CN112287615B - Multi-block structure meshing method for unsteady flow field of spring valve with complex structure - Google Patents

Multi-block structure meshing method for unsteady flow field of spring valve with complex structure Download PDF

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CN112287615B
CN112287615B CN202011160284.2A CN202011160284A CN112287615B CN 112287615 B CN112287615 B CN 112287615B CN 202011160284 A CN202011160284 A CN 202011160284A CN 112287615 B CN112287615 B CN 112287615B
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率志君
孟长霖
简洁
董烈祎
张相元
王曦
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Abstract

The invention aims to provide a multi-block structure meshing method for an unsteady flow field of a spring valve with a complex structure, which comprises the following steps: extracting a flow field model from the spring check valve design model; dividing a two-dimensional watershed simplified by an upper three-dimensional watershed model and a lower three-dimensional watershed model into a moving sub watershed and a non-moving sub watershed; calculating the height of a first layer of grids, and performing structural grid division on each two-dimensional sub-basin; generating three-dimensional structured grids from the upper and lower two-dimensional structured grids, and simultaneously generating three-dimensional unstructured grids of the watershed at the middle part; and integrating three-dimensional flow field grids of the upper part, the middle part and the lower part of the spring valve to obtain the whole flow field grid of the valve group. The invention can provide a grid which accords with the actual motion rule of the model and is convenient for the arrangement of the movable grid for the numerical simulation of the dynamic flow field of the spring valve, the moving subareas and the static subareas in the flow field are not influenced mutually, and the grid with the block structure is easy to generate an encrypted moving boundary layer grid, so that the accuracy of the numerical simulation is greatly improved.

Description

Multi-block structure meshing method for unsteady flow field of spring valve with complex structure
Technical Field
The invention relates to a fluid machinery technology, in particular to a method for simulating a dynamic numerical value of a steady flow field.
Background
The valve is used as an important control component in a pipeline system and can be used for regulating the flow and pressure of a medium in a pipeline or for cutting off and opening a flow-through medium passage. The main function of the non-return valve is to shut off the return flow of the medium. The spring check valve plays an important role in the pipeline system, and the functions of cutting off backflow and adjusting flow distribution are very important to the smooth operation of the pipeline system. The spring check valve is a valve which is automatically opened and closed according to the working pressure of a pipeline system, and is generally installed in a closed pipeline system to protect the safety of the system.
The study of the dynamic flow field characteristics of spring check valves is mainly based on the Computational Fluid Dynamics (CFD) method. The CFD method is characterized in that a computer is used for discretizing and solving a hydromechanical control equation by adopting a numerical method, so that the purpose of predicting the flow field regular characteristic is achieved, and the CFD method is an important branch of hydromechanics in the 21 st century. The first step of CFD calculation is preprocessing to generate a proper flow field grid, and a set of reasonable grids can not only reduce the cost of CFD trial calculation, but also improve the stability and accuracy of calculation. At present, the mainstream grid generation methods at home and abroad are mainly divided into three types, namely a structural grid, an unstructured grid and a mixed grid. The hybrid grid well combines the advantages of the structural grid and the non-structural grid, so that the hybrid grid has the advantages of accurately simulating boundary layer flow by the structural grid and also has the advantages of good adaptability and easiness in generation of the non-structural grid to a complex structure. For the drainage basin of a complex research object, a single-structure grid cannot be used for simply covering the drainage basin, and the drainage basin is divided into a plurality of blocks of structure grids. The multiple structural grids ensure the regularity of the structural grids and accord with the natural perception of human to the geometric structure, and the strong adaptability of the non-structural grids to the complex structure is fused.
When the flow field of the moving boundary needs to be numerically simulated, the application of the dynamic grid technology in the CFD calculation needs to be involved. The dynamic grid updating method in the widely used commercial CFD solver ANSYS Fluent mainly comprises the following steps: spring fairing methods, mesh reconstruction methods, and dynamic lamination methods. The principle of the spring fairing method is that all grids are regarded as a spring system, each grid line is regarded as a spring, the balance state of the spring, namely the current state, is under the constraint of given boundary conditions, and the grid position is updated by solving the balance of force.
