CN113221417B - Virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann - Google Patents
Virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann Download PDFInfo
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Abstract
The invention discloses a virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann, which comprises the following steps of: s1, constructing a discrete-continuous coupling numerical model of the virtual triaxial sample; s2, applying equal compressive stress in each direction, and carrying out discrete-continuous coupling numerical simulation in the consolidation process; s3, maintaining lateral confining pressure and applying axial loading speed, and carrying out discrete-continuous coupling numerical simulation in the shearing process; s4, slicing the virtual sample in the triaxial shearing process, processing the slice image, calculating the volume strain of the sample, and establishing a corresponding lattice Boltzmann numerical model; s5, aiming at the established lattice Boltzmann numerical model, carrying out lattice Boltzmann numerical simulation of seepage, and calculating the permeability of the sample. The method is simple in principle, convenient to implement and stable in calculation, and the feasibility and the superiority of the virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann are verified through a virtual triaxial penetration test simulation example.
Description
Technical Field
The invention relates to the technical field of particle material virtual test simulation in the field of geotechnical engineering, in particular to a virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann.
Background
Excavation, filling and the like that frequently take place among engineering construction such as highway, railway, water conservancy, mine can make the ground body take place to warp, and this kind of deformation can arouse the change of ground body inner structure, leads to its permeability to change then to probably induce engineering problems such as side slope slip, tunnel gush water, dam seepage. Therefore, the research on the change of the permeability of the rock-soil body in the deformation process is of great significance.
The triaxial compression test is the most common means for researching the stress-strain characteristics of rock-soil mass, and is also a common permeability coefficient measuring means, which not only can easily prevent the side wall leakage in the conventional permeability test, but also can simulate different stress states. Therefore, some students study the triaxial compression deformation of the rock-soil body and the change rule of the permeability coefficient in the deformation process through an indoor triaxial penetration test. However, the indoor triaxial penetration test is time-consuming and difficult to know the change of the microscopic structure and the seepage field inside the sample, and the mechanism exploration of the rock-soil mass shear deformation and the penetration change in the deformation process is hindered.
With the development of computers and numerical calculation methods, virtual test simulation technology has been widely applied in the field of geotechnical engineering. Aiming at the triaxial penetration test, on one hand, the rock-soil body is a discrete particle medium which generates larger shear deformation or cracking under the action of triaxial stress, and the latex film on the lateral side of the sample is a typical continuous medium which generates elastic deformation in the test, so that the triaxial deformation of the rock-soil body is difficult to effectively simulate by singly adopting a finite element of continuous medium mechanics and a finite difference method or a particle discrete element method of discontinuous medium mechanics. In recent years, partial scholars simulate a rock-soil triaxial test based on a discrete-continuous coupling method, and obtain better effects, but have defects, such as failure to obtain volume strain of a sample and the like. On the other hand, rock-soil mass belongs to complex porous media, a high-quality three-dimensional grid is difficult to generate when seepage of the rock-soil mass is simulated by adopting a conventional finite volume method or a finite element method, and the like, and the grid Boltzmann method adopts regular cubic grid units, so that seepage in the complex porous media can be conveniently simulated. Therefore, a virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann is needed to be provided, so that technical support is provided for subsequent scientific research and engineering application.
Disclosure of Invention
The invention aims to solve the technical problem of providing a virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann, aiming at the defects in the prior art, and the method realizes effective simulation of stress-strain, volume change and permeability change characteristics of a virtual rock-soil body sample in the triaxial test process by combining a discrete-continuous coupling calculation method in the field of solid mechanics and a lattice Boltzmann method in the field of fluid mechanics, and can provide powerful support for mechanical seepage characteristic research of rock-soil and other granular materials.
