CFD-Based Leakage Analysis of Immersed Tube Joints and Foundation Starting Discrimination Method

Abstract

The present invention discloses a method for analyzing leakage of immersed tube joints and discriminating foundation startup based on CFD, which includes: establishing a seepage physical model to calculate the resistance coefficient of the crushed stone bed; establishing a three-dimensional geometric model of the immersed tube joint and the crushed stone bed and carrying out grid refinement; using the CFD numerical simulation method to calculate the flow field distribution and the characteristics of flow velocity and shear stress; performing magnification calculation on the local flow velocity and shear stress of the calculation results; comparing the magnified shear stress with the critical startup shear stress of sediment to judge the startup situation of foundation soil. By simulating the local flow velocity and shear stress distribution at the bottom of the crushed stone bed, quantitatively analyzing the influence of different anti-seepage measures on the leakage flow field, optimizing the measure design, and improving the reliability of the foundation soil stability evaluation.

Description

Technical Field

[0001] The present invention relates to the technical field of ocean engineering, and particularly relates to an analysis method for the leakage hydraulic characteristics of a immersed tube joint based on CFD and a method for judging the initiation of foundation soil. Background Art

[0002] Since the joints of immersed tube tunnels in the marine environment usually adopt flexible or semi-rigid designs to adapt to uneven settlement and deformation. However, when leakage occurs at the joint, water flow will penetrate through the rubble bed, forming a complex seepage field, resulting in a significant increase in local flow velocity and shear stress, which may trigger the initiation of foundation soil and further affect the stability of the immersed tube.

[0003] The existing method for judging whether sediment is initiated mainly adopts the empirical formula calculation method of sediment incipient velocity. The empirical formula method judges whether sediment is initiated by calculating the critical incipient velocity of sediment and comparing it with the actual flow velocity. However, this method is applicable to the flow condition where the water flow is parallel to the sand bed. After leakage occurs in the immersed tube, the water flow scours the sand bed vertically or obliquely after seeping through the rubble bed, and it is not applicable to the sediment critical incipient velocity formula.

[0004] For the analysis of the leakage hydraulic characteristics of the immersed tube, physical model tests are mostly used. Although physical model tests can simulate the leakage hydraulic characteristics, there are scale effect influences, and it is difficult to fully restore the actual working conditions. In addition, the test cost is high, the cycle is long, and it cannot quickly adapt to various complex engineering requirements. Since the position for measuring the flow velocity is at the bottom of the rubble bed, the measurement difficulty is high and it is difficult to judge the initiation of sediment, resulting in insufficient universality and credibility of the results. Therefore, these technical limitations make it difficult for the existing methods to effectively evaluate the influence of the leakage of the immersed tube joint on the foundation soil. Summary of the Invention

[0005] Therefore, the purpose of the present invention is to provide an analysis method for the leakage hydraulic characteristics of a immersed tube joint based on CFD and a method for judging the initiation of foundation soil. By simulating the local flow velocity and shear stress distribution at the bottom of the rubble bed, quantitatively analyzing the influence of different anti-seepage measures on the leakage flow field, optimizing the measure design, and improving the reliability of the foundation soil stability evaluation.

[0006] In order to achieve the above purpose, an analysis method for the leakage hydraulic characteristics of a immersed tube joint based on CFD and a method for judging the initiation of foundation soil of the present invention includes the following steps:

[0007] S1. Establish a seepage physical model and calculate the resistance coefficient of the rubble bed;

[0008] S2. Establish a three-dimensional geometric model of the immersed tube joint and perform grid refinement;

[0009] S3. Use the CFD numerical simulation method to calculate the flow field distribution and the characteristics of flow velocity and shear stress;

[0010] S4. Perform local velocity and shear stress amplification calculations on the calculation results;

[0011] S5. Compare the amplified shear stress with the critical shear stress for sediment incipient motion to judge the incipient motion situation of the foundation soil.

[0012] Further preferably, the seepage physical model is arranged according to a water supply system, a test pipe section, pressure sensors, and a data collector. Homogeneous gravel is filled in the test pipe section, the inlet velocity is adjusted, and the pressure gradient at different velocities is recorded.

