Numerical calculation method for mechanical properties of wet granular medium based on CFD-DEM multiphase flow coupling

Abstract

The invention discloses a wet particle medium mechanical property numerical calculation method based on CFD-DEM multiphase flow coupling, and relates to the technical field of discrete element simulation technology and computer fluid dynamics, and the method comprises the following steps: firstly, obtaining initial information of a wet particle medium; secondly, establishing a geometric model of wet particles through a DEM solver; then, on the basis of the JKR theory, interaction parameters of the wet particles are obtained; then, on the basis of the geometric model and the interaction parameters of the wet particles, a fluid dynamics and discrete element method CFD-DEM coupling simulation model is built through a VOF coupling interface, and analogue simulation is conducted on the movement process of the robot foot paddle on the wet particle medium ground. Therefore, by adopting the numerical calculation method for the mechanical properties of the wet particle medium based on CFD-DEM multiphase flow coupling, the interaction among the wet particles can be accurately simulated, the simulation analysis precision and reliability of the wet particle medium are improved, and the dynamic simulation of the behavior of the wet particle medium is realized.

Description

Technical Field

[0001] The invention relates to the field of discrete element simulation technology and computer fluid dynamics technology, and in particular to a numerical calculation method for mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling. Background Art

[0002] In offshore environments, dry and wet granular media surfaces exhibit properties different from those of solids and fluids. As tasks in amphibious environments become increasingly complex, the performance requirements for amphibious robots are gradually increasing. The basic theory of the interaction mechanism between wet particles is an important basis for studying the characteristics of foot-ground interaction.

[0003] At present, the simplified wheel-ground model based on mechanics regards the ground as a deformable whole, and the interaction between the wheels and the ground only occurs on the surface. However, it is not suitable for exploring the situation of legged robots operating in amphibious environments. In this case, the feet destroy the original state of the ground and enter its interior to undergo complex interactions.

[0004] Most of the existing studies rely on experiments to obtain optimized parameters to simulate wet particle simulation environments under dry particle conditions, and only consider the inertial force, friction and elastic collision between particles for the DEM-CFD model of dry particle media. However, the existing numerical simulation methods do not consider the interaction force effects and attribute parameters between wet particles, which makes it impossible to accurately construct the solid-fluid-particle coupling simulation environment of wet particle media.

[0005] Therefore, it is necessary to provide a solid-fluid-particle multi-physics coupling simulation environment for wet granular media based on the VOF interface, which can effectively simulate the ground effect of wet granular media and provide a theoretical basis for the study of the mechanical properties between the robot and the wet granular ground. Summary of the invention

[0006] The purpose of the present invention is to provide a numerical calculation method for the mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling, which can more realistically simulate the behavior of wet particles when interacting with each other and enhance the accuracy and reliability of simulation results.

[0007] To achieve the above object, the present invention provides a numerical calculation method for mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling, comprising the following steps:

[0008] S1. Obtaining the initial information of the wet granular medium, including the physical parameters, morphological parameters, wall effect, moisture content and volume fraction of the particles;

[0009] S2, based on the initial information of the wet granular medium, a geometric model of the wet granular medium is established by using a DEM solver;

[0010] S3. Introduce JKR theory, measure the repose angle of various wet granular cone piles through experiments, and obtain the change law of the repose angle, and then obtain the interaction parameters of the wet granules;

[0011] S4. Based on the geometric model and interaction parameters of wet particles, a fluid dynamics and discrete element method CFD-DEM coupling simulation model is built through the VOF coupling interface to simulate the movement process of the robot foot paddle on the wet granular medium ground, and analyze and calculate the mechanical properties of the wet particles.

[0012] Preferably, establishing a geometric model of wet particles comprises the following steps:

[0013] S21. Add a geometry as a box for holding particles, set the attribute to solid, and delete the top surface, leaving only the four side walls and the bottom surface;

[0014] S22. According to the depth of the granular medium layer, add a quadrilateral plane for generating particles. The size of the quadrilateral is exactly the same as the bottom of the box, and the attribute is set to virtual;

[0015] S23, setting particle properties based on initial properties of the wet particle medium, including particle radius, Poisson's ratio, and friction coefficient;

[0016] S24, setting the number and time of particle generation, and setting a vertical downward speed for all particles, so that the generated particles can be quickly accumulated downward;

[0017] S25. Set the solution function and mesh division, and select the number of CPU cores and GPU computing power to be called;

[0018] S26, export the generated particles and set the time to 0s.

