CN116227176A - Combined structure and design method of square column vortex induced oscillation suppression with flexible plate behind - Google Patents

Abstract

The invention relates to a square column vortex-induced vibration suppression combined structure of a rear flexible plate and a design method thereof, wherein the combined structure comprises a square column and a flexible plate arranged at the rear part of the square column; the design method comprises the following steps: s1, single square column vortex-induced vibration numerical simulation in the bypass flow; s2, vortex-induced vibration numerical simulation of a square post rear-mounted flexible plate combined structure in the flowing around; s3, comparing vortex-induced vibration kinematics and dynamic response of the single square column and the square column rear flexible plate combined structure; s4, obtaining model parameters of the square post rear flexible plate combined structure with the best vibration reduction effect. The combined structure can greatly reduce the vortex-induced oscillation amplitude and the timely uniform resistance of the structure, thereby effectively improving the stability and the safety of the structure; the numerical analysis method for the parameter design and optimization of the post-side flexible plate provided by the invention can design the structural shape and size more economically, efficiently and accurately, and is not limited by the flexible flat plate material parameters, the size, the deformation shape, the specific placement position and the complexity of the flow field environment.

Description

Combined structure and design method of square column vortex-induced oscillation suppression with rear flexible plate

technical field

The invention relates to the technical field of marine engineering, in particular to a square column vortex-induced oscillation suppression combined structure with a rear flexible plate and a design method thereof.

Background technique

The vortex-induced oscillation of the square column is the vortex that alternately falls off on both sides and the tail during the flow around the square column. The periodic force generated by the vortex shedding makes the elastic or elastically supported square column forced to vibrate, and the vibration of the square column will affect the flow rate of the square column. The characteristics of the wake field form a complex fluid-solid coupling problem with passive motion. When the resonance phenomenon occurs, that is, when the vibration frequency of the square column is close to the frequency of the vortex shedding, the structure will vibrate greatly, seriously affecting the stability and safety of the structure.

The research on the vortex-induced oscillation of the square column shows that when the fluid flows through the square column, the square column will be subjected to the resistance of the flow direction and the lift force perpendicular to the flow direction. The lift force is caused by the pulsation generated by vortex shedding, and the resistance is divided into square Pressure difference resistance caused by pressure difference before and after the column and viscous resistance caused by viscous action. In vortex-induced oscillation, the vibration is mainly manifested as the oscillation of the square column in the direction of lift force. In engineering, the long-term vortex-induced oscillation of offshore platform supports and bridges may not only damage the structural stability, but also cause structural damage if resonance occurs. Therefore, vortex-induced oscillation suppression has important application value and research significance.

When studying the coupling motion of the flexible plate and the cylinder in the flow field, it is found that the flexible structure is passively deformed under the action of the incoming flow, and the flow field at the tail forms a complex multi-frequency vortex, which interacts with the flexible structure and has a significant impact on the dynamic characteristics of the structure. Significantly, the flexible structure in the vortex street can absorb energy from the vortex street, which can reduce the force on the system to a certain extent. Therefore, if the square column can be improved based on the characteristics of the flexible structure to reduce the alternating force of the flow field on the structure, it will be of great benefit to theoretical research and engineering applications.

Contents of the invention

The technical problem to be solved by the present invention is to provide a square column vortex induced oscillation suppression combination structure and its design method with a rear flexible plate for the large vibration generated during the vortex induced oscillation resonance of the square column, so as to effectively suppress the combined structure oscillation.

The technical scheme that the present invention adopts for solving the technical problem of above-mentioned proposal is:

A combined structure for suppressing vortex-induced oscillations of a square column with a rear flexible plate, comprising a square column and a flexible plate arranged at the rear of the square column, the flexible plate is arranged perpendicular to the surface of the square column, and placed on the side of the square column The location of the midpoint of the side length of the section.

In the above solution, the flexible board is hinged to the square column.

In the above solution, the flexible board is made of lightweight and corrosion-resistant materials.