The technical scheme adopted by the invention for solving the technical problems is as follows:
the invention provides a virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann, which comprises the following steps:
s1, constructing a discrete-continuous coupling numerical model of the virtual triaxial sample; the method comprises the following steps:
s1.1, establishing a cylindrical continuous medium mechanical unit simulating a lateral emulsion film and a coupling boundary attached to the surface of the unit according to the size of a virtual triaxial sample, and generating top and bottom loading plates;
s1.2, establishing a discrete particle aggregate of the sample according to the particle gradation of the virtual triaxial sample;
s1.3, fixing the lateral membrane units and the top and bottom loading plates, and performing discrete-continuous coupling numerical calculation to balance the model;
s2, applying equal compressive stress in each direction, and carrying out discrete-continuous coupling numerical simulation in the consolidation process; the method comprises the following steps:
s2.1, fixing the lateral membrane units on the uppermost layer and the lowermost layer, enabling other membrane units to be free, resetting the contact force among the particles to be zero, and setting an updating mode of the normal contact force among the particles to be an increment mode;
s2.2, applying lateral compressive stress which is perpendicular to the unit surface and points to the inside of the sample on the lateral membrane unit, applying vertical corresponding pressure with the same size through loading plates at the top and the bottom, and performing coupling simulation of an anisotropic consolidation process;
s3, maintaining lateral confining pressure and applying axial loading speed, and carrying out discrete-continuous coupling numerical simulation in the shearing process; the method comprises the following steps:
s3.1, maintaining the confining pressure on the lateral membrane unit unchanged, setting the axial loading speed of a top loading plate and a bottom loading plate, starting discrete-continuous coupling simulation of a shearing process, and recording the axial stress and the axial strain of a sample at regular intervals;
s3.2, when the axial strain reaches a specified target value, stopping test loading, and storing a test result;
s4, slicing the virtual sample in the triaxial shearing process, processing the slice image, calculating the volume strain of the sample, and establishing a corresponding lattice Boltzmann numerical model; the method comprises the following steps:
s4.1, cutting the triaxial sample by a series of sections with equal intervals smaller than a certain threshold value along the axial direction at intervals of certain axial strain, and respectively deriving sample boundaries and slice images of particles at each section;
s4.2, carrying out batch processing on the derived slice images of the sample boundaries, identifying the sample boundaries in the slice images, marking the inner and outer boundaries as different colors, marking the regions outside the sample boundaries and inside the picture boundaries as the same color as the particles on the particle slices, representing solid regions, accumulating the number of pixels inside the sample boundaries on all the slices, multiplying the total number of pixels obtained by accumulation by the actual area represented by 1 pixel to obtain the total area in the sample boundary range on all the slices, and multiplying the total area by the distance between the adjacent slices, wherein the value obtained can be approximate to the volume of the sample at the time; obtaining the volume of the sample when each axial strain occurs in the triaxial shearing process, and further obtaining the volume strain of the sample;
s4.3, splicing the sample particle slice image derived from the same position and the sample boundary slice image with the sample boundary identified, and taking the side length of a pixel unit of the spliced image as the vertical spacing distance of adjacent slices, so that the three-axis sample is divided by adopting a cubic lattice unit, and a lattice Boltzmann numerical model corresponding to the three-axis sample is established;
s5, carrying out lattice Boltzmann numerical simulation of seepage aiming at the established lattice Boltzmann numerical model, and calculating the permeability of the sample; the method comprises the following steps:
s5.1, setting fluid calculation parameters aiming at the established lattice Boltzmann numerical model, wherein the fluid calculation parameters comprise: performing iterative calculation of lattice Boltzmann migration and collision on a fluid particle collision model, a discrete velocity model and a fluid boundary condition;
and S5.2, stopping iterative calculation after the seepage field converges to a stable state, and calculating the permeability of the sample by utilizing Darcy' S law according to the applied water pressure difference.
Further, in step S1 of the present invention, the constructed discrete-continuous coupled numerical model of the virtual triaxial sample is implemented based on the coupled numerical platform of the three-dimensional particle discrete element method PFC3D and the finite difference method FLAC 3D; the lateral latex membrane is formed by arranging and combining Zone units or Shell structural units in FLAC3D in a cylindrical manner, rock-soil particles are simulated by balls or columns in PFC3D, top and bottom loading plates are simulated by walls in PFC3D, and the coupling of the membrane units and the particles is carried out through coupling walls attached to the membrane units; before the coupling calculation, the large strain mode of FLAC3D is turned on, and PFC3D and FLAC3D are set to be the same analysis time step by using a time step scaling function; in the calculation iteration process, the speed obtained by solving FLAC3D is transmitted to a PFC3D model part through coupling Wall, so that PFC model response is updated, acting force generated by the coupling Wall is transmitted to a corresponding node of a FLAC3D unit through a mapping principle to serve as a boundary condition for updating FLAC3D model mechanical response, and the like, and the coupling calculation is performed step by step.