Most of the current researches are more inclined to the theoretical research of a novel dynamic grid updating mode or the research of the application of a dynamic grid technology in various engineering fields; and the research on generating the flow field grids suitable for the dynamic grid technology in the preprocessing is less, and especially the research on a multi-block structure grid dividing method is less. The traditional mesh division method generally adopts single-form meshes in the same computational domain, mostly adopts triangular or tetrahedral meshes for complex structures, ignores the treatment of boundary layer meshes at the near wall surface, and often has the defects of large mesh quantity, poor local mesh quality and difficult movable mesh setting.
Disclosure of Invention
The invention aims to provide a multi-block structural grid division method for an unsteady flow field of a spring valve with a complex structure, which can provide a grid which accords with the actual motion rule of a model and is convenient for movable grid arrangement for numerical simulation of a dynamic flow field of the spring valve.
The purpose of the invention is realized by the following steps:
the invention discloses a multi-block structure grid division method for an unsteady flow field of a spring valve with a complex structure, which is characterized by comprising the following steps of:
(1) extracting a flow field model from a spring check valve design model;
(2) simplifying the upper and lower three-dimensional watershed models into two-dimensional models, and dividing the two-dimensional watershed into moving and non-moving sub watersheds;
(3) calculating the height of a first layer of grids by using a boundary layer thickness formula, dividing structural grids of each two-dimensional sub-basin, encrypting grids of the boundary layer area close to the wall surface, and establishing an interface between grids of each two-dimensional sub-basin;
(4) generating three-dimensional structured grids by the upper and lower two-dimensional structured grids through a rotary sweeping method, and simultaneously generating a three-dimensional unstructured grid of a middle drainage basin;
(5) integrating three-dimensional flow field grids of the upper part, the middle part and the lower part of the spring valve, and establishing interface interfaces among the grids of each part to obtain the whole flow field grid of the valve group.
The present invention may further comprise:
1. the step (1) of extracting the flow field model from the design model of the spring check valve specifically comprises the following steps: the method comprises the steps of simplifying the integral entity structure of the spring check valve, obtaining a spring valve flow field required by CFD calculation by utilizing Boolean subtraction operation, and dividing a three-dimensional model of the integral flow field of the spring valve into an upper part, a middle part and a lower part, wherein the step is determined based on the motion state of the spring valve in the opening and closing processes, the middle part of the flow field of the spring check valve is of an asymmetric structure, and the upper part and the lower part of the flow field of the spring check valve are of an axisymmetric structure.
2. Simplifying the upper and lower three-dimensional watershed models into two-dimensional models, and dividing the two-dimensional watershed into moving and non-moving sub-watersheds, wherein the specific steps are as follows: simplifying three-dimensional models of upper and lower flow fields of the spring valve into two dimensions, and obtaining a structure through rotation transformation, wherein the two-dimensional simplified model is half of a section of the three-dimensional model in the plumb direction; for the structure obtained by stretching, the two-dimensional simplified model is the end surface shape of the three-dimensional model, the two-dimensional flow areas of the upper spring valve and the lower spring valve are divided into the moving sub-flow areas and the non-moving sub-flow areas, and for the division of the sub-flow areas, the principle that the actual movement rule is met and the moving sub-flow areas and the non-moving sub-flow areas are not mutually influenced is followed.
3. Step (3) calculating the height of a first layer of grids by using a boundary layer thickness formula, carrying out structural grid division on each two-dimensional sub-basin, carrying out encryption processing on the grids of the boundary layer at the position close to the wall surface, and establishing interface interfaces among the grids of each two-dimensional sub-basin, which specifically comprises the following steps: carrying out structural grid division on each two-dimensional sub-basin of the upper part and the lower part according to a formula
Figure GDA0003627358150000031
Rex＝ρu∞The first layer of grid height obtained by x/mu is used for encrypting the boundary layer grids of each two-dimensional sub-basin of the upper part and the lower part, deltaxIs the thickness of the boundary layer, x is the inner diameter of the tube, RexIs Reynolds number, ρ is fluid density, u∞Mu is the dynamic viscosity coefficient of water under the normal temperature condition, and is the stable flow rate far away from the wall surface; CFD calculation is carried out by using the generated flow field grid, then whether the boundary layer treatment of the set of grid meets the requirement of the selected turbulence model is judged according to the wall surface Y + value parameter in the post-treatment, and if the boundary layer treatment of the set of grid meets the requirement, subsequent numerical values can be carried outAnd (4) simulating work, if the sub-regions do not meet the requirement, returning to the initial position of the step (3) to reprocess the boundary layer grids, and establishing interface interfaces between the grids of the upper and lower two-dimensional sub-domains to ensure that the sub-domains mutually circulate, so as to ensure that the adjacent sub-domains are shared and overlapped and realize block butt joint.
