Coupling methods of aerodynamics and vehicle dynamics applicable to high-speed trains

Abstract

This invention discloses a coupling method for aerodynamics and vehicle dynamics applicable to high-speed trains, relating to the interdisciplinary field of aerodynamics and vehicle dynamics. The method includes: establishing separate aerodynamic and vehicle dynamic models for the high-speed train; exchanging data between the two models using a dual-time-step time-progression method, improving computational time accuracy, step size, and stability. While comprehensively considering the interaction between the high-speed train and its surrounding flow field, the influence of track irregularities, and tunnel effects, a multi-region mesh generation method is adopted, improving the quality and efficiency of mesh generation. Furthermore, the overlapping mesh method avoids mesh reconstruction problems. Simultaneously, the use of six-degree-of-freedom motion velocity to reflect the attitude changes of the high-speed train simplifies the coupling steps. The entire process saves time on mesh generation, calculating the displacement of mesh nodes around the train, mesh reconstruction, and computational iterations, enabling the coupling method to be implemented quickly and on a large scale.

Description

Technical Field

[0001] This application relates to the interdisciplinary field of aerodynamics and vehicle dynamics, and more specifically, to a coupling method of aerodynamics and vehicle dynamics applicable to high-speed trains. Background Technology

[0002] High-speed trains offer a convenient, fast, environmentally friendly, punctual, and safe mode of transportation, making them a priority for many countries. With social progress and economic development, the flow of people between cities will experience explosive growth. Coupled with the uneven distribution of population and resources in my country, increasing train speeds is crucial for alleviating the pressure on passenger and freight transport and promoting economic and cultural exchange. High-speed trains operate on fixed tracks, have a much larger aspect ratio than other modes of transportation, resulting in a stronger ground effect. Furthermore, increased speed brings a series of new challenges, such as more pronounced aerodynamic effects around the train and intensified wheel-rail interaction. In addition, my country is a mountainous country with a relatively high ratio of railway tunnels, resulting in numerous tunnels along the lines. Passing through tunnels generates tunnel entrance / exit effects, aerodynamic drag, and sudden pressure changes. These factors affect the dynamic performance of trains during operation, and fluid-structure interaction becomes increasingly significant.

[0003] Currently, aerodynamic loads are often applied as time-varying external loads, without considering the effect of vehicle attitude changes on the surrounding flow field, and thus failing to reflect the interaction between aerodynamics and vehicle dynamics. High-speed train coupled systems include wheel-rail interaction exacerbated by track irregularities, fluid-structure interaction caused by traveling winds and ambient winds, and pantograph-catenary interaction. With increased train speeds, aerodynamic effects become more pronounced, and responses to various disturbances become more sensitive. The interactions between high-speed trains and the track, air, and overhead contact system intensify, necessitating in-depth research using new methods and approaches. Developing rapid, large-scale aerodynamic-vehicle dynamics coupling methods that consider the interaction between the vehicle and its surrounding air, the impact of track irregularities, and tunnel effects is essential.

[0004] Existing methods for coupling aerodynamics and vehicle dynamics for high-speed trains still have some shortcomings. For example, the patent application "A tight coupling method for aerodynamics and multibody dynamics for high-speed trains" (application number: 201910521024.4) proposes a method that can couple the two disciplines for calculation. However, it uses displacement to reflect changes in train attitude. The coupling process requires not only the conversion from the vehicle's six-degree-of-freedom displacement to the fluid's three-degree-of-freedom displacement, but also the prior selection of control points to calculate the displacement of all flow field grid nodes. The coupling process is cumbersome and not conducive to large-scale coupling calculations. Moreover, the selected dynamic mesh method is a combination of the layered dynamic mesh method and the viscous mesh deformation method. Not only is the motion process complex, but there is also the problem of mesh reconstruction, resulting in a long calculation time. Summary of the Invention

[0005] In view of this, this application provides a coupling method for aerodynamics and vehicle dynamics applicable to high-speed trains. While comprehensively considering the interaction between the high-speed train and the surrounding flow field, the influence of track irregularities, and tunnel effects, it adopts multi-region mesh generation to decompose the complex geometric problem into a simple system with overlapping meshes. This makes it easier to control mesh quality, improves the quality and efficiency of mesh generation, and eliminates the mesh reconstruction problem. The mesh quality remains unchanged during mesh movement, and the calculation is easy to converge. At the same time, it uses the six-degree-of-freedom motion velocity to reflect the attitude change of the high-speed train, avoiding the conversion between the vehicle's six-degree-of-freedom displacement and the fluid's three-degree-of-freedom displacement. It eliminates the need to pre-select control points and use the displacement of control points to calculate the flow field mesh node data around the train, simplifying the coupling steps and saving the time overhead of mesh generation, calculating the displacement of mesh nodes around the train, mesh reconstruction, and calculation iteration, enabling the coupling method to be performed quickly and on a large scale.

[0006] In a first aspect, this application provides a coupling method for aerodynamics and vehicle dynamics applicable to high-speed trains, comprising:

[0007] An aerodynamic model of a high-speed train is established, which includes component flow field and background flow field. Component flow field is the flow field around the high-speed train, and background flow field is the flow field inside and outside the tunnel.

[0008] A vehicle dynamics model for high-speed trains is established, and the excitations in the vehicle dynamics model include track irregularities and aerodynamic loads.

[0009] The data between the aerodynamic model and the vehicle dynamics model is exchanged using a dual-time-step time advancement method.

[0010] The method of exchanging data between the aerodynamic model and the vehicle dynamics model using a dual-time-step time advancement includes:

[0011] Divide a physical time step into sub-iterations with a predetermined precision or a predetermined number of iterations;

[0012] In each sub-iteration, the aerodynamic model is calculated to obtain the aerodynamic load, and the aerodynamic load is transferred to the vehicle dynamics model;

[0013] Calculate the six-degree-of-freedom motion velocity of the vehicle dynamics model under aerodynamic loads, and transfer the six-degree-of-freedom motion velocity to the aerodynamic model;

[0014] The aerodynamic model is updated using a mesh motion method that combines the overlapping mesh method and the custom dynamic mesh method, and then the calculation proceeds to the next sub-iteration.

[0015] When the computational accuracy reaches the predetermined accuracy, or the number of iterations reaches the predetermined number, proceed to the next physical time step.

