CFD (computational fluid dynamics)-based immersed tube joint leakage hydraulic characteristic analysis and foundation soil starting judgment method

Abstract

The invention discloses a CFD (computational fluid dynamics)-based immersed tube joint leakage hydraulic characteristic analysis and foundation soil starting judgment method. Comprising the steps of establishing a seepage physical model and calculating a resistance coefficient of a gravel foundation bed; establishing a three-dimensional geometric model of the immersed tube joint and the gravel foundation bed, and carrying out mesh refinement; calculating flow field distribution, flow velocity and shear stress characteristics by using a CFD numerical simulation method; carrying out amplification calculation of local flow velocity and shear stress on a calculation result; and comparing the amplified shear stress with the sediment critical starting shear stress, and judging the starting condition of the foundation soil. The influence of different anti-seepage measures on a leakage flow field is quantitatively analyzed by simulating the local flow velocity and shear stress distribution of the bottom of the gravel foundation bed, the measure design is optimized, and the reliability of foundation soil stability evaluation is improved.

Description

Technical Field

[0001] The invention relates to the field of marine engineering technology, and in particular to a CFD-based immersed pipe joint leakage hydraulic characteristic analysis and foundation soil start-up judgment method. Background Art

[0002] Since the joints of immersed tube tunnels in marine environments are usually designed with flexibility or semi-rigidity to adapt to uneven settlement and deformation, when the joints leak, the water will penetrate the gravel bed, forming a complex seepage field, resulting in a significant increase in local flow velocity and shear stress, which may cause the foundation soil to start, thus affecting the stability of the immersed tube.

[0003] The existing method for judging whether sediment has started to move mainly uses the empirical formula calculation method of sediment starting flow rate. The empirical formula method calculates the critical sediment starting flow rate and compares it with the actual flow rate to judge whether sediment has started to move. However, this method is only applicable to the flow condition where the water flow is parallel to the sand bed. After the immersed tube leaks, the water flows through the gravel base bed and then vertically or obliquely scours the sand bed, which is not applicable to the formula of critical sediment starting flow rate.

[0004] Physical model tests are often used to analyze the hydraulic characteristics of immersed tube leakage. Although physical model tests can simulate the hydraulic characteristics of leakage, they are affected by the scale effect and are difficult to fully restore the actual working conditions. In addition, the test costs are high and the cycle is long. It cannot quickly adapt to various complex engineering needs. Since the flow velocity is measured at the bottom of the gravel bed, the measurement is difficult and the judgment of sediment initiation is difficult, resulting in insufficient universality and credibility of the results. Therefore, these technical limitations make it difficult for existing methods to effectively evaluate the impact of immersed tube joint leakage on foundation soil. Summary of the invention

[0005] Therefore, the purpose of the present invention is to provide a CFD-based method for analyzing the hydraulic characteristics of immersed tube joint leakage and identifying the start-up of foundation soil. By simulating the local flow velocity and shear stress distribution at the bottom of the gravel base bed, the influence of different anti-seepage measures on the leakage flow field is quantitatively analyzed, the design of measures is optimized, and the reliability of foundation soil stability assessment is improved.

[0006] In order to achieve the above object, a CFD-based method for analyzing hydraulic characteristics of immersed pipe joint leakage and identifying foundation soil initiation includes the following steps: S1. Establish a physical model of seepage and calculate the resistance coefficient of the gravel bed; S2. Establish a three-dimensional geometric model of the immersed tube joint and perform mesh refinement; S3. Use CFD numerical simulation method to calculate flow field distribution, flow velocity and shear stress characteristics; S4, performing amplification calculation of local flow velocity and shear stress on the calculation results; S5. Compare the amplified shear stress with the critical starting shear stress of sediment to determine the starting condition of foundation soil.

[0007] Further preferably, the seepage physical model includes arranging according to a water supply system, a test pipe section, a pressure sensor and a data collector, filling the test pipe section with homogeneous gravel, adjusting the inlet flow rate, and recording the pressure gradient at different flow rates.

[0008] Further preferably, according to the pressure gradient obtained at different inlet flow rates, the relationship between the pressure gradient and the flow rate is fitted using the Forchheimer equation to obtain the viscous resistance coefficient C1 and the inertial resistance coefficient C2; ΔP = μ(C2 / ε)U + ρ(C1 / ε²)U² Among them, ΔP is the pressure gradient, μ is the fluid dynamic viscosity, ε is the porosity of the gravel, ρ is the fluid density, and U is the average flow velocity.

