CN111159941B - Method for simulating flow field transient state numerical value in automobile hydraulic torque converter - Google Patents
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Abstract
The invention discloses a method for simulating the transient state numerical value of a fluid field in an automobile hydraulic torque converter, which divides structured grids for all flow channels of all working wheels of the hydraulic torque converter, and a turbulence model selects two equationsThe method comprises the steps of selecting a Green-Gauss Node Based discrete mode for solving a unit central variable gradient in a parameter, selecting a PRESTO pressure interpolation format, selecting a first-order windward format for interpolation methods of flow items including momentum, turbulence energy and specific dissipation rate, performing transient numerical simulation on a flow field in the hydraulic torque converter by adopting a sliding grid method, obtaining pressure pulsation data of each monitoring point by creating a series of monitoring points, performing time-frequency domain analysis, quantitatively capturing transient characteristics of the hydraulic torque converter in the working process, and providing a new basis for the optimization design of the hydraulic torque converter. The invention has the advantages of improving the accuracy and stability of the numerical simulation of the flow field in the hydraulic torque converter and more accurately simulating the flow state in the hydraulic torque converter.
Description
Technical Field
The invention relates to a numerical simulation method for an internal flow field of an automobile hydraulic torque converter, in particular to a method for numerical simulation of an internal flow field transient state of the automobile hydraulic torque converter.
Background
Torque converters are used as important hydrodynamic transmission elements in motor vehicles, which are arranged between the engine and the transmission for the transmission and conversion of the output power of the engine. The working oil in the hydraulic torque converter is circulated between different impellers during operation, and the flow belongs to complex three-dimensional incompressible viscous turbulence. The flowing state of working oil directly influences the hydraulic performance of the hydraulic torque converter, so that the key performances such as the dynamic performance, the economical efficiency and the like of a transmission system and even the whole vehicle are influenced. Therefore, the analysis of the flow field in the hydraulic torque converter becomes the key content of the research of the hydraulic torque converter, and an important basis is provided for the optimal design of the hydraulic torque converter.
The hydraulic torque converter consists of three or more running wheels, and the running wheel flow channels have relative movement in the working process, so that the calculation of the internal flow field belongs to the numerical coupling problem of multiple calculation fields. There are three main approaches to solving such problems in computational fluid dynamics (Computational Fluid Dynamics, CFD): multi-reference frame method (Multiple Reference Frame, MRF), mixed-plane method (Mixing Planes), and Sliding Mesh method (slip Mesh). Wherein the multi-reference frame method and the mixed surface method are two steady state approximation methods, and the main difference is that the interface processing methods are different; while the slipping grid law is used to address non-steady state problems. The first two methods are widely adopted for analyzing the flow field in the hydraulic torque converter at present, and the third method is rarely applied.
The existing method has insufficient numerical simulation precision on the flow field in the hydraulic torque converter. Factors influencing the CFD numerical simulation precision are many, such as the quality of grid division, the selection of a turbulence model, the setting of solving parameters and the like. In order to ensure higher accuracy and obtain convergence results, these factors need to be set reasonably.
The multi-reference coordinate system method and the mixed plane method are basically steady-state approximation methods, the relative reference coordinate system is used for solving in the solving process, and the grid model does not move in the solving process. The hydraulic torque converter belongs to high-speed rotating turbine machinery, transient effects can be generated in the process of high-speed rotation of each working wheel, different-degree pulsation can be generated at each point pressure in an internal flow field, and the transient effects and the pressure pulsation can not be captured by the method.
The above is where the present application requires significant improvement.
Disclosure of Invention
The invention aims to provide a method for simulating the flow field transient state numerical value in an automobile hydraulic torque converter, which improves the accuracy and stability of numerical simulation and more accurately simulates the flow state in the hydraulic torque converter.