[0013] Further preferably, according to the pressure gradients obtained at different inlet velocities, the Forchheimer equation is used to fit the relationship between the pressure gradient and the velocity; the viscous resistance coefficient C1 and the inertial resistance coefficient C2 are obtained;

[0014] ΔP = μ(C2 / ε)U + ρ(C1 / ε²)U²

[0015] Where, ΔP is the pressure gradient, μ is the dynamic viscosity of the fluid, ε is the gravel porosity, ρ is the fluid density, and U is the average velocity.

[0016] Further preferably, in S3, the CFD simulation includes the following settings and boundary condition definitions:

[0017] Fluid domain setting:

[0018] Define the fluid domain of the immersed tube joint and the gap area as pure water, and its properties include density, dynamic viscosity, etc., and set it as a Newtonian fluid according to the physical properties of the fluid;

[0019] Set the calculation range of the gravel bed as a porous medium domain, and input the viscous resistance coefficient C1 and the inertial resistance coefficient C2;

[0020] Inlet boundary condition: Specify the inlet pressure value to simulate the situation of leakage water flowing into the joint gap, and its magnitude is the total water pressure inside the immersed tube;

[0021] Outlet boundary condition: Specify the outlet pressure value to simulate the condition of water flowing out of the model, and its magnitude is the total water pressure at the current water depth outside the immersed tube;

[0022] Wall boundary condition: Set the surface of the immersed tube and the gravel bed area as no-slip conditions;

[0023] By inputting the fluid density, dynamic viscosity, gravel bed resistance coefficient, and model geometric characteristics, use CFD software to solve the continuity equation and momentum equation to obtain the velocity and shear stress distributions in the immersed tube joint gap and gravel bed area.

[0024] Further preferably, the amplification of the flow velocity and shear stress of the gravel bed and the gap of the immersed tube includes calculating the amplified flow velocity and shear stress according to the following formula.

[0025] u_local = n * u_calculated

[0026] _local = n * _calculated

[0027] Wherein, the amplification factor n = 1 / ε, which represents the inverse ratio relationship between the local flow velocity and the porosity in the porous medium model; u_calculated is the flow velocity calculated by the model, _calculated is the shear stress calculated by the model, and u_local is the local flow velocity inside the gravel after amplification _local is the local shear stress inside the gravel bed after amplification.

[0028] Further preferably, the reference value of the critical incipient shear stress of the sediment is calculated by the following formula:

[0029]

[0030] Where is 0.0164, is the seawater density 1025 g / cm 3 , d * = 10mm, d ’ = 0.5mm, is the dry density of the sediment 1.1 g / cm 3 , is the dry bulk density of the sediment on the bed surface, is the stable dry bulk density of the sediment particles, taking =1, is the comprehensive cohesive force parameter, the thickness parameter of the film water = 2.31×10 -5 cm, is the acceleration of gravity, taking 9.81 m / s², is the water depth.

[0031] Further preferably, when judging whether there is incipient motion of the foundation soil, the judgment is made according to the following rules:

[0032] When _local > _c, it is determined that the sediment starts to move;

[0033] When _local ≤ If it is less than _c, it is determined that the sediment does not start to move.

[0034] The method for analyzing the hydraulic characteristics of the leakage of the immersed tube joint and discriminating the initiation of the foundation soil based on CFD disclosed in this application has at least the following advantages compared with the prior art:

[0035] Aiming at the fact that the empirical formula for the starting velocity of sediment is only applicable to the condition where the water flow is parallel to the sand bed and cannot accurately describe the situation where the water flow obliquely or vertically scours the sand bed through the gravel bed after the leakage of the immersed tube, the present invention provides a method for discriminating the initiation of sediment that is more in line with the actual working conditions by changing to the critical starting shear stress of sediment as the judgment criterion and combining the accurate calculation of the leakage water flow field.

[0036] By introducing CFD numerical simulation technology, the present invention constructs a three-dimensional hydraulic model of the leakage of the immersed tube joint, accurately analyzes the gap flow velocity, bed shear stress and flow field characteristics, solves the problem of result deviation caused by the scale effect in traditional physical model tests, and ensures the accuracy and universality of the analysis results.