Correspondingly, the present invention also proposes a design method of the above-mentioned square column vortex induced oscillation suppression combined structure with a rear flexible plate, including the following steps:

S1. Numerical simulation of vortex-induced oscillation of a single square column in the flow around it, specifically including the following steps:

S1.1. Establish the structure and flow field analysis model of the square column;

S1.2. Establish a structure-spring-damping model for the longitudinal motion of the square column, and carry out numerical experiments on the vortex-induced oscillation characteristics of a single square column at a set Reynolds number Re by means of computational fluid dynamics, and calculate the vortex-induced oscillation process of the square column The lift coefficient, drag coefficient and dimensionless amplitude of square column oscillation received in

S2. Numerical simulation of the vortex-induced oscillation of the composite structure with a flexible plate behind the square column in the flow, including the following steps:

S2.1. Select flexible plates with different lengths and different degrees of flexibility to establish a structural and flow field analysis model for the combined structure of the square column with flexible plates behind it;

S2.2. Calculate the vortex-induced oscillation motion of the combined structure model of the square column rear flexible plate at the same Reynolds number Re. Through numerical experiments, the lift coefficient, drag coefficient, and combination of the flexible plate with different lengths and different flexibility can be obtained. Dimensionless magnitude of structural oscillations;

S3. Comparing the vortex-induced oscillation kinematics and dynamic response of a single square column and a square column rear-mounted flexible plate composite structure:

According to the calculation results of steps S1 and S2, compare the lift coefficient, drag coefficient, and dimensionless amplitude of oscillation of the single square column and the rear flexible plate composite structure of the lower column with different lengths and different flexibility, and analyze the performance of the rear flexible plate composite structure Vibration reduction and drag reduction effect;

S4. Obtaining the model parameters of the combined structure with the square column and rear flexible plate with the best vibration reduction and drag reduction effects.

In the above method, in step S1.2, the movement of the structure-spring-damper model of the square column in the flow field is controlled by the longitudinal motion equation (1)

分别表示方柱无量纲速度、方柱无量纲加速度；In the formula: Y is the dimensionless amplitude of the vibration of the square column, Y=y/D, y is the longitudinal displacement of the square column, D is the side length of the square column,

Respectively represent the dimensionless velocity of the square column and the dimensionless acceleration of the square column;

ξ is the damping ratio;

U ^* is the reduction speed, U ^* = U _∞ /f _n D, f _n is the natural frequency of the square column, U _∞ is the fluid flow rate;

F_L为方柱受到的升力，ρ_f为流体密度；C _L is the lift coefficient,

F _L is the lift force on the square column, ρ _f is the fluid density;

m ^* =m/ρ _f D ² is the mass ratio of square column and fluid, m is the quality of square column;

The fluid motion is solved by solving the N-S equation or by the lattice Boltzmann method; among them, the viscous incompressible flow field motion uses the N-S equation:

In the formula: ρ _f is fluid density, p is fluid pressure, u is velocity vector, t is time, μ is fluid dynamic viscosity coefficient, f is force density;

The lattice Boltzmann method is to discretely solve the lattice Boltzmann equation, and the governing equation is as follows:

f _α (x+e _α δ _t ,t+δ _t )=f _α (x,t)+Φ _α (4)

In the formula: δ _t is the time step, α represents the discrete lattice direction, e _α is the lattice velocity vector, Φ _α represents the collision term and external force term, f _α is the density distribution function, x is the position coordinate of the Euler point, t for time;

The coupling between the square column and the flow field adopts the immersed boundary method, using the Euler grid to describe the flow field, and the Lagrangian grid to describe the structural boundary. By converting the effect of the complex boundary into the force source item on the Euler grid, and using The Delta function δ(x-X(s,t)) in formula (5) and formula (6) converts the force and velocity between Lagrangian point and Euler point:

In the formula: x is the position coordinate of Euler point, X is the position coordinate of Lagrangian point, s is the coordinate label of Lagrangian point, ds is the line segment length of Lagrangian boundary, u is the velocity vector, f(x ,t) is the force density at the corresponding time position, F(s,t) is the force on the solid boundary point;

By coupling and solving the square column oscillation equation such as formula (1) and the flow field equation such as formula (2)-(3) or (4), the lift coefficient C _L and drag coefficient C _D of the square column are calculated, and the square column oscillation is dimensionless Amplitude Y.

