary anchoring structure until the calculation end time is reached , stop calculation.

[0020] Further, the parameters of the JONSWAP spectrum include the significant wave height, the spectral peak period, the spectral peak enhancement factor and the average wave period.

[0021] Further, the sea spectrum spectral density is expressed as follows:

[0022] ;

[0023] In the formula, is the sea spectrum energy density, is the significant wave height, is the spectral peak period, is the frequency, is the spectral peak enhancement factor, is the spectral peak width parameter, is the scale parameter.

[0024] Further, the dispersion equation is expressed as:

[0025] ;

[0026] In the formula, is the wave circular frequency, is the gravity acceleration, is the wave number, is the water depth.

[0027] Further, the fluid domain geometry model of the area where the mooring structure is located is set as a cuboid.

[0028] Further, in the local coordinate system of the background water tank, the coordinate system origin is the air-sea interface, is the wave surface transmission direction, is the water tank depth direction, is the water tank width direction; in the catenary anchoring structure coordinate system, the coordinate system origin is the centroid of the anchoring structure, is the length direction, is the height direction, is the width direction.

[0029] Further, the steps of the grid model initialization include:

[0030] Step 1, determine the wave surface position according to the position vector field function, take the irregular wave theory model as a linear superposition wave model, and set the initial time t=0 of the wave equation according to the sub-wave parameters information of the sub-wave determined in step one, preview the generated superposition wave form and wave height time sequence consistent;

[0031] Step 2, the grid model of the water pool and the target catenary anchoring structure is initialized based on the VOF method, the grid model interface between the background water pool and the target catenary anchoring structure is activated, and then data exchange is carried out through interpolation;

[0032] Step 3, the catenary is initialized and set, so that the force at t=0 is 0;

[0033] Step 4, based on the overlapping grid theory, the grid model of the fluid domain is processed by digging holes, and invalid solid regions are identified and removed; the information interpolation between the grid models of the background water pool and the target catenary anchoring structure is defined as the second order accuracy.

[0034] Therefore, the irregular wave influence under the anchoring aquaculture net cage motion response solving method has the following technical effects:

[0035] (1) The method of the present application uses local grid encryption and wave boundary reconstruction by wave superposition to generate irregular waves with known wave height time series. The control of the generated wave surface is more accurate than the traditional wave making method, has the characteristics of fast, simple wave surface generation, easy data collection and high success rate;

[0036] (2) The present application uses overlapping grids, which can more accurately describe the details of the catenary anchoring aquaculture net cage model, reduce the risk of divergence of the calculation results caused by the traditional deformed grid method, and can realize the attitude motion evaluation of irregular waves on deep sea structures;

[0037] (3) The present application realizes the stable emergence of irregular waves with known wave height time series by performing fast Fourier transform on the wave height time series, effectively solves the problem of non-reproducible random irregular waves generated by traditional methods, and can reduce the error of the comparison test.

[0038] The technical solutions of the present application will be further described in detail below with the help of the drawings and examples. DESCRIPTION OF DRAWINGS

[0039] Figure 1 It is a fluid domain geometry model in an irregular wave influence under anchoring aquaculture net cage motion response solving method embodiment;

[0040] Figure 2 It is a structure diagram of an overlapping grid in an irregular wave influence under anchoring aquaculture net cage motion response solving method embodiment;

[0041] Figure 3 It is an overlapping boundary grid diagram based on the overlapping grid theory in an irregular wave influence under anchoring aquaculture net cage motion response solving method embodiment;

[0042] Figure 4 is a comparison result of irregular waves and a known wave height time sequence in an embodiment of a method for solving the motion response of a mooring net cage under the influence of irregular waves;

[0043] Figure 5 is a velocity vector diagram of a flow field in which the mooring structure is subjected to irregular wave load in an embodiment of a method for solving the motion response of a mooring net cage under the influence of irregular waves;

[0044] Figure 6 is a planar three-degree-of-freedom variation diagram of a catenary mooring structure in an embodiment of a method for solving the motion response of a mooring net cage under the influence of irregular waves, wherein (a) is surge, (b) is pitch, and (c) is heave. DETAILED DESCRIPTION

[0045] The present application can be explained in more detail by the following examples, and the purpose of disclosing the present application is to protect all variations and improvements within the scope of the present application, and the present application is not limited to the following examples.