[0037] The present invention replaces part of the physical model tests with numerical simulations, avoids repeated construction of experimental facilities and time-consuming test processes, and at the same time quickly simulates various complex working conditions through virtual tests, significantly reducing costs and shortening the analysis cycle.

[0038] By simulating the local flow velocity and shear stress distribution at the bottom of the gravel bed and combining the judgment criterion of the critical starting shear stress of sediment, the present invention quantitatively analyzes the influence of different anti-seepage measures (such as steel backing plates, gravel beds, etc.) on the leakage flow field, optimizes the measure design, and improves the reliability of the evaluation of the stability of the foundation soil.

[0039] The present invention provides a set of standardized methods for calculating hydraulic characteristics and discriminating the initiation of sediment, which is convenient for popularization and application in different engineering scenarios, and provides a theoretical basis and technical support for the design, construction and leakage risk assessment of immersed tube joints. Description of the Drawings

[0040] Figure 1 It is a schematic flow chart of the method for analyzing the hydraulic characteristics of the leakage of the immersed tube joint and discriminating the initiation of the foundation soil based on CFD provided by the present invention.

[0041] Figure 2 It is a schematic connection diagram of the immersed tube joint provided by the present invention;

[0042] Figure 3 It is a schematic structural diagram of the immersed tube joint provided by the present invention;

[0043] Figure 4 It is a schematic diagram of the seepage experiment device;

[0044] Figure 5 It is a velocity contour map at the bottom of the bed;

[0045] Figure 6 is the velocity contour map of the gap and the foundation bed;

[0046] Figure 7 is the partially enlarged view of the velocity contour map of the gap and the foundation bed;

[0047] Figure 8 is the velocity contour map of the shear stress at the bottom of the foundation bed.

[0048] In the figure: 1. Water supply device, 2. Valve, 3. Pressure sensor, 4. Data collector, 5. Test pipe section, 6. Flow measurement water tank. Specific implementation mode

[0049] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation modes.

[0050] As Figure 1 shown, an analysis method for leakage hydraulic characteristics of immersed tube joints and discrimination of foundation soil initiation based on CFD provided by an embodiment of the present invention on the one hand includes the following steps:

[0051] S1. Establish a seepage physical model and calculate the resistance coefficient of the gravel foundation bed; when building the seepage physical model, a simple seepage test device is adopted as Figure 4 shown. The test device mainly consists of three parts: a water supply system 1, a test pipe section 5, and a flow measurement water tank 6. The test pipe section is made of plexiglass, with a length of 1.5 m and a cross-sectional diameter of 0.1 m. Three pressure sensors 3 are arranged on the pipe wall to monitor the pressure change. Among them, the distances from two pressure points to the two ends of the test pipe are 0.5 m, and the distance between each pressure measurement point is 0.25 m. The outlet of the water supply system 1 is connected to the valve 2, and after the valve is opened, the water flows to the test pipe section 5; the pressure sensor 3 collects data at the diversion point of the test pipe section 5; the data collector 4 collects and aggregates the data.

[0052] Test steps:

[0053] - Fill the test pipe section with homogeneous gravel with a particle size of 0.03 m.

[0054] - Adjust the inlet flow velocity, and the flow velocity values are 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 gradients at different flow velocities.

[0055] - According to the Forchheimer equation, fit the relationship between the pressure gradient and the flow velocity, and calculate the viscous resistance coefficient C1 and the inertial resistance coefficient C2 of the gravel foundation bed.

[0056] According to the pressure gradients obtained at different inlet flow velocities, fit the relationship between the pressure gradient and the flow velocity using the Forchheimer equation; obtain the viscous resistance coefficient C1 and the inertial resistance coefficient C2;

[0057] ΔP = μ(C2 / ε)U + ρ(C1 / ε²)U²

[0058] Where ΔP is the pressure gradient, μ is the hydrodynamic viscosity of the fluid, ε is the porosity of the crushed stone, ρ is the fluid density, and U is the average flow velocity. In this embodiment, C1 = 333333 and C2 = 466.