In the above method, in step S2.2, the solution to the oscillation and flow field of the square column in the composite structure with a flexible plate behind the square column is the same as the numerical solution method for the vortex induced oscillation around a single square column in S1.2. The oscillation equation is shown in formula (1), and the flow field equation is shown in formula (2)-(3) or (4); the passive motion of the flexible plate is solved by the flexible plate motion equation, and the flexible plate motion equation is shown in formula (7), Equation (8) expresses the non-stretch condition of the flexible board:

表示物理量沿板切线方向的导数，t为时间，T为拉伸张力，K_b为弯曲刚度，F_f为流体流对板施加的力；In the formula: ρ _s is the linear density of the flexible board, X _s is the position coordinate of the flexible board,

Indicates the derivative of the physical quantity along the tangential direction of the plate, t is the time, T is the tensile tension, K _b is the bending stiffness, F _f is the force exerted by the fluid flow on the plate;

The coupling of the combined structure with the flexible plate behind the square column and the flow field adopts the immersed boundary method, and the Lagrangian point and Transformation of forces and velocities between Euler points;

Solve the oscillation equation of the square column by coupling as shown in equation (1), the flow field equation is shown in equation (2)-(3) or (4), and the motion equation of the flexible plate is shown in equation (7). The lift coefficient C′ _L and the drag coefficient C′ _D of the rear flexible plate composite structure, and the dimensionless amplitude Y′ of the composite structure oscillation.

In the above method, in step S3, an average flow field is used to analyze flow field characteristics, and the average flow field is an average of the flow field within one oscillation cycle.

The beneficial effects of the present invention are:

1. The vortex-induced oscillation suppression combined structure of the square column rear flexible plate designed by the present invention can greatly reduce the amplitude and average resistance of the vortex-induced oscillation of the structure, thereby effectively improving the stability and safety of the structure. The numerical analysis method for the design and optimization of the parameters of the rear flexible plate of the square column proposed by the present invention can design the structural shape and size more economically, efficiently and accurately, and is not limited to the material parameters, size, deformation shape, and specific placement position of the flexible plate, and The complexity of the flow field environment.

2. In the optimal solution designed according to the present invention, the dimensionless oscillation amplitude of the square column can be reduced by up to 76.83%, the vibration amplitude of the lift coefficient can be reduced by 85.90%, and the average value of the drag coefficient can be reduced by 33.31%. . During use, the specific parameters of the vortex-induced oscillation suppression structure with the flexible plate behind the square column can be adjusted to the optimal state according to the CFD design method used in the present invention, which can effectively suppress the vortex-induced oscillation of the square column.

3. The structure designed according to the present invention has a wide range of applications and can be used in ocean engineering, construction and other fields.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing and embodiment, in the accompanying drawing:

Fig. 1 is a structural schematic diagram of a square column vortex-induced oscillation suppression combined structure with a rear flexible plate of the present invention;

Fig. 2 is the structure and the flow field analysis model of the square column set up in the design method of the present invention;

Fig. 3 is the structure-spring-damping model of the square column set up in the design method of the present invention;

Fig. 4 is the structure and the flow field analysis model of the square post rear flexible plate combination structure that is set up in the design method of the present invention;

Fig. 5 is the average flow field vorticity diagram of a single square column in the embodiment of the present invention;

Fig. 6 is the average flow field vorticity nephogram of the combined structure of the square column rear L=1.5D, w ^* =1.5 in the embodiment of the present invention;

Fig. 7 is the average flow field pressure nephogram of a single square column in the embodiment of the present invention;

Fig. 8 is a cloud diagram of the average flow field pressure of the combined structure of the square column rear L = 1.5D, w ^* = 1.5 flexible plate in the embodiment of the present invention.

