In-situ 3D printing shield secondary lining constructability optimization method based on CFD

Abstract

The invention relates to a CFD-based in-situ 3D printing shield secondary lining constructability optimization method, and belongs to the technical field of tunnel construction. The method comprises the following steps: constructing a joint simulation platform, calibrating the size of a single-layer secondary lining, establishing a CFD simulation geometric model, setting boundary conditions of a physical field, simulating a printing process, and optimizing a construction process based on a simulation result. According to the method, the quality source items of concrete layer-by-layer stacking in the in-situ 3D printing secondary lining process are loaded and simulated by configuring the UDF, and the single-layer secondary lining size calibration result is synthesized, so that the function relationship between the quality source items and absolute coordinates in a closed calculation domain is established; according to the in-situ 3D printing shield secondary lining construction method, the effect of multi-layer printing of fresh concrete along with movement of a spray head is simulated, the overall efficiency of modeling simulation is improved, good universality is achieved, and an in-situ 3D printing shield secondary lining construction scheme with higher constructability can be planned according to different tunnels.

Description

Technical Field

[0001] The invention belongs to the technical field of tunnel construction, and particularly relates to an in-situ 3D printing shield secondary lining constructability optimization method based on CFD. Background Technique

[0002] The secondary lining is a permanent support structure built along the circumferential direction of the tunnel ring surface with materials such as reinforced concrete to prevent the surrounding rock from deforming or collapsing during shield tunneling. The structural stability and mechanical durability of the secondary lining are the prerequisites for ensuring that the underground tunnel can withstand external forces such as long-term loads and seismic impacts. The secondary lining is usually divided into forms such as cast-in-situ secondary lining, shotcrete secondary lining, and precast secondary lining. The precast secondary lining has been widely used in shield tunneling due to its low cost and reliable load-bearing capacity. However, before the final support is formed, the precast secondary lining needs to go through multiple links such as steel cage welding, concrete pouring, lining transportation, and lining assembly, and is easily restricted by problems such as production cycle, transportation cost, and assembly proficiency, thus limiting the improvement of the construction efficiency of underground tunnels. In order to optimize the secondary lining construction process, 3D printing concrete technology has been introduced in the project to realize the in-situ manufacturing of tunnel secondary lining. 3D concrete printing (3DCP) is a new emerging additive manufacturing technology that combines 3D printing technology and concrete material technology. Its potential advantages include template-free manufacturing, high design flexibility, and low construction operation cost, and it is expected to become an important development direction in the future construction industry. At present, 3DCP technology has been used in many fields such as on-site manufacturing of houses, prefabrication of concrete components, and rapid construction of underground works. The most common 3DCP technology at present is to extrude the fresh concrete with a short setting time and fast strength development along the set printing path through a nozzle and stack it layer by layer to form a high-quality customizable structure. By inversely solving the spatial motion curve of the printing module from the target secondary lining size and surface topography, and then controlling printing parameters such as printing module speed, extrusion speed, nozzle diameter, and nozzle height, high-precision and high-quality manufacturing of the secondary lining is realized, which greatly shortens the time and space distance from production to assembly of the secondary lining.

[0003] The printing quality of 3D printed concrete depends on multiple influencing factors, including the rheological properties of the material, printing process parameters, and nozzle geometry. Any changes in the rheological properties and printing parameters during the pumping or extrusion process may lead to instability in the shape or size of the extruded layer. To achieve the desired printing layer thickness and geometry, existing 3DCP applications often involve a large number of trial-and-error attempts and conservative designs. These physical tests are both expensive and time-consuming, making it difficult to support rapid iterative design solutions and delaying the process from concept to actual deployment, which clearly limits the practical application and wide promotion of 3D printed concrete in tunnel engineering. In addition, the tunnel secondary linings produced with different printing parameters show significant differences in load-bearing capacity, durability, and stability. This difference is mainly due to the fact that different printing parameters will result in different degrees of micro-pores and micro-cracks inside the concrete. Compared with traditional cast-in-place concrete, 3D printed concrete lacks the process of vibration compaction and cannot effectively fill these micro-pores and micro-cracks, which may affect the overall performance and durability of the structure. In tunnel engineering, whether the in-situ 3D printed tunnel secondary lining can withstand the pressure of the surrounding rock must be verified after actual construction. This not only increases the time cost during the tunnel construction process but also may pose potential safety hazards. Summary of the Invention

[0004] To solve the problems in the prior art, the present invention proposes an optimization method for the constructability of in-situ 3D printed shield secondary lining based on CFD.