In order to solve the technical problems, the invention provides a method for simulating the transient state numerical value of the flow field in an automobile hydraulic torque converter, wherein structured grids are divided for the full flow passage of each working wheel of the hydraulic torque converter, and a turbulence model selects two equationsThe method comprises the steps of selecting a Green-Gauss Node Based discrete mode for solving a unit central variable gradient in a parameter, selecting a PRESTO pressure interpolation format, selecting a first-order windward format for interpolation methods of flow items including momentum, turbulence energy and specific dissipation rate, performing transient numerical simulation on a flow field in the hydraulic torque converter by adopting a sliding grid method, obtaining pressure pulsation data of each monitoring point by creating a series of monitoring points, performing time-frequency domain analysis, quantitatively capturing transient characteristics of the hydraulic torque converter in the working process, and providing a new basis for the optimization design of the hydraulic torque converter.
The invention relates to a method for simulating the flow field transient state numerical value in an automobile hydraulic torque converter, which comprises the following steps of pretreatment, solving and setting and post-treatment:
step 1: the pretreatment is to obtain a finite element model for flow field calculation, and comprises the following specific steps:
(1) establishing geometric models of all working wheels of the hydraulic torque converter, namely geometric models of a pump wheel, a turbine and a guide wheel in three-dimensional modeling software CATIA, and obtaining full-runner models of three working wheels through geometric extraction;
(2) dividing a full-runner model of the three working wheels into a plurality of single-runner models according to the number of blades, and then geometrically repairing the single-runner models;
(3) the establishment of the topological relation is completed by establishing and modifying the Block and the mapping geometrical relation, and a bridge between the geometric model and the Block is built by the mapping geometrical relation;
(4) defining a distribution rule of nodes according to the grid size requirement to obtain a single-flow-channel grid, arranging a grid encryption layer near the surface of the blade, and then defining a rotation period parameter and rotation period nodes to obtain a full-flow-channel grid;
(5) checking the quality of the grid, and if the quality does not meet the requirement, returning to carry out grid light smoothing;
if the requirement is met, the msh grid file is exported.
Step 2: solving and setting: after obtaining a finite element model for flow field calculation, importing the finite element model into flow field analysis software Fluent to set corresponding boundary conditions and related solving parameters for solving, wherein the method comprises the following specific steps of:
reading an msh grid file, and defining physical parameters and boundary conditions;
wherein, the physical parameters refer to physical parameters of working oil of the torque converter, including density and viscosity;
when the sliding grid method is used for solving, boundary conditions of rotary motion are applied to the pump impeller and the turbine runner grid according to different speed ratios, the pump impeller rotating speed is set to be a constant value, and the turbine rotating speed is set to be the product of the pump impeller rotating speed and the speed ratio. A grid interface is also required to be established, and for interfaces of the sliding grid model for transmitting data information, the principle that the set interface area is smaller before larger is followed to reduce errors caused by interpolation;
turbulent flow model is selected asThe model is used for the production of the model,
wherein,the model belongs to a two-equation vortex-induced viscosity model, wherein +.>Representing turbulent kinetic energy>Representing the specific dissipation ratio;
the saidThe model has the characteristics that:
(1) By using a mixing function, in the near-wall regionModel, use of transformed ++in far field region>A model;
(2) At the position ofDamping cross diffusion derivative terms are added into the transport equation;
(3) Modifying the definition of the turbulence viscosity;
setting and solving a solving parameter, wherein the solving parameter comprises the following contents:
a) The solver selects a pressure base coupling solver, and the pressure speed coupling mode selects SIMPLEC;
b) Selection of a discrete format: the variable gradient discrete mode of the unit center selects Green-Gauss Node Based; PRESTO is selected in a pressure interpolation format; the method for interpolating the convection item comprises momentum, turbulence energy and specific dissipation rate, wherein a first-order windward format is selected;
c) Determining a time step: time stepThe formula of (2) is as follows:
wherein,the number of blades of the pump wheel; />Is the rotation speed of the pump wheel; />The number of time steps required for solving the corresponding angle of the pump impeller single flow channel is expressed by a self-defined integer;
d) Step-by-step transient solution: initializing the whole flow field, and then solving and calculating;
the solving process is carried out in three steps:
the first step is to obtain a converged result; if not, returning to modify the solving parameters;
if the convergence is carried out, carrying out post-processing;
the second step is to observe the change process of each monitoring variable along with time, so as to ensure that the solving result reaches periodic time variation;
and a third step of: firstly, creating a series of periodically distributed monitoring points in a calculation domain and monitoring variables of pressure at the points; then, the time step number is selected to obtain the data of the static pressure change of the monitoring point along with time.
