Load out-of-cabin process numerical simulation method based on dynamic nested grid motion

Abstract

The invention discloses a load out-of-cabin process numerical simulation method based on dynamic nested grid motion. Self-adaptive control is carried out on grid node motion by defining a UDF program of grid motion. The method has the advantages that the flow characteristics, especially the flow field characteristics of a boundary layer, in the load cabin-out process can be simulated more accurately by adopting a dynamic nested grid motion control technology; through self-adaptive control of grid node movement, the method effectively solves the problem of isolated units caused by hole digging processing, the improvement remarkably improves the grid quality and the numerical solution precision, it is ensured that the grid quality is kept in numerical simulation of the load cabin-out process, meanwhile, the accuracy and reliability of a simulation result are improved, and the method is suitable for large-scale popularization and application. The problem of solution distortion caused by isolated units at a boundary layer in a traditional nested grid technology is solved.

Description

Technical Field

[0001] The present invention relates to a numerical simulation method for the load out-of-cabin process based on dynamic nested grid motion, belonging to the technical field of air separation technology. Background Art

[0002] The load out-of-cabin system is a type of air multi-body separation system, and its specific forms include an unmanned aerial vehicle releasing a load, a spacecraft releasing a small satellite, and a rocket multi-stage separation, etc. This process involves complex coupling of aerodynamics and kinematics. The dynamic changes in the relative positions and velocities among multiple bodies may trigger various complex flow phenomena, such as shock wave interference, the interaction between shock waves and boundary layers, boundary layer separation, and vortex flow, etc. These phenomena result in highly nonlinear and unsteady characteristics of flow field parameters, causing significant interference to the aerodynamic and motion characteristics of the aircraft and its separation components, making the separation process a crucial and high-risk link in the flight mission. Although the separation process is very short, it determines the flight attitudes and velocities of the carrier and the load after separation, having a significant impact on whether the subsequent work can be successfully carried out. To ensure the safety of the flight mission, it is crucial to conduct a comprehensive and in-depth analysis of multi-body separation and motion problems. By carefully studying the flow field characteristics during the separation process, the motion characteristics of the separation components can be grasped more accurately, and the action mechanisms and influence laws of each key factor can be deeply understood.

[0003] For the problem of air multi-body separation with moving boundaries, the existing CFD numerical simulation methods are mainly implemented based on the dynamic grid technology. The traditional methods for dealing with moving boundaries generally include the DMS (Dynamic Modeling and Simulation) dynamic modeling and simulation method, the MDM (Moving / Deforming Mesh) moving / deforming mesh method, and the nested grid technology.

[0004] The DMS method calculates the steady-state aerodynamic forces by selecting some representative positions and attitudes during the separation process, fits the obtained discrete aerodynamic data into a continuous aerodynamic data curve through methods such as correction coefficients, and then inputs it into the dynamic model for rigid body motion simulation. However, although the DMS method has relatively low requirements for computing resources, it does not consider the unsteady effects brought by multi-body motion during the separation process. Moreover, the aerodynamic database of the DMS method is fitted from discrete steady-state data, while the actual process of air separation is a continuous transient process, and the calculation accuracy of this method is difficult to meet the overall quantitative research of the separation process.

[0005] The MDM method needs to frequently adjust the mesh during the boundary movement process to match the changing geometry. This may not only lead to a decline in mesh quality, such as mesh distortion or excessive stretching, but also trigger the emergence of negative volume elements, which may result in the failure of numerical solutions. In addition, the implementation of the MDM method is relatively complex, involving dynamic mesh updates, data transfer, and real-time adjustment of boundary conditions, all of which increase the computational difficulty and the demand for computing resources. Due to the dynamic changes of the mesh, additional numerical instabilities may also be introduced.

[0006] The idea of nested meshes is as follows: Meshes are independently generated for different components of the model. The meshes can move rigidly with the components, and the meshes of different components overlap with each other, and are coupled and solved in the form of interpolation. Nested meshes have strong independence, high quality, and fast generation speed. However, automatically and efficiently establishing the interpolation relationship of nested meshes is the key to the nested mesh method.