[0005] The present invention adopts the following technical solutions:

[0006] On the one hand, the present invention provides an optimization method for the constructability of in-situ 3D printed shield secondary lining based on CFD, including:

[0007] S1: Construct a joint simulation platform, which is composed of modeling software, printing path generation software, and 3D printed concrete simulation software;

[0008] S2: Calibrate the geometric dimensions of a single-layer secondary lining;

[0009] S3: Use the modeling software to construct a CFD simulation geometric model and perform mesh division. The CFD simulation geometric model includes an extrusion nozzle, a tunnel torus, and a closed computational domain;

[0010] S4: Combine the calibration results and use the modeling software and printing path generation software to generate the movement path of the extrusion nozzle;

[0011] S5: Import the CFD simulation geometric model and the movement path of the extrusion nozzle into the 3D printing simulation software, set the physical field boundary conditions; perform the printing process simulation of the secondary lining in the 3D printing simulation software; and correct the simulation results;

[0012] S6: Repeat S1 - S5 after adjusting the process parameters to make the finally corrected simulation results meet the expectations, obtain the optimal construction process, and use the optimal construction process for the secondary lining construction.

[0013] On the other hand, the present invention provides an in - situ 3D printing shield secondary lining constructability optimization system based on CFD for implementing the above - mentioned method.

[0014] The beneficial effects of the present invention are as follows:

[0015] (1) By constructing an in - situ 3D printing shield secondary lining simulation model, the present invention realizes the numerical simulation of the in - situ 3D printing process of the shield secondary lining, and solves the problem that a large number of trial - and - error attempts and conservative designs are necessary in the traditional construction process to achieve the desired printed layer and geometric shape.

[0016] (2) The present invention replaces the actual physical tests in the traditional secondary lining construction process with a numerical simulation method, allows multiple virtual tests in a short time for rapid iterative design, and while satisfying the visualization of the in - situ 3D printing of the shield secondary lining, greatly reduces the construction cost and avoids potential hazards.

[0017] (3) Through the detailed analysis of the hydration reaction, rheological properties, and mechanical property growth mechanism in the 3D printing process of the secondary lining, while introducing appropriate simplifications for the printing process, the present invention ensures the reduction degree of the simulation, and overall considers the calculation efficiency and accuracy of the numerical model.

[0018] (4) The CFD simulation geometric model and the concrete 3D printing process simulation method designed by the present invention can be applied to the printing processes under various manufacturing conditions such as material rheological properties, printing process parameters, and nozzle geometric shapes, and have good universality, which helps to seek optimal process parameters and optimize construction strategies for different tunnels.

[0019] (5) By comprehensively calibrating the results of the single - layer secondary lining dimensions, and by means of configuring user - defined functions (UDF) to load the mass source term of the concrete layer - by - layer stacking during the numerical simulation of the in - situ 3D printing of the secondary lining, a functional relationship is established between the mass source term within the closed computational domain and the absolute coordinates, thereby simulating the effect of the freshly mixed concrete being printed in multiple layers as the nozzle moves, and solving the problem that the Ansys standard module cannot directly achieve the layer - by - layer stacking of concrete.

[0020] (6) The present invention corrects the simulation results by comprehensively considering the scale factor after calibrating the actual printed layer, so as to achieve the purpose of considering the possible influence of many external factors such as equipment vibration, environmental temperature, and air humidity in the actual printing process on the printing results, solve the problem of the difference between the actual printing results and the simulation results, and accurately simulate the layer - by - layer stacking process of the secondary lining.

[0021] (7) The numerical simulation process of the present invention is realized through a co-simulation platform, and the geometric model, printing path, and simulation data can be efficiently transmitted, effectively improving the overall efficiency of modeling and simulation. At the same time, the present invention provides excellent expansion space for the performance verification of new composite 3D printing nozzles and complex printing paths, and provides great help for the efficient and safe operation of the shield secondary lining in engineering practice. Description of the Drawings

[0022] Figure 1 is the flowchart of the method according to an embodiment of the present invention;

[0023] Figure 2 is the architecture diagram of the co-simulation platform according to an embodiment of the present invention;

[0024] Figure 3 is the CFD simulation geometric model diagram constructed according to an embodiment of the present invention;

[0025] Figure 4 is the schematic diagram of the printing process parameter setting according to an embodiment of the present invention;

[0026] Figure 5 is the mesh division result according to an embodiment of the present invention;

[0027] Figure 6 is the three-dimensional printing path and off-line point taking result according to an embodiment of the present invention;

[0028] Figure 7 is the printing morphology and cross-section pressure condition of the single-layer and multi-layer secondary linings according to an embodiment of the present invention.