[0016] Optionally, where:

[0017] An aerodynamic model of the high-speed train is established, which includes the component flow field and the background flow field. The component flow field is the flow field surrounding the high-speed train, and the background flow field is the flow field inside and outside the tunnel, including:

[0018] Establish a three-dimensional fluid model of the high-speed train;

[0019] The component flow fields and background flow fields of the three-dimensional fluid model of the high-speed train are meshed as follows:

[0020] A hybrid mesh generation method was used to mesh the flow field around the high-speed train, which was then used as a component mesh.

[0021] The flow field inside and outside the tunnel was meshed using a hexahedral meshing method, which served as the background mesh.

[0022] Assemble the component grid and background grid, and define the overlapping grid interface of the component grid and background grid;

[0023] The mesh motion method for component meshes and background meshes is defined as a combination of overlapping mesh method and custom dynamic mesh method;

[0024] Complete the aerodynamic model of the high-speed train.

[0025] Optionally, where:

[0026] A hybrid mesh generation method is used to mesh the flow field surrounding the high-speed train. The specific component mesh is as follows:

[0027] The boundary layer mesh is obtained by meshing the part of the flow field around the high-speed train near the surface of the high-speed train using the prismatic meshing method; the outermost boundary layer mesh is obtained by meshing the part of the flow field around the high-speed train near the inside and outside of the tunnel using the hexahedral meshing method; and the outer boundary layer mesh is obtained by meshing the remaining part of the flow field around the high-speed train using the tetrahedral meshing method.

[0028] The boundary layer mesh, the outermost boundary layer mesh, and the outermost boundary layer mesh together constitute the component mesh;

[0029] The boundary layer mesh has 6 to 10 mesh layers, and the outermost mesh of the boundary layer has 6 to 10 mesh layers.

[0030] Optionally, where:

[0031] The mesh motion method for defining the component mesh and the background mesh is a combination of overlapping mesh method and custom dynamic mesh, including:

[0032] The mesh movement method of the component mesh relative to the background mesh is defined as the overlapping mesh method;

[0033] A custom mesh motion method for component meshes is achieved through motion velocities with six degrees of freedom.

[0034] Optionally, where:

[0035] The three-dimensional fluid model also includes a three-dimensional model of the high-speed train, establishing a vehicle dynamics model for the high-speed train. The excitations in the vehicle dynamics model include track irregularities and aerodynamic loads.

[0036] Based on the three-dimensional model of the high-speed train, vehicle dynamics parameters, and track dynamics parameters, a multi-level substructure modeling method is used to establish the basic vehicle dynamics model of the high-speed train.

[0037] By applying track irregularities as excitations to the track and aerodynamic loads as force elements to the high-speed train, the basic model of vehicle dynamics of the high-speed train is improved.

[0038] Complete the establishment of a vehicle dynamics model for high-speed trains.

[0039] Optionally, where:

[0040] In each sub-iteration, the aerodynamic load is calculated from the aerodynamic model, and then transferred to the vehicle dynamics model. Specifically:

[0041] In each sub-iteration, the aerodynamic model is calculated to obtain the aerodynamic loads, and the aerodynamic loads are saved to a data file;

[0042] Vehicle dynamics model reads data file.

[0043] Optionally, where:

[0044] The calculation of the six-degree-of-freedom motion velocity of the vehicle dynamics model under aerodynamic loads, and the transfer of the six-degree-of-freedom motion velocity to the aerodynamic model, are specifically as follows:

[0045] Calculate the six-degree-of-freedom motion velocity of the vehicle dynamics model under aerodynamic loads and save the six-degree-of-freedom motion velocity to a data file;

[0046] The aerodynamic model reads the data file.

[0047] Optionally, where:

[0048] Aerodynamic loads are permutation variables, and the vehicle dynamics model reads the data file by overriding the permutation variables.

[0049] Compared with existing technologies, the coupling method for aerodynamics and vehicle dynamics of high-speed trains provided in this application achieves at least the following beneficial effects:

[0050] (1) The aerodynamic and vehicle dynamic coupling method for high-speed trains provided in this application embodiment is to add the influence of the change of the high-speed train vehicle attitude on the surrounding flow field to the coupling calculation by exchanging data between the aerodynamic model and the vehicle dynamic model of the high-speed train; by setting the flow field inside and outside the tunnel as the background flow field, and adding track irregularities and aerodynamic loads as excitations to the vehicle dynamic model of the high-speed train, the tunnel effect and track irregularities are added to the coupling calculation. It can be seen that the embodiment of this application comprehensively considers the interaction between the high-speed train and the surrounding air, the influence of track irregularities and tunnel effects, and is more comprehensive, which is convenient for studying the interaction between the high-speed train and various excitations such as air and track after the speed increase.

[0051] (2) The aerodynamic and vehicle dynamics coupling method for high-speed trains provided in this application adopts multi-region mesh generation, decomposing complex geometric problems into simple systems with overlapping meshes, making it easier to control mesh quality and improving the quality and efficiency of mesh generation. During mesh movement, each component mesh only has overall six degrees of freedom motion, and there is no relative motion between internal meshes, so there is no mesh reconstruction problem and the mesh quality will not be reduced. This has obvious advantages for large-scale problems. The flow field mesh has high quality and is easy to converge during calculation, which helps to reduce the number of calculation iterations and save calculation time, thereby realizing fast and large-scale coupled calculation.

[0052] (3) The aerodynamic and vehicle dynamics coupling method for high-speed trains provided in this application embodiment uses the six-degree-of-freedom motion speed of the high-speed train to reflect the attitude change of the high-speed train, and directly transfers the six-degree-of-freedom motion speed between the aerodynamic model and the vehicle dynamics model. Compared with the existing method of using displacement to reflect the attitude change of the high-speed train, it avoids the conversion between the six-degree-of-freedom displacement of the vehicle and the three-degree-of-freedom displacement of the fluid. At the same time, it does not require the prior selection of control points, nor does it require the calculation of the displacement of all flow field grid nodes around the train using the displacement of control points. It only gives the six-degree-of-freedom motion speed to each component grid as a whole, which simplifies the coupling method of high-speed train aerodynamics and vehicle dynamics, greatly reduces the time required for coupling calculation, and has obvious advantages for large-scale problems, further realizing fast and large-scale coupling calculation.

