CN103337097A - Multiple Cartesian grid generation method applicable to LBM) - Google Patents

Abstract

The invention discloses a multiple Cartesian grid generation method suitable for the lattice Boltzmann method. This method can generate multiple Cartesian grids for the interior or exterior of any complex geometric object, and the steps are as follows: A. Set the parameters of multiple Cartesian grids according to user requirements; B. Calculate and generate the coarsest single grid , and use the ray method to judge the relative position between the center of the grid unit and the object. If it is inside, delete the grid unit. If it is on the boundary of the object, calculate the distance from the center of the grid unit to the boundary (this distance will be used in the LBM method ); C. Encrypt the grid from the surface of the object layer by layer, and the generated sub-grid unit also needs to judge the relative position with the object. If it is inside, delete the sub-grid unit, and if it is on the boundary of the object, calculate The distance from the center of the subgrid cell to the boundary, and finally search for the neighbor information that forms the subgrid. Through the above steps, a multiple Cartesian grid is finally formed.

Description

Multiple Cartesian Grid Generation Applicable to Lattice Boltzmann Method

technical field

The invention relates to the fields of computational fluid dynamics and computers, and proposes a multiple Cartesian grid generation method suitable for the lattice Boltzmann method.

Background technique

Lattice Boltzmann Method (Lattice Boltzmann Method, LBM) discretizes the particle velocity distribution into a limited number of values, and simulates the solution of the incompressible Navier-Stokes equation through the movement and collision process of molecules along the finite velocity direction between lattice nodes . LBM has attracted widespread attention because of its simple algorithm, high computational efficiency, good parallelism, and the ability to handle complex boundary conditions. It has developed rapidly in the past decade. On the one hand, the application of LBM has been successful in many fields, including complex turbulent flow, multiphase flow, reactive flow, aerodynamic noise, phase interface and phase transition, flow in porous media, natural convection heat transfer, flow with free surface , flow with high Knudsen number, prediction of diffusion coefficient of miscible liquid, unsteady flow, etc. On the other hand, LBM has also made great progress in numerical simulation processing methods and techniques, such as the processing of irregular geometric shapes, processing of complex boundary conditions, non-uniform lattice and stability analysis, etc. However, due to the negative value of the existing equilibrium state density distribution function in the case of high-speed flow, the stability of the LBM scheme cannot be guaranteed, so it is difficult to apply LBM to calculate compressible flow.

In recent years, another calculation method based on the BGK model based on the Boltzmann equation has been developed. The basic idea is to start from the Boltzmann equation in statistical physics, in which the collision item is simplified by the BGK model, and based on this, a differential scheme of macroscopic quantities is established, which is called the gas-kinetic BGK scheme (Gas-Kinetic BGK Scheme). The method has been applied in chemical reaction flow, multiphase flow, rarefied gas flow, compressible flow, water wave motion, and magnetic fluid. These applications show that the method has rich physical connotations and can solve some complex flow problems, but the method requires a large amount of calculation for three-dimensional problems.

As the LBM method has been developed, its application has become more and more widespread, but the method requires the use of a computational grid called a Cartesian grid. At present, no mature Cartesian grid generation software suitable for LBM method has been seen, only appeared in the large-scale commercial computing software PowerFLOW, so it is very necessary to invent a Cartesian grid generation method suitable for LBM method.

Contents of the invention

The object of the present invention is, in order to solve the above-mentioned problem and provide a kind of Cartesian grid generation method applicable to LBM method, this method can generate multiple Cartesian grids to the inside or outside of any complex geometry object, has avoided only in the past The limitation of being able to mesh a specific simple object is that the method can well adapt to objects with complex geometries, so it has high adaptability.

In order to achieve the above-mentioned purpose, the idea of the present invention is: if the outflow calculation is performed, first surround the object with a calculation area, and use the coarsest grid size to carry out grid division for this area (if the inflow calculation is performed, directly Divide the inside of the object, and then only explain the situation of the outflow in this patent, and the inside flow will not be repeated), then delete the grid whose center is located inside the object, and finally encrypt the grid layer by layer from the surface of the object , eventually forming a multiple Cartesian grid.

According to the above inventive concept, for the calculation area outside an object, the present invention adopts the following technical solutions:

A. Set the parameters of multiple Cartesian grids according to user requirements; the grid type (such as D3Q15, D3Q19 or D3Q27, etc.), grid size, grid multiplicity, and grid layers of the LBM method;

B. Calculate and generate the thickest single grid, and use the ray method to judge the relative position between the center of the grid unit and the object. If it is inside, delete the grid unit. If it is on the boundary of the object, calculate the center of the grid unit to the boundary The distance (this distance will be used in the LBM method) to generate the neighbor information of the grid;

C. Build a multi-grid layer by layer from coarse to fine through encryption: the grid is encrypted layer by layer from the surface of the object, and the generated sub-grid unit also needs to judge the relative position of the object. If it is inside, delete the sub-grid If the grid unit is on the boundary of the object, the distance from the center of the sub-grid unit to the boundary is calculated, and finally the neighbor information forming the sub-grid is searched, and finally a new Cartesian grid is formed.

