Method for simulating sediment erosion around offshore wind turbine foundation through ISPH combined with corrected k-epsilon turbulence model

Abstract

The invention discloses a method for simulating sediment erosion around an offshore wind turbine foundation by combining ISPH with a modified k-epsilon turbulence model. The method comprises the following steps: establishing an ISPH discrete algorithm for solving an average mass conservation equation and a momentum conservation equation of a fluid dynamic ensemble; correcting the standard k-epsilon turbulence model, and calculating the turbulence vortex viscosity vt in the Reynolds stress tensor expression; a critical starting theory on a slope is introduced, a traditional two-dimensional sediment starting model is corrected and expanded, and a three-dimensional sediment starting model is established; establishing a local scouring experiment water tank model of the offshore wind turbine single pile foundation; establishing a corresponding ISPH numerical water tank model; the experimental water tank model and the numerical water tank model respectively simulate the characteristics of the scouring process of sediment around the cylinder, and a numerical simulation result and an experimental result are compared, verified and analyzed. By means of the method, the sediment erosion process around the offshore wind turbine single-pile foundation can be well simulated, the calculation time is short, and the requirements of actual engineering can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of local scouring around an offshore wind turbine foundation, and in particular relates to a method for simulating sediment erosion around an offshore wind turbine foundation by combining ISPH with a modified k-ε turbulence model. Background Art

[0002] As a clean, pollution-free and renewable energy, wind energy has received extensive attention and has been vigorously promoted by the international community. Offshore wind energy is superior to onshore wind power generation in terms of stability, power generation, environmental impact and grid access convenience, and is increasingly valued and applied. The Global Offshore Wind Power Report 2023 released by the Global Wind Energy Council shows that by the end of 2022, the global offshore wind power installed capacity will reach 64.3 million kilowatts, of which Europe's offshore wind power installed capacity is 30 million kilowatts. In 2022, the global new offshore wind power installed capacity will reach 8.8 million kilowatts, continuing to lead the growth of the global offshore wind power market. As of June 30, 2023, China's offshore wind power installed capacity has maintained a global leading position for many years, and achieved more than 5 million kilowatts of new installed capacity in 2022. This achievement further consolidates China's dominant position in the global offshore wind power market. Despite the good prospects of the offshore wind turbine power generation market, offshore wind power still faces many problems and challenges. The most important of these is the lack of mature wind turbine foundation technology, which is an important component of offshore wind turbines. According to different factors such as water depth and load, a variety of offshore wind turbine foundation forms have emerged, such as large-diameter single pile foundation, gravity structure foundation, tripod foundation, pipe frame foundation and suction bucket foundation. The most widely used is the large-diameter single pile foundation. Currently, more than 81.5% of offshore wind turbine foundations in Europe are single pile foundations, and single pile foundations are recommended by the DNV specification, which is the most widely used in the offshore wind power development industry, as the optimal foundation form for offshore wind turbines in water depths below 30m. Most of my country's wind turbines are located in shallow offshore waters, which are more suitable for the use of single pile foundations.

[0003] In the marine environment, the wind turbine single pile foundation can be approximately regarded as a pile column structure. The existence of the pile column structure will change the original flow field characteristics, especially in the area around the structure, which may form a strong vortex effect and fluid dynamic disturbance, and these factors will aggravate the scouring phenomenon in the local sea area. The deepening of the scouring pit means that the buried depth of the pile column originally fixed on the seabed is reduced. On the one hand, it may lead to a decrease in the stability of the pile column, and on the other hand, it may also cause changes in the horizontal load borne by the pile column, such as an increase in eccentricity, which poses a potential threat to the structural safety of the entire wind turbine. In recent years, the local scouring problem around the offshore wind turbine foundation has become a key challenge faced by the marine engineering and offshore power generation fields. For offshore wind turbines, the stability of their foundation determines the safety of the entire wind turbine. In the ocean, the wind turbine will be coupled with horizontal loads such as wind, waves, and currents. At the same time, under the action of waves and currents, the wind turbine single pile foundation will also be locally scoured, forming a scouring pit at the bottom of the single pile, so that the seabed cannot provide sufficient support for the wind turbine, forming a safety hazard, and even causing the wind turbine to become unstable and damaged, causing major safety accidents. Therefore, revealing the local scouring process and sediment movement laws around the single pile foundation of offshore wind turbines has become a key issue in the field of marine engineering. At the same time, studying the local scouring characteristics and maximum scouring depth around the wind turbine foundation has become a top priority in wind turbine construction.