[0009] Further preferably, in S3, the CFD simulation includes the following settings and definitions of boundary conditions: Fluid Domain Settings: The immersed tube joint and gap area are defined as the fluid domain of pure water, and its properties include density, dynamic viscosity, etc., which are set as Newtonian fluid according to the physical properties of the fluid; The calculation range of the gravel bed is set to the porous medium domain, and the viscous resistance coefficient C1 and the inertial resistance coefficient C2 are input; Inlet boundary condition: Specify the inlet pressure value to simulate the situation where the leaking water flows into the joint gap. The value is equal to the total water pressure in the immersed tube. Outlet boundary condition: specifies the outlet pressure value, which is used to simulate the condition of water flow leaving the model. The value is the total water pressure outside the immersed tube at the current water depth. Wall boundary conditions: The surface of the immersed tube and the gravel bed area are set to no-slip conditions; By inputting the fluid density, dynamic viscosity, gravel bed resistance coefficient and model geometric characteristics, the continuity equation and momentum equation were solved using CFD software to obtain the flow velocity and shear stress distribution in the slot of the immersed tube joint and the gravel bed area.

[0010] Further preferably, the flow velocity and shear stress of the obtained gravel base bed and the immersed tube gap are amplified, including calculating the amplified flow velocity and shear stress according to the following formula: u_local = n * u_calculated _local = n * _calculated Where, the magnification factor n = 1 / ε, indicating the inverse relationship between the local flow velocity and the porosity in the porous media model; u_calculated is the model calculated flow velocity, _calculated is the shear stress calculated by the model, u_local is the amplified local flow velocity in the gravel, _local is the magnified local shear stress in the gravel bed.

[0011] Further preferably, the critical starting shear stress reference value of sediment is calculated using the following formula: in is 0.0164, The density of seawater is 1025 g / cm 3 , = 10mm, = 0.5mm, The dry density of sediment is 1.1 g / cm 3 , is the dry bulk density of bed sediment, is the stable dry bulk density of sediment particles, =1, It is the comprehensive adhesion parameter, film water thickness parameter = 2.31 × 10 -5 cm, is the acceleration due to gravity, take 9.81 m / s², For water depth, Represents the median particle size of sediment.

[0012] Further preferably, when judging whether there is a foundation soil movement, the following rules are used for judgment: when _local> _c, it is determined that the sediment is starting; when _local ≤ _c, it is determined that the sediment is not moving.

[0013] The CFD-based immersed tube joint leakage hydraulic characteristics analysis and foundation soil initiation judgment method disclosed in this application has at least the following advantages over the prior art: The empirical formula for the sediment starting flow velocity is only applicable to the condition that the water flow is parallel to the sand bed, and cannot accurately describe the situation in which the water flows through the gravel base bed obliquely or vertically to scour the sand bed after the immersed tube leaks. The present invention uses the critical starting shear stress of sediment as the judgment criterion, combined with the precise calculation of the leakage water flow field, to provide a sediment starting judgment method that is more in line with actual working conditions.

[0014] The present invention introduces CFD numerical simulation technology to construct a three-dimensional hydraulic model of immersed tube joint leakage, accurately analyzes the gap flow velocity, bed shear stress and flow field characteristics, solves the problem of result deviation caused by scale effect in traditional physical model tests, and ensures the accuracy and universality of the analysis results.

[0015] The present invention replaces part of the physical model test with numerical simulation, avoiding repeated construction of experimental facilities and time-consuming test processes. At the same time, it quickly simulates a variety of complex working conditions through virtual testing, significantly reducing costs and shortening analysis cycles.

[0016] The present invention simulates the local flow velocity and shear stress distribution at the bottom of the gravel base bed, combines the judgment criteria of the critical starting shear stress of sediment, quantitatively analyzes the influence of different anti-seepage measures (such as steel pads, gravel base beds, etc.) on the leakage flow field, optimizes the design of measures, and improves the reliability of foundation soil stability assessment.

[0017] The present invention provides a set of standardized hydraulic characteristic calculation and sediment initiation judgment methods, which are easy to promote and apply in different engineering scenarios, and provide a theoretical basis and technical support for the design, construction and leakage risk assessment of immersed tube joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of the flow chart of the CFD-based immersed tube joint leakage hydraulic characteristic analysis and foundation soil initiation judgment method provided by the present invention; Figure 2 A schematic diagram of the connection of the immersed tube joint provided by the present invention; Figure 3 A schematic diagram of the structure of the immersed tube joint provided by the present invention; Figure 4 This is a schematic diagram of the seepage experiment device; Figure 5 It is the velocity cloud map of the bottom of the bed; Figure 6 It is the velocity cloud diagram of the gap and the bed; Figure 7 This is a partial enlarged view of the velocity cloud map of the gap and the bed; Figure 8 It is the shear stress velocity cloud diagram at the bottom of the base bed.