Further, in the step S2 of the present invention, the lateral compressive stress is slowly applied step by step, and the vertical consolidation pressure of the top and bottom loading plates is applied by a servo mechanism in discrete elements.
Further, in said step S3.1 of the invention, the axial stress σ1And axial strain epsilonaThrough a middle die of PFC3DThe load plates at the top and the bottom of the model are obtained by Wall calculation, and the calculation formula is as follows:
wherein, FztopAnd FzbotTotal vertical contact force, Z, to which the top and bottom load plates Wall are subjected, respectivelytopAnd ZbotVertical coordinates of the Wall positions of the top and bottom loading plates, H0Height of the sample after consolidation and before shearing, A0Is the initial cross-sectional area of the sample; because the lateral film units on the uppermost layer and the lowermost layer are kept fixed in the process of solidifying and shearing the sample, the cross sectional areas of the top and the bottom of the sample are always equal to the initial cross sectional area.
Furthermore, in the step S4.3, after the sample particle slice image and the sample boundary slice image are pieced together, marking of the solid region and the pore region is realized; pixels of the solid area correspond to solid cells of the lattice Boltzmann model, and pixels of the pore area correspond to fluid cells of the lattice Boltzmann model; the solid area is further divided into a flow-solid boundary unit and a solid internal unit, and the specific method is as follows: if the adjacent units around a certain solid unit are all solid units and comprise corresponding adjacent units on adjacent slices, the solid unit is marked as a solid internal unit; if one or more adjacent cells are pore cells, the cell is labeled as a flow-solid boundary cell.
Further, in the step S5.1 of the present invention, a single relaxation time BGK collision model is adopted as the fluid particle collision model; a discrete velocity model, which adopts a D3Q19 model; the peripheral wall surfaces of the model and the surfaces of the solid particles are non-slip flow-solid boundaries, a rebound method is adopted for processing, the flow direction of the fluid is along the axial direction of the sample and is driven by the pressure difference of an inlet and an outlet, and the pressure boundary is processed by a Zou/He method; and no calculation is carried out on the solid internal unit in the seepage grid Boltzmann simulation process, so that the calculation time is reduced.
Further, in step S5.2 of the present invention, after the seepage field converges to a steady state, the iterative calculation is stopped, and the permeability K of the sample is calculated according to the applied water pressure difference by using darcy' S law, where the calculation formula is:
wherein rho is the density of the fluid, mu is the dynamic viscosity of the fluid, upsilon is the kinematic viscosity of the fluid,
in order to be the average flow rate,
is the pressure difference, l is the length of the model in the direction of flow, pinAnd poutInlet and outlet pressures, respectively.
The invention has the following beneficial effects: the virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann has the advantages of simple principle, convenient implementation and stable calculation, can realize effective simulation of the stress-strain, volume change and penetration change characteristics of a virtual rock-soil body sample in the triaxial test process, can conveniently obtain the microscopic internal structure and the corresponding seepage field of the sample in the triaxial shear deformation process, and can provide technical support for the multi-scale research of the mechanical seepage characteristics of rock soil and other granular materials.
Drawings
The invention will be further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a general flow chart of a virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a virtual three-axis sample discrete-continuous coupling numerical model according to an embodiment of the present invention;
FIG. 3 is a graph of the relationship of partial stress to axial strain for an embodiment of the present invention;
FIG. 4 is a schematic diagram of a post-shear model of a virtual triaxial sample according to an embodiment of the present invention;
FIG. 5 is a schematic view of slice image processing according to an embodiment of the present invention;
FIG. 6 is a graph of volumetric strain versus axial strain for an embodiment of the present invention;
FIG. 7 is a graph of permeability versus axial strain for an embodiment of the present invention;
fig. 8 is a cloud view of a flow field of a virtual triaxial sample after shearing in accordance with an embodiment 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 is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
As shown in fig. 1, an embodiment of the present invention discloses a virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann, including the following steps:
and S1, constructing a discrete-continuous coupling numerical model of the virtual triaxial sample. The method comprises the following specific steps:
s1.1, establishing a cylindrical continuous medium mechanical unit simulating a lateral emulsion film and a coupling boundary attached to the surface of the cylindrical continuous medium mechanical unit according to the size of a virtual triaxial sample, and generating top and bottom loading plates;
s1.2, establishing a discrete particle aggregate of the sample according to the particle gradation of the virtual triaxial sample;
s1.3, fixing the lateral membrane units and the loading plates at the top and the bottom, and performing discrete-continuous coupling numerical calculation to balance the model.
