Fluid-structure interaction method in offshore oil spill simulation emergency system

Abstract

The invention discloses a fluid-solid coupling interaction method in an offshore oil spill simulation emergency system. The fluid-solid coupling interaction method comprises the following steps: tracking and updating an interface of oil spill and an obstacle; calculating a volume fraction field occupied by an obstacle in a fluid-solid interaction area in the grid by using a wedge cutting method; and calculating the fluid-solid interaction force and updating the velocity field by using an immersed boundary method according to the volume fraction field occupied by the obstacle. According to the method, the immersed boundary method is adopted, calculation can be carried out only by using a simple regular grid, meanwhile, the calculation precision can be accurately controlled through the grid near the immersed boundary, and the calculation efficiency and the flexibility can be improved to a great extent through the immersed boundary method and the immersed boundary method. According to the method, the level set is adopted to provide high-precision interface description, the combination of the level set and the interface description is excellent in performance in a complex fluid-solid coupling scene, a high-precision real-time numerical simulation result can be provided in a dynamic complex fluid scene, and remarkable efficiency improvement is brought to the technical field of offshore oil spill simulation emergency systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of offshore oil spill simulation emergency systems, and particularly relates to a fluid-structure interaction method in an offshore oil spill simulation emergency system, which can be applied to the simulation of the initial stage of oil spill diffusion around and collision with various obstacles. Background Technique

[0002] In recent years, with the rapid development of ocean development and maritime transportation, offshore oil spill accidents occur frequently, posing a serious threat to the marine ecological environment and human society. Therefore, it has become a top priority to strengthen the emergency training of offshore oil spill response personnel. Using an offshore oil spill simulation emergency system for emergency training of staff is a good choice with low cost and high efficiency at present. However, a large amount of computing resources is required for fluid-structure coupling in offshore oil spill simulation, especially when the oil film spreads rapidly around in the initial stage of the simulation and contacts a large number of obstacles, the gap in computing resources becomes more prominent.

[0003] At present, the mainstream technologies for real-time simulation of fluid-structure coupling between oil spills and obstacles in offshore oil spill simulation mainly fall into two categories: traditional physical methods [1][2] and data-driven methods [3][4]. Although these methods perform well in specific fields, they still face many challenges when dealing with complex geometries, multi-scale dynamic interactions, and real-time requirements.

[0004] The deficiencies of traditional physical methods are as follows:

[0005] 1. Low computational efficiency: In dynamic boundary scenarios, grids need to be frequently regenerated to adapt to geometric changes, resulting in high computational costs, especially poor performance in real-time simulation.

[0006] 2. Lack of real-time performance: Traditional numerical methods are usually only applicable to offline simulation and cannot meet the real-time requirements of offshore oil spill simulation.

[0007] The deficiencies of data-driven methods are as follows:

[0008] 1. Lack of physical authenticity: Data-driven methods rely on training data, with insufficient performance in unseen scenarios, and the generated results may lack physical consistency.

[0009] 2. Weak adaptability: It is difficult to handle scenarios with complex geometric dynamic changes and multi-scale fluid-structure interactions, and both the accuracy and generalization ability are limited.

[0010] References

[0011] (1)Ferziger, J.H., & Peric, M. (2002). Computational Methods for Fluid Dynamics. Springer. https: / / doi.org / 10.1007 / 978-3-642-56026-2

[0012] (2)Turek, S., & Hron, J. (2006). Proposal for Numerical Bench - marking of Fluid - Structure Interaction between an Elastic Object and Laminar Incompressible Flow. Lecture Notes in Computational Science and Engineering, 53, 371 - 385.

[0013] (3)Fan, X., & Wang, J.X. (2024). Differentiable hybrid neural modeling for fluid - structure interaction. Journal of Computational Physics. [DOI: 10.1016 / j.jcp.2023.111233]

[0014] (4)Miyanawala, T.P., & Jaiman, R.K. (2019). A hybrid data - driven deep learning technique for fluid - structure interaction. Proceedings of the ASME 2019 International Conference on Offshore Mechanics and Arctic Engineering. [DOI: 10.1115 / OMAE2019 - 95071] Summary of the Invention

[0015] In order to solve the problem of insufficient computational efficiency of the offshore oil spill simulation emergency system, the present invention proposes a fluid - structure interaction method in the offshore oil spill simulation emergency system with high computational efficiency, high interaction accuracy, and good numerical stability.

[0016] A fluid - structure interaction method in an offshore oil spill simulation emergency system includes the following steps:

[0017] A. Tracking and updating the interface between the oil spill and obstacles

[0018] A1. Divide the offshore oil spill simulation area into rectangular grids.