[0004] In order to evaluate the safety of wind turbine foundations, the sediment erosion process and scouring depth around the foundation must be accurately predicted. The physical model experiments and field observations used in the past are difficult to observe the sediment erosion process in detail. At the same time, the grid-based model causes serious numerical dissipation when dealing with complex free surface deformations such as water falling during tsunami wave overtopping, due to the need to solve the convection term in the equation. In recent years, with the rapid development of computer technology, numerical simulation has been valued for its low investment cost, strong adaptability to various fluids and complex terrains, and high flexibility. The incompressible smoothed particle hydrodynamics (ISPH) method has a strong advantage in dealing with complex free surface and water-sand interface problems. Although there are many studies on the numerical simulation of local scouring of single pile foundations of offshore wind turbines, there are few studies that use the incompressible smoothed particle hydrodynamics (ISPH) method combined with the sediment initiation model and the modified turbulence model to simulate the local scouring of single pile foundations. Summary of the invention

[0005] In view of the above problems, the present invention proposes a method for simulating sediment erosion around offshore wind turbine foundations. Based on the ISPH method combined with the sediment initiation model and the modified k-ε turbulence model, the three-dimensional local scour problem of the single pile foundation cylinder is studied. The sediment erosion process around the single pile foundation cylinder of the offshore wind turbine can be well simulated, aiming to reveal the law of the sediment erosion process around the wind turbine foundation and provide safety guidance and suggestions for the safe design and installation of the wind turbine.

[0006] The present invention is implemented as follows: a method for simulating sediment erosion around an offshore wind turbine foundation by using ISPH combined with a modified k-ε turbulence model, wherein the method uses ISPH combined with a sediment initiation model and a modified k-ε turbulence model to simulate the flow field characteristics and local scouring process around an offshore wind turbine foundation, and the method steps include:

[0007] S1, establish the ISPH discretization algorithm for solving the ensemble average mass conservation equation and momentum conservation equation (i.e., NS equation) of fluid dynamics. The NS equation is expressed as follows:

[0008]

[0009]

[0010] Where ρ0 is the fluid density, t is the time, u is the ensemble average velocity, g is the gravitational acceleration, ν0 is the kinematic viscosity coefficient, and p is the pressure. is the Reynolds stress tensor;

[0011] S2, the standard k-ε turbulence model is modified to calculate the turbulent eddy viscosity ν in the Reynolds stress tensor expression t ;

[0012] S3, introduce the critical sediment initiation criterion on the slope, modify and expand the traditional two-dimensional sediment initiation model, and establish a three-dimensional sediment initiation model;

[0013] S4, establish a physical model of the water tank for the scour test of the single pile foundation of the offshore wind turbine;

[0014] S5, establish the corresponding ISPH numerical tank model;

[0015] S6, the experimental flume model and the numerical flume model respectively simulate the characteristics of the sediment scouring process around the cylinder, and the numerical simulation results are compared with the experimental results for verification and analysis.

[0016] Furthermore, in step S2, the turbulent kinetic energy k is defined at each particle i. i and energy dissipation rate ε i , for the turbulence existing in the ocean, the standard k-ε turbulence closed model has the following form:

[0017]

[0018] In the formula, σ k =1.0,σ ε =1.3, C ε1 =1.44, C ε2 =1.92, Ρ represents the generation term of turbulent kinetic energy, t is time, and the turbulent eddy viscosity ν t To make corrections:

[0019]

[0020] In the formula, is the corrected dissipation rate, taking λ3=0.1, in represents the deformation rate tensor, represents the rotation rate tensor of the ensemble average flow field. The subscript 'initial' represents the time average and depth average of the first wave cycle in the potential flow region. 0initial =1.24s -2 , p Ωinitial =1.18×10 -8 s -2 , C μ , C1 and C2 are empirical constants. In this model, C μ =0.09, C1=1.33, C2=1.92.

[0021] Furthermore, S3 modifies and expands the traditional two-dimensional sediment initiation model, and the expanded equation is:

[0022]

[0023] In the formula, is the ratio of the critical shear stress of slope sediment to the horizontal critical shear stress, α is the angle between the S plane and the S' plane on the yz plane, β is the angle between the S plane and the S' plane on the xz plane, φ is the sediment repose angle, η is taken as 0.85, if the ratio of the calculated shear stress of the bottom particles to the horizontal sediment critical shear stress obtained by the Shields curve, It is considered that the bottom particles have reached the critical start-up state and will be transformed into turbid water particles and participate in the calculation of the NS equation. bi is the calculated shear stress of sediment:

[0024]

[0025] In the formula, κ is the Karman coefficient, which is 0.4, D is the diameter of the particle, and u is bi is the tangential velocity component of particle i.