Fig. 2 shows a virtual triaxial sample numerical model constructed based on a PFC3D-FLAC3D coupled numerical platform. The virtual triaxial sample has a diameter of 50mm and a height of 125 mm. The lateral emulsion membrane is formed by arranging and combining Shell structural units in FLAC3D according to a cylinder shape, the axial direction of the lateral emulsion membrane is divided into 12 sections, the annular direction of the lateral emulsion membrane is divided into 24 sections, the Young modulus is 1.25MPa, the Poisson ratio is 0.2, the thickness is 0.3mm, and the density is 930kg/m3. The top and bottom load plates were simulated by a square Wall in PFC3D, with a side length of60mm, normal stiffness 1X 107N/m, and the tangential stiffness and friction coefficient are 0. The sample soil particles were simulated by Ball in PFC3D, and had a diameter of 4mm and a density of 2600kg/m3Normal stiffness and tangential stiffness are 1X 10 respectively6And 5X 105N/m, coefficient of friction 0.4. The coupling of the membrane Shell cell to the particle Ball is performed by the coupling Wall attached to the membrane Shell cell, the large strain mode of FLAC3D is turned on before the coupling calculation and the analysis time steps of both PFC3D and FLAC3D are set to 1 with the time step scaling function.
And S2, applying the pressure stress with equal directions to carry out the discrete-continuous coupling numerical simulation of the consolidation process.
The method comprises the following specific steps:
s2.1, keeping the fixing of the lateral membrane units on the uppermost layer and the lowermost layer, enabling other membrane units to be free, resetting the contact force among the particles to be zero, and setting an updating mode of the normal contact force among the particles to be an increment mode;
and S2.2, applying confining pressure which is perpendicular to the unit surface and points to the inside of the sample on the lateral membrane unit, applying vertical corresponding pressure with the same size through loading plates at the top and the bottom, and performing coupling simulation of an anisotropic pressure consolidation process.
In this example, the consolidation confining pressure of the sample is 50kPa, the lateral compressive stress is gradually and slowly applied in steps of 1kPa/2000steps, and the vertical compressive stress is gradually applied by continuously adjusting the speeds of the top and bottom loading plates Wall through the Wall servo mechanism in PFC 3D. In order to avoid the 'explosion' of the sample caused by small compressive stress when loading is started, the contact force between particles is reset to 0 and the contact force between particles is calculated and adjusted to be in an increment mode before consolidation stress is applied.
And S3, maintaining the lateral confining pressure and applying the axial loading speed, and carrying out discrete-continuous coupling numerical simulation of the shearing process. The method comprises the following specific steps:
s3.1, maintaining the confining pressure on the lateral membrane unit unchanged, setting the axial loading speed of a top loading plate and a bottom loading plate, starting discrete-continuous coupling simulation of a shearing process, and recording the axial stress and the axial strain of a sample at regular intervals;
and S3.2, stopping test loading when the axial strain reaches a specified target value, and storing a test result.
In this example, the top and bottom load plates Wall were set to have speeds of 1 × 10-7m/s, the top Wall moves downwards, the bottom Wall moves upwards, the target value of the axial strain is 15%, and a bias stress-axial strain relation curve obtained by triaxial test simulation is shown in fig. 3. Fig. 4 shows a virtual sample with an axial strain of 15% obtained by the triaxial test simulation of the present embodiment, and it can be seen that the sample undergoes non-uniform bulging deformation in the lateral direction after being compressed, which is consistent with the failure mode of the sample in the indoor triaxial test.