[0053] (4) The aerodynamic and vehicle dynamic coupling method for high-speed trains provided in this application uses the overlapping mesh method and the custom dynamic mesh method as the mesh motion method. Compared with other mesh motion methods, the custom dynamic mesh method can be well matched with the local mesh motion of the air around the train caused by the attitude change of the high-speed train. This application embodiment makes full use of the advantages of the two mesh motion methods, and can quickly and well complete the mesh motion that combines the overall large-scale longitudinal motion and local small-scale attitude change of the high-speed train in aerodynamics.

[0054] (5) The coupling method for aerodynamics and vehicle dynamics of high-speed trains provided in this application is more likely to converge than the direct coupling method of solving large equations of two disciplines at the same time, and meets the engineering requirements when aerodynamics and structure interact violently. Compared with unidirectional coupling and loose coupling methods, the coupling method provided in this application has higher accuracy. When the aerodynamics and vehicle dynamics solutions reach second-order accuracy, the accuracy of the coupling calculation is also second-order, which can improve the accuracy of first-order calculation time.

[0055] (6) The coupling method of aerodynamics and vehicle dynamics for high-speed trains provided in this application is not only applicable to the coupling calculation of aerodynamics and vehicle dynamics of high-speed trains, but also applicable to other large-scale situations where fluid and rigid body interactions are intense.

[0056] Of course, any product implementing this application does not necessarily need to achieve all of the technical effects described above at the same time.

[0057] Other features and advantages of this application will become clear from the following detailed description of exemplary embodiments with reference to the accompanying drawings. Attached Figure Description

[0058] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments of the present application and, together with their description, serve to explain the principles of the present application.

[0059] Figure 1 The diagram shown is a flowchart of a coupling method for aerodynamics and vehicle dynamics applicable to high-speed trains provided in an embodiment of this application;

[0060] Figure 2 The diagram shown is a schematic representation of the aerodynamic model provided in an embodiment of this application;

[0061] Figure 3 The diagram shown is a schematic representation of the background grid and component grid provided in an embodiment of this application;

[0062] Figure 4 The diagram shown is a schematic diagram of the component mesh provided in an embodiment of this application;

[0063] Figure 5 The diagram shown is a schematic representation of the vehicle dynamics model provided in an embodiment of this application;

[0064] Figure 6 The diagram shown is a topology diagram of the main model of the high-speed train provided in an embodiment of this application.

[0065] Figure 7 The diagram shows a flowchart of data exchange between the aerodynamic model and the vehicle dynamics model provided in this embodiment of the application.

[0066] Figure 8 The figure shows the aerodynamic lift variation curves under different coupling methods provided in the embodiments of this application;

[0067] Figure 9 The figure shows the variation curves of vertical acceleration under different coupling methods provided in the embodiments of this application;

[0068] Figure 10 The figure shows the vertical displacement variation curves under different coupling methods provided in the embodiments of this application;

[0069] Figure 11 The figure shows the variation curves of lateral displacement under different coupling methods provided in the embodiments of this application. Detailed Implementation

[0070] Various exemplary embodiments of the present application will now be described in detail with reference to the accompanying drawings. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the present application.

[0071] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the scope of this application and its application or use.

[0072] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0073] In all the examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0074] It should be noted that similar labels and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be discussed further in subsequent figures.

[0075] High-speed trains offer a convenient, fast, environmentally friendly, punctual, and safe mode of transportation, making them a priority for many countries. With social progress and economic development, the flow of people between cities will experience explosive growth. Coupled with the uneven distribution of population and resources in my country, increasing train speeds is crucial for alleviating the pressure on passenger and freight transport and promoting economic and cultural exchange. High-speed trains operate on fixed tracks, have a much larger aspect ratio than other modes of transportation, resulting in a stronger ground effect. Furthermore, increased speed brings a series of new challenges, such as more pronounced aerodynamic effects around the train and intensified wheel-rail interaction. In addition, my country is a mountainous country with a relatively high ratio of railway tunnels, resulting in numerous tunnels along the lines. Passing through tunnels generates tunnel entrance / exit effects, aerodynamic drag, and sudden pressure changes. These factors affect the dynamic performance of trains during operation, and fluid-structure interaction becomes increasingly significant.

[0076] Currently, aerodynamic loads are often applied as time-varying external loads, without considering the effect of vehicle attitude changes on the surrounding flow field, and thus failing to reflect the interaction between aerodynamics and vehicle dynamics. High-speed train coupled systems include wheel-rail interaction exacerbated by track irregularities, fluid-structure interaction caused by traveling winds and ambient winds, and pantograph-catenary interaction. With increased train speeds, aerodynamic effects become more pronounced, and responses to various disturbances become more sensitive. The interactions between high-speed trains and the track, air, and overhead contact system intensify, necessitating in-depth research using new methods and approaches. Developing rapid, large-scale aerodynamic-vehicle dynamics coupling methods that consider the interaction between the vehicle and its surrounding air, the impact of track irregularities, and tunnel effects is essential.

[0077] To address the aforementioned technical problems, this application proposes a coupling method for aerodynamics and vehicle dynamics applicable to high-speed trains. While comprehensively considering the interaction between the high-speed train and its surrounding flow field, the influence of track irregularities, and tunnel effects, it employs multi-region mesh generation, decomposing the complex geometric problem into a simpler system with overlapping meshes. This makes it easier to control mesh quality, improving the quality and efficiency of mesh generation. Furthermore, the overlapping mesh method avoids mesh reconstruction problems, ensuring mesh quality remains constant during mesh movement and facilitating computational convergence. Simultaneously, it utilizes a six-degree-of-freedom (6DOF) velocity to reflect the attitude changes of the high-speed train, avoiding the conversion from the vehicle's 6DOF displacement to the fluid's 3DOF displacement. This eliminates the need for pre-selecting control points and calculating the flow field mesh node data around the train using control point displacements, simplifying the coupling steps and saving time on mesh generation, calculating the displacements of mesh nodes around the train, mesh reconstruction, and computational iterations. This allows the coupling method to be implemented quickly and on a large scale. The following detailed description, in conjunction with the accompanying drawings and specific embodiments, provides further insights.