[0026] Furthermore, the total length of the experimental flume model described in step S4 is 35m, the width is 7m, and the depth is 1.6m. A 9m-long area is set in the central position of the flume as a scouring test area, and a layer of experimental fine sand with a depth of 0.60m is pre-filled in the scouring test area; a cylindrical model with a diameter of 0.6m is set in the center of the experimental area; an inflow area is set on the left side of the experimental flume, and an outflow area is set on the right side to maintain water level balance; the number of experimental groups is not less than 5, the experimental water depth is 0.3m, and the average flow rate is 0.157-0.216m / s.

[0027] Furthermore, in step S5, in order to improve calculation efficiency and facilitate focusing on key areas, the calculation area of ​​the numerical flume model is set to a length of 5.45m, which is 9 times the diameter of the cylindrical model, and the width is set to 5m, which is 8.3 times the diameter of the cylindrical model; the particle diameter in the numerical model is set to 0.02m; an inflow area is set on the left side of the model, and a uniform flow rate is applied according to the actual experimental data to simulate the inflow of upstream water; an outflow area is set on the right side of the model to keep the water level in the entire model relatively constant, and a series of numerical flow velocity measurement points are set according to different water depth positions along the longitudinal section of the cylinder in the direction of water flow to collect flow velocity data at corresponding positions during the simulation process.

[0028] Furthermore, step S6 verifies and analyzes the flow field distribution characteristics around the cylinder, including velocity distribution, vortex structure and turbulent kinetic energy distribution; and then verifies and analyzes the characteristics of the sediment scouring process around the cylinder.

[0029] The beneficial effects of the present invention are:

[0030] (1) Compared with the existing technology, a three-dimensional numerical flume was established to simulate the flow field around the offshore wind turbine foundation under the action of unidirectional water flow by using ISPH in combination with the sediment initiation erosion model and the modified k-ε turbulence model. The numerical simulation results were compared with the experimental results to verify the accuracy of the flow field calculation.

[0031] (2) The results of the numerical model are highly consistent with the experimental data. The established numerical water flume can effectively simulate the flow field around the wind turbine single pile foundation and can more accurately reflect the flow field distribution characteristics around the wind turbine single pile foundation. The descending water flow in front of the single pile, the streamline contraction on both sides of the single pile, and the tail vortex behind the single pile are all well simulated, verifying the accuracy and applicability of the numerical model of the present invention.

[0032] (3) Based on the physical model experimental results of scouring around a cylinder, the method of the present invention can well simulate the sediment erosion process around the single pile foundation of an offshore wind turbine and meet the needs of actual engineering.

[0033] The present invention is further explained in detail below in conjunction with the accompanying drawings and specific implementation methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of three-dimensional slope;

[0035] Figure 2 Angular relationship diagram between local coordinates and global coordinates;

[0036] Figure 3 Schematic diagram of experimental water tank layout;

[0037] Figure 4 Numerical model setup;

[0038] Figure 5 Horizontal velocity diagram at different depths of the cross section;

[0039] Figure 6 Water depth variation diagram of the numerical model at x = 1.5m;

[0040] Figure 7 Velocity trend chart at different water depths;

[0041] Figure 8 Vorticity diagram around a cylinder;

[0042] Fig. 9 Streamline diagram around a cylinder;

[0043] Fig.10 Diagram of the horseshoe vortex formation process;

[0044] Fig.11 Turbulent kinetic energy distribution cloud map;

[0045] Fig.12 Plan view of sediment erosion process;

[0046] Fig.13 Longitudinal section of sediment erosion process;

[0047] Fig.14 Comparison of the experimental and simulation results of the scour depth in front of the cylinder;

[0048] Fig.15 Comparison of the experimental and simulation results of the scour depth behind the cylinder;

[0049] Fig.16 Comparison of the cylinder lateral scour depth test and simulation results; DETAILED DESCRIPTION

[0050] The present invention provides a method for simulating sediment erosion around an offshore wind turbine foundation by combining ISPH with a modified k-ε turbulence model. The method uses ISPH in combination with a sediment initiation model and a modified k-ε turbulence model to simulate the flow field characteristics and local scouring process around the offshore wind turbine foundation cylinder. Incompressible Smoothed Particle Hydrodynamic, abbreviated as ISPH in English, is to discretize a continuous fluid domain into a series of particles with a certain mass. These particles represent a part of the fluid and carry various physical properties of the fluid, such as density, velocity, pressure, etc., and move according to the NS equation.