[0019] Preferably, obtaining the interaction parameters of the wet particles comprises the following steps:

[0020] S31, preparing wet particles to be tested, keeping the moisture content of each wet particle the same, and stirring until the moisture is evenly distributed;

[0021] S32, placing the wet particles on a pre-cleaned surface to form a conical pile, and adjusting the pile height of the particles to allow them to flow naturally and eventually stop;

[0022] S33, after the particle movement stops, use a high-precision camera to photograph the state of the particle pile from the side, record the contour of the particle pile, and accurately measure the contact angles of the top and bottom of the pile;

[0023] S34. Conduct simulation tests on the particle properties of static friction coefficient, dynamic friction coefficient, and surface energy to obtain the variation law between the angle of repose and each parameter, and then use the measured contact angle to calibrate the interaction parameters of the wet particles.

[0024] Preferably, the VOF coupling interface includes opening a multiphase flow model and a discrete phase, and using implicit equations to solve the volume fraction of each phase.

[0025] Preferably, the discretized form of the implicit equation is as follows:

[0026]

[0027] Where n+1 is the index of the current time step, n is the index of the previous time step, is the unit value of volume fraction at the n+1th time step, is the unit value of volume fraction at the nth time step, is the nominal value of the volume fraction at step n+1, is the volume flux through the surface at step n+1, S αq is the source phase, It represents the mass transfer between phases, and V is the unit volume.

[0028] Preferably, a fluid dynamics and discrete element method CFD-DEM coupling simulation model is built, including establishing a three-dimensional model of the robot's foot paddle, setting the speed of the foot paddle according to the simulation requirements; dividing the fluid area grid, selecting the grid type and density, and processing the particle area coupling grid; simulating the physical process of two-phase flow through geometric reconstruction and using standard interpolation methods; setting a donor-acceptor scheme, balancing the source terms within the control volume and the convection and diffusion fluxes on the control volume surface, and simulating phase interactions; setting boundary conditions, flow field materials and turbulence equations, adjusting the model size, and solving through non-steady state.

[0029] Therefore, the present invention adopts the above-mentioned numerical calculation method of mechanical properties of wet granular medium based on CFD-DEM multiphase flow coupling, which has the following technical effects:

[0030] (1) Enhanced simulation accuracy: By introducing surface energy properties based on the JKR theory, the interaction between wet particles can be simulated more accurately, overcoming the neglect of adhesion and surface energy effects in traditional particle models, thereby improving the reliability of simulation results.

[0031] (2) Scientific parameter calibration and verification: Through the combination of experimental and numerical methods, the physical parameters of wet particles are systematically calibrated, including density, surface energy, and friction coefficient. This provides solid data support for simulation analysis and ensures the authenticity and accuracy of the model.

[0032] (3) Comprehensively improve fluid coupling simulation capabilities: The VOF interface is used to realize solid-liquid-particle multiphase flow coupling, which can dynamically simulate the behavior of wet granular media under the conditions of liquid and particle interaction, enhancing the understanding and prediction of particle motion characteristics in complex environments.

[0033] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a flow chart of a numerical calculation method of mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling;

[0035] Figure 2 It is a graph of the test results of the repose angle calibration in an embodiment of a numerical calculation method for mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling;

[0036] Figure 3 It is a FLUENT-EDEM coupling simulation flow chart in an embodiment of a numerical calculation method for mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling;

[0037] Figure 4 It is a granular medium simulation scene diagram of an embodiment of a numerical calculation method for mechanical properties of wet granular medium based on CFD-DEM multiphase flow coupling;

[0038] Figure 5 It is a granular medium simulation scene diagram of a conventional method in an embodiment of a numerical calculation method for mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling. DETAILED DESCRIPTION

[0039] The present invention can be explained in more detail by the following examples. The purpose of disclosing the present invention is to protect all changes and improvements within the scope of the present invention. The present invention is not limited to the following examples.

[0040] like Figure 1 As shown, the present invention provides a numerical calculation method for mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling, comprising the following steps:

[0041] S1. Obtain the initial information of the wet granular medium, including the physical parameters, morphological parameters, moisture content and volume fraction of the particles.

[0042] Specifically, the radius of the wet particles is obtained by laser particle size instrument detection, and the density and other parameters of the wet particles are obtained by density experiment detection, so as to construct a geometric model of the wet particles in subsequent simulation. In this embodiment, the initial simulation parameters of the particles are shown in Table 1.