[0078] Figure 1 The diagram shown is a flowchart of a coupling method for aerodynamics and vehicle dynamics applicable to high-speed trains provided in an embodiment of this application; Figure 2 The diagram shown is a schematic representation of the aerodynamic model provided in an embodiment of this application; Figure 3 The diagram shown is a schematic representation of the background grid and component grid provided in an embodiment of this application; Figure 4 The diagram shown is a schematic diagram of the component mesh provided in an embodiment of this application.

[0079] Please see Figures 1-4 The coupling method for aerodynamics and vehicle dynamics of high-speed trains proposed in this application includes:

[0080] S10. Establish an aerodynamic model for the high-speed train. The aerodynamic model includes component flow fields and background flow fields. The component flow fields are the flow fields surrounding the high-speed train, and the background flow fields are the flow fields inside and outside the tunnel.

[0081] Optionally, an aerodynamic model of the high-speed train 30 is established. The aerodynamic model includes the component flow field and the background flow field. The component flow field is the flow field surrounding the high-speed train 30, and the background flow field is the flow field inside and outside the tunnel, including:

[0082] Establish a three-dimensional fluid model of high-speed train 30;

[0083] The component flow fields and background flow fields of the three-dimensional fluid model of high-speed train 30 are meshed as follows:

[0084] A hybrid mesh generation method was used to mesh the flow field around the high-speed train 30, which was then used as component mesh 20.

[0085] The flow field inside and outside the tunnel was meshed using a hexahedral meshing method, and the background mesh 10 was used.

[0086] Assemble component grid 20 and background grid 10, and define the overlapping grid interface of component grid 20 and background grid 10;

[0087] The mesh motion method for component mesh 20 and background mesh 10 is defined as a combination of overlapping mesh method and custom dynamic mesh method;

[0088] Complete the aerodynamic model of the high-speed train 30.

[0089] Based on this, please refer to Figures 1-4 When establishing the three-dimensional fluid model of the high-speed train 30, due to the large size of the train body, the aerodynamic load has a significant impact on its attitude change but a relatively small impact on its deformation. Therefore, the train body can be considered as a rigid body, taking into account the interaction between air and the attitude change of the train body, ignoring the deformation of the solid region, and only considering the motion of the flow field mesh caused by the attitude change. For a single train body, motion in six degrees of freedom is considered, which facilitates the direct transfer of motion velocity with six degrees of freedom between the aerodynamic model and the vehicle dynamics model. When meshing the component flow field and background flow field of the three-dimensional fluid model of the high-speed train 30, a hybrid meshing method can be used to mesh the flow field around the high-speed train 30. This allows meshes at different locations in the component mesh 20 to be meshed using different meshing methods, which can fully consider surface viscous forces, better simulate boundary layer effects, and facilitate mesh generation and control. The flow field inside and outside the tunnel has a regular and uniform shape, making it easy to generate high-quality hexahedral meshes. Therefore, a hexahedral meshing method can be used to mesh the area, resulting in a more regular mesh that is easier to control. Next, component mesh 20 and background mesh 10 are assembled, and an overlapping mesh interface is defined to avoid the generation of isolated cells. Component mesh 20 and background mesh 10 achieve data interaction through interface interpolation. Finally, the motion method of component mesh 20 and background mesh 10 is defined as a mesh motion method combining the overlapping mesh method and the custom dynamic mesh method. The overlapping mesh method avoids the problem of reduced computational efficiency after updating the mesh using the reconstruction method, ensuring the quality of the mesh. The custom dynamic mesh method is more compatible with the local mesh motion of the air around the train caused by the attitude change of the high-speed train 30. It comprehensively utilizes the advantages of the two mesh motion methods, and effectively and quickly completes the mesh motion combining the overall large-scale longitudinal motion and local small-scale attitude change of the high-speed train 30 in aerodynamics.

[0090] For example, please refer to Figure 2When establishing a three-dimensional fluid model of a high-speed train 30, a three-car train 3-car train model can be used, including the head car, middle car and tail car, all of which are rigid bodies. The flow field around the three cars is used as the component mesh 20, and the flow field inside and outside the tunnel is used as the background mesh 10 for assembly.

[0091] Optionally, please refer to Figure 4 The flow field surrounding the high-speed train 30 is meshed using a hybrid mesh generation method. The specific component mesh 20 is as follows:

[0092] A prismatic meshing method is used to mesh the part of the flow field around the high-speed train 30 that is close to the surface of the high-speed train 30, resulting in boundary layer mesh 201; a hexahedral meshing method is used to mesh the part of the flow field around the high-speed train 30 that is close to the flow field inside and outside the tunnel, resulting in outermost boundary layer mesh 203; a tetrahedral meshing method is used to mesh the remaining part of the flow field around the high-speed train 30, resulting in outer boundary layer mesh 202.

[0093] Boundary layer mesh 201, outermost boundary layer mesh 203, and outer boundary layer mesh 202 together form component mesh 20;

[0094] Among them, the boundary layer grid 201 has 6 to 10 grid layers, and the outermost boundary layer grid 203 has 6 to 10 grid layers.

[0095] Based on this, please refer to Figure 4When using a hybrid meshing method to mesh the flow field surrounding the high-speed train 30, a prismatic meshing method can be used near the surface of the high-speed train 30, i.e., the portion close to the surface of the high-speed train 30. The resulting boundary layer mesh 201 is a prismatic boundary layer, which can better describe the boundary layer motion and also consider surface viscous forces. For the outermost part of the high-speed train 30, i.e., the portion of the flow field surrounding the high-speed train 30 that is close to the flow field inside and outside the tunnel, a hexahedral meshing method can be used. This results in a regular shape and easy control of the outermost boundary layer mesh 203. Since the flow field inside and outside the tunnel is also meshed using a hexahedral meshing method, the outermost boundary layer mesh 203 can better match the tunnel flow field mesh, and the generation of isolated elements can be avoided when assembling the component mesh 20 and the background mesh 10. Due to the relatively complex shape of the high-speed train 30, a tetrahedral meshing method can be used for the remaining portion of the flow field surrounding the high-speed train 30, making it more convenient to mesh the outer boundary layer mesh 202. Boundary layer mesh 201, outermost boundary layer mesh 203, and outer boundary layer mesh 202 together form component mesh 20, which is then assembled with the pre-defined background mesh 10 to define the interface. The outermost boundary layer mesh 203 has at least 6 to 10 mesh layers. Sufficient layers of the outermost boundary layer mesh 203 ensure sufficient overlap between component mesh 20 and background mesh 10. Boundary layer mesh 201 also has 6 to 10 mesh layers. The size of the outer boundary layer mesh 202 can be determined based on the tunnel space to plan the size of the background mesh 10. This size is obtained by subtracting the sizes of boundary layer mesh 201 and outermost boundary layer mesh 203 from the total size of the background mesh 10. Simultaneously, the total thickness of the three boundary layer meshes 201, 202, and 203—that is, the thickness of component mesh 20—must maintain a certain distance from the tunnel wall.