Compared with the existing grid generation technology, the multi-Cartesian grid generation method applicable to the lattice Boltzmann method of the present invention has the following prominent substantive features and significant advantages:

1. The multi-Cartesian grid established by this method is suitable for the grid unit types of various LBM methods, avoiding the lack of dedicated grid generators for previous LBM methods;

2. This method can use parallel meshing on each computing node of the cluster computer, avoiding the previous meshing that was performed on one node and then transmitted to each computing node;

3. With the present invention, in the future calculation of the flow field, the local grid can be adaptively refined according to the calculation result of the flow field.

Description of drawings

FIG. 1 is a flow chart of the multiple Cartesian grid generation method applicable to the lattice Boltzmann method of the present invention.

FIG. 2 is a specific flow chart of encrypting the grid on the upper layer described in step C in FIG. 1 .

FIG. 3 is a flow chart of encrypting a grid unit described in C2 in FIG. 2 .

Fig. 4 is the geometric shape of the tested wing-body assembly of the specific implementation example.

Fig. 5 is a partial rendering of the multi-Cartesian grid finally generated by the present invention.

Detailed ways

Preferred embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

Embodiment one:

Referring to Fig. 1, this multi-Cartesian grid generation method applicable to the lattice Boltzmann method is characterized in that: it can generate a multi-Cartesian grid applicable to the lattice Boltzmann method for the interior or exterior of any object with complex geometry; specific operation steps as follows:

A. Set the parameters of multiple Cartesian grids according to user requirements: grid type D3Q15, D3Q19 or D3Q27, grid size, grid multiplicity and grid layer number of grid Boltzmann method;

B. Calculate and generate the thickest single grid, and use the ray method to judge the relative position between the center of the grid unit and the object. If it is inside, delete the grid unit. If it is on the boundary of the object, calculate the center of the grid unit to the boundary The distance to generate the neighbor information of the grid;

Build a multi-grid layer by layer from coarse to fine through encryption: the grid is encrypted layer by layer from the surface of the object, and the generated sub-grid unit also needs to judge the relative position of the object. If it is inside, delete the sub-grid If the unit is on the boundary of the object, the distance from the center of the sub-grid unit to the boundary is calculated, and finally the neighbor information forming the sub-grid is searched, and finally a new Cartesian grid is formed.

Embodiment two:

Referring to Fig. 1-Fig. 3, this embodiment is basically the same as Embodiment 1, and the special features are as follows:

The specific steps for calculating and generating the thickest single grid in the step B are as follows:

B1. Make a ray from the center of the unit along a certain direction, and calculate the number of intersections between the ray and the surface of the object. If the number of intersections is even, it means that the center of the unit is located outside the object, otherwise it is located inside;

B2. Calculate the shortest distance from the center of the grid to the surface of the object. If the distance is less than the size of the grid, the grid intersects the surface of the object, otherwise it does not intersect;

B3. If the center of the grid is inside the object and does not intersect with the surface of the object, delete the grid;

B4. According to the grid type of the lattice Boltzmann method, search for the information of the neighbor units forming the grid.

The specific method of constructing the multi-grid layer by layer from coarse to fine by encrypting described in the step C is:

C1. Scan the nth grid, encrypt the grid unit intersecting with the surface of the object, and use the same method as step B to judge and process the position of the generated sub-grid unit, and finally the encrypted parent unit Marking, the grid marked as encrypted will not participate in the calculation in the lattice Boltzmann method in the future;

C2. Scan the nth grid multiple times. The number of scans depends on the number of grid layers set by the user. The neighbors of the grid marked as encrypted are encrypted, so that the formed sub-grid does not need to judge its position;

C3, finally generate the n +1th heavy grid, and search for the information of the neighbor units forming the sub-grid;

C4. Steps C1-C3 are repeated to form multiple Cartesian grids layer by layer.

Embodiment three:

In this embodiment, the multi-Cartesian grid generation method applicable to the lattice Boltzmann method is written in standard C language and can run under Windows and Linux operating systems.

The geometric description of the object adopts the stl file format. The multi-Cartesian grid generation method applicable to the lattice Boltzmann method of the present invention, as shown in Figures 1-3, includes the following steps:

A. Initialization of the algorithm, the specific steps include:

A1. Read the stl file describing the geometry of the object, and the user sets the flow field area;

A2. Set the grid type, grid size, grid multiplicity and layer number of the LBM method.