[0043] Table 1 Wet particle simulation calculation parameters

[0044] Serial number property Numeric 1 Particle radius 1.5mm 2 Poisson's ratio 0.25 3 density <![CDATA[2500kg / m 3 ]]> 4 Shear modulus 1e+08Pa 5 Coefficient of restitution 0.5 6 Static friction coefficient 0.5 7 Rolling friction coefficient 0.01

[0045] In addition, in the research related to granular media, moisture content is a key factor, which will have a significant impact on the physical and mechanical properties of granular media. Therefore, this embodiment predetermines the saturated moisture content and ensures that this saturated moisture content is reached in each test, providing a stable and consistent starting condition for subsequent experiments, thereby ensuring the reliability of the experimental results, making different batches of experiments comparable under the same granular medium type and volume fraction, and facilitating scientific data analysis and law summary.

[0046] First, the moisture content of the saturated wet granular medium is calculated to ensure that the conditions are consistent for each test. Specifically, 125 kg of sand is placed in the trough and the surface of the sand is scraped flat to the mark 100 mm from the bottom of the box with a scraper to ensure that the volume fraction φ is the same for each test, which is 0.73. Among them, φ is defined as follows:

[0047] φ=V a / V GM ;

[0048] Where V a 、V GM are the absolute compressed volume of the granular medium and the volume of the granular medium respectively.

[0049] Second, before testing the wet granular media, slowly add water to the trough until the water level is just above the surface of the sand, and then calculate the moisture content W as follows:

[0050] W=m water / m GM ;

[0051] In the formula, m water 、m GM are the mass of added water and the mass of added granular medium respectively.

[0052] After three tests, the moisture content was 21.6%, 21% and 21.5% respectively, which shows that under the conditions of the same type of sand and the same volume fraction, the saturated moisture content is approximately fixed. In this embodiment, the arithmetic mean of the three measurement results is taken as the fixed saturated moisture content, which is about 21.4%.

[0053] Then, in the wet granular medium test, multiple groups of tests are conducted to avoid the accidental nature of the experimental results. In each test, after reaching the average saturated moisture content, water is continuously injected until the water surface is 50 mm higher than the sand surface to ensure that the granular medium has been saturated and has reached the absorption limit. Then the volume fraction φ, moisture content W, and volume V of the saturated wet granular medium are calculated.GM As the simulation input parameters. Among them, V GM The calculation formula is as follows:

[0054] V GM =M wet / [ρ GM (1+W)];

[0055] Where M wet is the mass of the water-wet granular medium, ρ GM is the density of the granular medium.

[0056] On the other hand, when the moving parts are close to the side wall of the slot during the test, the force measurement results will be affected by the wall effect. Therefore, it is necessary to calibrate the effective distance between the plate and the wall during the test, that is, the wall effect calibration.

[0057] Existing studies have shown that when the distance between the plate and the wall exceeds 30 mm during the test of dry granular media, the influence of the wall effect can be avoided. Based on this, a preliminary calibration is first performed before testing wet granular media. Specifically, the microelement plate is measured 5 times from 430 mm to 30 mm from the side wall, the distance between the test position n and the test position n+1 is 80 mm, the measurement depth is 30 mm, and the speed is 30 mm / s.

[0058] The measurement results show that when testing in a wet granular medium, if the distance between two tests is greater than 80 mm and the distance from the side wall exceeds 30 mm, there is no mutual interference between multiple measurements and they are not affected by the near-wall effect. The measurement results are credible, which also proves that the conclusions in previous studies have great reference value.

[0059] Then, in order to better grasp the characteristics of the wet granular medium, this embodiment performs a more detailed secondary calibration on the effective distance of the wall effect, as follows:

[0060] The plate was measured 7 times from 10mm to 70mm from the side wall, the distance between test position n and test position n+1 was 10mm, the measurement depth was 10mm, and the speed was 30mm / s. The measurement results show that when testing in a saturated wet granular medium, if the distance between the plate and the side wall exceeds 40mm, the force during the movement is not affected by the wall effect, and the measurement results are reliable.

[0061] S2. Based on the initial properties of the wet granular medium, a preliminary geometric model of the wet granular medium is established using the simulation software model and version EDEM 2018. The steps are as follows:

[0062] S21. Add a geometric body as a box to hold the particles in the software, set the properties to solid, and delete the top surface, leaving only the four side walls and the bottom surface.

[0063] S22. According to the depth of the granular medium layer, add a quadrilateral plane for generating particles. The size of the quadrilateral is exactly the same as the bottom of the box, and the attribute is set to virtual.