Step 3: and the post-processing comprises the steps of drawing a cloud chart of the distribution of each field quantity, analyzing the frequency spectrum and drawing the frequency spectrum chart, so as to analyze the transient flow characteristic of the hydraulic torque converter in the actual working process.
The invention has the advantages that:
1) The grid division adopts a structured grid, so that the convergence speed is improved;
2) The turbulence model taking two equationsThe model is used for more accurately simulating the flow of the near-wall area;
3) The discrete mode of solving the unit central variable gradient in parameter setting selects Green-Gauss Node Based, the pressure interpolation format selects PRESTO, the interpolation methods of the convection items all select a first-order windward format, the accuracy and stability of flow field numerical simulation in the hydraulic torque converter are improved, and the flow state in the hydraulic torque converter is simulated more accurately;
4) And carrying out transient solving calculation by adopting a slipping grid technology, thereby capturing the transient characteristics of a flow field in the hydraulic torque converter.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention. In the drawings:
FIGS. 1a and 1b are diagrams of a pump grid model and a partial enlarged view according to an embodiment of the present invention;
FIGS. 2a and 2b are graphs of pressure pulsations at zero speed ratio and 0.2 speed ratio at a monitoring point selected in accordance with an embodiment of the present invention;
FIG. 3 is a cloud chart of the speed distribution of the inlet and outlet surfaces of the pump wheel according to the embodiment of the invention;
FIG. 4 is a cloud chart of the pressure distribution of the inlet and outlet surfaces of a pump wheel according to an embodiment of the invention;
FIGS. 5a and 5b are graphs showing the frequency spectrum of a monitoring point selected at a zero speed ratio and a 0.2 speed ratio according to the embodiment of the present invention;
FIG. 6 is a flow chart of a flow field transient numerical simulation in a torque converter of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail with reference to the accompanying drawings and examples. The specific embodiments described herein are to be considered in an illustrative sense only and are not intended to limit the invention.
As shown in FIG. 6, the invention provides a method for simulating the flow field transient state numerical value in an automobile torque converter, which divides structured grids for all runners of each running wheel of the torque converter, and selects two equations for a turbulence modelA model, a discrete mode of solving the unit center variable gradient in the parameter is selected from Green-Gauss Node Based,the pressure interpolation format selects PRESTO, the interpolation methods of the flow items including momentum, turbulence energy and specific dissipation rate all select a first-order windward format, a slippage grid method is adopted to carry out transient numerical simulation on the flow field in the hydraulic torque converter, a series of monitoring points are created to obtain pressure pulsation data of each monitoring point to carry out time-frequency domain analysis, transient characteristics of the hydraulic torque converter in the working process are quantitatively captured, and a new basis is provided for the optimal design of the hydraulic torque converter.