Detailed ways

In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation manners of the present invention will now be described in detail with reference to the accompanying drawings.

As shown in Figure 1, the present invention proposes a square column vortex-induced oscillation suppression combined structure with a rear flexible plate, including a square column and a lightweight, corrosion-resistant flexible plate arranged at the rear of the square column, the square column and the flexible plate The hinge connection, the flexible plate is arranged perpendicular to the surface of the square column, and placed at the midpoint of the side length of the cross section of the square column. The square column and the flexible plate are a combined structure; wherein, the cross section of the square column is a square, its side length is D, and the mass ratio of the square column is m ^* =2; the length of the flexible plate is represented by L, and the thickness of the flexible plate is compared to Its length is very small and can be ignored. In the design of combined structure for vortex-induced oscillation suppression, plates with different lengths and different flexibility are selected for numerical experiments and comparative analysis, so as to determine its structural parameters.

Correspondingly, the present invention also proposes a design method for the above-mentioned square column vortex-induced oscillation suppression combined structure with a flexible plate at the rear. In order to determine the vortex-induced oscillation suppression effect of the combined structure, first perform computational fluid dynamics ( CFD) numerical experiments, and then carry out high-fidelity numerical simulation on the designed square column rear flexible plate composite structure, compare the lift coefficient, drag coefficient, and oscillation amplitude of the structure, and select the square column rear with the best vortex induced oscillation suppression effect. Structural parameters of flexible board composite structure. The design method specifically includes the following steps:

S1. Numerical simulation of vortex-induced oscillation of a single square column in the flow around it, specifically including the following steps:

S1.1. Establish the structure and flow field analysis model of the square column. As shown in Figure 2, the geometric modeling of the square column is carried out for the problem of the oscillating square column in the flow field, and the size of the calculation domain is determined according to the size of the square column. On this basis, the structure and flow field analysis model of the oscillating square column is established . It should be noted that U _∞ in Figure 2 represents the fluid velocity, D represents the side length of the square column, and y on the left represents the displacement in the y direction of the square column.

S1.2. Since the vortex-induced oscillation of the square column is mainly longitudinal (perpendicular to the flow direction, that is, the y direction shown in the figure), only the longitudinal motion is considered, and a structure-spring-damping model is established for the longitudinal motion of the square column , through computational fluid dynamics (CFD) method to carry out numerical experiments on the vortex-induced oscillation characteristics of a single square column at a set Reynolds number Re, and calculate the lift coefficient, drag coefficient, and dimensionless vibration of the square column during the vortex-induced oscillation process magnitude. Among them, the lift coefficient C _L =2F _L /(ρ _f U _∞ ² D), the drag coefficient C _D =2F _D /(ρ _f U _∞ ² D), and F _L is the lift force received during the vortex induced oscillation of the square column, F _D is the resistance suffered by the square column during vortex-induced oscillation, U _∞ is the fluid velocity, and ρ _f is the fluid density. The dimensionless amplitude of the vibration of the square column Y=y/D, that is, the ratio of the amplitude y to the side length D of the square column.

The established spring-damper model is shown in Figure 3, and its motion in the flow field is controlled by the longitudinal motion equation (1)

分别表示方柱无量纲速度、方柱无量纲加速度；In the formula: Y is the dimensionless amplitude of the vibration of the square column, Y=y/D, y is the longitudinal displacement of the square column, D is the side length of the square column,

Respectively represent the dimensionless velocity of the square column and the dimensionless acceleration of the square column;

ξ is the damping ratio;

U ^* is the reduction speed, U ^* = U _∞ /f _n D, f _n is the natural frequency of the square column, U _∞ is the fluid flow rate;

F_L为方柱受到的升力，ρ_f为流体密度；C _L is the lift coefficient,

F _L is the lift force on the square column, ρ _f is the fluid density;

m ^* =m/ρ _f D ² is the mass ratio of the square column to the fluid, and m is the mass of the square column.