[0029] In the figure: 1 - extrusion nozzle; 2 - tunnel toroidal surface; 3 - closed computational domain. Detailed Embodiments

[0030] The present invention will be further described and explained below in conjunction with the detailed embodiments. The embodiments are only examples of the present disclosure and do not delimit the scope of limitation. The technical features of each embodiment of the present invention can be combined correspondingly without conflict.

[0031] The optimization method for the constructability of the in-situ 3D printed shield secondary lining based on CFD is as Figure 1 shown. The complete process mainly includes the calibration of the single-layer secondary lining size, the establishment of the CFD simulation geometric model, mesh division, generation of the printing path, setting of the physical field boundary conditions, control of the printing process, and correction of the simulation results. The above steps are integrated and controlled through a co-simulation platform, and finally, the parameters and the model are optimized based on the corrected simulation results.

[0032] To achieve efficient geometric modeling, path generation, and numerical simulation, the present invention constructs a joint simulation platform based on software such as Solidworks, RoboDK, and Ansys to enable efficient transmission of geometric models, printing paths, and simulation data. The architecture of the joint simulation platform is as shown in Figure 2 . Solidworks, as the underlying designer, outputs the CFD simulation geometric model and visualizes the establishment of a three-dimensional printing path. RoboDK, as the path generator, completes offline automatic point extraction of the printing path and generates the UDF printing path language. Ansys, as the simulation executor, completes the simulation calculation task according to the printing path and displays the simulation results. The geometric model is transmitted between the underlying designer and the path generator through a built-in interaction plug-in. The discrete coordinate points are identified in the form of code between the path generator and the simulation executor. The simulation data will be saved as a text file to facilitate modification of the model part.

[0033] During the in-situ 3D printing of the shield secondary lining, the 3D printing nozzle will use concrete materials to perform multi-layer printing on the surface of the excavated tunnel ring to achieve the expected structural strength. The size of the single-layer secondary lining is one of the key parameters for numerical modeling and simulation, indicating the geometric shape size of each layer of material during the 3D printing process of the secondary lining. It reflects the comprehensive effects of parameters such as the rheological properties of the material, printing process parameters, and nozzle geometry during the actual printing process. By calibrating the size of the single-layer secondary lining, it can help study and adjust the simulation parameter settings to make the simulation results match the actual process. The calibration of the size of the single-layer secondary lining is specifically as follows: the geometric shape size of the single-layer secondary lining 3D printing is verified through pre-experiments, and the outer contour and area size of the longitudinal section are calibrated using a three-dimensional scanner. A standard rectangle is designed to simplify and replace the cross-sectional shape of the irregular printing filament. The irregular printing filament is simplified to a rectangular cross-section swept body, and the designed standard rectangle encloses the cross-sectional area of the printing filament at the corresponding position.

[0034] The CFD simulation geometric model can be directly constructed and imported through Solidworks software. The CFD simulation geometric model is as shown in Figure 3 . It mainly includes an extrusion nozzle 1, a tunnel ring surface 2, and a closed calculation domain 3. The tunnel ring surface can be represented as the tunnel crown, arch waist, or arch bottom in combination with different gravity directions. The printing process parameter settings are as shown in Figure 4 . They mainly include the nozzle diameter D, nozzle height H, volume flux U, and nozzle moving speed V. Among them, the nozzle diameter and nozzle height are determined during the establishment stage of the CFD simulation geometric model, and the volume flux and nozzle moving speed are determined during the physical field boundary condition setting stage.