[0051] The method steps include:

[0052] S1, establish the ISPH discretization algorithm for solving the ensemble average mass conservation equation and momentum conservation equation (i.e., NS equation) of fluid dynamics. The NS equation is expressed as follows:

[0053]

[0054] Where ρ0 is the fluid density, t is the time, u is the ensemble average velocity, g is the gravitational acceleration, ν0 is the kinematic viscosity coefficient; p is the pressure, is the Reynolds stress tensor.

[0055] S2, the standard k-ε turbulence model is modified to calculate the turbulent eddy viscosity ν in the Reynolds stress tensor expression t .

[0056] In the ISPH method, the turbulent kinetic energy k is defined at each particle i. i and energy dissipation rate ε i , for the turbulence existing in the ocean, the k-ε turbulence closed model has the following form:

[0057]

[0058] In the formula, σ k =1.0,σ ε =1.3, C ε1 =1.44, C ε2 =1.92, Ρ represents the generation term of turbulent kinetic energy, t is time, and the turbulent eddy viscosity ν t To make corrections:

[0059]

[0060] In the formula, is the corrected dissipation rate, taking λ3=0. in represents the deformation rate tensor, represents the rotation rate tensor of the ensemble average flow field. The subscript 'initial' represents the time average and depth average of the first wave cycle in the potential flow region. 0initial =1.24s -2 , p Ωinitial =1.18×10 -8 s -2 , C μ , C1 and C2 are empirical constants. In this model, C μ =0.09, C1=1.33, C2=1.92.

[0061] S3, introduce the critical sediment initiation criterion on the slope, modify and expand the traditional two-dimensional sediment initiation model, and establish a three-dimensional sediment initiation model. In the three-dimensional sediment initiation model, especially on the slope terrain, the influence of gravity on the sediment will lead to significant differences in the sediment initiation shear stress between the positive slope (along the direction of the water flow) and the reverse slope (against the direction of the water flow). On the positive slope, the sediment is more easily washed away by the water flow, so the initiation shear stress is relatively small; on the reverse slope, due to the gravity counteracting the water flow, the sediment requires a larger shear stress to be initiated. In order to solve this problem, the critical sediment initiation theory on the slope is introduced into the three-dimensional sediment initiation model, and the traditional two-dimensional model is modified and expanded.

[0062] In general, the critical shear stress of horizontal sediment is τ c The calculation method is: Calculation parameters The value of d is the actual sediment particle size, ν is the viscosity, γ s and γ are the bulk densities of sediment and water, respectively. In the Shields curve, τ * = 0.1; draw a straight line with a slope of 2 through this point to intersect the curve. The ordinate of the intersection is the critical Shields number τ *c , and then substitute into the equation τ *c =τ c / (γ s -γ)d, the critical starting shear stress τ can be obtained c .

[0063] In order to accurately simulate the movement and erosion of sediment on slope terrain, firstly, for each bed surface particle in the neighborhood particle set within its smooth radius, an approximate plane (i.e., representing the slope surface S') is fitted using the least squares method. Figure 1 is the local coordinate system of the slope surface S' where any particle on the bed is located. The angle relationship between the local coordinate and the global coordinate is as follows: Figure 2 The expanded equation for slope terrain is:

[0064]

[0065] In the formula, is the ratio of the critical shear stress of slope sediment to the horizontal critical shear stress, α is the angle between the S plane and the S' plane on the yz plane, β is the angle between the S plane and the S' plane on the xz plane, φ is the sediment repose angle, η is taken as 0.85, if the ratio of the calculated shear stress of the bottom particles to the horizontal sediment critical shear stress obtained by the Shields curve, It is considered that the bottom particles have reached the critical start-up state and will be transformed into turbid water particles and participate in the calculation of the NS equation. bi is the calculated shear stress of sediment:

[0066]

[0067] In the formula, κ is the Karman coefficient, which is 0.4, D is the diameter of the particle, and u is bi is the tangential velocity component of particle i.

[0068] S4, establish a physical model of the local scour test tank for the single pile foundation of offshore wind turbines. Figure 3 As shown in the figure, the total length of the experimental flume model is 35m, the width is 7m, and the depth is 1.6m. A 9m long area is set in the center of the flume as the scour test area. A layer of experimental fine sand with a depth of 0.60m is pre-filled in the scour test area. A cylindrical model is set in the center of the experimental area. An inflow area is set on the left side of the experimental flume and an outflow area is set on the right side to maintain water level balance. The number of experimental groups is not less than 5. The diameter of the cylindrical model is 0.6m, the water depth is 0.3-0.5m, and the average flow rate is 0.157-0.216m / s.