And S4, slicing the virtual sample in the triaxial shearing process, processing the slice image, calculating the volume strain of the sample, and establishing a corresponding grid Boltzmann numerical model. The method comprises the following specific steps:
s4.1, cutting the triaxial sample by a series of sections with small and equal intervals in the axial direction at intervals of certain axial strain, and respectively deriving sample boundaries and slice images of particles at each section;
and S4.2, carrying out batch processing on the derived sample boundary slice images, identifying the sample boundaries in the slice images, marking the inner and outer boundaries as different colors (the regions outside the sample boundaries and inside the picture boundaries are marked as the same color as the particles on the particle slices and represent solid regions), accumulating the number of pixels inside the sample boundaries on all the slices, multiplying the total number of pixels obtained by accumulation by the actual area represented by 1 pixel to obtain the total area in the sample boundary range on all the slices, and multiplying the total area by the distance between the adjacent slices to obtain a value which can approximate the volume of the sample at the time. Therefore, the sample volume when each axial strain is generated in the triaxial shearing process can be obtained, and the change of the sample volume strain along with the axial strain can be further obtained.
And S4.3, splicing the sample particle slice image led out from the same position with the sample boundary slice image with the sample boundary identified, and taking the side length of a pixel unit of the spliced image as the vertical spacing distance of adjacent slices, so that the three-axis sample is divided by adopting a cubic lattice unit, and a lattice Boltzmann numerical model corresponding to the three-axis sample is established.
Fig. 5 is a schematic diagram illustrating a process of processing a slice image of a virtual specimen. In this embodiment, the initial sample before shearing after consolidation and the model corresponding to each 1.5% axial strain in the shearing process are derived into horizontal slices, the vertical interval of the slices is 0.625mm, and the actual length corresponding to the side length of the slice image is 800 mm. And (3) carrying out batch processing on the derived sample boundary slice images and sample particle slice images according to steps S4.2 and S4.3 based on MATLAB, wherein the sample boundaries on the slice images are irregular after the sample is deformed, identifying through an edge function (operator selection log) in the MATLAB, filling the areas except the identified boundaries into black (the particles are black), calculating the volume of the sample, and further obtaining the volume strain of the sample when the axial strain is different in the shearing process, as shown in FIG. 6.
Further, as shown in fig. 5, pixels (corresponding to solid cells of the lattice Boltzmann model) of solid regions (black regions including solid particles and regions outside the sample boundary) on the slice image after the particles and the boundary are pieced together are classified into two types, i.e., a flow-solid boundary cell and a solid internal cell, and specifically, the method includes: if adjacent units (including corresponding adjacent units on adjacent slices) around a certain solid unit are all solid units, the solid unit is marked as a solid internal unit; if one or more adjacent cells are pore cells, the cell is labeled as a flow-solid boundary cell. Therefore, in the subsequent seepage grid Boltzmann simulation process, the calculation of the solid internal unit is not carried out, and the calculation time can be obviously saved.
S5: and carrying out the lattice Boltzmann numerical simulation of seepage aiming at the established lattice Boltzmann numerical model, and calculating the permeability of the sample. The method comprises the following specific steps:
s5.1, aiming at the established lattice Boltzmann numerical model, setting a fluid particle collision model, a discrete velocity model, a fluid boundary condition and related fluid calculation parameters, and performing iterative calculation of lattice Boltzmann migration and collision.
And S5.2, stopping iterative calculation after the seepage field converges to a stable state, and calculating the permeability of the sample by utilizing Darcy' S law according to the applied water pressure difference.
The seepage grid Boltzmann simulation of this embodiment is implemented by performing parallel computation on an open-source distributed computing platform palebos using 8 computing threads. Fluid particle collisions employ a common single relaxation time BGK collision model. The discrete velocity model uses a commonly used D3Q19 model. The peripheral wall surfaces of the model and the surfaces of the solid particles are non-slip fluid-solid boundaries and are treated by a rebound method. The fluid flow direction is along the sample axis, driven by the pressure difference between the inlet and outlet, and the pressure boundary is processed by the Zou/He method. The side length of the lattice unit is the side length of the pixel after the image processing in the above step S4, that is, the spacing distance between slices is 0.625mm, the relaxation time τ is 0.8, and the pressure difference is 1 × 10-7(grid unit). Fig. 7 is a graph showing the relationship between permeability and axial strain obtained by simulation, and fig. 8 is a flow velocity field cloud chart obtained after the seepage of a triaxial sample is simulated by using a lattice Boltzmann when the axial strain is 15%.