[0019] A2. Use the level set function to mark the oil spill area, obstacle area, and the superimposed area of the interaction between the oil spill and obstacles in the grid cells. At the same time, depict the interface between the oil spill and obstacles through the zero level set method of the level set function. The obstacles include ships, offshore platforms, or islands. Consider the oil spill area as the fluid area, the obstacle area as the solid area, and the superimposed area of the interaction between the oil spill and obstacles as the fluid-structure interaction area.

[0020] A3. Solve the level set equation according to the level set function to update the interface.

[0021] B. Use the wedge cutting method to calculate the volume fraction field of obstacles in the fluid-structure interaction area in the grid.

[0022] C. Calculate the fluid-structure interaction force and update the velocity field using the immersed boundary method according to the volume fraction field of obstacles. The steps are as follows:

[0023] C1. Calculate the interaction force between the oil spill and obstacles according to the volume fraction field, and correct the fluid velocity field by the discrete forcing method.

[0024] C2. Update the position of the obstacles.

[0025] Furthermore, the definition of depicting the interface between the oil spill and obstacles through the zero level set method of the level set function in step A2 is as follows:

[0026] If φ(x, y, z, t) > 0, then the grid cell belongs to the oil spill and is defined as the fluid area;

[0027] If φ(x, y, z, t) < 0, then the grid cell belongs to the obstacle and is defined as the solid area;

[0028] If φ(x, y, z, t) = 0, then the grid cell is exactly located at the interface between the oil spill and obstacles.

[0029] Among them, φ(x, y, z, t) is the level set function, the x coordinate represents the horizontal direction of the simulation area, the y coordinate represents the vertical direction of the simulation area, the z coordinate represents the direction perpendicular to the x - y plane of the simulation area, and t represents the discrete time step at different times.

[0030] Furthermore, the level set equation for solving the level set equation according to the level set function to update the interface in step A3 is as follows:

[0031]

[0032] where \(v\) is the velocity field and \(\nabla\varphi\) is the gradient of the level set function, representing the change of the level set function over time.

[0033] Furthermore, the method for calculating the volume fraction field of obstacles in the superimposed area of oil spill and obstacles in the grid described in step B is as follows:

[0034] In Cartesian coordinates, the superimposed area of these oil spills and obstacles is cut into regular wedges according to the wedge cutting method, and the proportion of each side of the wedge in the grid cell is calculated using the zero level set and Cartesian coordinates, so as to approximately calculate the volume fraction field \(\alpha\) of the obstacles occupied s .

[0035] Furthermore, the method for calculating the fluid-structure interaction force and updating the velocity field using the immersed boundary method according to the volume fraction field of the obstacles occupied described in step C is as follows:

[0036] C1. After obtaining the volume fraction field in step B, calculate the interaction force between the oil spill and the obstacle according to the following formula:

[0037]

[0038] where \(f\) s is the interaction force between the oil spill and the obstacle. \(\alpha\) s is the volume fraction field of the obstacles occupied. is the velocity on the surface of the obstacle at the time step, representing the motion state of the obstacle surface at the current time step. is the predicted velocity field at time step \(n\). \(\Delta t\) is the time interval between two consecutive time steps.

[0039] Correct the fluid velocity by the discrete forcing method according to the following formula:

[0040]

[0041] where \(u\) n+1 is the velocity field at time step \(n + 1\). is the predicted velocity field at time step \(n\). C2. Calculate the force of the oil spill on the obstacle according to the following formula:

[0042] \(f\) n \(=-\rho\) f \(\int f\) s \(dV\)

[0043] Update the motion velocity of the obstacle according to the following formula:

[0044]

[0045] where \(f\) nThe force exerted by the oil spill on the obstacle at time step n, f n-1 The force exerted by the oil spill on the obstacle at time step n - 1, ρ f Is the density of the obstacle.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] 1. The present invention adopts the immersed boundary method, which only needs to use a simple regular grid for calculation. At the same time, the calculation accuracy can be precisely controlled through the grid near the immersed boundary, and both can greatly improve the calculation efficiency and flexibility. The present invention uses the level set to provide a high-precision interface description, and the combination of the two performs excellently in complex fluid-structure interaction scenarios, and can provide high-precision real-time numerical simulation results in dynamic complex fluid scenarios, bringing a significant improvement in efficiency to the technical field of offshore oil spill simulation emergency systems.

[0048] 2. The improved method of the present invention based on the wedge cutting method can simplify the calculation while ensuring a certain accuracy when calculating the volume fraction field at the junction of the oil spill and the obstacle, so as to achieve fast simulation. It can not only improve the calculation efficiency while retaining a high simulation accuracy, but also has strong adaptability and is applicable to more complex fluid-structure interaction problems, especially in real-time simulation and large-scale calculations.

[0049] 3. The implicit function form of the level set adopted by the present invention can naturally handle topological changes and avoid numerical instability, especially showing stability in scenarios of interface splitting and merging.