[0069] Table 1 Experimental groups and parameters

[0070]

[0071] S5, establish the corresponding ISPH numerical tank model. Figure 4As shown in the figure, according to the simulation requirements, in order to improve the calculation efficiency and facilitate the key areas to be observed, the size of the actual experimental area is adaptively adjusted. The initial parameters of the model are: the calculation area of ​​the numerical flume model is set to a length of 5.45m, which is 9 times the diameter of the cylindrical model, and the width is set to about 5m, which is 8.3 times the diameter of the cylindrical model. The particle diameter in the numerical model is set to 0.02m, and the inflow area is set on the left side of the model. A uniform flow rate is applied according to the actual experimental data to simulate the inflow of upstream water sources. On the right side of the model, an outflow area is configured to keep the water level in the entire model relatively constant. A series of numerical flow velocity measurement points are set according to different water depth positions along the cylindrical longitudinal section of the water flow direction to collect flow velocity data at corresponding positions during the simulation process.

[0072] S6, the experimental flume model and the numerical flume model respectively simulate the characteristics of the sediment scouring process around the cylinder. Using ISPH and combining the three-dimensional sediment starting model and the modified k-ε turbulence model, the numerical simulation results are compared with the experimental results to verify the accuracy of the flow field calculation and analyze the distribution of the flow field around the wind turbine single pile foundation. The flow field distribution characteristics around the cylinder are analyzed, including the flow velocity distribution, vortex structure and turbulent kinetic energy distribution. Finally, the characteristics of the sediment scouring process around the cylinder are verified and analyzed.

[0073] Specifically, the numerical model verification uses the water depth and average flow rate of the experimental group 1 of the experimental flume, and the scouring area at the bottom of the model is reduced to a prism shape, such as Figure 4 As shown in the red area in (b), the initial water level is preset above the bed surface, such as Figure 4 As shown in the blue part, the inflow area is set on the left side of the model, and the outflow area is set on the right side. At the same time, based on the experience of multiple debugging, 10 layers of particles are set in the inflow area and the outflow area respectively. The flow velocity adopts a vertical uniform inflow method. There is a sand pit in the central area for sediment erosion, and the depth of the sand pit is 0.6m.

[0074] The model verification is aimed at the longitudinal section of the cylinder along the water flow direction. A series of numerical flow velocity measurement points are set according to different water depth positions, as shown in Table 2, to record the flow velocity data at the corresponding positions during the simulation process.

[0075] Table 2 Flow velocity measurement point locations

[0076]

[0077] 1. Flow field verification

[0078] Figure 5The horizontal flow velocity at different depths of the cross section is shown. From the figure, it can be seen that the water flow velocity is significantly lower near the bottom, and the increasing trend of the flow velocity is roughly logarithmic with the water depth. The average flow velocity of the cross section is calculated to be 0.158m / s, which is close to the average flow velocity of 0.157m / s required by the experiment. Figure 5 The red dot is Figure 7 The specific flow rate changes are shown in Figure 2.

[0079] Figure 6 , Figure 7 The water depth variation curve and the flow velocity variation with time at different water depths are shown respectively. Figure 6 The water depth fluctuated greatly in the early and middle stages, which was caused by the instability of the water flow velocity when the numerical simulation was just started. However, the fluctuation gradually decreased after 5 seconds, and the water depth value stabilized after 15 seconds. In addition, the water depth remained above 0.3m after 5 seconds, which was consistent with the water depth required in the experimental data. Figure 7 (a) and (b) show the change of water velocity over time at a distance of 0.1m and 0.2m from the bottom, respectively. Figure 7 In the figure (a), the water velocity is relatively small because it is close to the bottom, which is consistent with the actual flow velocity. In the figure (b), the flow velocity fluctuation is relatively large at the beginning, but the fluctuation gradually decreases after 5s, and fluctuates around the flow velocity u = 0.157m / s. Although the fluctuation in the early stage of the numerical simulation is relatively large, the overall trend is consistent with the flow velocity conditions required by the experiment. Therefore, this numerical model can simulate a more realistic flow field, and the flow field simulated by this model can reproduce the flow field required by the experiment.

[0080] 2. Vorticity Verification

[0081] Vorticity is usually used to indicate the degree or speed of rotation of a fluid at its location. Under the action of water flow, the flow field before and after the cylinder will have a large structural difference. Due to the obstruction of the cylinder, a vortex will be formed around the cylinder, and the size and direction of the vortex are mainly expressed by the vorticity:

[0082]

[0083] Wherein, the two terms on the right side represent the partial derivative of the flow velocity u in the x direction in the y direction and the partial derivative of the flow velocity v in the y direction in the x direction respectively.