[0096] For example, the boundary layer mesh can have 6, 7, 8, 9 or 10 mesh layers, and the outermost mesh of the boundary layer can have 6, 7, 8, 9 or 10 mesh layers. This is just an example and is not a specific limitation.

[0097] Optionally, the mesh motion method for defining the component mesh and the background mesh can be a combination of overlapping meshing and custom dynamic meshing, including:

[0098] The mesh movement method of the component mesh relative to the background mesh is defined as the overlapping mesh method;

[0099] A custom mesh motion method for component meshes is achieved through motion velocities with six degrees of freedom.

[0100] Based on this, compared with the overlapping mesh method, other mesh motion methods, if using reconstruction to update the mesh, will reduce the computational efficiency during the calculation process. However, with the overlapping mesh method, there is no mesh reconstruction problem. During the mesh motion process, each component mesh only has the overall six degrees of freedom motion, and the internal meshes have no relative motion, so the mesh quality will not be reduced. This has a significant advantage for large-scale problems. The overlapping mesh method uses multi-region meshing, decomposing complex geometric problems into simple systems with overlapping meshes, making it easier to control the mesh quality and improving the quality and efficiency of mesh generation. Therefore, the overlapping mesh method has a significant advantage in simulating fluid-structure interaction of large models with multi-degree-of-freedom motion. The fluid mesh quality is high, and the calculation is easy to converge, which helps to reduce the number of calculation iterations and save computation time. The custom dynamic mesh method is suitable for considering the local mesh motion of the air around the train caused by changes in train attitude. In the embodiments of this application, the motion velocity of the component mesh itself can be defined by the six-degree-of-freedom motion velocity caused by changes in train attitude in vehicle dynamics. Based on this, the advantages of two mesh motion methods are combined, and a mesh motion method combining the overlapping mesh method and the custom dynamic mesh method is adopted. This method can quickly and effectively complete the mesh motion that combines the overall large-scale longitudinal motion and local small-scale attitude changes of the vehicle in aerodynamics, and realize fast and large-scale coupled calculation. In addition, both the overlapping mesh method and the custom dynamic mesh method are rigid body mesh motion methods. Under the premise that the high-speed train model is a rigid body model in the established three-dimensional fluid model of the high-speed train, the overlapping mesh method and the custom dynamic mesh method can be more applicable to the motion of rigid bodies.

[0101] Figure 5 The diagram shown is a schematic representation of the vehicle dynamics model provided in an embodiment of this application; Figure 6 The diagram shown is a topology diagram of the main model of the high-speed train provided in the embodiment of this application.

[0102] S20. Establish a vehicle dynamics model for high-speed trains. The excitations in the vehicle dynamics model include track irregularities and aerodynamic loads.

[0103] Optionally, please refer to Figure 5 and Figure 6 The three-dimensional fluid model also includes a three-dimensional model of the high-speed train 30, establishing a vehicle dynamics model of the high-speed train 30. The excitations in the vehicle dynamics model include track irregularities and aerodynamic loads.

[0104] Based on the three-dimensional model of the high-speed train 30, its vehicle dynamics parameters, and track dynamics parameters, a multi-level substructure modeling method is used to establish the basic vehicle dynamics model of the high-speed train 30.

[0105] Track irregularities are applied to the track as an excitation, and aerodynamic loads are applied to the high-speed train 30 as force elements to improve the basic vehicle dynamics model of the high-speed train 30.

[0106] Complete the vehicle dynamics model of high-speed train 30.

[0107] Therefore, when establishing the basic vehicle dynamics model, a multi-level substructure modeling method can be adopted. Compared with the traditional modeling method that requires building from individual parts to form the basic vehicle dynamics model, the multi-level substructure modeling method models through sub-assemblies, avoiding the repeated modeling of common parts between adjacent vehicle bodies in the basic vehicle dynamics model. An example is... Figure 6 As shown, the high-speed train 30 includes three cars. The model of one car includes the car body 301 and the left bogie 302 and right bogie 303 connected to the car body 301, avoiding repeated modeling of the common parts of the front and rear bogies of the three cars or one car. At the same time, when it is necessary to modify the substructure model, only the separately established sub-model needs to be modified. When there is a problem with the vehicle dynamics model, the location of the problem can be quickly located through each sub-model. Furthermore, by ensuring the correctness of each sub-model, the probability of problems in the main model can be reduced, which is conducive to improving the accuracy and efficiency of modeling. In response to the problem of severe train vibration caused by the increased wheel-rail coupling and enhanced coupling between the train and the surrounding air after the train speed is increased, after establishing the basic vehicle dynamics model, the basic vehicle dynamics model of the high-speed train 30 is improved by applying track irregularities as excitations to the track and aerodynamic loads as force elements to the high-speed train 30. The influence of track irregularities and aerodynamic forces is taken into account. It can be seen that the embodiments of this application comprehensively consider the interaction between the high-speed train 30 and the surrounding air, the influence of track irregularities and tunnel effects, which facilitates the study of the interaction between the high-speed train 30 and various disturbances such as air and track after the speed is increased.

[0108] Understandably, a subassembly refers to a component consisting of multiple parts, such as a wheelset, bogie, or a car section, and is not the final assembly.

[0109] Figure 7 The diagram shows a flowchart of data exchange between the aerodynamic model and the vehicle dynamics model provided in this embodiment.

[0110] S30 uses a dual-time-step time-progression method to exchange data between the aerodynamic model and the vehicle dynamics model.

[0111] Please refer to Figure 7 The data exchange between the aerodynamic model and the vehicle dynamics model is achieved using a dual-time-step time-progression method, including:

[0112] S301 divides a physical time step into sub-iterations with a predetermined precision or a predetermined number of iterations.