[0046] As shown in Figure 1 , the present application provides a method for solving the motion response of a mooring net cage under the influence of irregular waves, which includes generating a required wave height time sequence and performing fast Fourier transform (FFT) to obtain a subwave based on a VOF model and an overlapping grid method through known JONSWAP spectrum parameters; in a numerical simulation pool, a reproducible irregular wave load is simulated by subwave superposition, specifically by a turbulence model to generate the same irregular wave as the obtained wave height time sequence in a pool containing gas and liquid, and to detect the attitude influence of the generated irregular wave on a catenary anchoring structure; at the same time, by using local encryption grid in the key area of the flow field, the attitude response of the catenary mooring floating net cage structure model under the action of irregular wave load is simulated in the numerical pool by using the fluid-structure coupling method. Compared with the traditional push plate wave making and the pull plate wave making method, the accuracy of the generated wave surface is improved, and the stable reproduction of irregular waves is realized. The calculation time is shortened and the same irregular wave load can be applied to different targets, which provides a reference for the attitude control of the catenary mooring floating net cage structure.

[0047] In this embodiment, the JONSWAP spectrum parameters used are significant wave height = 3.5 m, spectrum peak period = 10.5 s, spectrum peak enhancement factor = 2.2, and average wave period = 8.7 s; the catenary anchoring structure used is a cuboid with a weight of 2050 kg, a length of 2 m, a width of 1 m, and a height of 2 m, and the initial position of the center of gravity is at the same height as the water surface, and the diagonal components of the inertia moment are [854.17, 1366.67, 854.17] kg-m 2 The catenary is defined in a pre-tightened state, the pre-tightening force is 100 N, the stiffness is 5000.0 N / m, and the mass per unit length is 0.005 kg / m.

[0048] Based on the above parameters, a method for solving the motion response of an irregular wave-affected mooring net cage is provided, and the specific implementation process includes:

[0049] Step one, first, according to the parameters of the sea state corresponding to the JONSWAP spectrum of the random irregular wave, the sea wave spectrum spectral density of the irregular wave is calculated through the JONSWAP spectrum formula.

[0050] Among them, the expression of the sea wave spectrum spectral density is as follows:

[0051] ;

[0052] In the formula, is the sea wave spectrum energy density, is the significant wave height, is the spectral peak period, is the frequency, is the spectral peak enhancement factor, is the spectral peak width parameter, is the scale parameter. When , = 0.07; when , = 0.09. Here = 0.27.

[0053] Secondly, the dispersion equation is solved by Newton iteration method to obtain the wave number of the irregular wave. Among them, is the wave circular frequency; is the gravity acceleration, taking the value of 9.81 m / s 2 ; is the water depth, taking the value of 30 m.

[0054] Then, according to the sea wave spectrum spectral density and the wave number of the irregular wave, the random phase is generated, and the component wave height is calculated, is the frequency interval, taking the value of 0.0011. The wave height time series is obtained by iteration wherein, is the initial spatial position of wave generation, usually x = 0; , is the number of frequency components, in the present embodiment = 450; the time series is , is the sampling frequency, in the present embodiment = 200 Hz.

[0055] Then, based on the fast Fourier transform, the wave height time series is decomposed into subwaves.

[0056] Step two, a fluid domain geometric model of the area where the mooring structure is located is established, which defines the continuous shape and physical properties of the calculation domain. The fluid domain geometric model is a cuboid, and the length, width and depth directions are 200 m, 2 m and 60 m respectively. The length direction is the propagation direction of the irregular wave, and the height direction is the water depth direction of the test tank. The water depth is 30 m. It is set that the upper and lower 5 m of the gas-liquid interface is the wave surface fluid domain grid area, which is determined according to the maximum wave height of the irregular wave. This area is used for subsequent grid encryption processing to reduce the risk of calculation divergence and error and increase the accuracy of generating irregular waves; according to the irregular wave and the mooring structure, the overlapping grid boundary is divided, and the overlapping fluid domain is generated by subtraction operation, that is, the fluid domain around the target structure, which can move after being affected by the irregular wave load, as shown in Figure 2 .

[0057] The fluid domain geometric model is distributed to the background tank as a static wave-making area; the overlapping fluid domain is distributed to the mooring structure area to generate the overlapping grid interface between the mooring structure area and the background tank, and the flow field information exchange between the two areas is established.

[0058] Step three, the grid model of the background tank and the fluid domain around the target structure (i.e. the overlapping fluid domain) is generated, the interface between the overlapping fluid domain and the background tank area is defined as the overlapping grid boundary condition, and the wave surface fluid domain grid area is locally encrypted based on the size of the overlapping area boundary element, and the grid volume growth rate is set to be slow, as shown in Figure 3 .

[0059] The irregular wave incoming boundary, wave absorbing boundary, tank front and back boundary, and tank bottom boundary are defined as velocity inlet boundary conditions, and the tank upper boundary is defined as a pressure outlet boundary condition. Compared with the traditional numerical simulation tank boundary condition setting method, the boundary reflection effect can be completely eliminated, and the wave field control logic is simplified.

[0060] The irregular wave numerical tank depth is defined, and the interface between seawater and air is defined as the irregular wave generating datum plane. Specifically, based on a VOF (Volume of Fluid) model, a Eulerian multiphase liquid (water) is added, and the concentration is defined as 997.561 kg / m 3 A Eulerian multiphase gas (air) is added, and the concentration is defined as 1.18415 kg / m 3 An internal solitary wave numerical tank is defined, the positions of the liquid and the gas are set along the depth direction of the tank, and the interface between the liquid and the gas is defined as the irregular wave generating datum plane.