The fluid motion is solved by the solution of the N-S equations or by the lattice Boltzmann method. Among them, the viscous incompressible flow field motion adopts the N-S equation:

In the formula: ρ _f is fluid density, p is fluid pressure, u is velocity vector, t is time, μ is fluid dynamic viscosity coefficient, f is force density.

The lattice Boltzmann method is to discretely solve the lattice Boltzmann equation, and the governing equation is as follows:

f _α (x+e _α δ _t ,t+δ _t )=f _α (x,t)+Φ _α (4)

In the formula: δ _t is the time step, α represents the discrete lattice direction, e _α is the lattice velocity vector, Φ _α represents the collision term and external force term, f _α is the density distribution function, x is the position coordinate of the Euler point, t for time.

The coupling between the square column and the flow field adopts the immersed boundary method, using the Euler grid to describe the flow field, and the Lagrangian grid to describe the structural boundary. By converting the effect of the complex boundary into the force source item on the Euler grid, and using The Delta function δ(x-X(s,t)) in formula (5) and formula (6) converts the force and velocity between Lagrangian point and Euler point:

In the formula: x is the position coordinate of Euler point, X is the position coordinate of Lagrangian point, s is the coordinate label of Lagrangian point, ds is the line segment length of Lagrangian boundary, u is the velocity vector, f(x ,t) is the force density at the corresponding time position, and F(,t) is the force on the solid boundary point.

By coupling and solving the square column oscillation equation such as formula (1) and the flow field equation such as formula (2)-(3) or (4), the lift coefficient C _L and drag coefficient C _D of the square column are calculated, and the square column oscillation is dimensionless Amplitude Y.

Due to the periodicity of the force on the square column, after comprehensive consideration, the vibration amplitude of the square column is analyzed by the dimensionless amplitude of the square column, and the hydrodynamic force acting on the square column is analyzed by the oscillation amplitude of the lift coefficient and the mean value of the drag coefficient.

In this embodiment, for the flow around a square column under Re=150, mass ratio m ^* =2, and reduction velocity U ^* =5, the dimensionless oscillation amplitude Y of a single square column is 0.46, and the oscillation amplitude of the lift coefficient is 1.963. The average resistance coefficient is 2.338.

S2. Numerical simulation of the vortex-induced oscillation of the composite structure with a flexible plate behind the square column in the flow, including the following steps:

S2.1. Select flexible plates with different lengths and different degrees of flexibility to establish a structural and flow field analysis model for the combined structure of the square column with flexible plates behind. As shown in Figure 4, based on the size of the square column and the calculation domain in step S1, design flexible plates of different lengths and different degrees of flexibility, and place the flexible plate behind the square column. On this basis, the square column in the flow field is established Structural flow field analysis model of flexible plate composite structure.

S2.2. Calculate the vortex-induced oscillation motion of the combined structure model of the square column rear flexible plate under the same Reynolds number Re. Through numerical experiments, the lift coefficient, drag coefficient, and dimensionless amplitude of the combined structure oscillation under the flexible plate with different parameters are obtained .

The solution to the oscillation and flow field of the square column in the composite structure with a flexible plate behind the square column is the same as the numerical solution of the vortex induced oscillation around a single square column in S1.2. The oscillation equation of the square column is shown in equation (1), The flow field equation is shown in formula (2)-(3) or (4); the passive motion of the flexible plate is solved through the equation of motion of the flexible plate, the equation of motion of the flexible plate is shown in formula (7), and the formula (8) expresses the non-stretch of the flexible plate condition:

表示物理量沿板切线方向的导数，t为时间，T为拉伸张力，K_b为弯曲刚度，F_f为流体流对板施加的力；In the formula: ρ _s is the linear density of the flexible board, X _s is the position coordinate of the flexible board,

Indicates the derivative of the physical quantity along the tangential direction of the plate, t is the time, T is the tensile tension, K _b is the bending stiffness, F _f is the force exerted by the fluid flow on the plate;

The coupling of the combined structure with the flexible plate behind the square column and the flow field adopts the immersed boundary method, and the Lagrangian point and Transformation of forces and velocities between Euler points;

Solve the oscillation equation of the square column by coupling as shown in equation (1), the flow field equation is shown in equation (2)-(3) or (4), and the motion equation of the flexible plate is shown in equation (7). The lift coefficient C′ _L and the drag coefficient C′ _D of the combined structure of the rear flexible plate, and the dimensionless amplitude Y′ of the combined structure oscillation.