[0059] S2. Establish a three-dimensional geometric model of the immersed tube joint and refine the mesh; as Figure 2 shown, the connection of the immersed tube joint is a flexible connection method using a Q-type waterstop. As Figure 3 shown, the structure of the immersed tube joint is that the immersed tube joint structure uses a Q-type waterstop for sealing and connection. The overall structural characteristics can be described as follows:

[0060] Immersed tube body structure:

[0061] The figure shows two adjacent immersed tube segments, and a Q-type waterstop is arranged at the joint between them. Each tube segment has a box-shaped cross-section, which is used to carry the upper load and meet the requirements of the internal clearance of the tunnel.

[0062] Position and function of the Q-type waterstop:

[0063] A Q-type waterstop is arranged at the joint surface between the two immersed tube segments. This waterstop is usually made of a flexible elastic material and has a "Q"-shaped cross-section (or a special cross-section shape with similar functional characteristics). The Q-type waterstop can provide reliable waterproof sealing for the structure at the joint and has a certain deformation ability to adapt to the small displacements and deformations that may occur between the immersed tubes due to water pressure, soil pressure or temperature changes, thereby ensuring the watertightness of the joint part and the long-term stability of the overall structure.

[0064] Upper protection and overburden layer:

[0065] There are multiple layers of protection and overburden layers above the immersed tube.

[0066] The top layer is a graded backfill of surface protection stones of a certain thickness (such as 200 - 300 kg boulders with h = 1000 mm, 10 - 50 kg boulders with h = 500 mm) to provide upper protection.

[0067] Below it, auxiliary structures such as an anti-seepage geotextile (400 g / m²) and a C20 concrete cushion (h = 500 mm) are provided to enhance the anti-seepage and stability performance of the top surface.

[0068] Side and bottom backfill and foundation:

[0069] On both sides of the immersed tube are backfilled with crushed stones and the surrounding rock slopes to ensure the lateral stability and bearing capacity of the tube body underwater; at the bottom, there are laid a copper plate, a crushed stone foundation bed (about 600 mm thick), a composite reinforced geotextile and other base layers to provide good bearing and anti-seepage foundation conditions, and effectively disperse the load and reduce the risk of settlement and deformation.

[0070] Use the SpaceClaim modeling tool to establish a three-dimensional geometric model of the immersed tube joint and its surrounding structures, including:

[0071] - Immersed tube gap: According to experience, the width range is taken as 0.01 m to 0.1 m, and in this embodiment, it is 0.1 m.

[0072] - Crushed stone foundation bed: Established according to the design drawings. In this embodiment, the thickness of the crushed stone foundation bed is 0.6 m.

[0073] To improve the simulation accuracy, the grid is refined in the joint gap and the crushed stone foundation bed area. In this embodiment, the grid size in the gap area is about 2 mm, and the grid size in the foundation bed area is about 20 mm. Through grid independence analysis and quality control, ensure that the grid quality in the calculation area meets the CFD solution accuracy requirements.

[0074] S3. Use the CFD numerical simulation method to calculate the flow field distribution and the characteristics of flow velocity and shear stress. Among them, the CFD simulation includes the following settings and the definition of boundary conditions:

[0075] Fluid domain settings:

[0076] Define the immersed tube joint and the gap area as the fluid domain of pure water, and its properties include density, dynamic viscosity, etc. Set it 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 , and the dynamic viscosity is 1.07×10 −3 N · s · m −2 ;

[0077] Set the calculation range of the crushed stone foundation bed as a porous medium domain and input its resistance coefficients (viscous resistance coefficient C1 and inertial resistance coefficient C2).

[0078] Inlet boundary condition: Specify the inlet pressure value to simulate the situation of leakage water flowing into the joint gap, and its magnitude is the total water pressure inside the immersed tube. In this embodiment, the inlet total pressure is 193366 Pa;

[0079] Outlet boundary condition: Specify the outlet pressure value to simulate the condition of water flowing out of the model, and its magnitude is the total water pressure at the current water depth outside the immersed tube. In this embodiment, the outlet total pressure is 152684 Pa;

[0080] Wall boundary condition: The surface of the immersed tube and the gravel bed area are set as no-slip conditions. In this embodiment, the roughness height is 0.003 m;

[0081] By inputting the fluid density, dynamic viscosity, gravel bed resistance coefficient, and model geometric characteristics, the continuity equation and momentum equation are solved using CFD software to obtain the velocity and shear stress distributions in the immersed tube joint gap and the gravel bed area.