The invention has the advantages that: the invention has the outstanding advantages and innovation points that when the spring check valve grids are divided by the method, the workload is smaller than that of the conventional structural grid division, the grid quality is higher than that of the conventional method, and the number of the divided grids is less than that of the conventional method. The method for dividing the grids with the multi-block structure is adopted, so that the local encryption of the boundary layer of the flow field grids is ensured, and the adaptability to the complex flow field structure is not lost. The method adopts a mixed grid division method to divide the valve into an upper part, a lower part (symmetrical structure) and a middle part (asymmetrical structure), a simpler symmetrical structure adopts a structured grid with high precision, the asymmetrical part with a complex structure adopts a mixed grid, and the mixed grid better combines the advantages of the structured grid and the non-structured grid, thereby having the advantages of the structural grid simulating the flow of a boundary layer and the non-structured grid having strong adaptability to the complex structure and being easy to generate. The height of the first layer of grids is controlled through design parameters of the spring valves, and boundary layer grids close to the wall surface are reasonably encrypted. The three-dimensional grid is generated by adopting a sweeping method, so that the grid division process is simpler and more controllable, the size of the grid of the boundary layer is convenient to adjust, and the organic combination of the two-dimensional and three-dimensional grid is realized. Dividing and processing the moving and static subareas in the spring valve non-return flow area to ensure that the moving and static subareas do not influence each other; in the simulation process, the watershed motion is consistent with the actual motion due to reasonable division of the grid area.
Compared with the prior art, the method provided by the invention has the advantages that the multiple structural grids of the spring valve are divided, the multiple structural grids are easy to generate encrypted boundary layer grids, the size of the boundary layer grids can be adjusted according to a turbulence model selected by simulation calculation, parameters such as finally obtained grid wall surface values and the like conform to the turbulence model used by calculation, the stability and accuracy of the simulation calculation are further improved, the time cost of repeated CFD trial calculation is reduced, and the calculation efficiency is increased. The number of the divided grids is small, the number of the grids of the whole set of valve groups is controlled to be about 280 ten thousands, the computing resources are saved, and the computing efficiency is improved. Complex watershed structures in the spring valve can be simplified, block processing is carried out, and the movable grid is simpler to set. Because the dynamic and static sub-regions are not influenced with each other, the negative volume grids can not appear during the updating of the dynamic grids in the opening and closing processes of the simulated spring valve, so that the updating fails; the motion in the numerical simulation is consistent with the motion in the reality, and the regular characteristic of the unsteady flow field in the spring valve can be better predicted. In addition, in the process of dividing the flow field grids of the spring check valve by using the method, two-dimensional grids corresponding to the three-dimensional grids can be obtained, and two-dimensional verification calculation can be performed before three-dimensional unsteady flow field numerical simulation, so that the problems of the calculation grids can be conveniently found in the verification calculation, the grid division is modified, and the workload of people and the calculation amount of a computer are greatly reduced.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a schematic diagram of a conventional method of meshing;
FIG. 3 is a schematic view of an upper two-dimensional motion non-motion sub-basin of the present invention;
FIG. 4 is a schematic view of an upper two-dimensional grid of the present invention;
fig. 5 is a schematic diagram of the overall three-dimensional grid of the present invention.