[0050] 4. Combined with GPU parallel acceleration, the present invention can meet the real-time simulation requirements while maintaining high precision when using Cartesian coordinates.

[0051] 5. In summary, the present invention has made a huge breakthrough in terms of efficiency while maintaining a certain accuracy in fluid-structure interaction problems, and is particularly suitable for real-time and efficient simulation in various simulation simulators. Description of the Drawings

[0052] Figure 1 Is the cutting method in which the regular solid boundary of the wedge shows a trapezoid in the two-dimensional case.

[0053] Figure 2 Is the cutting method in which the regular solid boundary of the wedge shows a triangle in the two-dimensional case.

[0054] Figure 3 Is the loss function cutting method shown by the irregular solid boundary of the wedge in the two-dimensional case. Detailed Embodiments

[0055] The present invention will be further described below in conjunction with the accompanying drawings. A fluid-structure interaction method in an offshore oil spill simulation emergency system includes the following steps:

[0056] A. First, divide the offshore oil spill simulation area into matrix grids, and then use the level set function to accurately describe the interface between the oil spill and the obstacles. A scalar field is defined in the form of an implicit function, where the zero level set is used to represent the surface of the obstacle, realizing the implicit definition of the interface position. This method is efficient and flexible, and can distinguish the obstacle area, the oil spill area and their interaction area through the sign and magnitude of the scalar field. During the simulation, in order to capture the movement or deformation caused by the force on the obstacle surface, the level set equation is solved through the level set function to dynamically update the interface position, so as to accurately describe the interaction behavior between the oil spill and the obstacle.

[0057] B. After determining the grid of the superimposed area where the oil spill and the obstacle interact, it is necessary to further calculate the volume fraction field of the obstacle contained in these superimposed areas. In these superimposed areas, the wedge-shaped fast cutting method is used to deal with the geometric segmentation problem of the grid. Specifically, by analyzing the distribution of the level set function inside the grid, it is judged whether the obstacle is penetrated by the zero level set, and it is divided into simpler geometric shapes, such as triangles or trapezoids.

[0058] The following will show an example of grid cutting in a unit superimposed area, using the fluid area to represent the oil spill and the solid area to represent the obstacle.

[0059] For regular solid shapes, the wedge cutting method can quickly calculate the boundary along the contour line and obtain high accuracy.

[0060] For irregular solid shapes, the calculation accuracy depends on the curvature of the solid boundary. In the case of large curvature, directly approximating the irregular boundary using the contour line may result in a certain loss of accuracy.

[0061] The main cutting idea is to calculate, according to the contour line where phi = 0, the area calculated by the proportion of the solid area on the four sides of the grid at the unit grid in Cartesian coordinates. The example cutting method is as Figure 1-2 shown:

[0062] For regular objects, the most common fluid-structure interaction in the superimposed area is often as shown in the figure. The gray solid area often interacts with at least one of the four sides of the unit Cartesian coordinates. Other contact methods can be generalized from these two methods.

[0063] And as shown in the figure, it is in the 2D case. It can also be generalized to 3D, but the situation will become complicated and it is necessary to calculate with multiple sides of the cube.

[0064] The specific calculation method is as follows:

[0065] As Figure 1 shown, the wedge of the fluid region appears as a trapezoid in 2D. Calculate the area of the solid trapezoid ABFE. For side CB of the unit grid in Cartesian coordinates, point F is where phi equals 0. So, |φB - φF| / |φB - φC| can quickly obtain the ratio of FB on side BC. Similarly, for side AD, point E is where phi is 0, and the ratio of AE on side AD can also be obtained in this way. Thus, EF can be quickly obtained based on the ratio, and together with AB, the solid volume fraction of the trapezoid ABFE in the unit grid can be obtained according to the trapezoid area formula.

[0066] As Figure 2 shown, the wedge of the fluid region appears as a triangle in 2D. Calculate the area of the fluid triangle AED. For side CD of the unit grid in Cartesian coordinates, point F is where phi equals 0. So, |φD - φE| / |φD - φC| can quickly obtain the ratio of DF on side DC, and then the ratio on side AE can be obtained according to the ratio. Thus, the ratio of triangle AED can be obtained. Subtract the area of the fluid domain from the area of square ABCD to obtain the solid volume fraction in the unit grid.

[0067] For the cutting method under irregular objects, as Figure 3 shown, in order to find a balance between simulation accuracy and computational efficiency, for such small irregular concave and convex regions, a loss function L is set. By connecting point I at the farthest distance of the isocontour, the area of triangle IHG directly approximates the concave and convex region, that is, the loss function L. Subtract this approximately loss function from the final calculated solid volume fraction part to approximate the true solid volume fraction as much as possible.