Due to the periodicity of the force on the square column, after comprehensive consideration, the vibration amplitude of the square column in the composite structure with a flexible plate behind the square column is analyzed by the dimensionless amplitude of the combined structure oscillation, and the square column is analyzed by the oscillation amplitude of the lift coefficient and the mean value of the drag coefficient. Kinematics and dynamics characteristics of rear flexible plate composite structure.

In this embodiment, flexible boards of different lengths and flexibility are designed. Perform high-fidelity numerical simulation on the designed composite structure model of multiple square columns with rear flexible plates according to the method of step S2 above. Then statistically analyze the lift coefficient oscillation amplitude, the drag coefficient average value and the dimensionless amplitude of the combined mechanism oscillation under the flexible plates with different length L and different flexibility w ^* .

K_b为弯曲系数，f_v为单个固定方柱的旋涡脱落频率。The flexibility of the plate w ^* = 2πf _v ω _n , where

K _b is the bending coefficient, and f _v is the vortex shedding frequency of a single fixed square column.

Among the design parameters of the flexible board, the length L is taken as: 1.2D, 1.5D, 3D; the flexibility w ^* is taken as: 1, 1.5, 2.

In this embodiment, under the same Reynolds number, the combined structure of flexible plates with different lengths and different pliability is designed.

Table 1 Combination structure of different schemes

S3. Comparing the kinematics and dynamic response of the vortex-induced oscillation between a single square column and a combined structure with a flexible plate behind the square column.

According to the calculation results of steps S1 and S2, compare the lift coefficient, drag coefficient, and dimensionless amplitude of oscillation of the single square column and the rear flexible plate composite structure of the lower column with different lengths and different flexibility, and analyze the performance of the rear flexible plate composite structure Vibration reduction and drag reduction effect.

The average flow field is used to analyze the flow field characteristics, where the average flow field is the average of the flow field within one oscillation period. When the fluid flows through a single square column, it can be seen from the average flow field pressure cloud diagram (Fig. 7) that the pressure difference between the front and rear of the square column is large and the vertical pressure range is large, resulting in a large pressure differential resistance around the square column. From the vorticity diagram of the average flow field of the column (Fig. 5), it can be seen that the area of the shedding vortex on both sides and the tail of the square column is relatively large, so the vertical vibration amplitude and force of the square column are relatively large. However, it can be seen from the average flow field pressure cloud diagram (Fig. 8) of the square column rear flexible plate composite structure that the pressure difference between the front and back of the square column is small and the vertical pressure distribution range is small. From the corresponding average flow field vorticity cloud diagram ( It can be seen from Fig. 6) that the scope of the shedding vortex on both sides and tail of the square column is also significantly reduced, so that the vertical vibration amplitude of the square column is reduced. From the perspective of kinematics and dynamics, it can be seen from the results of steps S1 and S2 that, compared with the vortex-induced oscillation of a single square column, the square column is placed behind the flexible plate, and the dimensionless oscillation amplitude, lift coefficient oscillation amplitude, and resistance of the square column The mean values of the coefficients were significantly reduced. It can be seen that the flexible plate behind the square column has excellent vibration and drag reduction effects.

S4. Obtaining the model parameters of the combined structure with flexible plate behind the square column, which has the best vibration reduction and drag reduction effect of the vortex-induced oscillation suppression structure.