The invention relates to a method for simulating the flow field transient state numerical value in an automobile hydraulic torque converter, which comprises the following steps of pretreatment, solving and setting and post-treatment:
step 1: the pretreatment is to obtain a finite element model for flow field calculation, and comprises the following specific steps:
(1) establishing geometric models of all working wheels of the hydraulic torque converter, namely geometric models of a pump wheel, a turbine and a guide wheel in three-dimensional modeling software CATIA, and obtaining full-runner models of three working wheels through geometric extraction;
(2) because each working wheel is periodically distributed in structure, in order to facilitate grid division, the full-flow channel model of the three working wheels is divided into a plurality of single-flow channel models according to the number of blades, and then the geometric repair is carried out on the single-flow channel models;
the geometric repair is to delete redundant geometric elements such as points, curves and the like, and close each adjacent curved surface so as to obtain a continuous and simplified flow channel model;
(3) the establishment of the topological relation is completed by establishing and modifying the Block and the mapping geometrical relation;
the Block is a foundation for generating a structured grid, a geometric model is embodied through the Block, and a bridge between the geometric model and the Block is built through mapping geometric relations. The mapping geometrical relationship refers to mapping from Vertex to Point, edge to Curve, and Face to Surface, wherein Vertex, edge, face corresponds to nodes, boundaries and surfaces on Block respectively; point, curve, surface correspond to points, curves and surfaces on the geometric model, respectively;
(4) defining a distribution rule of nodes according to the grid size requirement to obtain a single-channel grid; in order to more accurately simulate the flow of the area near the surface of the blade, a grid encryption layer is arranged near the surface of the blade; then, a full-runner grid is obtained by defining rotation period parameters including a rotation shaft, a rotation angle and rotation period nodes;
(5) checking the quality of the grid, and if the quality does not meet the requirement, returning to carry out grid light smoothing;
if the requirement is met, an msh grid file is exported;
the criterion for quality inspection uses the determinent (2 x 2) criterion, which represents the ratio of the minimum jacobian matrix to the maximum jacobian matrix Determinant, 1 indicating the best quality, and 0 indicating the worst quality. The minimum value of the grid quality of each work wheel is ensured to be more than 0.2;
the divided impeller mesh model is shown in fig. 1a and 1 b.
Step 2: solving and setting: after obtaining a finite element model for flow field calculation, importing the finite element model into flow field analysis software Fluent to set corresponding boundary conditions and related solving parameters for solving, wherein the method comprises the following specific steps of:
reading an msh grid file, and defining physical parameters and boundary conditions;
wherein, the physical parameters refer to physical parameters of working oil of the torque converter, including density and viscosity;
when the sliding grid method is used for solving, boundary conditions of rotary motion are applied to the pump impeller and the turbine runner grid according to different speed ratios, the pump impeller rotating speed is set to be a constant value, and the turbine rotating speed is set to be the product of the pump impeller rotating speed and the speed ratio. In order to realize the transmission of all field variables of the internal flow field among all the running wheels, grid interfaces are also required to be established, and for interfaces of the sliding grid model for transmitting data information, the principle that the arranged interface areas are smaller before larger is followed to reduce errors caused by interpolation. The interfaces set are shown in Table 1.
；
Turbulent flow model is selected asThe model is used for the production of the model,
wherein,the model belongs to a two-equation vortex-induced viscosity model, wherein +.>Representing turbulent kinetic energy>Representing the specific dissipation ratio;
the saidThe model has the characteristics that:
(1) By using a mixing function, in the near-wall regionModel, use of transformed ++in far field region>A model;
(2) At the position ofDamping cross diffusion derivative terms are added into the transport equation;
(3) The definition of turbulent viscosity is modified to take into account the transport of turbulent shear stresses.
In the model->And->The transport equation for (2) is as follows:
（1）
（2）
wherein,and->Respectively->And->Is a product of the steps; />And->Respectively->And->Diffusion coefficient of (a); />Andrespectively->And->Dissipation terms created by turbulence; />Is a cross diffusion term; />And->The source items are respectively customized by users, and the source items are taken as zero in the flow field calculation in the hydraulic torque converter.