[0007] For the moving boundary problem during the payload's out-of-capsule process, the complexity of the separation stage often requires a fine simulation of the flow characteristics. The existing patent CN117436305A - A Numerical Simulation Method for Payload Out-of-Capsule Process Based on Dynamic Nested Mesh Technology uses CFD-6DOF to solve the problems that the DMS method does not consider the unsteady effects brought by multi-body motion during the separation process, and the aerodynamic database of the DMS method is fitted from discrete steady-state data, while the actual process of in-air separation is a continuous transient process, resulting in the computational accuracy being difficult to meet the overall quantitative research requirements of the separation process. It has higher flexibility and stability, but there are also the following problems: During the separation process, due to the too-close boundary gap between components and the existence of shear layers, after the nested meshes are processed by "hole cutting" (minimizing the overlapping area), there is an interpolation relationship between components at the boundary layer. To accurately solve the flow field characteristics of the boundary layer, the mesh element resolution of the capsule component is relatively high, resulting in a large difference in the element resolution of the two-component meshes at the boundary layer. It is difficult for the receptor component to retrieve suitable donor elements for interpolation at the boundary layer, forming isolated elements. The isolated elements in the boundary layer have a significant negative impact on the flow field solution, mainly manifested in the reduction of numerical stability, the distortion of flow field characteristics, the difficulty in correctly applying boundary conditions, and the change of local flow characteristics. Since the boundary layer plays a decisive role in the viscous resistance and heat exchange of the fluid, the existence of isolated elements may lead to misjudgment of the overall flow characteristics, especially in the case of amplified local effects, this impact is more significant. In addition, the limitations of numerical methods in dealing with the boundary layer and the mesh dependence problem make the existence of isolated elements further exacerbate the difficulty of the solution. Summary of the Invention

[0008] In view of the problem of solution distortion caused by isolated cells in the boundary layer of the traditional nested grid technology during the simulation of the load out-of-cabin process using the Fluent commercial software in engineering applications, the present invention provides a numerical simulation method for the load out-of-cabin process based on the motion of dynamic nested grids. The adaptive control of the grid node motion is realized through the UDF program of define_grid_motion, which makes up for the deficiency of the nested grid method in solving multi-body separation.

[0009] The present invention is realized through the following scheme: a numerical simulation method for the load out-of-cabin process based on the motion of dynamic nested grids, which adaptively controls the grid node motion through the UDF program for defining grid motion.

[0010] It includes the following steps:

[0011] S1. Before the simulation starts, define a set of numbers;

[0012] S2. After S1, following the advancement of the simulation time, determine whether the calculation time step is the first time step. If it is, enter S3; if not, directly enter S5;

[0013] S3. Traverse the wall cells of the moving block and record the cell resolution;

[0014] S4. After S3, traverse the nodes and update the array to form the nested grid node coordinate twins;

[0015] S5. Update the nodes according to the basic motion and synchronously update the twin array;

[0016] S6. After S5, calculate the projection distance of the node to the rear end face of the cabin in the motion direction;

[0017] S7. After S6, find the relative coordinate difference between the twin and the static block boundary, and complete the node update based on the coordinate difference.

[0018] The set of arrays defined in S1 is used to store the nested grid node coordinate information.

[0019] The set of arrays defined in S1 is implemented through the UDF program, with the base being C++. A variable array with a fixed dimension is defined through the basic syntax, and the dimension is determined according to the number of unit nodes.

[0020] If the relative coordinate difference between the twin and the static block boundary in S7 is 0, the twin coordinates are directly assigned to the nodes to complete the node update.

[0021] When the relative coordinate difference between the twin and the static block boundary in S7 is 0, the nested grid nodes are on the rear end face of the static block.

[0022] If the absolute value of the relative coordinate difference between the twin of S7 and the static block boundary is greater than 0 and less than or equal to 1 / 2 of the unit resolution, the updated node matches the static block boundary, and the node update is completed.

[0023] If the relative coordinate difference between the twin of S7 and the static block boundary is greater than 1 / 2 and not greater than 1 unit resolution or not less than -1 and less than -1 / 2 unit resolution, the updated node matches the nodes of the inner layer of cells of the static block boundary.

[0024] If the relative coordinate difference between the twin of S7 and the static block boundary is greater than 1 and not greater than 3 / 2 unit resolution or not less than -3 / 2 and less than -1 unit resolution, the updated node matches the nodes of the outer layer of cells of the static block boundary.

[0025] If the absolute value of the relative coordinate difference between the twin of S7 and the static block boundary is greater than 3 / 2 unit resolution, the twin coordinates are directly assigned to the node, and the node update is completed.

[0026] The beneficial effects of the present invention are as follows:

[0027] 1. A numerical simulation method for the load out-of-cabin process based on dynamic nested grid motion according to the present invention can more accurately simulate the flow characteristics during the load out-of-cabin process, especially the flow field characteristics of the boundary layer, by adopting the dynamic nested grid motion control technology.

[0028] 2. A numerical simulation method for the load out-of-cabin process based on dynamic nested grid motion according to the present invention effectively solves the problem of isolated cells caused by the "hole digging" process by adaptively controlling the motion of grid nodes. This improvement significantly improves the quality of the grid and the accuracy of numerical solutions, ensuring that the numerical simulation of the load out-of-cabin process not only maintains the grid quality but also improves the accuracy and reliability of the simulation results, and solves the problem of solution distortion caused by isolated cells at the boundary layer in traditional nested grid technology. Brief Description of the Drawings

[0029] Figure 1 is the flow chart of the process steps of the present invention.