[0035] Mesh generation is a core process in numerical simulation. It involves dividing the continuous geometric space into finite discrete elements so that complex physical problems can be solved through numerical simulation. Usually, even if the absolute size of the mesh changes slightly, the total number of meshes in the simulation model will change significantly. Therefore, mesh generation needs to preserve the precise geometric shape of the nozzle, which directly affects the flow characteristics of the fluid, and also preserve the accurate size and shape of the closed computational domain to construct the actual spraying environment. Remove minor features and non-critical components that have little impact on the flow, such as small holes, chamfers, bolt holes, and brackets, etc., which have no impact on the flow characteristics but will increase the number of meshes and the computational workload. The mesh generation results are as Figure 5 shown, mainly including tetrahedral meshes and boundary layers. The refinement degree of the mesh is higher in the complex regions of the CFD simulation geometric model (regions with many structural changes and fluid-structure contact interfaces) to capture the detailed dynamics, while it can be appropriately coarser in regions with fewer changes, thus balancing the computational cost and the result accuracy. In order to accurately capture the velocity gradient phenomenon that exists near the solid boundary due to the influence of fluid viscosity, the boundary layer is used to encrypt the wall surface of the extrusion nozzle.

[0036] Printing path generation is a key prerequisite for controlling the movement of the extrusion nozzle along the target path. The printing path is controlled by the discrete coordinate points of the in-situ 3D printing curve of the secondary lining. Considering that manual point selection requires the person selecting the points to have a high spatial geometry foundation, the present invention directly visualizes and establishes a three-dimensional printing path based on Solidworks, imports the three-dimensional printing path into RoboDK for offline automatic point selection, and then generates a UDF printing path language that Ansys can recognize. As Figure 6 shown are the three-dimensional printing path established by visualization and the offline point selection results. Combining the single-layer secondary lining size calibration results, by means of configuring user-defined functions (UDF), the mass source term of the concrete layer-by-layer stacking during the in-situ 3D printing of the secondary lining in the numerical simulation is loaded, so that a functional relationship is established between the mass source term in the closed computational domain and the absolute coordinates, thereby simulating the effect of the freshly mixed concrete being printed in multiple layers as the nozzle moves. The user-defined function specifically specifies the mass of concrete generated per unit time at the position of each standard rectangle in the single-layer secondary lining size calibration result. Combining the user-defined function with the UDF printing path language gives the movement path of the extrusion nozzle including the mass source term.

[0037] The setting of physical field boundary conditions mainly includes setting the multiphase flow model, viscosity model, thermodynamics model, fluid material properties, inlet and outlet types, and velocities. In the present invention, only two phases are involved in the in-situ 3D printing of the secondary lining of shield tunnels, namely the concrete phase and the air phase. It is generally considered that there is an interface between these two immiscible fluids. The multiphase flow model is set in the "Multiphase Flow" module, and the VOF multiphase flow model is selected to realize the simulation of the in-situ 3D printing of the secondary lining of shield tunnels. The viscosity model is set in the "Viscosity" module. The in-situ 3D printing of the secondary lining of tunnels usually adopts the extrusion-based 3D printing concrete process. This process realizes the extrusion of concrete by driving the spiral extrusion rod to rotate by a motor, resulting in a stable and smooth concrete extrusion process, which conforms to the characteristics of the laminar flow model. Therefore, the viscosity model is set as the laminar flow model in this embodiment. The thermodynamics model is set in the "Solidification and Melting" module. To correctly simulate the endothermic hardening process in the hydration reaction of fresh concrete, the Ansys solidification and melting module is selected to perform direct thermal coupling on the model. By setting the paste region parameters, the material expansion or contraction caused by heat transfer is considered, which in turn affects the mechanical properties of the secondary lining structure. The fluid material properties are set in the "Material" module. The fluid material properties include the response characteristics of fresh concrete and air materials. The response characteristic of fresh concrete material is that shear flow deformation can only occur when the shear stress exceeds the material yield stress. The shear stress and shear rate of fresh concrete are obtained through material rheology experiments, and the Herschel-Bulkley model is synthesized to realize the material characterization of fresh concrete. The air phase is set according to the measured density and viscosity of air during the actual construction process. The setting of the inlet and outlet types and velocities is specifically as follows: the inlet type of the concrete nozzle is set as "Velocity-Inlet". The nozzle moves at a stable nozzle moving speed V, without considering the specific rotation of the screw inside the printing device. The volume flux U is derived from the cross-sectional area of the hopper and the target extrusion velocity, and an inlet boundary with a constant volume flux U is defined at the inlet of the extrusion nozzle to approximately simulate the extrusion flow. By default, the numerical simulation in this method does not consider the concrete slip behavior inside the nozzle. Therefore, the no-slip boundary condition is selected between the fluid and the solid.