Detailed Description
The invention is described in more detail below by way of example with reference to the accompanying drawings:
with reference to fig. 1-5, the method for partitioning a grid of a block structure provided by the invention is suitable for three dynamic grid updating methods in a commercial CFD solver ANSYS Fluent, separates a static area from a dynamic area in a basin, has strong adaptability to a basin with a complex structure, and can ensure better processing on boundary layer flow, thereby achieving the combination of simulation calculation accuracy and high efficiency. The method provided by the invention adopts a division mode of a sweep method, wherein the sweep method is a 2.5D structural grid division method, namely, a 3D grid is generated by rotating and stretching the 2D grid, and the method has the characteristics of strong controllability, convenience for boundary layer grid encryption and good generated grid orthogonality. The method is mainly realized by two types of general flow field preprocessing software, namely Hypermesh and ICEM CFD. When the dynamic mesh technology is used in the ANSYS Fluent, the requirement on the computational mesh is that all meshes in the same motion area are hexahedron or tetrahedron meshes, which brings certain difficulty to the processing of boundary layer meshes. Hypermesh has strong tetrahedral mesh generation capacity, and when the tetrahedral mesh is divided by adopting Hypermesh, a triangular prism (pentahedron) mesh can be generated at the boundary layer part of the Hypermesh, which obviously does not meet the requirement of a solver; if the boundary layer part grid is not encrypted, the grid of the near wall area hardly meets the Y + requirement of the turbulence model. Meanwhile, the ICEM generates quadrangles and hexahedrons more conveniently, and has strong advantages for processing boundary layer grids, so that the method integrates the advantages of two preprocessing software to realize reasonable division of the grids.
Since the fluid has viscosity and both the velocity gradient and the pressure gradient are large in the boundary layer near the wall surface, the respective physical quantities also change drastically near the wall surface. While according to a similar criterion, for one-dimensional x-direction flow, the boundary layer thickness δxReynolds number Re at xxIt is relevant. The subject spring check valve herein is in a turbulent flow regime, δxWith RexThe relationship between them is as follows:
Figure GDA0003627358150000051
in the formula I, Reynolds number RexIs defined by the following equation:
Rex＝ρu∞x/mu (two)
The flow in the spring check valve of the model belongs to the flow problem in the round pipe, wherein x is the inner diameter of the pipe, rho is the density of the fluid, u is the density of the fluid∞Mu is the dynamic viscosity coefficient of water under normal temperature condition for stable flow speed far away from the wall surface.
Reference will now be made in detail to the embodiments of the present invention, the following examples are given by way of illustration and are intended to better illustrate, but not limit, the methods of the present invention.
The method comprises the following steps of firstly, extracting a flow field model from a design model of the spring check valve, and dividing the whole flow field of the spring check valve into an upper part, a middle part and a lower part, wherein the middle part is in an asymmetric structure, and the upper part and the lower part are in an axisymmetric structure.
In the three-dimensional modeling software Creo, the integral entity structure of the spring check valve is simplified, and structures such as chamfers and the like which have small influence on CFD calculation are removed. And obtaining a spring valve flow field area required by CFD calculation by utilizing Boolean subtraction. The three-dimensional model of the integral flow area of the spring valve is divided into an upper part, a middle part and a lower part, and the step is determined based on the motion state of the spring valve in the opening and closing processes. The middle part of the flow field of the spring check valve is of an asymmetric structure, the upper part and the lower part of the flow field of the spring check valve are of an axisymmetric structure, and the study on the internal flow characteristics of the spring valve mainly focuses on the flow characteristics near the complex structures (valve rod and valve core) in the upper part of the valve, so that the spring valve is divided into an upper part, a middle part and a lower part, which are favorable for better separating moving and non-moving areas, and the division process of grids and the setting process of moving grids are facilitated.
And step two, simplifying the upper and lower three-dimensional watershed models into two-dimensional models, and dividing the two-dimensional watershed into a plurality of moving and non-moving sub watersheds.
The three-dimensional models of the upper and lower parts of the flow field of the spring valve are simplified into two dimensions, the structure is obtained by rotation transformation in the embodiment, and the two-dimensional simplified model is half of the section of the three-dimensional model in the plumb direction; and for a structure resulting from stretching, the two-dimensional simplified model should be the end shape of the three-dimensional model. The two-dimensional flow area of the upper spring valve and the lower spring valve is divided into a plurality of moving sub-flow areas and non-moving sub-flow areas, which is the most important and innovative step in the method. For the sub-watershed segmentation, the principle that the sub-watershed segmentation conforms to the actual motion rule and each non-motion sub-watershed segmentation does not influence each other needs to be followed, and under the condition that the structure of the watershed is simple and the watershed conforms to the actual motion rule, the number of segmentation times is required to be small. The division of the motion non-motion sub-watersheds is a contradictory process, and on one hand, the sub-watersheds are required to be reduced as much as possible, and on the other hand, the structure of the sub-watersheds is required to be simple.