[0030] Figure 2 is the schematic diagram of the two-dimensional grid division of the carrier in the embodiment of the present invention.

[0031] Figure 3 is the schematic diagram of the two-dimensional grid division of the load cabin in the embodiment of the present invention.

[0032] Figure 4 is the flow field pressure contour map at 0.1 s when the load out-of-cabin separation starts in the embodiment of the present invention.

[0033] Figure 5 is the flow field pressure contour map at 0.3 s when the load out-of-cabin separation starts in the embodiment of the present invention.

[0034] Figure 6 It is the flow field pressure contour map 0.5 s after the start of the separation of the payload out of the capsule in the embodiment of the present invention. Specific embodiments

[0035] The following will be combined with Figure 1-6 to further illustrate the present invention, but the protection scope of the present invention is not limited to the described content.

[0036] For clarity, not all features of the actual embodiments are described. In the following description, well-known functions and structures are not described in detail because they would obscure the present invention with unnecessary details. It should be considered that in the development of any actual embodiment, a large number of implementation details must be made to achieve the specific goals of the developer. For example, according to the relevant system or business limitations, changing from one embodiment to another. Additionally, it should be considered that such development work may be complex and time-consuming, but it is only routine work for those skilled in the art.

[0037] The present invention aims at the problem of solution distortion caused by isolated cells in the boundary layer of the traditional nested grid technology during the simulation of the payload out-of-capsule process using the fluent commercial software in engineering applications. A numerical simulation method for the payload out-of-capsule process based on the dynamic nested grid motion control technology is proposed. By defining the UDF program for grid motion, the motion of grid nodes is adaptively controlled to optimize the CFD-6DOF coupled solution of the payload out-of-capsule process. As Figure 1 、 Figure 2 and Figure 3 shown, the method of the present invention includes a background grid, a capsule nested grid, and a UDF program, and includes the following steps:

[0038] Step 1: Use DesignModeler to establish two-dimensional geometric models of the carrier and the payload capsule respectively. The carrier is 8 m long and 2 m wide; the payload capsule is 1.5 m long and 1 m wide.

[0039] Step 2: Use the nested grid method to divide the grid of the two-dimensional model. According to the motion of the separation system, the separation system is divided into two grid layers. Among them, the grid where the carrier is located is the background grid, and the grid where the payload capsule is located is the nested grid. Since the background grid has a lower priority in the calculation model and is responsible for providing receptor cells to receive the data interpolated and transmitted by the moving grid, the central area of the grid is the motion trajectory area of the separation system, and local encryption is performed to ensure the model accuracy. The grid size smoothly transitions from the wall surface to the far field and gradually becomes sparser.

[0040] Step 3: Based on the three-dimensional compressible unsteady Navier-Stokes equation and the SST K-Omega turbulence model, establish a physical model of the air flow during the separation process; it includes the following steps:

[0041] a: Establish a three-dimensional compressible unsteady N-S equation model, and its integral form of the governing equation is:

[0042]

[0043] Among them, W is the conserved variable, F, F v are the convective flux vector term and the dissipative flux vector term respectively, Ω is an arbitrary control volume, dS is the differential area, Re is the Reynolds number, and n is the outer normal vector of the control volume;

[0044] b: Use the finite volume method to discretely solve the flow governing equation, adopt the second-order upwind scheme for spatial discretization on the computational grid, and use the k-th order backward difference discretization in the time direction.

[0045] Step 4: Import the grid files of the pressure far field and the load cabin into Fluent respectively, and perform the following settings in sequence in Fluent for steady-state calculation:

[0046]

[0047] Step 5: After calculating 800 steps, judge whether the entire flow field converges according to whether the residuals and aerodynamic coefficients converge.

[0048] Step 6: After the steady-state calculation is completed, unsteady calculation needs to be carried out on the basis of the steady flow field of the steady-state calculation. At the same time, the written UDF file is imported for unsteady state setting. It is also possible to define the motion form of the required moving body by adding code in the UDF program. In this embodiment, it is defined that the load cabin moves away at a speed of 0.1 m / s relative to the carrier at the beginning of the unsteady calculation. As Figure 1 shown, the writing method of the UDF file is as follows:

[0049] S1. Before the simulation starts, define a set of numbers to store the nested grid node coordinate information;

[0050] S2. After S1, follow the simulation time advancement, judge whether the calculation time step is the first time step. If it is, enter S3. If not, directly enter S5;

[0051] S3. Traverse the wall cells of the moving block and record the cell resolution;

[0052] S4. After S3, traverse the nodes and update the array to form a nested grid node coordinate twin;

[0053] S5. Update the nodes according to the basic motion and synchronously update the twin array;

[0054] S6. After S5, calculate the projection distance of the node to the rear end face of the cabin in the motion direction;

[0055] S7. After S6, calculate the relative coordinate difference between the twin and the static block boundary, and complete the node update based on the coordinate difference.