The results of the embodiment show that the virtual triaxial penetration test simulation method based on the discrete-continuous coupling and the lattice Boltzmann can better simulate the triaxial compression deformation of the rock-soil body and the change characteristics of the penetration in the deformation process, and further verifies the advantages of the invention.
It should be understood that the above are only specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily made by those skilled in the art within the technical scope of the present invention disclosed herein should be covered within the scope of the present invention.
Claims (7)
1. A virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann is characterized by comprising the following steps:
s1, constructing a discrete-continuous coupling numerical model of the virtual triaxial sample; the method comprises the following steps:
s1.1, establishing a cylindrical continuous medium mechanical unit simulating a lateral emulsion film and a coupling boundary attached to the surface of the cylindrical continuous medium mechanical unit according to the size of a virtual triaxial sample, and generating top and bottom loading plates;
s1.2, establishing a discrete particle aggregate of the sample according to the particle gradation of the virtual triaxial sample;
s1.3, fixing the lateral membrane units and the top and bottom loading plates, and performing discrete-continuous coupling numerical calculation to balance the model;
s2, applying equal compressive stress in each direction, and carrying out discrete-continuous coupling numerical simulation in the consolidation process; the method comprises the following steps:
s2.1, fixing the lateral membrane units on the uppermost layer and the lowermost layer, enabling other membrane units to be free, resetting the contact force among the particles to be zero, and setting an updating mode of the normal contact force among the particles to be an increment mode;
s2.2, applying lateral compressive stress which is perpendicular to the unit surface and points to the inside of the sample on the lateral membrane unit, applying vertical compressive stress with the same size through loading plates at the top and the bottom, and performing coupling simulation of an anisotropic consolidation process;
s3, maintaining lateral confining pressure and applying axial loading speed, and carrying out discrete-continuous coupling numerical simulation in the shearing process; the method comprises the following steps:
s3.1, maintaining the confining pressure on the lateral membrane unit unchanged, setting the axial loading speed of a top loading plate and a bottom loading plate, starting discrete-continuous coupling simulation of a shearing process, and recording the axial stress and the axial strain of a sample at regular intervals;
s3.2, stopping test loading when the axial strain reaches a specified target value, and storing a test result;
s4, slicing the virtual sample in the triaxial shearing process, processing the sliced image, calculating the volume strain of the sample, and establishing a corresponding lattice Boltzmann numerical model; the method comprises the following steps:
s4.1, cutting the triaxial sample by a series of sections with equal intervals smaller than a certain threshold value along the axial direction at intervals of certain axial strain, and respectively deriving sample boundaries and slice images of particles at each section;
s4.2, carrying out batch processing on the derived slice images of the sample boundaries, identifying the sample boundaries in the slice images, marking the inner and outer boundaries as different colors, marking the regions outside the sample boundaries and inside the picture boundaries as the same color as the particles on the particle slices, representing solid regions, accumulating the number of pixels inside the sample boundaries on all the slices, multiplying the total number of pixels obtained by accumulation by the actual area represented by 1 pixel to obtain the total area in the sample boundary range on all the slices, and multiplying the total area by the distance between the adjacent slices, wherein the value obtained can be approximate to the volume of the sample at the time; obtaining the volume of the sample when each axial strain occurs in the triaxial shearing process, and further obtaining the volume strain of the sample;
s4.3, splicing the sample particle slice image led out from the same position and the sample boundary slice image with the sample boundary identified, and taking the side length of a pixel unit of the spliced image as the vertical spacing distance of adjacent slices, namely dividing a triaxial sample by adopting a cubic lattice unit and establishing a lattice Boltzmann numerical model corresponding to the triaxial sample;
s5, carrying out lattice Boltzmann numerical simulation of seepage aiming at the established lattice Boltzmann numerical model, and calculating the permeability of the sample; the method comprises the following steps:
s5.1, setting fluid calculation parameters aiming at the established lattice Boltzmann numerical model, wherein the fluid calculation parameters comprise: performing iterative calculation of lattice Boltzmann migration and collision on a fluid particle collision model, a discrete velocity model and a fluid boundary condition;
and S5.2, stopping iterative calculation after the seepage field converges to a stable state, and calculating the permeability of the sample by utilizing the Darcy law according to the applied water pressure difference.