[0019] In the figure: 1. water supply device, 2. valve, 3. pressure sensor, 4. data acquisition device, 5. test pipe section, 6. flow measuring water tank. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below through the accompanying drawings and specific embodiments.

[0021] like Figure 1As shown, an embodiment of the present invention provides a CFD-based method for analyzing hydraulic characteristics of immersed pipe joint leakage and determining foundation soil initiation, comprising the following steps: S1. Establish a seepage physical model and calculate the resistance coefficient of the gravel base bed. When building the seepage physical model, follow the following steps: Figure 4 The simple seepage test device shown in the figure is mainly composed of three parts: water supply system 1, test pipe section 5 and flow measurement system 6. The test pipe section is made of organic glass, with a length of 1.5 m and a cross-sectional diameter of 0.1 m. Three pressure sensors 3 are set on the pipe wall to monitor pressure changes, of which two pressure points are 0.5 m away from the two ends of the test pipe, and each pressure measurement point is 0.25 m apart. The outlet of the water supply system 1 is connected to the valve 2, and the valve is opened to flow to the test pipe section 5; the pressure sensor 3 collects the data at the diversion point of the test pipe section 5; the data collected is collected by the data collector 4. Test steps: - Fill the test pipe section with homogeneous gravel with a particle size of 0.03 m. - Adjust the inlet flow rate to 0.1 m / s, 0.2 m / s, 0.3 m / s, 0.4 m / s and 0.5 m / s, and record the pressure gradient at different flow rates. - The relationship between pressure gradient and flow velocity is fitted according to the Forchheimer equation, and the viscous resistance coefficient C1 and inertial resistance coefficient C2 of the gravel bed are calculated.

[0022] According to the pressure gradient obtained at different inlet flow rates, the Forchheimer equation is used to fit the relationship between the pressure gradient and the flow rate; the viscous resistance coefficient C1 and the inertial resistance coefficient C2 are obtained; ΔP = μ(C2 / ε)U + ρ(C1 / ε²)U² Wherein, ΔP is the pressure gradient, μ is the fluid dynamic viscosity, ε is the gravel porosity, ρ is the fluid density, and U is the average flow velocity. In this embodiment, C1=333333, and C2=466.

[0023] S2. Establish a three-dimensional geometric model of the immersed tube joint and refine the mesh; Figure 2 As shown, the immersed tube joint connection adopts a flexible connection method using a Q-type water stop. Figure 3 As shown, the immersed tube joint structure is a immersed tube joint structure that uses Q-type water stop for sealing and connection. The overall structural features can be described as follows: 1. Immersed tube structure: The figure shows two adjacent immersed tube sections with Q-type waterstops at the joints. Each section has a box-shaped cross section, which is used to carry the upper load and meet the clearance requirements inside the tunnel.

[0024] 2. Q-type waterstop location and function: Q-type waterstop is arranged at the joint surface between two immersed tube sections. The waterstop is usually made of flexible elastic material and has a "Q"-shaped cross section (or a special cross-sectional shape with similar functional characteristics). The Q-type waterstop can provide a reliable waterproof seal for the structure at the joint and has a certain deformation capacity to adapt to the small displacement and deformation that may occur between the immersed tubes due to water pressure, soil pressure or temperature changes, thereby ensuring the watertightness of the joint and the long-term stability of the overall structure.

[0025] 3. Upper protection and covering layer: There are multiple layers of protection and covering soil above the immersed tubes.

[0026] The top layer is graded backfill of facing stones of a certain thickness (e.g. 200-300 kg block stone h=1000mm, 10-50 kg block stone h=500mm) to provide upper protection.

[0027] Auxiliary structures such as anti-permeability geotextile (400g / m²) and C20 grade concrete cushion (h=500mm) are set underneath to enhance the anti-seepage and stability performance of the top surface.

[0028] Side and bottom backfill and foundation: The two sides of the immersed tube are backfilled with boulders and surrounding rock slopes to ensure the lateral stability and load-bearing capacity of the tube body underwater; the bottom is paved with copper plates, gravel base beds (about 600mm thick), composite reinforced geotextiles and other foundation layers to provide good bearing and anti-seepage foundation conditions, and effectively disperse the load and reduce the risk of settlement and deformation.

[0029] Use SpaceClaim modeling tools to build a 3D geometric model of the immersed tube joint and its surrounding structures, including: - Immersed tube gap: Based on experience, the width range is 0.01m~0.1m, and in this embodiment, it is 0.1m; - Gravel base bed: established according to the design drawings. In this embodiment, the thickness of the gravel base bed is 0.6m.