[0038] The key point of the printing process control lies in the setting of the solution time step. The simulation time step and the maximum number of iterations are adjusted manually according to the solution stability and convergence criteria. For the CFD simulation geometric model established in the present invention, the fluid flow control equation is transformed into a linear algebraic equation on the grid cells by the finite volume method. The discrete pressure and velocity variables are solved based on the pressure-velocity coupling algorithm. For a given pressure field, the discrete form of the momentum equation is solved to obtain the corrected velocity field, and the given pressure field is corrected based on the continuity equation. The above steps are repeated iteratively to gradually correct the velocity field and the pressure field until the changes in the velocity field and the pressure field are within the allowable error range. As Figure 7It shows the printing morphology and cross-sectional strain rate of single-layer and multi-layer secondary linings. Since there is strain in the in-situ 3D printed secondary lining under the extrusion pressure, and under the condition that other conditions remain the same, the strain rate is positively correlated with the extrusion pressure. Therefore, the strain rate is used as the characterization of the pressure distribution of the secondary lining.

[0039] After the printing process simulation of the secondary lining is carried out in Ansys software, the simulation results are obtained. The simulation results mainly include the characterization using Volume fraction, describing the outer surface contour shape and size of the filaments printed on the tunnel surface. The key to correcting the simulation results lies in comprehensively correcting the simulation results with the scale factor after calibrating the actual printed outer surface contour shape and size. Considering that many external factors such as equipment vibration, environmental temperature, and air humidity in the actual printing process may cause differences between the actual printing results and the simulation results, through calibrating the actual printed concrete outer surface contour shape and size, the calculation formula for the scale factor Δ between the measured result and the simulation result is obtained by fitting:

[0040] Δ = aD m + bH n + cU o + dV p + f

[0041] Among them, Δ is the scale factor between the measured result and the simulation result, D is the nozzle diameter, H is the nozzle height, U is the concrete volume flux, V is the nozzle moving speed, m, n, o, p are non-linear power terms, and f is the constant term. The simulation results are corrected by the comprehensive scale factor Δ to achieve the purpose of considering the possible impacts of many external factors in the actual printing process on the printing results, and solve the problem of differences between the actual printing results and the simulation results. The expression for correcting the simulation results is:

[0042] T2 = T1 × Δ

[0043] Among them, T1 is the outer surface contour shape and size of the printed concrete simulation before correction, and T2 is the outer surface contour shape and size of the printed concrete simulation after correction.

[0044] Finally, the construction process is optimized based on the corrected simulation results. If the gap between the corrected simulation results and the target results is too large, the construction process is adjusted and the above simulation process is repeated until the corrected simulation results are within the acceptable error range of the target results, and the optimal construction process is adopted for the construction of the secondary lining. The optimization of the construction process includes the optimization of the CFD simulation geometric model and the adjustment of printing process parameters and concrete material properties, etc.