[0056] If the relative coordinate difference is 0, directly assign the twin coordinates to the node to complete the node update.

[0057] When the relative coordinate difference is 0, the nested grid node is on the back face of the static block.

[0058] If the absolute value of the relative coordinate difference is greater than 0 and less than or equal to 1 / 2 of the element resolution, update the node to match the static block boundary to complete the node update.

[0059] If the relative coordinate difference is greater than 1 / 2 and not greater than 1 unit resolution or not less than -1 and less than -1 / 2 unit resolution, update the node to match the nodes of the inner layer of elements on the static block boundary.

[0060] If the relative coordinate difference is greater than 1 and not greater than 3 / 2 unit resolution or not less than -3 / 2 and less than -1 unit resolution, update the node to match the nodes of the outer layer of elements on the static block boundary.

[0061] If the absolute value of the relative coordinate difference is greater than 3 / 2 unit resolution, directly assign the twin coordinates to the node to complete the node update.

[0062] Step 7. Set the calculation time step to 0.01, and the calculation time step is 60 steps.

[0063] Step 8. After the calculation, check the variation characteristics of the flow field pressure during the hatch-opening process.

[0064] Figure 4 、 5 Figures 6 are the flow field pressure nephograms at 0.1 s, 0.3 s, and 0.5 s when the separation starts. In the early stage of separation, the carrier and the payload compartment are in the same pressure field. Due to the compression of air at the head of the carrier, a certain high-pressure area is formed. The air density on both sides of the carrier decreases, resulting in a decrease in pressure and the formation of a shock wave. As time progresses, the shock wave spreads rapidly, and the low-pressure area extends backward, covering the hatch-opening process.

[0065] The results show that the grid control technology based on the UDF program can well simulate the payload hatch-opening process; during the simulation process, no isolated elements are generated in the boundary layer, and the results are in good agreement with the actual situation.

[0066] Although the technical solutions of the present invention have been described in detail and listed, it should be understood that for those skilled in the art, making modifications to the above embodiments or adopting equivalent alternative solutions are obvious to those skilled in the art. These modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection required by the present invention.

Claims

1. A numerical simulation method for load out-of-cabin process based on dynamic nested grid motion, characterized in that: The mesh node motion is adaptively controlled by the UDF program that defines the mesh motion.

2. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 1 is characterized in that: The following steps are involved: S1. Before the simulation starts, define a set of numbers; S2, after S1, follow the simulation time to determine whether the calculation time step is the first time step, if so, enter S3, if not, directly enter S5; S3, traverse the wall units of the moving block and record the unit resolution; S4, after S3, traverse the nodes and update the array to form nested grid node coordinate twins; S5. Update the nodes according to the basic motion and update the twin array synchronously; S6. After S5, calculate the projection distance from the node to the rear end surface of the cabin in the direction of motion; S7. After S6, calculate the relative coordinate difference between the twin and the static block boundary, and complete the node update based on the coordinate difference.

3. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2 is characterized in that: The S1 defines a set of arrays used to store nested grid node coordinate information.

4. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2 is characterized in that: The S1 defines a set of arrays, which are implemented through a UDF program with a base of C++. A variable array with a fixed dimension is defined through basic syntax, and the dimension is determined according to the number of unit nodes.

5. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2 is characterized in that: The relative coordinate difference between the twin and the static block boundary of S7 is 0, so the twin coordinates are directly assigned to the node to complete the node update.

6. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2 is characterized in that: When the relative coordinate difference between the twin of S7 and the boundary of the quiet block is 0, the nested grid node is located on the rear end surface of the quiet block.

7. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2 is characterized in that: If the absolute value of the relative coordinate difference between the twin of S7 and the boundary of the static block is greater than 0 and less than and equal to 1 / 2 of the unit resolution, the node is updated to match the boundary of the static block, and the node update is completed.

8. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2 is characterized in that: If the relative coordinate difference between the twin of S7 and the boundary of the quiet block is greater than 1 / 2 and not greater than 1 unit resolution or not less than -1 and less than -1 / 2 unit resolution, the node is updated to match the unit node of one layer inside the boundary of the quiet block.

9. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2 is characterized in that: If the relative coordinate difference between the twin of S7 and the boundary of the quiet block is greater than 1 and not greater than 3 / 2 unit resolution or not less than -3 / 2 and less than -1 unit resolution, the node is updated to match the unit node of one layer outside the boundary of the quiet block.

10. The method for numerical simulation of load out-of-cabin process based on dynamic nested grid motion according to claim 2, characterized in that: If the absolute value of the relative coordinate difference between the twin and the static block boundary of S7 is greater than 3 / 2 unit resolution, the twin coordinates are directly assigned to the node to complete the node update.

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