[0030] To improve the simulation accuracy, the mesh of the joint gap and the gravel bed area is encrypted. In this embodiment, the mesh size of the gap area is about 2 mm, and the mesh size of the bed area is about 20 mm. Through mesh independence analysis and quality control, it is ensured that the mesh quality of the calculation area meets the CFD solution accuracy requirements.

[0031] S3. Use CFD numerical simulation method to calculate flow field distribution, flow velocity and shear stress characteristics. CFD simulation includes the following settings and boundary conditions: Fluid Domain Settings: The immersed tube joint and the gap area are defined as the fluid domain of pure water. Its properties include density, dynamic viscosity, etc. It is set as a Newtonian fluid according to the physical properties of the fluid. In this embodiment, the density is the density of seawater, 1025 kg / m 3 , the dynamic viscosity is 1.07×10 −3 N·s·m −2 ; The calculation scope of the gravel base bed is set to the porous media domain, and its resistance coefficients (viscous resistance coefficient C1 and inertial resistance coefficient C2) are input.

[0032] Inlet boundary condition: specify the inlet pressure value, which is used to simulate the situation where the leaking water flows into the joint gap. The value is the total water pressure in the immersed tube. In this embodiment, the inlet total pressure is 193366Pa; Outlet boundary condition: specifies the outlet pressure value, which is used to simulate the condition of water flow leaving the model. The value is the total water pressure outside the immersed tube at the current water depth. In this embodiment, the outlet total pressure is 152684Pa; Wall boundary conditions: The surface of the immersed tube and the gravel bed area are set to a no-slip condition. In this embodiment, the roughness height is 0.003m; By inputting the fluid density, dynamic viscosity, gravel bed resistance coefficient and model geometric characteristics, the continuity equation and momentum equation were solved using CFD software to obtain the flow velocity and shear stress distribution in the slot of the immersed tube joint and the gravel bed area.

[0033] S4, amplifying the flow velocity and shear stress of the gravel base bed and the slot of the immersed tube to obtain the flow velocity and shear stress measurement values; including amplifying the flow velocity and shear stress of the gravel base bed and the slot of the immersed tube, including calculating the amplified flow velocity and shear stress according to the following formula u_local = n * u_calculated _local = n * _calculated Where, the magnification factor n = 1 / ε, indicating the inverse relationship between the local flow velocity and the porosity in the porous media model; u_calculated is the model calculated flow velocity, _calculated is the shear stress calculated by the model, u_local is the amplified local flow velocity in the gravel, _local is the magnified local shear stress in the gravel bed.

[0034] S5. Compare the obtained flow velocity and shear stress measurement values ​​with the reference value of sediment critical starting shear stress to determine whether there is foundation soil starting condition.

[0035] The reference value of the critical starting shear stress of sediment is calculated using the following formula: in is 0.0164, The density of seawater is 1025 g / cm 3 , is the median particle size of sediment, = 10mm, =0.5mm, The dry density of sediment is 1.1 g / cm 3 , is the dry bulk density of bed sediment, is the stable dry bulk density of sediment particles, =1, It is the comprehensive adhesion parameter, film water thickness parameter = 2.31 × 10 -5 cm, is the acceleration due to gravity, take 9.81 m / s², is the water depth. In this embodiment, = 1.75 cm 3 / s 2 , water depth h =15.1m, dry density is 1.1g / cm 3 , The median particle size of the sediment is taken as 0.075 mm; the calculated value of the critical starting shear stress is 0.24 Pa.

[0036] When judging whether there is foundation soil movement, make the judgment according to the following rules: when _local> _c, it is determined that the sediment is starting; when _local ≤ _c, it is determined that the sediment is not moving.

[0037] like Figure 5-8 As shown, Figure 5 As shown in the figure, after the water flows out of the gap, it seeps and diffuses in the gravel bed, and the flow rate gradually decays. Figure 6 and Figure 7 As shown in the figure, the flow velocity reaches its maximum value in the area near the gap at the bottom, that is, the red or yellow area in the center of the figure is the area with the maximum flow velocity; Figure 8As shown in the figure, as the distance increases, the velocity gradually diffuses to both sides and decreases. This law is clearly reflected in the gradient change from the red high velocity area to the green and blue low velocity areas in the velocity distribution cloud map. At the same time, the shear stress distribution at the bottom also shows a similar law. The shear stress is the largest in the area near the gap, and gradually diffuses outward and decays. In this embodiment, without setting a steel pad and using a gravel bed for anti-seepage protection, the local maximum shear stress at the bottom of the flow velocity flowing out of the gap is 0.83Pa, which is greater than the critical starting shear stress of sediment, and there is sediment initiation.