After the square column is equipped with a flexible plate, the dimensionless amplitude of vibration of the square column, the oscillation amplitude of the lift coefficient, and the mean value of the drag coefficient are all significantly lower than those of a single square column. The vibration reduction and drag reduction effects of flexible plates with different flexibility are different. Numerical experiment results show that, considering the range of design parameters now, the vibration reduction and drag reduction effects are the best under the condition of w ^* = 1.5, L = 1.5D and the rear flexible plate of the square column. In the optimal scheme, the vortex-induced oscillation suppression structure with the flexible plate behind the square column can reduce the dimensionless oscillation amplitude of the square column by 76.83%, the amplitude of the lift coefficient oscillation by 85.90%, and the average value of the drag coefficient by 33.31%.

Therefore, according to the design method of the present invention, the specific parameters of the vortex-induced oscillation suppression combined structure of the square column rear flexible plate designed in conjunction with Figure 1 are as follows: the side length of the square column is D, the length of the flexible plate is L=1.5D, and the flexibility w ^* = 1.5.

During use, the specific parameters of the vortex-induced oscillation suppression structure with the flexible plate behind the square column can be adjusted to the optimal state according to Fig. 1 and the CFD design method used in the present invention, which can effectively suppress the vortex-induced oscillation of the square column.

Embodiments of the present invention have been described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementations, and the above-mentioned specific implementations are only illustrative, rather than restrictive, and those of ordinary skill in the art will Under the enlightenment of the present invention, many forms can also be made without departing from the gist of the present invention and the protection scope of the claims, and these all belong to the protection of the present invention.

Claims (7)

1. A square column vortex-induced oscillation suppression combination structure with a rear flexible plate, characterized in that it comprises a square column and a flexible plate arranged at the rear of the square column, and the flexible plate is arranged perpendicular to the surface of the square column, And place it at the midpoint of the side length of the cross section of the square column. 2 . The combined structure of square column vortex induced oscillation suppression with a rear flexible plate according to claim 1 , wherein the flexible plate is hinged to the square column. 3 . 3. The combined structure of square column vortex-induced oscillation suppression with a rear flexible board according to claim 1, characterized in that the flexible board is made of lightweight and corrosion-resistant materials. 4. The design method of the square column vortex-induced oscillation suppression combined structure of the rear flexible plate according to claim 1, it is characterized in that, comprising the following steps: S1. Numerical simulation of vortex-induced oscillation of a single square column in the flow around it, specifically including the following steps: S1.1. Establish the structure and flow field analysis model of the square column; S1.2. Establish a structure-spring-damping model for the longitudinal motion of the square column, and carry out numerical experiments on the vortex-induced oscillation characteristics of a single square column at a set Reynolds number Re by means of computational fluid dynamics, and calculate the vortex-induced oscillation process of the square column The lift coefficient, drag coefficient and dimensionless amplitude of square column oscillation received in S2. Numerical simulation of the vortex-induced oscillation of the composite structure with a flexible plate behind the square column in the flow, including the following steps: S2.1. Select flexible plates with different lengths and different degrees of flexibility to establish a structural and flow field analysis model for the combined structure of the square column with flexible plates behind it; S2.2. Calculate the vortex-induced oscillation motion of the combined structure model of the square column rear flexible plate at the same Reynolds number Re. Through numerical experiments, the lift coefficient, drag coefficient, and combination of the flexible plate with different lengths and different flexibility can be obtained. Dimensionless magnitude of structural oscillations; S3. Comparing the vortex-induced oscillation kinematics and dynamic response of a single square column and a square column rear-mounted flexible plate composite structure: According to the calculation results of steps S1 and S2, compare the lift coefficient, drag coefficient, and dimensionless amplitude of oscillation of the single square column and the rear flexible plate composite structure of the lower column with different lengths and different flexibility, and analyze the performance of the rear flexible plate composite structure Vibration reduction and drag reduction effect; S4. Obtaining the model parameters of the combined structure with the square column and rear flexible plate with the best vibration reduction and drag reduction effects. 5. The design method of the square column vortex induced oscillation suppression combined structure with the rear flexible plate according to claim 4, characterized in that, in step S1.2, the structure-spring-damping model of the square column in the flow field Motion is governed by the longitudinal motion equation (1)

In the formula: Y is the dimensionless amplitude of the vibration of the square column, Y=y/D, y is the longitudinal displacement of the square column, D is the side length of the square column,