[0113] S302, calculates the aerodynamic load in each sub-iteration using the aerodynamic model and transfers the aerodynamic load to the vehicle dynamics model;

[0114] S303 calculates the six-degree-of-freedom motion velocity of the vehicle dynamics model under aerodynamic loads and transfers the six-degree-of-freedom motion velocity to the aerodynamic model.

[0115] S304, using a mesh motion method combining overlapping mesh method and custom dynamic mesh method to update the aerodynamic model, and enter the calculation of the next sub-iteration;

[0116] S305: When the calculation accuracy reaches the predetermined accuracy or the number of iterations reaches the predetermined number, proceed to the next physical time step.

[0117] Based on this, a dual-time-step time-progression method can be used to exchange the calculation results between the aerodynamic model and the vehicle dynamics model multiple times within a physical time step until a predetermined accuracy or a predetermined number of iterations is reached before proceeding to the next physical time step. After multiple iterations, the set number of iterations is reached, completing the entire calculation process. At this point, the data exchanged between the two disciplines can be approximated as the current step data, which can reduce the time error of the current step calculation using the results of the previous step, improving the calculation step size and calculation stability. Simultaneously, when calculating the vehicle motion obtained from the vehicle dynamics model under aerodynamic loads, the attitude change of the high-speed train can be reflected by the six-degree-of-freedom motion speed of the high-speed train. Existing technologies exist for transferring displacements from the vehicle dynamics model to the aerodynamic model, but the displacements in vehicle dynamics are six, while those in aerodynamics are three. Therefore, a conversion from the six-degree-of-freedom vehicle displacement to the three-degree-of-freedom fluid displacement is required during the transfer. The embodiments of this application utilize velocity to reflect the attitude changes of high-speed trains. In vehicle dynamics, there are six velocities, and in aerodynamics, the acceptable rigid body velocities are also six, so no conversion is needed. The aerodynamic model can directly update the mesh using the motion velocity with six degrees of freedom, avoiding the conversion between the vehicle's six-degree-of-freedom displacement and the fluid's three-degree-of-freedom displacement. At the same time, it is not necessary to select control points in advance, nor is it necessary to use the displacement of control points to calculate the displacement of all flow field mesh nodes around the train. Only the six-degree-of-freedom motion velocity is given to each component mesh as a whole, which simplifies the coupling calculation process and greatly reduces the time required for coupling calculation. This further realizes the fast and large-scale coupling of aerodynamics and vehicle dynamics of high-speed trains.

[0118] Optionally, in each sub-iteration, the aerodynamic load is calculated from the aerodynamic model, and the aerodynamic load is transferred to the vehicle dynamics model as follows:

[0119] In each sub-iteration, the aerodynamic model is calculated to obtain the aerodynamic loads, and the aerodynamic loads are saved to a data file;

[0120] Vehicle dynamics model reads data file.

[0121] Based on this, after calculating the aerodynamic load in each sub-iteration, the aerodynamic load can be saved to a data file. Then, the vehicle dynamics model can read the data file, apply the aerodynamic load as a force element to the high-speed train, and calculate the vehicle dynamics model of the high-speed train based on this. Data transfer between the two disciplines can be achieved through data files, which is not only simple, fast, and easy to implement, but also ensures the reliability of the data.

[0122] Optionally, the vehicle dynamics model is calculated to have six degrees of freedom of motion velocity under aerodynamic loads, and the six degrees of freedom of motion velocity is transferred to the aerodynamic model as follows:

[0123] Calculate the six-degree-of-freedom motion velocity of the vehicle dynamics model under aerodynamic loads and save the six-degree-of-freedom motion velocity to a data file;

[0124] The aerodynamic model reads the data file.

[0125] Based on this, after the vehicle dynamics model reads the aerodynamic loads from the data file and calculates the six-degree-of-freedom (DOF) velocity of the high-speed train under the influence of these aerodynamic loads, the six-DOF velocity can be saved in the data file. Then, the aerodynamic model reads this six-DOF velocity from the data file and, based on it, updates the flow field mesh after the high-speed train's motion using a mesh motion method combining overlapping meshes and a custom dynamic mesh method, starting the next sub-iteration of aerodynamic load calculation. When the calculation accuracy reaches a predetermined level or the predetermined number of iterations, the sub-iteration calculation is completed, and the flow field information and structural position for the next physical time step are updated. This process is repeated until the entire calculation is complete. Using data files to achieve data transfer between the two disciplines is not only simple, fast, and easy to implement, but also ensures data reliability.

[0126] Optionally, aerodynamic loads are used as permutation variables, and the vehicle dynamics model reads the data file by overriding the permutation variables. Based on this, to establish a vehicle dynamics calculation model based on fluid-structure interaction, all aerodynamic loads can be set as permutation variables. By continuously updating the aerodynamic loads by overriding the permutation variables, both the aerodynamic model and the vehicle dynamics model can exchange fluid-structure interaction data by reading and writing text files, thus realizing data exchange between the aerodynamic model and the vehicle dynamics model of the high-speed train.

[0127] The following will illustrate the coupling method of aerodynamics and vehicle dynamics for high-speed trains provided in the embodiments of this application with specific examples.

[0128] First, the calculation condition was set as a single-vehicle tunnel passage, using a standard Chinese single-track tunnel. Since the main objective of this study was to apply the proposed aerodynamics-vehicle dynamics coupling method, the high-speed train was simplified in the fluid calculations, ignoring equipment such as air conditioning, pantographs, and bogies. The flow field around the high-speed train was a three-dimensional, viscous, compressible, unsteady turbulent flow field. The flow medium was set as air (ideal gas), and the turbulence model was a shear stress transfer (SST) model. The boundary conditions of the flow field were set as follows: the car body surface was set as a frictional, no-slip boundary; the ground and tunnel walls were fixed walls; and the outer boundary of the computational domain was set as a pressure outlet, with the outlet pressure set to one standard atmosphere.

[0129] Figure 8 The figure shows the aerodynamic lift variation curves under different coupling methods provided in the embodiments of this application; Figure 9 The figure shows the variation curves of vertical acceleration under different coupling methods provided in the embodiments of this application; Figure 10 The figure shows the vertical displacement variation curves under different coupling methods provided in the embodiments of this application; Figure 11 The figure shows the variation curves of lateral displacement under different coupling methods provided in the embodiments of this application.