[0064] S23. Setting particle properties based on initial properties of the wet granular medium, including particle radius, Poisson's ratio, friction coefficient, etc.

[0065] S24. Set the number and time of particle generation, and set a vertical downward speed for all particles so that the generated particles can accumulate downward quickly.

[0066] S25. Set the solution function and mesh division, and select the number of CPU cores and GPU computing power to be called.

[0067] S26. Export the generated particles as "Input deck" and set the time to 0s. All subsequent simulations for this granular medium can directly call the particles without the need to generate the environment again, which improves research efficiency.

[0068] S3. At present, the discrete element software EDEM can simulate the movement of dry particles, but due to the limitations of the software itself, it is impossible to add water to the dry particle medium environment to form a wet particle ground effect. The basic principle of the JKR theory can be explained as follows: when the pressure applied to two objects in contact with each other is completely removed, the contact radius between the two objects does not become zero, but a certain tensile load needs to be applied to completely separate the two surfaces, which is consistent with the characteristics of the wet particle medium ground. Therefore, the JKR theory is introduced in this embodiment to take into account the adhesion and surface energy effects between wet particles to make up for the shortcomings of the discrete element software EDEM.

[0069] However, when calling the JKR model function in EDEM, several key parameters of the particles need to be set, including the static friction coefficient, dynamic friction coefficient and surface energy. For wet granular media, the surface energy is unknown and needs to be measured through experiments. In this embodiment, the repose angle of the wet granular medium used in the experiment is calibrated by the discharge method to simulate the interaction parameters of the wet particles, as follows:

[0070] S31. Prepare the wet particles to be tested, ensure that the moisture content of each particle remains the same, and stir evenly to achieve uniform moisture distribution;

[0071] S32. Place the wet particles on a pre-cleaned surface to form a conical pile. Adjust the height of the particles so that they flow naturally and eventually stop. Note that during this process, the particles will gradually be discharged to form a free particle pile.

[0072] S33. After the particle movement stops, use a high-precision camera to photograph the state of the particle pile from the side. At the same time, record the outline of the particle pile and accurately measure the contact angles of the top and bottom of the pile.

[0073] S34, in EDEM, the static friction coefficient is 0.3-0.6, the dynamic friction coefficient is 0.06-0.12, and the surface energy is 1J / m 2 ~5J / m 2 The particle properties of the particles were simulated and tested, and the experimental design is shown in Table 2.

[0074] Table 2 Design and results of repose angle calibration test

[0075] Simulation Group <![CDATA[JKR surface energy (J / m 2 )]]> Static friction coefficient Dynamic friction coefficient Angle of repose (rad) 1 1 0.3 0.06 0.30 2 1 0.3 0.12 0.34 3 1 0.6 0.06 0.30 4 1 0.6 0.12 0.34 5 3 0.3 0.06 0.31 6 3 0.3 0.12 0.37 7 3 0.6 0.06 0.35 8 3 0.6 0.12 0.47 9 5 0.3 0.06 0.38 10 5 0.3 0.12 0.45 11 5 0.6 0.06 0.39 12 5 0.6 0.12 0.48

[0076] It can be seen from Table 2 that the test results under different particle parameters are obviously different. The angle of repose increases with the increase of surface energy and the increase of dynamic friction coefficient, but the change with static friction coefficient is not obvious.

[0077] like Figure 2 As shown in the figure, the repose angle of the wet granular medium is 0.37 rad, which is consistent with the test results of Group 6 in Table 2. Therefore, the JKR surface energy of 3 J / m 2 , static friction coefficient of 0.3, and dynamic friction coefficient of 0.12 are used as input parameters for wet particle simulation in EDEM to simulate the movement of the robot on the wet granular medium ground.

[0078] S4, such as Figure 5 As shown in the figure, combining the wet particle model with the calibrated wet particle interaction parameters, a fluid dynamics and discrete element method CFD-DEM coupling simulation model is built through the coupling interface to simulate the robot's motion process on the wet particle medium ground and analyze and calculate the mechanical properties of the particles, as follows:

[0079] Set up the wet particle model in EDEM: First, create a new particle material and set the contact parameters between particles according to the properties of the wet particles (Table 1); then, import the geometric model of the wet particles, and set the size distribution according to the wet particle property parameters; then, EDEM will automatically calculate the mass, volume and other properties of the particles, providing a calculation basis for the subsequent study of the collision and interaction forces between the paddle and the wet particles. At the same time, establish a particle factory to generate the required particles, and add corresponding contact models for different contacts.