This step is performed in Hypermesh software, which is described in detail below in connection with fig. 3. Fig. 3 shows the upper two-dimensional moving non-moving sub-flow field structure of the spring valve, in the corresponding solid structure, the right side of the line a-b-c-d-e-f-g-h is the moving area, so in the corresponding flow field structure, the sub-flow fields 1, 3 and 5 do not move, and the sub-flow fields 2, 4 and 6 move. The degree of freedom is larger in the actual cutting process, more than one cutting scheme is provided theoretically, but the invention provides a cutting rule, and the cutting can be carried out reasonably, simply and efficiently. The principle can be briefly described as that the moving and non-moving areas are cut along the plumb direction (in this example, the moving direction is the plumb direction), and a-b, c-d, e-f and g-h in fig. 4 are all cut along the plumb direction; and cutting according to the original boundary of the structure in the horizontal direction.
And thirdly, calculating the height of the first layer of grids by using a boundary layer thickness formula, dividing a plurality of blocks of structural grids of each two-dimensional sub-basin, encrypting grids of the boundary layer area close to the wall surface, and establishing interface interfaces among the grids of each two-dimensional sub-basin.
And carrying out multi-block structural grid division on each two-dimensional sub-basin of the upper part and the lower part in ICEM CFD software. The ICEM CFD has great advantages in dividing the structured grids compared with other commercial flow field preprocessing software in the market, and the divided structured grids are regular and good in controllability. The division results are shown in fig. 4. And (3) encrypting the boundary layer grids of the two-dimensional sub-domains at the upper part and the lower part according to the first layer grid height obtained by the first formula and the second formula, wherein the step is still carried out in ICEM CFD software, and the characteristic of good adjustability is embodied by adjusting the nodes on the Edge. The adjustment of the boundary layer grid size according to the boundary conditions and the turbulence model is actually a feedback adjustment process. In the actual grid dividing and numerical simulation process, firstly, the height of the first-layer grid is calculated by applying a first formula and a second formula according to the boundary condition required by an actual problem, and the grid is divided in advance by applying the calculated theoretical value. And then after the operation of the steps, performing CFD calculation by using the generated flow field grid, and then judging whether the boundary layer processing of the set of grids meets the requirements of the selected turbulence model or not according to the wall surface Y + value parameter in post-processing. If the requirement is met, the subsequent numerical simulation work can be carried out, and if the requirement is not met, the step three is returned to the initial position to carry out retreatment on the boundary layer grid. An interface is established between the grids of each two-dimensional sub-basin on the upper part and the lower part, so that the sub-basins can mutually circulate, the sharing and the overlapping of the adjacent sub-basins are ensured, the block butt joint is realized, and the subsequent calculation can be carried out.
And fourthly, generating three-dimensional multi-block structured grids by the upper and lower parts of the two-dimensional multi-block structured grids through a rotary sweeping method, and simultaneously generating a three-dimensional non-structured grid of the middle part of the watershed.
And generating the three-dimensional multi-block structural grids by rotating and sweeping the upper part and the lower part of the two-dimensional multi-block structural grids. During the rotary sweep, attention is paid to the selection control of the number of meshes generated per revolution. Too few grids in each circle can lead to too large length-width ratio of the flow field grid, which is not beneficial to subsequent CFD calculation, and too many grids in each circle can lead to rapid increase of numerical simulation calculation amount. In the study on the unsteady flow field characteristics of the spring check valve in the opening and closing process, the characteristics of the valve element part are mainly focused, and the structure of the flow field around the valve element (i.e. the structure of the upper flow field) is also relatively complex, so that the embodiment divides the upper flow field and the lower flow field into a plurality of structured grids. And the intermediate part is not a focus research part and has an asymmetric structure, so that the intermediate part flow field grid is subjected to unstructured grid division. Therefore, a certain workload can be reduced, and the advantages of the hybrid grid method are also reflected.
And fifthly, integrating the three-dimensional flow field grids of the upper part, the middle part and the lower part of the spring valve, and establishing interface interfaces among the grids of each part to obtain the whole flow field grid of the valve group.
Integrating three-dimensional flow field grids of the upper part, the middle part and the lower part of the spring valve, and establishing interface interfaces among the grids of all the parts to obtain the whole flow field grid of the valve group so as to carry out subsequent numerical simulation research. The division results are shown in fig. 5.