Respectively represent the dimensionless velocity of the square column and the dimensionless acceleration of the square column; ξ is the damping ratio; U ^* is the reduction speed, U ^* = U _∞ /f _n D, f _n is the natural frequency of the square column, U _∞ is the fluid flow rate; C _L is the lift coefficient,

F _L is the lift force on the square column, ρ _f is the fluid density; m ^* =m/ρ _f D ² is the mass ratio of square column and fluid, m is the quality of square column; The fluid motion is solved by solving the N-S equation or by the lattice Boltzmann method; among them, the viscous incompressible flow field motion uses the N-S equation:

In the formula: ρ _f is fluid density, p is fluid pressure, u is velocity vector, t is time, μ is fluid dynamic viscosity coefficient, f is force density; The lattice Boltzmann method is to discretely solve the lattice Boltzmann equation, and the governing equation is as follows: f _α (x+e _α δ _t ,t+δ _t )=f _α (x,t)+Φ _α (4) In the formula: δ _t is the time step, α represents the discrete lattice direction, e _α is the lattice velocity vector, Φ _α represents the collision term and external force term, f _α is the density distribution function, x is the position coordinate of the Euler point, t for time; The coupling between the square column and the flow field adopts the immersed boundary method, using the Euler grid to describe the flow field, and the Lagrangian grid to describe the structural boundary. By converting the effect of the complex boundary into the force source item on the Euler grid, and using The Delta function δ(x-X(s,t)) in formula (5) and formula (6) converts the force and velocity between Lagrangian point and Euler point:

In the formula: x is the position coordinate of Euler point, X is the position coordinate of Lagrangian point, s is the coordinate label of Lagrangian point, ds is the line segment length of Lagrangian boundary, u is the velocity vector, f(x ,t) is the force density at the corresponding time position, F(s,t) is the force on the solid boundary point; By coupling and solving the square column oscillation equation such as formula (1), and the flow field equation such as formula (2)-(3) or (4), the lift coefficient C _L and drag coefficient C _D of the square column are calculated, and the square column oscillation is dimensionless Amplitude Y. 6. The design method of the square column vortex-induced oscillation suppression combined structure with a rear flexible plate according to claim 5, characterized in that, in step S2.2, the vibration of the square column in the rear flexible plate combined structure of the square column And the solution of the flow field is the same as the numerical solution of the vortex-induced oscillation around a single square column in S1.2. The square column oscillation equation is shown in equation (1), and the flow field equation is shown in equation (2)-(3) or ( 4); the passive motion of the flexible board is solved by the equation of motion of the flexible board, the equation of motion of the flexible board is shown in formula (7), and the formula (8) represents the non-stretch condition of the flexible board:

In the formula: ρ _s is the linear density of the flexible board, X _s is the position coordinate of the flexible board,

Indicates the derivative of the physical quantity along the tangential direction of the plate, t is the time, T is the tensile tension, K _b is the bending stiffness, F _f is the force exerted by the fluid flow on the plate; The interaction between the combined structure of the square column and the flexible plate behind the flow field is coupled by the immersed boundary method, the flow field is described by the Euler grid, and the structure boundary is described by the Lagrangian grid, and the complex boundary is transformed into an Eulerian grid The force source item on the grid, and use the Delta function δ(x-X(s,t)) in formula (5) and formula (6) to convert the force and velocity between Lagrangian point and Euler point; Solve the oscillation equation of the square column by coupling as shown in equation (1), the flow field equation is shown in equation (2)-(3) or (4), and the motion equation of the flexible plate is shown in equation (7). The lift coefficient C _L ^′ , the drag coefficient C _D ^′ of the combined structure of the rear flexible plate, and the dimensionless amplitude Y ^′ of the combined structure oscillation. 7. The design method of the square column vortex-induced oscillation suppression composite structure with rear flexible plate according to claim 4, characterized in that, in step S3, the flow field characteristics are analyzed by using the average flow field, and the average flow field is The flow field is averaged over one oscillation period.

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