[0130] Based on the above conditions, a high-speed single-car passage through a tunnel was conducted, considering vertical and lateral track irregularities, but not the effects of wind load, aerodynamic load as known loads, and different coupling methods such as aerodynamic and vehicle dynamic coupling on the car body coupling characteristics. Figures 8-11 As shown. By comparing and analyzing these curves, the following main conclusions are drawn:

[0131] (1) Please refer to Figure 8Compared to simple aerodynamic calculations, the aerodynamic lift changes and directions of each car are basically consistent during coupling. The lead car experiences downward pressure, the middle car experiences sometimes downward pressure and sometimes upward buoyancy, and the tail car experiences primarily upward buoyancy, with relatively large values ​​but much larger fluctuations, increasing sequentially from the lead car to the middle car and then to the tail car. For example, at the tunnel exit, the lift abrupt changes are 3.4kN, 4.4kN, and 10.7kN in simple aerodynamic calculations, but 5.8kN, 4.5kN, and 12.6kN in coupling. When the lead car exits the tunnel, the lift of the middle and tail cars both experience abrupt changes, with fluctuations of 5.25kN and 18.6kN respectively, especially the tail car, which exceeds the fluctuation amplitude when the tail car exits the tunnel.

[0132] (2) Please refer to Figure 9 The acceleration of the middle car, lead car, and tail car generally increases sequentially. At the tunnel exit, the vertical acceleration of the three cars at the lead car is relatively large, while the middle and tail cars have relatively large vertical accelerations at their respective exits. However, at 3.05s, the tail car reaches its maximum vertical acceleration of 0.296 m / s², but this is not the point of maximum lift. This shows that aerodynamic lift is an important factor influencing vertical acceleration, but it is not the only one.

[0133] (3) Please refer to Figure 10 Compared to when wind load is not considered, when aerodynamic behavior is known and coupled, the vertical displacement of the lead car and tail car increases significantly, while the change in the middle car is relatively small. For example, around 2.35s, the vertical displacement of the tail car increases from 0.0065m to 0.0154m and 0.016m respectively; around 2.65s, the vertical displacement of the lead car increases from 0.0077m to 0.0136m and 0.0128m respectively; and around 3.0s, the vertical displacement of the middle car increases from 0.0066m to 0.0068m and 0.0073m respectively. Compared with the case where the aerodynamic load is known, the vertical displacement of each car sometimes increases and sometimes decreases when coupled. This shows that coupling makes the calculation results closer to the objective reality, and does not necessarily increase them. For example, at 4.5s, the vertical displacement of the tail car decreases from 0.0112m to 0.0105m, and at 2.65s, the vertical displacement of the head car decreases from 0.0136m to 0.0128m.

[0134] (4) Please refer to Figure 11Compared to when wind load is not considered, when aerodynamic behavior is known under load and coupling, the changes in lateral displacement of the lead and tail cars are much smaller than the changes in vertical displacement. For example, around 2.35s, the vertical displacement of the tail car changes from 0.0065m to 0.0154m and 0.016m respectively; around 2.31s, the lateral displacement of the tail car changes from 0.0031m to 0.0032m and 0.0033m respectively; around 2.65s, the vertical displacement of the lead car changes from 0.0077m to 0.0136m and 0.0128m respectively; and around 1.8s, the lateral displacement of the lead car changes from 0.0031m to 0.0032m and 0.0025m respectively.

[0135] In summary, the coupling method for aerodynamics and vehicle dynamics of high-speed trains provided in this application achieves at least the following beneficial effects:

[0136] (1) The aerodynamic and vehicle dynamic coupling method for high-speed trains provided in this application embodiment is to add the influence of the change of the high-speed train vehicle attitude on the surrounding flow field to the coupling calculation by exchanging data between the aerodynamic model and the vehicle dynamic model of the high-speed train; by setting the flow field inside and outside the tunnel as the background flow field, and adding track irregularities and aerodynamic loads as excitations to the vehicle dynamic model of the high-speed train, the tunnel effect and track irregularities are added to the coupling calculation. It can be seen that the embodiment of this application comprehensively considers the interaction between the high-speed train and the surrounding air, the influence of track irregularities and tunnel effects, and is more comprehensive, which is convenient for studying the interaction between the high-speed train and various excitations such as air and track after the speed increase.

[0137] (2) The aerodynamic and vehicle dynamics coupling method for high-speed trains provided in this application adopts multi-region mesh generation, decomposing complex geometric problems into simple systems with overlapping meshes, making it easier to control mesh quality and improving the quality and efficiency of mesh generation. During mesh movement, each component mesh only has overall six degrees of freedom motion, and there is no relative motion between internal meshes, so there is no mesh reconstruction problem and the mesh quality will not be reduced. This has obvious advantages for large-scale problems. The flow field mesh has high quality and is easy to converge during calculation, which helps to reduce the number of calculation iterations and save calculation time, thereby realizing fast and large-scale coupled calculation.

[0138] (3) The aerodynamic and vehicle dynamics coupling method for high-speed trains provided in this application embodiment uses the six-degree-of-freedom motion speed of the high-speed train to reflect the attitude change of the high-speed train, and directly transfers the six-degree-of-freedom motion speed between the aerodynamic model and the vehicle dynamics model. Compared with the existing method of using displacement to reflect the attitude change of the high-speed train, it avoids the conversion between the six-degree-of-freedom displacement of the vehicle and the three-degree-of-freedom displacement of the fluid. At the same time, it does not require the prior selection of control points, nor does it require the calculation of the displacement of all flow field grid nodes around the train using the displacement of control points. It only gives the six-degree-of-freedom motion speed to each component grid as a whole, which simplifies the coupling method of high-speed train aerodynamics and vehicle dynamics, greatly reduces the time required for coupling calculation, and has obvious advantages for large-scale problems, further realizing fast and large-scale coupling calculation.

[0139] (4) The aerodynamic and vehicle dynamic coupling method for high-speed trains provided in this application uses the overlapping mesh method and the custom dynamic mesh method as the mesh motion method. Compared with other mesh motion methods, the custom dynamic mesh method can be well matched with the local mesh motion of the air around the train caused by the attitude change of the high-speed train. This application embodiment makes full use of the advantages of the two mesh motion methods, and can quickly and well complete the mesh motion that combines the overall large-scale longitudinal motion and local small-scale attitude change of the high-speed train in aerodynamics.