[0038] Obviously, the above embodiments are only examples for clear explanation, and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from them are still within the protection scope of the invention.

Claims

1. A CFD-based method for analyzing the hydraulic characteristics of immersed pipe joint leakage and determining the start-up of foundation soil, characterized in that: The following steps are involved: S1. Establish a physical model of seepage and calculate the resistance coefficient of the gravel bed; S2. Establish a three-dimensional geometric model of the immersed tube joint and the gravel bed, and refine the mesh; S3. Use CFD numerical simulation method to calculate flow field distribution, flow velocity and shear stress characteristics; S4, performing amplification calculation of local flow velocity and shear stress on the calculation results; S5. Compare the amplified shear stress with the critical starting shear stress of sediment to determine the starting condition of foundation soil.

2. The CFD-based immersed tube joint leakage hydraulic characteristics analysis and foundation soil initiation judgment method according to claim 1 is characterized in that: In S1, the seepage physical model includes arranging a water supply system, a test pipe section, a pressure sensor and a data collector, filling the test pipe section with homogeneous gravel, adjusting the inlet flow rate, and recording the pressure gradient at different flow rates.

3. The CFD-based immersed tube joint leakage hydraulic characteristics analysis and foundation soil initiation judgment method according to claim 2 is characterized in that: According to the pressure gradient obtained at different inlet flow rates, the Forchheimer equation is used to fit the relationship between the pressure gradient and the flow rate; the viscous resistance coefficient C1 and the inertial resistance coefficient C2 are obtained; ΔP = μ(C2 / ε)U + ρ(C1 / ε²)U² Among them, ΔP is the pressure gradient, μ is the fluid dynamic viscosity, ε is the porosity of the gravel, ρ is the fluid density, and U is the average flow velocity.

4. The CFD-based immersed tube joint leakage hydraulic characteristics analysis and foundation soil initiation judgment method according to claim 1 is characterized in that: In S3, when using CFD numerical simulation, set the following boundary conditions: Fluid Domain Settings: Define the immersed tube joint and gap area as the fluid domain of pure water, set the properties: density, dynamic viscosity; set it as Newtonian fluid according to the physical properties of the fluid; The calculation range of the gravel bed is set to the porous medium domain, and the viscous resistance coefficient C1 and the inertial resistance coefficient C2 are input; Inlet boundary condition: specify the inlet pressure value to simulate the situation where the leaking water flows into the joint gap. The inlet pressure is the total water pressure in the immersed tube. Outlet boundary condition: Specify the outlet pressure value to simulate the condition of water flow leaving the model. The outlet pressure is the total water pressure at the current water depth outside the immersed tube. Wall boundary conditions: The surface of the immersed tube and the gravel bed area are set to no-slip conditions; By inputting the fluid density, dynamic viscosity, gravel bed resistance coefficient and model geometric characteristics, the continuity equation and momentum equation were solved using CFD software to obtain the flow velocity and shear stress distribution in the slot of the immersed tube joint and the gravel bed area.

5. The CFD-based immersed tube joint leakage hydraulic characteristics analysis and foundation soil initiation judgment method according to claim 1 is characterized in that: In S4, the amplified flow velocity and shear stress are calculated according to the following formula; u_local = n * u_calculated _local = n * _calculated Where, the magnification factor n = 1 / ε, indicating the inverse relationship between the local flow velocity and the porosity in the porous media model; u_calculated is the model calculated flow velocity, _calculated is the shear stress calculated by the model, u_local is the amplified local flow velocity in the gravel, _local is the magnified local shear stress in the gravel bed.

6. The CFD-based immersed tube joint leakage hydraulic characteristics analysis and foundation soil initiation judgment method according to claim 1 is characterized in that: The reference value of the critical starting shear stress of sediment is calculated using the following formula: in is 0.0164, The density of seawater is 1025 g / cm 3 , = 10mm, = 0.5mm, The dry density of sediment is 1.1 g / cm 3 , is the dry bulk density of bed sediment, is the stable dry bulk density of sediment particles, =1, It is the comprehensive adhesion parameter, film water thickness parameter = 2.31 × 10 -5 cm, is the acceleration due to gravity, take 9.81 m / s², For water depth, Represents the median particle size of sediment.

7. The CFD-based immersed tube joint leakage hydraulic characteristics analysis and foundation soil initiation judgment method according to claim 6 is characterized in that: When judging whether there is foundation soil movement, make the judgment according to the following rules: when _local > _c, it is determined that the sediment is starting; when _local ≤ _c, it is determined that the sediment is not moving.

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