[0080] In FLUENT, the multiphase flow model and discrete phase are enabled, and the VOF coupling interface is established to realize the dynamic simulation of liquid distribution in wet granular media, thereby more comprehensively describing the movement characteristics of particles in a wet environment. Specifically, it includes establishing a three-dimensional model of the foot-paddle, setting the rotation speed of the foot-paddle according to the simulation requirements (for example, defining the initial velocity and acceleration of the foot-paddle); dividing the fluid area grid, selecting the appropriate grid type and density, and processing the particle area coupling grid; through geometric reconstruction, using standard interpolation methods, obtaining the surface flux when a unit is completely filled with one or the other phase, so as to simulate the physical process of the two-phase flow more accurately, improve the calculation accuracy, better handle complex phase interfaces, and adapt to the simulation of various physical processes; setting a donor-acceptor scheme, for the cells located near the interface between the two phases, through special interpolation processing, balancing the source terms in the control volume and the convection and diffusion fluxes on the control volume surface, which is used to determine the direction and mode of transfer of physical quantities (such as mass, momentum, energy, etc.) between phases, accurately simulate the interaction between phases, ensure numerical stability and convergence, and adapt to different types of two-phase flow problems; setting boundary conditions, flow field materials and turbulence equations, adjusting the model size, and solving through unsteady state.

[0081] There is also a most important step: establishing a coupling interface between EDEM and FLUENT. At present, the PDM model is usually used in the commonly used CFD-DEM model to establish a coupling interface, but this model is only applicable to the case where the volume fraction of the particle phase is less than 10%. For the saturated wet granular medium ground in an amphibious environment, the volume fraction of sand in the water-sand mixed part is obviously greater than 10%, and the PDM model is difficult to meet the requirements of wet particle simulation under this condition. Based on this, this embodiment selects the VOF model as the coupling interface between EDEM and FLUENT to better simulate the motion characteristics of the wet granular medium.

[0082] Specifically, the VOF model can model two or more immiscible fluids by solving a set of momentum equations and tracking the volume fraction of each fluid in the entire computational domain. For each immiscible additional fluid phase, the volume fraction of the phase is introduced to track the interface between the phases in the computational domain. The volume fraction of the phase refers to the proportion of each phase in the whole, usually represented by the symbol α, and the sum of the volume fractions of all phases is 1. If the volume fraction of the i-th term in the multiphase fluid is α i , there are three situations:

[0083] (1)α i =0, the unit does not contain the fluid item;

[0084] (2)α i =1, then all the cells are this fluid item;

[0085] (3) 0<αi <1, in which case the cell contains the interface between the fluid term and one or more other fluid phases.

[0086] The interface tracking of phases is essentially accomplished by solving the continuity equation of the volume fraction of one (or more) phases. The expression of the continuity equation is as follows:

[0087]

[0088] In the formula, the subscripts p and q represent the two interacting phases. Represents the mass transfer between phases, S αq is the source phase, which is usually taken as 0, but a constant can be specified for each phase according to the specific characteristics of the fluid phase.

[0089] The volume fraction equation of the initial phase is not solved, but is calculated according to the constraints as follows:

[0090]

[0091] In the solution process based on the VOF model, there are two different time discretization methods: implicit equations and explicit equations. The implicit equations iteratively solve the transport equations in each time step, while the explicit equations do not iteratively solve the transport equations in each time step. In this embodiment, since the coupled simulation of the two simulation platforms in FLUENT and EDEM depends on the mutual transfer and solution of parameters between the two software in each time step, the implicit equations are selected for solution in the entire coupled simulation process. At this time, the discretized form of the volume fraction equation is as follows:

[0092]

[0093] Where n+1 is the index of the current time step, n is the index of the previous time step, is the unit value of volume fraction at the n+1th time step, is the unit value of volume fraction at the nth time step, is the nominal value of the volume fraction at step n+1, is the volume flux through the surface at step n+1, and V is the unit volume. Since the volume fraction in the current time step is a function of other quantities in the current time step, the scalar transport equation for each second phase volume fraction is iteratively solved in each time step. The surface flux is interpolated using the selected spatial discretization format, and the implicit formula can be used for both time-varying and steady-state calculations.