[0084] Figure 8Figure 2 is a diagram of the vorticity field around the cylinder, in which the red particles represent the positive vorticity and the blue particles represent the negative vorticity. On the rear side of the cylinder, the existence of the cylinder changes the flow path of the fluid, and the water flow is forced to converge to the inner side behind the column. This process is accompanied by a loss of kinetic energy and a decrease in flow velocity. When the water flows through the cylinder, its kinetic energy is not enough to maintain the original straight-line flow, and it will gradually break away from the cylinder and slowly spread out. At the same time, the water flow continuously replenishes the rear of the cylinder to fill the "blank" area formed by the obstruction of the cylinder, thus forming a tail vortex behind the column. At the same time, due to the inconsistent distribution of the water flow velocity in the vertical direction, the development of the tail vortex in the vertical direction is also different, resulting in the phenomenon that the tail vortex is small at the bottom and large at the upper layer, such as Figure 8 As shown in the cloud diagram of the vorticity around the cylinder, the axis of the tail vortex will bend backward.

[0085] Fig. 9 It is a streamline diagram at different times, showing the process of the tail vortex from generation to shedding. It can be seen that in the process of the fluid bypassing the cylinder, the vortex will first be generated near the side of the cylinder. As the fluid flows further downstream, these initially generated vortices will develop backward along the surface of the cylinder and form a distinct tail vortex structure behind the column. The tail vortex behind the column will go through a series of development stages: starting from the small-scale vortex initially formed, it will gradually increase as the distance from the cylinder increases. In this process, the vortex may also merge and break up, forming a series of vortex structures of different scales. Eventually, the larger vortices will gradually weaken and disappear due to internal friction and turbulent dissipation, and as they dissipate, new vortices will be generated due to the continuous replenishment of water flow to the space behind the column.

[0086] Fig.10 The formation process of the horseshoe vortex is shown. It can be clearly seen from the figure that there is a downward water flow in front of the cylinder. The flow velocity direction changes in the bottom area in front of the cylinder, forming a horseshoe vortex. It is the existence of the horseshoe vortex that causes strong fluid disturbance and high shear stress in the area in front of the obstacle. The formation of the horseshoe vortex shows that this numerical simulation can well simulate the vortex structure of the flow field and accurately reproduce the real flow field characteristics required by the experimental conditions, which provides an analytical basis for the subsequent sediment scour simulation process.

[0087] 3. Verification of turbulent kinetic energy

[0088] Turbulence generally refers to the irregular and disordered motion state of a fluid caused by various factors during its flow. In turbulence, the motion trajectory of fluid particles is extremely complex, and physical quantities such as velocity and pressure vary dramatically and randomly in space and time. It is generally believed that a fluid is in a turbulent state when the Reynolds number Re>4000. Turbulence phenomena involve the interaction between large-scale flow structures and small-scale vortices. The process of energy transfer from large scale to small scale is extremely complex, and the turbulent kinetic energy needs to be analyzed. This embodiment uses the above-mentioned modified k-ε turbulence model to perform numerical calculations of turbulence.

[0089] Fig.11 is a cloud diagram of turbulent kinetic energy distribution. The changing trend of turbulent kinetic energy is consistent with the changing trend of vorticity in the area in front of the cylinder, that is, with the development of water flow, vorticity and turbulent kinetic energy first increase, and with the development of the recirculation zone and the further flow of the fluid, vorticity and turbulent kinetic energy decrease accordingly. On the rear side of the cylinder, the distribution of turbulent kinetic energy is mainly affected by the shedding of the tail vortex. When the fluid bypasses the cylinder, a series of vortex structures will be formed behind the column. These vortices produce strong turbulence and high turbulent kinetic energy in the process of continuous development, rupture and reorganization. Therefore, the turbulent kinetic energy area behind the cylinder usually has strong turbulence, and due to the instability and periodic shedding of the tail vortex, the turbulent kinetic energy in this area is usually higher, and the impact on phenomena such as bed scouring is also more significant. With the development of water flow, due to the shedding of the tail vortex, the turbulent kinetic energy behind the column will gradually approach the rear of the column, such as Fig.11 As shown in Figures (e) and (f).