[0082] S4. Magnify the velocity and shear stress of the obtained gravel bed and the immersed tube gap to obtain the measured values of velocity and shear stress; Magnifying the velocity and shear stress of the obtained gravel bed and the immersed tube gap includes calculating the magnified velocity and shear stress according to the following formula

[0083] u_local = n * u_calculated

[0084] _local = n * _calculated

[0085] where the magnification factor n = 1 / ε, which represents the inverse relationship between the local velocity and the porosity in the porous medium model; u_calculated is the velocity calculated by the model, _calculated is the shear stress calculated by the model, u_local is the local velocity inside the magnified gravel, _local is the local shear stress inside the magnified gravel bed.

[0086] S5. Compare the obtained measured values of velocity and shear stress with the reference value of the critical incipient shear stress of sediment to determine whether there is incipient motion of the foundation soil.

[0087] Among them, the reference value of the critical incipient shear stress of the sediment is calculated using the following formula:

[0088]

[0089] where is 0.0164, is the seawater density of 1025 g / cm 3 , d is the median grain size of the sediment, d * = 10 mm, d ’ = 0.5 mm, is the dry density of the sediment of 1.1 g / cm 3 , is the dry bulk density of the sediment on the bed surface, is the stable dry bulk density of the sediment particles, taking = 1, is the comprehensive bonding force parameter and the film water thickness parameter = 2.31×10 -5 cm is the acceleration due to gravity, taking 9.81 m / s² is the water depth. In this embodiment = 1.75 cm 3 / s 2 , the water depth = 15.1 m, the dry density is 1.1 g / cm 3 , d is the median grain size of the sediment, taking 0.075 mm; the calculated value of the critical incipient shear stress is 0.24 Pa

[0090] When judging whether there is incipient motion of the foundation soil, the judgment is made according to the following rules

[0091] When _local > _c, it is determined that the sediment starts to move

[0092] When _local ≤ _c, it is determined that the sediment does not start to move

[0093] As Figures 5 - 8 shown, as Figure 5 shown, after the water flow flows out of the gap, seepage diffusion occurs in the crushed stone bed, and the flow velocity gradually decays. As Figure 6 and Figure 7 shown, in the area near the bottom of the gap, the flow velocity reaches the maximum value, that is, the central red or yellow area in the figure is the area with the maximum flow velocity; as Figure 8 shown, as the distance increases, the flow velocity gradually diffuses to both sides and decreases, and this law is clearly reflected by the gradient change from the red high-flow velocity area to the green and blue low-flow velocity areas in the velocity distribution contour map. At the same time, the distribution of the bottom shear stress also shows a similar law, with the maximum shear stress in the area near the gap, gradually diffusing outward and decaying. In this embodiment, under the condition of using a crushed stone bed for anti-seepage protection without setting steel backing plates, the local maximum shear stress of the flow velocity flowing out of the bottom through the gap is 0.83 Pa, which is greater than the critical incipient shear stress of the sediment, and there is sediment incipient motion

[0094] Obviously, the above embodiments are only examples clearly described and not limitations on the implementation manners. For those of ordinary skill in the art, 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 manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention

Claims

1. A CFD-based immersed tube joint leakage analysis and foundation start-up judgment method, characterized in that: The following steps are involved: S1. Establish a seepage physical model and calculate the resistance coefficient of the gravel bed; the seepage physical model includes arranging the water supply system, the test pipe section, the pressure sensor and the data acquisition device, filling the test pipe section with homogeneous gravel, adjusting the inlet flow rate, and recording the pressure gradient at different flow rates; 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 Where 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; 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 analysis and foundation start-up determination method according to claim 1, 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.

3. The CFD-based immersed tube joint leakage analysis and foundation start-up determination method according to claim 1, 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.

4. The CFD-based immersed tube joint leakage analysis and foundation start-up determination method according to claim 1, 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 , d * = 10mm, d ' = 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, , 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, and d is the median particle size of the sediment.

5. The CFD-based immersed tube joint leakage analysis and foundation start-up determination method according to claim 4, 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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