B. According to the grid parameters set by the user, the entire calculation area is divided into the thickest single grid, and the position of the divided grid unit is judged. The specific steps include:

B1. Make a ray from the center of the unit along a certain direction (such as the X direction), and calculate the number of intersections between the ray and the surface of the object. If the number of intersections is even, it means that the center of the unit is located outside the object, otherwise it is located inside;

B2. Calculate the shortest distance from the center of the grid to the surface of the object. If the distance is less than the size of the grid, the grid intersects the surface of the object, otherwise it does not intersect;

B3. If the center of the grid is inside the object and does not intersect with the surface of the object, delete the grid;

B4. According to the grid type of the LBM method, search for the information of the neighbor units forming the grid.

C. Build a multi-grid layer by layer from coarse to fine through encryption. The specific steps include:

C1. Scan the nth grid, encrypt the grid unit intersecting with the surface of the object, and use the same method as step B to judge and process the position of the generated sub-grid unit, and finally the encrypted parent unit Marking (the grid marked as encrypted will not participate in the calculation in the LBM method in the future);

C2. Scan the nth grid multiple times (the number of scans depends on the number of grid layers set by the user), and encrypt the neighbors of the grid marked as encrypted, so that the formed sub-grid does not need to judge its position. ;

C3, finally generate the n+1th heavy grid, and search for the information of the neighbor units forming the sub-grid;

C4. Repeat C1-C3 to form multiple Cartesian grids layer by layer.

Referring to FIG. 4 , it shows the geometric shape of the wing-body assembly of the specific implementation example of the multiple Cartesian grid generation method applicable to the lattice Boltzmann method of the present invention. Referring to FIG. 5 , it shows an effect diagram of the multi-Cartesian grid finally generated by the present invention.

The description herein in conjunction with the accompanying drawings and specific embodiments is only used to help understand the method and core idea of the present invention. The method described in the present invention is not limited to the examples described in the specific implementation manner, and other implementation manners obtained by those skilled in the art based on the method and idea of the present invention also belong to the technical innovation scope of the present invention. The contents of this description should not be construed as limiting the present invention. the

Claims (3)

1. be applicable to the multiple cartesian grid generation method of lattice Boltzmann method, it is characterized in that: can be to inside or the outside multiple cartesian grid that is applicable to lattice Boltzmann method that generates of the object of any complex geometry; The concrete operations step is as follows:

A, the parameter of multiple cartesian grid is set according to customer requirements: trellis-type D3Q15, the D3Q19 of lattice Boltzmann method or D3Q27, size of mesh opening, grid tuple and the grid number of plies;

B, calculating generate the thickest substance grid, and utilize rays method to judge the relative position of grid cell center and object, if just delete this grid cell in inside, and if on object boundary the computing grid unit center to the distance on border, the neighbor information of generating mesh;

C, successively make up multi grid from coarse to fine by encrypting: begin grid is encrypted from body surface layer by layer, the sub-grid unit that generates also needs to judge the relative position with object, if just delete this sub-grid unit in inside, if on object boundary then calculate the sub-grid unit center to the distance on border, final search forms the neighbor information of sub-grid, finally forms new one heavy cartesian grid.

2. the multiple cartesian grid generation method that is applicable to lattice Boltzmann method according to claim 1 is characterized in that, it is as follows that described step B calculates the concrete steps that generate the thickest substance grid:

B1, do a ray from unit center along certain direction, calculate the crossing number of times of this ray and body surface, be that even number shows that unit center is positioned at the object outside if intersect number of times, otherwise be positioned at inside;

B2, computing grid center be to the minimum distance of body surface, if this distance less than size of mesh opening then grid and body surface are crossing, otherwise non-intersect;

If the B3 grid element center is positioned at interior of articles and non-intersect with body surface, then delete this grid;

B4, according to the trellis-type of lattice Boltzmann method, search forms neighbours' unit information of grid.

3. the multiple cartesian grid generation method that is applicable to lattice Boltzmann method according to claim 1 is characterized in that, passing through described in the described step C encrypted the concrete grammar that successively makes up multi grid from coarse to fine and be:

C1, scanning the nHeavy grid, the grid cell that intersects with body surface is encrypted, and the employing method identical with step B judged the position of the sub-grid unit that generates and handled, at last encrypted father unit is marked, the grid that is noted as encryption does not participate in calculating in the future in lattice Boltzmann method;

C2, repeatedly scan nHeavy grid, the number of times of scanning is decided according to the grid number of plies that the user arranges, and the neighbours of the grid that is labeled as encryption are encrypted, and the sub-grid of Xing Chenging needn't be judged its position more like this;

C3, generate the th at last n+ 1 heavy grid, search forms neighbours' unit information of sub-grid;

C4, repeating step C1-C3 successively form multiple cartesian grid.

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