[0090] 4. Erosion process verification

[0091] The three-dimensional sediment initiation model was used to verify the sediment erosion process around the cylindrical foundation by taking the experimental group 1 of the experimental flume as an example. Fig.12 The detailed process of sediment erosion around the cylinder is shown. Fig.12 As shown in Figure (a) at t = 1.0s, the sediment erosion process of the cylinder first starts from the sides of the cylinder. This is because there is a streamline contraction effect on both sides of the cylinder. The water flow velocity here is greater than the flow velocity at other locations. Then it gradually spreads from both sides of the cylinder to the front of the cylinder. The sediment erosion phenomenon is roughly butterfly-shaped and symmetrically distributed on both sides of the cylinder until t = 4.0s when they meet in front of the cylinder and then begin to develop together. At the same time, due to the shielding effect of the cylinder at t = 4.0s and t = 5.0s, the degree of erosion behind the cylinder is very small. At t = 5.0s, the maximum sediment erosion position is located in the front side of both sides of the cylinder.

[0092] like Fig.13The longitudinal profile of the sediment erosion process shown in the figure shows that the sediment erosion process continues to deepen with the passage of time, but the scour depth in front of the cylinder at t=4.0s and t=5.0s is greater than the scour depth behind the cylinder. This is because due to the shielding effect of the cylinder, a downward water flow is formed in front of the cylinder, and the downward water flow will lead to the formation of a horseshoe vortex, which promotes the formation of a scour pit.

[0093] Comparing the results of numerical simulation with the experimental scour topography, it can be found that the numerical simulation results are in good agreement with the experimental results. The scour first occurs on both sides of the cylinder and then gradually develops to the front of the cylinder. Therefore, this model can better simulate the scour process around the cylinder.

[0094] The experimental group 2 of the experimental flume was used as the research object, and the numerical simulation results of the development of the scour depth in front, behind and to the side of the cylinder over time were compared with the experimental measurement data, such as Figures 14 to 16 As shown in the figure, it can be seen that the sediment erosion process of numerical simulation develops very quickly. In the numerical simulation results, the sediment erosion process basically reaches the maximum scouring depth in only 6 seconds and is in a balanced state, while it takes about 10,000 seconds to reach the same scouring depth in the experimental results. Therefore, the application of numerical simulation can greatly save time. Figures 14 to 16 It can be found that when the scouring time is only one-third of the time taken to reach the equilibrium state, the scouring depth is about half of the maximum scouring depth. As the scouring progresses, the scouring pit continues to develop. When the scouring time is 70% of the time taken to reach the equilibrium state, the scouring is basically completed. However, as the scouring process continues, the change in scouring depth becomes smaller and smaller, gradually tending to a stable state. Therefore, whether in front of, behind or to the side of the cylinder, the maximum scouring depth obtained by numerical simulation shows good consistency with the actual experimental results, and the gap between the two is within an acceptable range, showing that the numerical simulation method can well reproduce the sediment erosion process.

[0095] The method of simulating sediment erosion around offshore wind turbine foundations by combining ISPH with a modified k-ε turbulence model of the present invention well simulates the sediment erosion process around the single pile cylinder of offshore wind turbines, and analyzes the formation process of scour pits and the influence of scour pits on the vortex field, etc. Through ISPH numerical simulation, it is found that the scour process first starts on both sides of the cylinder and spreads to the front of the cylinder. In the early stage of scour, the contraction of streamlines on both sides of the cylinder causes the flow velocity on both sides of the cylinder to accelerate, which is the main reason for the formation of scour pits. In the scour development stage, the development of scour pits is mainly affected by the joint action of the horseshoe vortex in front of the column and the contraction of streamlines on both sides of the cylinder. In the equilibrium stage, the influence of the horseshoe vortex in front of the column and the contraction of streamlines on both sides of the cylinder on the scour pit is weakened, but the rear of the cylinder is still affected by the tail vortex behind the column, so that the scour pit behind the cylinder continues to develop.

[0096] It should be noted that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. It should be noted that those skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention, and therefore all improvements and modifications made without departing from the scope of the present invention are also within the protection scope of the present invention.