[0094] On the other hand, in order to verify the effectiveness of the method of this embodiment, a comparison is also made with the wet particle simulation effect diagram of the conventional method. Figure 4 and Figure 5As shown, when the foot paddle leaves the granular medium and moves upward, the particles carried up by the wet granular medium in the method of this embodiment are in a viscous aggregated state, while the wet granular medium in the conventional method is more discrete, which shows that the fluid does play a role in the fluid-solid-particle coupling simulation, further proving the feasibility of the method, and more intuitively demonstrating that the method of this embodiment is more in line with the physical phenomenon of the wet granular medium.

[0095] Therefore, the present invention adopts the above-mentioned numerical calculation method of the mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling, which can analyze and calculate the mechanical properties of wet particles, such as the interaction force between particles, the influence law of the motion between the paddle and particles, and the particle motion behavior, thereby improving the simulation analysis accuracy of the wet granular medium and providing a basis for the subsequent optimization of robot structure design, motion stability and motion planning.

[0096] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A numerical calculation method for mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling, characterized in that: The following steps are involved: S1. Obtaining the initial information of the wet granular medium, including the physical parameters, morphological parameters, wall effect, moisture content and volume fraction of the particles; S2, based on the initial information of the wet granular medium, a geometric model of the wet granular medium is established by using a DEM solver; S3. Introduce JKR theory, measure the repose angle of various wet granular cone piles through experiments, and obtain the change law of the repose angle, and then obtain the interaction parameters of the wet granules; S4. Based on the geometric model and interaction parameters of wet particles, a fluid dynamics and discrete element method CFD-DEM coupling simulation model is built through the VOF coupling interface to simulate the movement process of the robot foot paddle on the wet granular medium ground, and analyze and calculate the mechanical properties of the wet particles.

2. The numerical calculation method of mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling according to claim 1 is characterized in that: The geometric model of wet particles is established, including the following steps: S21. Add a geometry as a box for holding particles, set the attribute to solid, and delete the top surface, leaving only the four side walls and the bottom surface; S22. According to the depth of the granular medium layer, add a quadrilateral plane for generating particles. The size of the quadrilateral is exactly the same as the bottom of the box, and the attribute is set to virtual; S23, setting particle properties based on initial properties of the wet particle medium, including particle radius, Poisson's ratio, and friction coefficient; S24, setting the number and time of particle generation, and setting a vertical downward speed for all particles, so that the generated particles can be quickly accumulated downward; S25. Set the solution function and mesh division, and select the number of CPU cores and GPU computing power to be called; S26, export the generated particles and set the time to 0s.

3. The numerical calculation method of mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling according to claim 1 is characterized in that: Obtaining the interaction parameters of wet particles includes the following steps: S31, preparing wet particles to be tested, keeping the moisture content of each wet particle the same, and stirring until the moisture is evenly distributed; S32, placing the wet particles on a pre-cleaned surface to form a conical pile, and adjusting the pile height of the particles to allow them to flow naturally and eventually stop; S33, after the particle movement stops, use a high-precision camera to photograph the state of the particle pile from the side, record the contour of the particle pile, and accurately measure the contact angles of the top and bottom of the pile; S34. Conduct simulation tests on the particle properties of static friction coefficient, dynamic friction coefficient, and surface energy to obtain the variation law between the angle of repose and each parameter, and then use the measured contact angle to calibrate the interaction parameters of the wet particles.

4. The numerical calculation method of mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling according to claim 1 is characterized in that: The VOF coupling interface, including turning on the multiphase flow model and discrete phases, uses implicit equations to solve the volume fraction of each phase.

5. The numerical calculation method of mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling according to claim 4 is characterized in that: The discretized form of the implicit equation is as follows: Where n+1 is the index of the current time step, n is the index of the previous time step, is the unit value of volume fraction at the n+1th time step, is the unit value of volume fraction at the nth time step, is the nominal value of the volume fraction at step n+1, is the volume flux through the surface at step n+1, S αq is the source phase, and It represents the mass transfer between phases, and V is the unit volume.

6. The numerical calculation method of mechanical properties of wet granular media based on CFD-DEM multiphase flow coupling according to claim 1 is characterized in that: Build a fluid dynamics and discrete element method CFD-DEM coupling simulation model, including building a three-dimensional model of the robot's foot paddle and setting the speed of the foot paddle according to simulation requirements; Divide the fluid region grid, select the grid type and density, and process the particle region coupling grid; simulate the physical process of two-phase flow through geometric reconstruction and standard interpolation method; Set up a donor-acceptor scheme to balance the source terms within the control volume and the convection and diffusion fluxes on the control volume surface to simulate phase interactions; set boundary conditions, flow field materials and turbulence equations, adjust the model size, and solve through unsteady state.

Images