2. The virtual triaxial penetration test simulation method based on discrete-continuous coupling and lattice Boltzmann according to claim 1, wherein in the step S1, the discrete-continuous coupling is implemented based on a coupling numerical platform of a three-dimensional particle discrete element method PFC3D and a finite difference method FLAC 3D; the lateral latex membrane is formed by arranging and combining Zone units or Shell structural units in FLAC3D in a cylindrical manner, rock-soil particles are simulated by balls or columns in PFC3D, top and bottom loading plates are simulated by walls in PFC3D, and the coupling of the membrane units and the particles is carried out through coupling walls attached to the membrane units; before coupling calculation, the large strain mode of FLAC3D is turned on, and PFC3D and FLAC3D are set to be the same analysis time step by using a time step scaling function; in the calculation iteration process, the speed obtained by solving the FLAC3D is transmitted to the PFC3D model part through the coupling Wall, so that the PFC model response is updated, the acting force generated by the coupling Wall is transmitted to the corresponding node of the FLAC3D unit through the mapping principle to serve as the updated boundary condition of the FLAC3D model mechanical response, and the like, and the coupling calculation is carried out step by step.
3. The method for virtual triaxial penetration test simulation based on discrete-continuous coupling and lattice Boltzmann according to claim 1, wherein in the step S2, the lateral compressive stress is slowly applied step by step, and the vertical consolidation compressive stress of the top and bottom loading plates is applied by a servo mechanism in the discrete elements.
4. The method for simulating the virtual triaxial penetration test based on the discrete-continuous coupling and lattice Boltzmann of claim 1, wherein the axial stress σ in the step S3.1 is1And axial strain epsilonaThe model is obtained by the calculation of load plates Wall at the top and the bottom of the model in PFC3D, and the calculation formula is as follows:
wherein, FztopAnd FzbotTotal vertical contact force, Z, to which the top and bottom load plates Wall are subjected, respectivelytopAnd ZbotVertical coordinates of the Wall positions of the top and bottom loading plates, H0Height of the sample after consolidation and before shearing, A0Is the initial cross-sectional area of the sample; because the lateral film units on the uppermost layer and the lowermost layer are kept fixed in the process of solidifying and shearing the sample, the cross sectional areas of the top and the bottom of the sample are always equal to the initial cross sectional area.
5. The simulation method for the virtual triaxial penetration test based on discrete-continuous coupling and lattice Boltzmann according to claim 1, wherein in the step S4.3, after the sample particle slice image and the sample boundary slice image are spliced, the marking of the solid area and the pore area is realized; pixels of the solid area correspond to solid cells of the lattice Boltzmann model, and pixels of the pore area correspond to fluid cells of the lattice Boltzmann model; the solid area is further divided into a flow-solid boundary unit and a solid internal unit, and the specific method is as follows: if the adjacent units around a certain solid unit are all solid units and comprise corresponding adjacent units on adjacent slices, the solid unit is marked as a solid internal unit; if one or more adjacent cells are pore cells, the cell is labeled as a flow-solid boundary cell.
6. The simulation method for the virtual triaxial penetration test based on discrete-continuous coupling and lattice Boltzmann according to claim 1, wherein in the step S5.1, a single relaxation time BGK collision model is adopted as a fluid particle collision model; a discrete velocity model, which adopts a D3Q19 model; the fluid boundary condition is that the wall surfaces around the model and the surface of the solid particles are non-slip fluid-solid boundaries, a rebound method is adopted for processing, the fluid flow direction is along the axial direction of the sample and is driven by the pressure difference of an inlet and an outlet, and a Zou/He method is adopted for processing the pressure boundary; and no calculation is carried out on the solid internal unit in the seepage grid Boltzmann simulation process, so that the calculation time is reduced.
7. The method for simulating the virtual triaxial penetration test based on discrete-continuous coupling and lattice Boltzmann according to claim 1, wherein in the step S5.2, after the penetration field converges to a steady state, the iterative calculation is stopped, and the permeability K of the sample is calculated according to the applied water pressure difference by using darcy' S law, wherein the calculation formula is as follows:
wherein rho is the density of the fluid, mu is the dynamic viscosity of the fluid, upsilon is the kinematic viscosity of the fluid,
in order to be the average flow rate,
is the pressure difference, l is the length of the model in the direction of flow, pinAnd poutInlet and outlet pressures, respectively.
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