The definition of two production terms is as follows:
（3）
the two diffusion terms are defined as follows:
（4）
wherein the method comprises the steps ofAnd->Respectively->And->Is defined as follows:
（5）
the definition of the two dissipation terms is as follows:
（6）
the cross diffusion term is defined as follows:
（7）
the constant values in the above formula are as follows:
setting and solving a solving parameter, wherein the solving parameter comprises the following contents:
a) The solver selects a pressure-based coupling solver; in order to accelerate the convergence speed, the pressure speed coupling mode is selected from SIMPLEC;
b) Selection of a discrete format: the variable gradient discrete mode of the unit center selects Green-Gauss Node Based; the PRESTO is selected in a pressure interpolation format which is suitable for high rotational flow, including surfaces with abrupt pressure gradient changes, and thus suitable for calculation of flow fields in a torque converter; because the calculation amount is large when the sliding grid technology is applied, the interpolation methods of the convection items including momentum, turbulence energy and specific dissipation rate all select a first-order windward format, which is beneficial to accelerating the convergence speed and shortening the calculation time;
c) Determining a time step: since the rotational speed of the pump wheel is unchanged under each calculation condition, the pump wheel is used as a reference object to determine the time step, and the time stepThe formula of (2) is as follows:
wherein,the number of blades of the pump wheel is 31 in the present embodiment; />The rotational speed of the pump wheel, in this example 2000rpm; />The number of time steps required for solving the corresponding angle of the pump impeller single flow channel is expressed as a self-defined integer, and the larger the value is, the smaller the time step is, the larger the frequency range is when the frequency spectrum analysis is carried out on the pressure pulsation data. However, the larger the total number of solving steps, the longer the time required for solving. Considering the required frequency range and computational efficiency in combination, in this embodiment +.>Taken as 11 and the maximum frequency is 11358.33Hz. Substituting the parameters into the expression to calculate the time step of 8.798 ×10 -5 s；
d) Step-by-step transient solution: initializing the whole flow field, and then solving and calculating;
the solving process is carried out in three steps:
the number of time steps in the first step is set to be the number of steps used for two rotations of the pump impellerStep, in order to obtain a convergence result; if not, returning to modify the solving parameters;
if the convergence is carried out, carrying out post-processing;
the time step number in the second step is set as 341 steps which are used for one revolution of the pump wheel, so as to observe the change process of each monitoring variable along with time and ensure that the solving result reaches periodic time variation;
and a third step of: firstly, creating a series of periodically distributed monitoring points in a calculation domain and monitoring variables of pressure at the points; then the time step number is set as 1364 steps which are used for the pump wheel to rotate around, so as to obtain the data of the static pressure change of the monitoring point along with the time. The more steps, the higher the frequency resolution of the spectral analysis of the pulsating pressure, in this case 8.33Hz. The pressure pulsation curves of a certain selected monitoring point under the working condition of zero speed ratio and the working condition of 0.2 speed ratio are shown in figures 2a and 2b respectively.
Step 3: post-treatment:
post-processing includes two ways: the first mode is to draw a distribution cloud chart of each field quantity such as speed and pressure by taking the solving result of the last step as a reference so as to observe the distribution characteristic of each field quantity in space, thereby analyzing the steady-state flow characteristic of the flow field in the hydraulic torque converter. The velocity distribution cloud patterns of the inlet and the outlet of the pump wheel and the pressure distribution cloud patterns of the surface of the pump wheel blade are respectively shown in fig. 3 and fig. 4. The second mode aims at transient solving, frequency spectrum analysis is carried out on the obtained pressure pulsation data of the monitoring points through Fourier transformation, and frequency characteristics of pressure pulsation on each monitoring point are obtained, so that transient flow characteristics of the hydraulic torque converter in the actual working process are analyzed, and frequency spectrograms of a selected monitoring point under a zero speed ratio working condition and a 0.2 speed ratio working condition are respectively shown in fig. 5a and 5 b.
The above embodiments are preferred embodiments of the present invention, and the present invention is not limited to the above embodiments, but changes, modifications, substitutions, combinations, and simplifications made within the spirit and basic principles of the present invention should be included in the scope of the present invention.