[0045] An in-situ 3D printing shield secondary lining constructability optimization method based on CFD proposed by the present invention focuses on numerically simulating the in-situ 3D printing process of the shield secondary lining, and predicting the effects of factors such as material rheological properties, printing process parameters, and nozzle geometry on the printing layer thickness, geometry, and overall mechanical properties of the secondary lining from the virtual test level. Compared with existing methods, the present invention introduces the following advantages: First, CFD provides detailed visualization of the 3D printing of the secondary lining and related physical phenomena (such as pressure and temperature distribution), which helps researchers to deeply understand the complex flow field behavior, which is often difficult to observe in traditional experimental methods; Second, detailed simulation experiment analysis is carried out on aspects such as hydration reaction, rheological characteristics, and mechanical property growth during the 3D printing process of the secondary lining from an analytical perspective. While introducing appropriate simplifications for the printing process, the reduction degree of the simulation is ensured, and the calculation efficiency and accuracy of the numerical model are considered as a whole; Third, it is applicable to the printing process simulation of various manufacturing conditions such as material rheological properties, printing process parameters, and nozzle geometry, and has good universality, which helps to seek optimal process parameters and optimize construction strategies for different tunnels. Fourth, compared with traditional actual physical tests, numerical simulation allows multiple virtual tests to be carried out in a short time to quickly iterate the design scheme, so as to identify and solve potential problems before physical construction, greatly reducing the development and test costs, and at the same time avoiding potential dangerous scenarios or extreme environments as much as possible. Fifth, by synthesizing the calibration results of the single-layer secondary lining size, the mass source term of the concrete layer stacking during the in-situ 3D printing of the secondary lining in the numerical simulation is loaded by means of configuring a user-defined function (UDF), so as to establish a functional relationship between the mass source term in the closed calculation domain and the absolute coordinates, thereby simulating the effect of multi-layer printing of the fresh concrete as the nozzle moves, and solving the problem that the Ansys standard module cannot directly realize the concrete layer stacking. In addition, the present invention builds a joint simulation platform based on Solidworks, RoboDK, and Ansys. The geometric modeling of the printing mechanism, the offline generation of the printing path, and the numerical simulation of the printing process are realized through the joint simulation platform, so that the geometric model, the printing path, and the simulation data can be efficiently transmitted, effectively improving the overall efficiency of modeling and simulation. To sum up, the present invention helps to deeply understand the in-situ 3D printing mechanism of the secondary lining and optimize the material rheological properties, printing process parameters, nozzle geometry and other factors for the target printing layer thickness, geometry and overall mechanical properties of the secondary lining, and has great application potential in improving the construction efficiency of the shield tunnel secondary lining and reducing the material loss and time cost during the secondary lining construction process.

[0046] Example 2

[0047] In this embodiment, a CFD-based in-situ 3D printing shield secondary lining constructability optimization system is also provided, which is used to implement the above embodiment. The terms "module", "unit", etc. used hereinafter can be a combination of software and / or hardware that can achieve a predetermined function. Although the system described in the following embodiments is preferably implemented in software, implementation in hardware, or a combination of software and hardware is also possible.

[0048] The CFD-based in-situ 3D printing shield secondary lining constructability optimization system in this embodiment includes:

[0049] A platform construction module, which is used to construct a co-simulation platform;

[0050] A dimension calibration module, which is used to calibrate the geometric dimensions of a single-layer secondary lining;

[0051] A model construction module, which is used to construct a CFD simulation geometric model, perform mesh division, and generate the movement path of the extrusion nozzle;

[0052] A boundary setting module, which is used to set the physical field boundary conditions;

[0053] A simulation execution module, which is used to execute the printing process simulation of the secondary lining;

[0054] A result processing module, which is used to obtain the scale factor between the actual printing result and the simulation result, and correct the simulation result based on the scale factor;

[0055] An optimization control module, which is used to optimize the construction process according to the corrected simulation result.

[0056] For the system embodiment, since it basically corresponds to the method embodiment, the relevant parts can refer to the partial description of the method embodiment, and the implementation methods of the remaining modules will not be elaborated here. The system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place, or may be distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of the present invention. Those of ordinary skill in the art can understand and implement it without creative work.

[0057] Embodiments of the system of the present invention can be applied to any device with data processing capabilities, and such a device with data processing capabilities can be a device or apparatus such as a computer. The system embodiments can be implemented by software, or by hardware or a combination of software and hardware. Taking software implementation as an example, as a logically meaningful device, it is formed by the processor of any device with data processing capabilities reading the corresponding computer program instructions in the non-volatile memory into the memory for operation.

[0058] The above embodiments only represent several implementation manners of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the patent of the present invention. For those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. An in-situ 3D printing shield secondary lining constructability optimization method based on CFD, characterized in that Including: S1: Construct a co-simulation platform, which consists of a modeling software, a printing path generation software, and a 3D printing concrete simulation software; S2: Calibrate the geometric dimensions of the single-layer secondary lining; S3: Use the modeling software to construct a CFD simulation geometric model and perform mesh division. The CFD simulation geometric model includes an extrusion nozzle, a tunnel torus, and a closed computational domain; S4: Combine the calibration results and use the modeling software and the printing path generation software to generate the movement path of the extrusion nozzle; S5: Import the CFD simulation geometric model and the movement path of the extrusion nozzle into the 3D printing simulation software, and set the physical field boundary conditions; perform the printing process simulation of the secondary lining in the 3D printing simulation software; correct the simulation results; S6: Repeat S1 - S5 after adjusting the process parameters until the finally corrected simulation results meet the expectations, obtain the optimal construction process, and use the optimal construction process to construct the secondary lining.