[0061] Step four, a local coordinate system of the background tank and a catenary anchoring structure coordinate system are constructed. In the local coordinate system of the background tank, the coordinate system origin is located at the interface between air and seawater, which is set at the midpoint of the interface between the liquid and the gas at the wave-making boundary in this embodiment, the direction is the wave face transmission direction, the direction is the tank depth direction, the direction is the tank width direction. In the catenary anchoring structure coordinate system, the coordinate system origin is the centroid of the anchoring structure, the direction is the length direction, the direction is the height direction, the direction is the width direction.

[0062] The surface boundary of the catenary anchoring structure is defined as a wall boundary condition, and the face control is defined to prohibit the generation of the prismatic layer grid at the interface between the overlapping fluid domain and the background tank, so that the prismatic layer grid is only attached to the surface of the subsurface marker.

[0063] A fluid-structure interaction module is defined to simulate the dynamic and bidirectional interaction between the fluid and the solid target. The diagonal components of the inertia tensor of the target catenary model with respect to the catenary anchoring structure coordinate system are set as [854.17, 1366.67, 854.17] kg m 2 , and the mass of the target catenary model is 2050 kg.

[0064] According to the local coordinate system of the background tank and the catenary anchoring structure coordinate system, the relative positions of the two ends of the catenary are determined, the catenary is set in a pre-tightened state, the pre-tightening force is 100 N, the stiffness is 5000.0 N / m, and the unit length mass is 0.005 kg / m.

[0065] Step five, the sub-wave parameters obtained by decomposing step one are added to the sub-waves of the superimposed wave to generate a reproducible irregular wave load. Specifically, the parameters of the first fifty sub-waves with the largest amplitude obtained by decomposing step one, including phase, amplitude, and period, are added one by one to the sub-waves of the superimposed wave, as shown in Figure 4 .

[0066] The damping wave is activated at the wave-eliminating boundary to eliminate the non-physical reflection of irregular waves at the numerical simulation pool boundary, ensuring the accuracy of the results; the length of the wave-eliminating damping wave is defined as twice the wavelength of the sub-wave with the maximum wavelength, which is 121 m in this case, and the velocities of the wave-eliminating boundary, the front and rear boundaries of the pool, and the bottom boundary of the pool are set to 0 m / s.

[0067] Step six, first, initialize the grid model, as follows:

[0068] (1) Set the liquid area range according to the position vector field function, determine the location of the sea surface (i.e., determine the wave surface position), and use the linear superposition wave model as the irregular wave theoretical model. Preview the wave shape generated by the superposition wave boundary wave-making method to ensure that the recorded wave height sequence is consistent with the wave height time sequence obtained in step one, avoiding the waste of computing resources caused by the inconsistency of the wave shape after the calculation starts. In this process, set the initial time t = 0 of the wave shape equation, and the wave shape of the irregular wave propagates forward with time, and the wave height sequence is recorded by the wave height meter.

[0069] (2) Based on the VOF method, initialize the grid model of the background pool and the target catenary anchoring structure, so that the interface between the grid model of the background pool and the target catenary anchoring structure is activated to realize data exchange.

[0070] (4) Initialize the catenary and set its force to 0 at t = 0.

[0071] (5) Based on the overlapping grid theory, perform hole processing on the fluid domain grid model, identify and exclude invalid solid regions, ensure that the solver only calculates in the fluid region, avoid redundant calculation of invalid regions, and reduce the amount of calculation. In addition, define the information interpolation between the grid models of the background pool and the target catenary anchoring structure as second-order accuracy to improve the accuracy of data transmission and reduce numerical dissipation and dispersion errors.

[0072] Then, based on the initial state of the target catenary anchoring structure's sway, heave, and roll, create reports, monitors, and drawings of x-direction movement, z-direction movement, and y-axis rotation at different times, set the time step for calculation to 0.025 s, and perform flow field calculation on the grid model of the target catenary anchoring model until the calculation end time is reached , the calculation is stopped, and then the velocity vector variation diagram of the entire simulation process flow field and the planar three-degree-of-freedom motion simulation results of the target under the influence of the generated irregular waves can be obtained, as shown in Figure 5 and Figure 6 .

[0073] Therefore, the application adopts the above irregular wave influence under the mooring net cage motion response solving method, can stably reproduce the irregular wave of the known wave height time sequence, improves the accuracy and controllability of the irregular wave generation, and realizes the numerical simulation of the ocean irregular wave load on the mooring net cage structure.

[0074] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that: it can still modify or equivalently replace the technical solutions of the present application, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

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 transmission, 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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