Claims

1. A method for simulating sediment erosion around offshore wind turbine foundations using ISPH combined with a modified k-ε turbulence model, characterized in that: The method is to use ISPH in combination with a sediment initiation model and a modified k-ε turbulence model to simulate the flow field characteristics and local scouring process experiment around an offshore wind turbine foundation. The method steps include: S1, establish the ISPH discretization algorithm for solving the ensemble average mass conservation equation and momentum conservation equation (i.e., NS equation) of fluid dynamics. The NS equation is expressed as follows: Where ρ0 is the fluid density, t is the time, u is the ensemble average velocity, g is the gravitational acceleration, ν0 is the kinematic viscosity coefficient, and p is the pressure. is the Reynolds stress tensor; S2, the standard k-ε turbulence model is modified to calculate the turbulent eddy viscosity ν in the Reynolds stress tensor expression t ; S3, introduce the critical sediment initiation criterion on the slope, modify and expand the traditional two-dimensional sediment initiation model, and establish a three-dimensional sediment initiation model; S4, establish a physical model of the water tank for the scour test of the single pile foundation of the offshore wind turbine; S5, establish the corresponding ISPH numerical tank model; S6, the experimental flume model and the numerical flume model respectively simulate the characteristics of the sediment scouring process around the cylinder, and the numerical simulation results are compared with the experimental results for verification and analysis.

2. The method for simulating sediment erosion around an offshore wind turbine foundation using ISPH combined with a modified k-ε turbulence model according to claim 1, characterized in that: In step S2, the turbulent kinetic energy k is defined at each particle i. i and energy dissipation rate ε i , for the turbulence existing in the ocean, the standard k-ε turbulence closed model has the following form: In the formula, σ k =1.0,σ ε =1.3, C ε1 =1.44, C ε2 =1.92, P represents the generation term of turbulent kinetic energy, t is time, and the turbulent eddy viscosity ν t To make corrections: In the formula, is the corrected dissipation rate, taking λ3=0.1, in represents the deformation rate tensor, represents the rotation rate tensor of the ensemble average flow field. The subscript 'initial' represents the time average and depth average of the first wave cycle in the potential flow region. 0initial =1.24s -2 , p Ωinitial =1.18×10 -8 s -2 , C μ , C1 and C2 are empirical constants. In this model, C μ =0.09, C1=1.33, C2=1.

92.

3. The method for simulating sediment erosion around an offshore wind turbine foundation using ISPH combined with a modified k-ε turbulence model according to claim 1, characterized in that: The S3 modifies and expands the traditional two-dimensional sediment initiation model, and the expanded equation is: In the formula, is the ratio of the critical shear stress of slope sediment to the horizontal critical shear stress, α is the angle between the S plane and the S' plane on the yz plane, β is the angle between the S plane and the S' plane on the xz plane, φ is the sediment repose angle, η is 0.85, if the ratio of the calculated shear stress of the bottom particles to the horizontal sediment critical shear stress obtained by the Shields curve is It is considered that the bottom particles have reached the critical start-up state and will be transformed into turbid water particles and participate in the calculation of the NS equation. b i is the calculated shear stress of sediment: In the formula, κ is the Karman coefficient, which is 0.4, D is the diameter of the particle, and u is bi is the tangential velocity component of particle i.

4. The method for simulating sediment erosion around an offshore wind turbine foundation using ISPH combined with a modified k-ε turbulence model according to claim 1, characterized in that: The total length of the experimental flume model described in step S4 is 35m, the width is 7m, and the depth is 1.6m. A 9m-long area is set in the central position of the flume as a scour test area. The scour test area is pre-filled with a layer of experimental fine sand with a depth of 0.60m. A cylindrical model with a diameter of 0.6m is set in the center of the experimental area; an inflow area is set on the left side of the experimental flume, and an outflow area is set on the right side to maintain water level balance; the number of experimental groups is not less than 5, the experimental water depth is 0.3m, and the average flow rate is 0.157-0.216m / s.

5. The method for simulating sediment erosion around an offshore wind turbine foundation using ISPH combined with a modified k-ε turbulence model according to claim 1, characterized in that: In step S5, in order to improve the calculation efficiency and facilitate the key area to be observed, the calculation area of ​​the numerical flume model is set to a length of 5.45m, which is 9 times the diameter of the cylindrical model, and the width is set to about 5m, which is 8.3 times the diameter of the cylindrical model. The particle diameter in the numerical model is set to 0.02m. An inflow area is set on the left side of the model, and a uniform flow rate is applied according to the actual experimental data to simulate the inflow of upstream water sources. On the right side of the model, an outflow area is configured to keep the water level in the entire model relatively constant. A series of numerical flow velocity measurement points are set according to different water depth positions along the longitudinal section of the cylinder in the direction of water flow to collect flow velocity data at corresponding positions during the simulation process.

6. The method for simulating sediment erosion around an offshore wind turbine foundation using the ISPH method according to claim 1 in combination with a modified k-ε turbulence model, characterized in that: The verification and analysis of step S6 include velocity distribution, vortex structure and turbulent kinetic energy distribution; and then the characteristics of the sediment scouring process around the cylinder are verified and analyzed.

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