2. The in-situ 3D printing shield secondary lining constructability optimization method based on CFD according to claim 1, characterized in that, The calibration of the geometric dimensions of the single-layer secondary lining specifically includes: conducting a pre-experiment on the single-layer 3D printing of the secondary lining, using a 3D scanner to scan the outer surface contour shape and dimensions of the secondary lining obtained by the single-layer 3D printing, and using rectangles to simplify and replace the cross-sections of the printed filaments at various locations. The rectangles enclose the cross-sectional areas of the printed filaments at the corresponding positions.

3. A method for optimizing the constructability of the in-situ 3D printed shield secondary lining based on CFD according to claim 1, characterized in that, During the mesh division process, tetrahedral meshes and boundary layers are used. The refinement degree of the tetrahedral meshes is higher in the regions with complex structures and fluid-solid contact interfaces and lower in other regions; the boundary layer is used to encrypt the wall surface of the extrusion nozzle.

4. The optimality method for constructability of in-situ 3D printed shield secondary lining based on CFD according to claim 2, characterized in that, The generation of the movement path of the extrusion nozzle using the modeling software and the printing path generation software specifically includes: initially establishing a three-dimensional printing path based on the modeling software, importing the three-dimensional printing path into the printing path generation software to automatically obtain points, and then generating a UDF printing path language; combining the single-layer secondary lining size calibration results, configuring a user-defined function to simulate the mass source term of the concrete layer-by-layer stacking during the in-situ 3D printing of the secondary lining. The user-defined function represents the mass of concrete generated per unit time by the extrusion nozzle at each position of the rectangle in the single-layer secondary lining size calibration results; combining the user-defined function with the UDF printing path language to obtain the movement path of the extrusion nozzle including the mass source term.

5. The in-situ 3D printing shield secondary lining constructability optimization method based on CFD according to claim 1, characterized in that The setting of the physical field boundary conditions specifically includes: setting the multiphase flow model as the VOF multiphase flow model in the 3D printing simulation software. The multiphase flow model only involves the concrete phase and the air phase. The Herschel-Bulkley model is used to characterize the concrete phase. An inlet boundary with a constant volume flux is defined at the inlet of the extrusion nozzle to approximately simulate the extrusion flow, and a no-slip boundary condition is selected between the fluid and the solid.

6. The optimality method for constructability of in-situ 3D printing shield secondary lining based on CFD according to claim 1, wherein During the printing process simulation of the secondary lining, repeatedly perform iterative solutions and update the flow field velocity field and pressure field in the CFD simulation geometric model until the changes in the velocity field and pressure field are within the allowable error range.

7. A method for optimizing the constructability of the in-situ 3D printed shield secondary lining based on CFD according to claim 1, characterized in that, The correction of the simulation results specifically includes: By performing actual printing tests and simulation tests under the same process parameters in advance, calculating the scale factor between the actual test results and the simulation results, and multiplying the scale factor by the simulation results to obtain the corrected simulation results, the calculation formula for the scale factor is: Δ = aD m + bH n + cU o + dV p + f where Δ is the scale factor between the measured result and the simulation result, D is the nozzle diameter, H is the nozzle height, U is the concrete volume flux, V is the nozzle moving speed, m, n, o, p are non-linear power terms, and f is a constant term.

8. A method for optimizing the constructability of the in-situ 3D printing shield secondary lining based on CFD according to claim 1, characterized in that The construction process includes a CFD simulation geometric model, volume flux, nozzle moving speed, and concrete material properties, where the volume flux and the nozzle moving speed are the physical field boundary conditions.

9. An in-situ 3D printing shield secondary lining constructability optimization system based on CFD, characterized in that, It includes: A platform construction module for constructing a joint simulation platform; A dimension calibration module for calibrating the geometric dimensions of the single-layer secondary lining; A model construction module for constructing a CFD simulation geometric model, performing mesh division, and generating the movement path of the extrusion nozzle; A boundary setting module for setting physical field boundary conditions; A simulation execution module for executing the printing process simulation of the secondary lining; A result processing module for obtaining the scale factor between the actual printing result and the simulation result, and correcting the simulation result based on the scale factor; An optimization control module for optimizing the construction process according to the corrected simulation result.

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