[0140] (5) The coupling method for aerodynamics and vehicle dynamics of high-speed trains provided in this application is more likely to converge than the direct coupling method of solving large equations of two disciplines at the same time, and meets the engineering requirements when aerodynamics and structure interact violently. Compared with unidirectional coupling and loose coupling methods, the coupling method provided in this application has higher accuracy. When the aerodynamics and vehicle dynamics solutions reach second-order accuracy, the accuracy of the coupling calculation is also second-order, which can improve the accuracy of first-order calculation time.

[0141] (6) The coupling method of aerodynamics and vehicle dynamics for high-speed trains provided in this application is not only applicable to the coupling calculation of aerodynamics and vehicle dynamics of high-speed trains, but also applicable to other large-scale situations where fluid and rigid body interactions are intense.

[0142] While specific embodiments of this application have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of this application. Those skilled in the art should understand that modifications can be made to the above embodiments without departing from the scope and spirit of this application. The scope of this application is defined by the appended claims.

Claims

1. A coupling method for aerodynamics and vehicle dynamics applicable to high-speed trains, characterized in that, include: An aerodynamic model of a high-speed train is established, which includes component flow field and background flow field. The component flow field is the flow field around the high-speed train, and the background flow field is the flow field inside and outside the tunnel. A vehicle dynamics model for a high-speed train is established, wherein the excitations of the vehicle dynamics model include track irregularities and aerodynamic loads; The data between the aerodynamic model and the vehicle dynamics model is exchanged using a dual-time-step time advancement method. The method of exchanging data between the aerodynamic model and the vehicle dynamics model using a dual-time-step time advancement method includes: Divide a physical time step into sub-iterations with a predetermined precision or a predetermined number of iterations; In each sub-iteration, the aerodynamic model is calculated to obtain the aerodynamic load, and the aerodynamic load is transferred to the vehicle dynamics model; Calculate the six-degree-of-freedom motion velocity of the vehicle dynamics model under the aerodynamic load, and transfer the six-degree-of-freedom motion velocity to the aerodynamic model; The aerodynamic model is updated using a mesh motion method that combines the overlapping mesh method and the custom dynamic mesh method, and then the calculation proceeds to the next sub-iteration. When the calculation accuracy reaches the predetermined accuracy, or the number of iterations reaches the predetermined number, proceed to the next physical time step; The aerodynamic model of the high-speed train is established, which includes component flow fields and background flow fields. The component flow fields are the flow fields surrounding the high-speed train, and the background flow fields are the flow fields inside and outside the tunnel, including: Establish a three-dimensional fluid model of the high-speed train; The component flow field and background flow field of the three-dimensional fluid model of the high-speed train are meshed, specifically as follows: A hybrid mesh generation method is used to mesh the flow field around the high-speed train, which is then used as a component mesh. The flow field inside and outside the tunnel is meshed using a hexahedral meshing method, which serves as the background mesh. Assemble the component mesh and the background mesh, and define the overlapping mesh interface of the component mesh and the background mesh; the component mesh and the background mesh achieve data interaction through interface interpolation; The mesh motion method between the component mesh and the background mesh is defined as a combination of overlapping mesh method and custom dynamic mesh method; including: defining the mesh motion method of the component mesh relative to the background mesh as overlapping mesh method; and customizing the mesh motion method of the component mesh by the motion velocity with six degrees of freedom. Complete the aerodynamic model of the high-speed train.

2. The coupling method for aerodynamics and vehicle dynamics of high-speed trains according to claim 1, characterized in that, The method of using a hybrid mesh generation technique to divide the flow field around the high-speed train into meshes, specifically as component meshes, is as follows: A prismatic meshing method is used to mesh the portion of the flow field around the high-speed train that is close to the surface of the high-speed train, resulting in a boundary layer mesh. A hexahedral meshing method is used to mesh the portion of the flow field around the high-speed train that is close to the flow field inside and outside the tunnel, resulting in the outermost boundary layer mesh. A tetrahedral meshing method is used to mesh the remaining portion of the flow field around the high-speed train, resulting in the outer boundary layer mesh. The boundary layer mesh, the outermost boundary layer mesh, and the outermost boundary layer mesh together constitute the component mesh; The boundary layer mesh has 6 to 10 mesh layers, and the outermost mesh of the boundary layer has 6 to 10 mesh layers.

3. The coupling method for aerodynamics and vehicle dynamics of high-speed trains according to claim 1, characterized in that, The three-dimensional fluid model also includes a three-dimensional model of the high-speed train. The establishment of the high-speed train's vehicle dynamics model includes disturbances such as track irregularities and aerodynamic loads. Based on the three-dimensional model of the high-speed train, vehicle dynamics parameters, and track dynamics parameters, a multi-level substructure modeling method is used to establish the basic vehicle dynamics model of the high-speed train. Track irregularities are applied to the track as an excitation, and the aerodynamic loads are applied to the high-speed train as force elements to improve the basic vehicle dynamics model of the high-speed train. Complete the establishment of the vehicle dynamics model of the high-speed train.

4. The coupling method for aerodynamics and vehicle dynamics of high-speed trains according to claim 1, characterized in that, The specific steps of calculating the aerodynamic load from the aerodynamic model in each sub-iteration and transferring the aerodynamic load to the vehicle dynamics model are as follows: In each sub-iteration, the aerodynamic model is calculated to obtain the aerodynamic load, and the aerodynamic load is saved to a data file; The vehicle dynamics model reads the data file.

5. The coupling method for aerodynamics and vehicle dynamics of high-speed trains according to claim 4, characterized in that, The calculation of the six-degree-of-freedom motion velocity of the vehicle dynamics model under the aerodynamic load, and the transfer of the six-degree-of-freedom motion velocity to the aerodynamic model, specifically involves: Calculate the six-degree-of-freedom motion velocity of the vehicle dynamics model under the aerodynamic load, and save the six-degree-of-freedom motion velocity to a data file; The aerodynamic model reads the data file.

6. The coupling method for aerodynamics and vehicle dynamics of high-speed trains according to claim 4, characterized in that, The aerodynamic load is a permutation variable, and the vehicle dynamics model reads the data file by overriding the permutation variable.

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