Method for solving motion response of moored aquaculture cage under irregular wave action

A method for solving a motion response of a moored aquaculture cage under an irregular wave action, comprising:

• step I, according to parameters of a JONSWAP spectrum corresponding to a sea state reached by a random irregular wave, obtaining a spectral density of an ocean wave spectrum of the irregular wave using a JONSWAP spectrum formula;
• solving a dispersion equation via a Newton iteration method, and obtaining a wavenumber of the irregular wave;
• calculating a wave height of each component based on the spectral density of the ocean wave spectrum and wavenumber of the irregular wave, generating a random phase, and obtaining a wave height time series through iteration;
• and
• decomposing the wave height time series into wavelets based on FFT, and determining parameter information of the wavelets;
• step II, establishing a fluid domain geometric model of an area where a mooring structure is located as a background tank region;
• in the background tank region, according to a maximum wave height of the irregular wave, determining a grid region of a wave surface fluid domain, and subtracting a target mooring structure by a subtraction operation to generate an overlapping fluid domain;
• establishing an interface between the overlapping fluid domain and the background tank as an overlapping grid interface, and establishing a flow field information exchange between the two regions;
• wherein the fluid domain geometry model defines a continuous shape and physical properties of a computational domain, a length direction is a propagation direction of the irregular wave, and a height direction is a water depth direction of the background tank;
• step III, based on a VOF model, constructing a grid model of the background tank and a catenary mooring structure, and performing a local encryption processing on the grid region of the wave surface fluid domain in the background tank, and setting a slow grid volume growth rate;
• and
• defining an irregular wave inlet boundary, a wave absorption boundary, and a front and rear boundary of the tank, defining a bottom boundary of the tank as a velocity inlet boundary condition, defining a top boundary of the tank as a pressure outlet boundary condition;
• defining a depth of an irregular wave numerical wave tank;
• and defining a seawater-air interface as a reference plane for an irregular wave generation;
• step IV, constructing a local coordinate system of the background tank and a coordinate system of the catenary mooring structure;
• defining a surface boundary of a target catenary mooring structure as a wall boundary condition;
• defining a fluid-solid interaction module to simulate a dynamic and bidirectional interaction between fluid and solid targets;
• setting an inertia moment of a target catenary model with respect to the coordinate system of the catenary mooring structure, along with a mass of the target catenary mooring structure;
• and
• determining relative positions of two ends of the catenary according to the local coordinate system of the background tank and the coordinate system of the catenary mooring structure, and setting a relaxation length and a stiffness of the catenary;
• step V, incorporating the wavelet parameters obtained from the decomposition in step I into the wavelet of a superimposed wave using a superimposed-wave boundary wave-generation method, so as to generate a reproducible irregular wave load, and defining a length of a wave-absorbing damping wave to be twice a wavelength of the wavelet with the maximum wavelength;
• and setting velocities of the wave absorption boundary, and the front and rear boundary of the tank, and the bottom boundary of the tank to 0 m/s;
• and
• step VI, initializing the grid model, and taking initial states of sway, heave, and pitch motions of the target catenary mooring structure as a reference, performing a flow field calculation on the grid model of the target catenary mooring structure until reaching an end time tend of the calculation, and terminating the calculation.