[0068] C. After obtaining the volume fraction field of the obstacle, use the immersed boundary method to calculate the interaction force between the oil spill and the obstacle and update the velocity field through the discrete forcing method. First, calculate the interaction force between it and the oil spill according to the volume fraction of the obstacle and the velocity difference between the obstacle and the oil spill. Subsequently, in the fluid-structure interaction region, obtain the discrete forcing term through the volume fraction field, and directly apply the discrete forcing term to the grid velocity field. After updating the velocity field, calculate the motion state of the obstacle and feedback it to its boundary position to ensure the accuracy of dynamic changes.

[0069] The present invention is not limited to this embodiment. Any equivalent concept or change within the technical scope disclosed in the present invention shall be included in the protection scope of the present invention.

Claims

1. A fluid-solid coupling interaction method in a marine oil spill simulation emergency system, characterized in that: The following steps are involved: A. Track and update the interface between the oil spill and the obstacle A1. Divide the offshore oil spill simulation area into rectangular grids; A2. Use the level set function to mark the oil spill area, the obstacle area, and the superposition area of ​​the interaction between the oil spill and the obstacle in the grid cells. At the same time, the interface between the oil spill and the obstacle is depicted by the zero level set method of the level set function. The obstacle includes a ship, an offshore platform or an island. The oil spill area is regarded as a fluid area, the obstacle area is regarded as a solid area, and the superposition area of ​​the interaction between the oil spill and the obstacle is regarded as a fluid-solid interaction area. A3. Solve the level set equation according to the level set function to update the interface; B. Use the wedge cutting method to calculate the volume fraction field occupied by obstacles in the fluid-solid interaction area of ​​the grid; C. Calculate the fluid-solid interaction force and update the velocity field using the immersed boundary method based on the volume fraction field occupied by the obstacle. The steps are as follows: C1. Calculate the interaction force between the oil spill and the obstacle based on the volume fraction field, and correct the fluid velocity field by discrete forcing method; C2. Update the position of the obstacle.

2. According to claim 1, a fluid-solid coupling interaction method in a marine oil spill simulation emergency system is characterized by: The definition of describing the interface between the oil spill and the obstacle by the zero level set method of the level set function in step A2 is as follows: If φ(x,y,z,t)>0, the grid cell belongs to the oil spill and is defined as the fluid region; If φ(x,y,z,t)<0, the grid cell belongs to an obstacle and is defined as a solid area; If φ(x, y, z, t) = 0, the grid cell is located exactly at the interface between the oil spill and the obstacle; Among them, φ(x, y, z, t) is the level set function, the x-coordinate represents the horizontal direction of the simulation area, the y-coordinate represents the vertical direction of the simulation area, the z-coordinate represents the direction of the simulation area perpendicular to the xy plane, and t represents the discrete time step at different times.

3. According to claim 1, a fluid-solid coupling interaction method in a marine oil spill simulation emergency system is characterized by: The level set equation for updating the interface by solving the level set equation according to the level set function in step A3 is as follows: Where v is the velocity field, ▽φ is the gradient of the level set function, Represents the change of the level set function over time.

4. According to claim 1, a fluid-solid coupling interaction method in a marine oil spill simulation emergency system is characterized by: The method for calculating the obstacle volume fraction field in the superposition area where the oil spill and the obstacle interact in the grid as described in step B is as follows: In Cartesian coordinates, the overlapping areas of the oil spill and the obstacle are cut into regular wedges according to the wedge cutting method. The proportion of each edge of the wedge in the grid unit is calculated using the zero level set and Cartesian coordinates, so as to approximate the volume fraction field α occupied by the obstacle. s .

5. According to claim 1, a fluid-solid coupling interaction method in a marine oil spill simulation emergency system is characterized by: The method described in step C for calculating the fluid-solid interaction force and updating the velocity field using the immersed boundary method according to the volume fraction field occupied by the obstacle is as follows: C1. Calculate the interaction force between the oil spill and the obstacle using the following formula after obtaining the volume fraction field in step B: In the formula, f s is the interaction force between oil spill and obstacle; α s is the volume fraction field occupied by obstacles; is the velocity of the obstacle surface at the time step, indicating the motion state of the obstacle surface at the current time step; is the predicted velocity field at time step n; Δt is the time interval between two consecutive time steps; The fluid velocity is corrected by the discrete forcing method according to the following formula: In the formula, u n+1 is the velocity field at time step n+1; is the predicted velocity field at time step n; C2. Calculate the force of the oil spill on the obstacle according to the following formula: f n =-ρ f ∫f s dV Update the obstacle's movement speed according to the following formula: In the formula, f n is the force exerted by the oil spill on the obstacle at time step n, f n-1 is the force exerted by the oil spill on the obstacle at time step n-1, ρ f is the obstacle density.

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