Claims (3)

1. A multi-block structure grid division method for an unsteady flow field of a spring valve with a complex structure is characterized by comprising the following steps:
(1) extracting a flow field model from a spring check valve design model;
(2) simplifying the upper and lower three-dimensional watershed models into two-dimensional models, and dividing the two-dimensional watershed into moving and non-moving sub watersheds;
(3) calculating the height of a first layer of grids by using a boundary layer thickness formula, carrying out structural grid division on each two-dimensional sub-basin, carrying out encryption processing on the grids of the boundary layer area close to the wall surface, and establishing interface interfaces among the grids of each two-dimensional sub-basin;
(4) generating three-dimensional structured grids by the upper and lower two-dimensional structured grids through a rotary sweeping method, and simultaneously generating a three-dimensional unstructured grid of a middle drainage basin;
(5) integrating three-dimensional flow field grids of an upper part, a middle part and a lower part of the spring valve, and establishing interface interfaces among the grids of each part to obtain the whole flow field grid of the valve group;
step (3) calculating the height of a first layer of grids by using a boundary layer thickness formula, carrying out structural grid division on each two-dimensional sub-basin, carrying out encryption processing on the grids of the boundary layer at the position close to the wall surface, and establishing interface interfaces among the grids of each two-dimensional sub-basin, which specifically comprises the following steps: carrying out structural grid division on each two-dimensional sub-basin of the upper part and the lower part according to a formula
Figure FDA0003627358140000011
Rex＝ρu∞The first layer of grid height obtained by x/mu is used for encrypting the boundary layer grids of each two-dimensional sub-basin of the upper part and the lower part, deltaxIs the thickness of the boundary layer, x is the inner diameter of the tube, RexIs Reynolds number, ρ is the fluid density, u∞The flow rate is stable far away from the wall surface, and mu is the dynamic viscosity coefficient of water under the normal temperature condition; using generated flow field meshesAnd (3) carrying out CFD calculation on the grids, judging whether the boundary layer processing of the set of grids meets the requirements of the selected turbulence model or not according to the wall surface Y + value parameters in the post-processing, carrying out subsequent numerical simulation if the boundary layer processing meets the requirements, returning to the initial part of the step (3) to carry out reprocessing on the boundary layer grids if the boundary layer processing does not meet the requirements, and establishing interface interfaces between the grids of the upper and lower two-dimensional sub-domains to enable the sub-domains to mutually circulate, so that the adjacent sub-domains are shared and overlapped to realize block butt joint.
2. The method for meshing the unsteady flow field multi-block structure of the spring valve with the complex structure as recited in claim 1, wherein the method comprises the following steps: the step (1) of extracting the flow field model from the design model of the spring check valve specifically comprises the following steps: the method comprises the steps of simplifying the integral entity structure of the spring check valve, obtaining a spring valve flow field required by CFD calculation by utilizing Boolean subtraction operation, and dividing a three-dimensional model of the integral flow field of the spring valve into an upper part, a middle part and a lower part, wherein the step is determined based on the motion state of the spring valve in the opening and closing processes, the middle part of the flow field of the spring check valve is of an asymmetric structure, and the upper part and the lower part of the flow field of the spring check valve are of an axisymmetric structure.
3. The unsteady flow field multi-block structural meshing method for the spring valve with the complex structure as claimed in claim 1, wherein the method comprises the following steps: simplifying the upper and lower three-dimensional watershed models into two-dimensional models, and dividing the two-dimensional watershed into moving and non-moving sub-watersheds, wherein the specific steps are as follows: simplifying three-dimensional models of upper and lower flow fields of the spring valve into two dimensions, and obtaining a structure through rotation transformation, wherein the two-dimensional simplified model is half of a section of the three-dimensional model in the plumb direction; for the structure obtained by stretching, the two-dimensional simplified model is the end surface shape of the three-dimensional model, the two-dimensional flow areas of the upper spring valve and the lower spring valve are divided into the moving sub-flow areas and the non-moving sub-flow areas, and for the division of the sub-flow areas, the principle that the actual movement rule is met and the moving sub-flow areas and the non-moving sub-flow areas are not mutually influenced is followed.
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