Claims (2)
1. A method for flow field transient numerical simulation in an automotive torque converter, characterized by: the method comprises the steps of preprocessing, solving and setting and post-processing; wherein:
step S1: the pretreatment comprises the steps of dividing a structured grid for all runners of each working wheel of the hydraulic torque converter, and comprises the following specific steps:
step S11: establishing geometric models of all working wheels of the hydraulic torque converter, namely geometric models of a pump wheel, a turbine and a guide wheel in three-dimensional modeling software, and obtaining a full-runner model of the working wheels through geometric extraction;
step S12: dividing a full-runner model of the running wheel into a plurality of single-runner models according to the number of blades, and then geometrically repairing the single-runner models;
step S13: the establishment of the topological relation is completed by establishing and modifying the Block and the mapping geometrical relation, and a bridge between the geometric model and the Block is built by the mapping geometrical relation;
step S14: defining a distribution rule of nodes according to the grid size requirement to obtain a single-flow-channel grid, arranging a grid encryption layer near the surface of the blade, and then defining a rotation period parameter and rotation period nodes to obtain a full-flow-channel grid;
step S15: checking the quality of the grid, and if the quality does not meet the requirement, returning to carry out grid light smoothing; if the requirement is met, an msh grid file is exported;
step S2: the solving and setting comprises the steps of selecting a turbulence model to obtain a k-omega SST model of two equations, selecting a Green-Gauss Node Based discrete mode of unit central variable gradient in solving parameters, selecting a PRESTO format, selecting a first-order windward format by interpolation methods of the flow items including momentum, turbulence energy and specific dissipation rate, performing transient numerical simulation on the flow field in the hydraulic torque converter by adopting a sliding grid method, and obtaining a pressure pulsation curve of the monitoring points by creating a series of monitoring points, wherein the method comprises the following specific steps;
step S21: reading in an msh grid file, and defining physical parameters and boundary conditions;
wherein, the physical parameters refer to physical parameters of working oil of the torque converter, including density and viscosity;
applying boundary conditions of rotary motion to the pump impeller and the turbine runner grid according to different speed ratios, wherein the pump impeller rotating speed is set to be a constant value, and the turbine rotating speed is set to be the product of the pump impeller rotating speed and the speed ratio; a grid interface is also established, and the principle that the arranged interface area is firstly small and then large is followed;
step S22: the turbulence model is chosen to be a k- ωsst model,
wherein the k-omega SST model belongs to a two-equation vortex-induced viscosity model, wherein k represents turbulent kinetic energy, and omega represents specific dissipation ratio;
step S23: setting and solving parameters, wherein the method comprises the following steps:
step S231: the solver selects a pressure base coupling solver, and the pressure speed coupling mode selects SIMPLEC;
step S232: selection of a discrete format: the variable gradient discrete mode of the unit center selects Green-Gauss Node Based; PRESTO is selected in a pressure interpolation format; the method for interpolating the convection item comprises momentum, turbulence energy and specific dissipation rate, wherein a first-order windward format is selected;
step S233: determining a time step: the time step τ is calculated as follows:
wherein b is the number of blades of the pump wheel; n is the rotation speed of the pump wheel; a represents the number of time steps required for solving the corresponding angle of the pump impeller single flow channel;
step S234: step-by-step transient solution: initializing the whole flow field, solving and calculating, and creating a series of periodically distributed monitoring points in a calculation domain and pressure monitoring variables on the points; selecting the time step number to obtain data of static pressure change of the monitoring point along with time;
step S3: the post-processing is to acquire pressure pulsation data of each monitoring point for time-frequency domain analysis and quantitatively capture transient characteristics of the hydraulic torque converter in the working process.
2. The method for flow field transient numerical simulation in an automotive torque converter of claim 1, wherein: the post-processing comprises the steps of drawing a cloud chart of distribution of each field quantity, carrying out spectrum analysis and drawing a spectrum chart, and analyzing transient flow characteristics of the hydraulic torque converter in the actual working process.
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