A CFD-based LNG ship C-type tank fluid sloshing load calculation method

Abstract

The application discloses a kind of ship multi-C type liquid tank fluid sloshing load calculation method based on CFD, which belongs to the technical field of ship and ocean engineering.The application is aimed at the calculation problem of multi-C type liquid tank fluid sloshing load arranged on the upper deck, based on STAR-CCM+ simulation technology, the coupling effect of ship motion and multi-liquid tank fluid sloshing is analyzed when the ship sails in the wave close to real sea conditions.The size of fluid load on the liquid tank wall surface under different parameters is output through simulation results, and then the safety of the liquid tank structure is analyzed.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ship and ocean engineering, and particularly relates to a CFD-based LNG ship multi-C-type liquid tank fluid sloshing load calculation method. BACKGROUND

[0002] With the increasing demand for clean energy worldwide, LNG is widely used as a main fuel in the field of ship and sea. During the sailing process of the LNG ship, the consumption of fuel will cause partial loading of the C-type liquid tank, and the liquid in the tank will produce sloshing due to the movement of the ship, causing impact load on the tank body. Since the filling ship needs to carry more fuel, the volume of the C-type tank is often larger, and the value of the sloshing impact load is also higher. Liquid tank sloshing not only reduces the stability of the ship, but also can cause serious fatigue damage to the liquid tank structure, and even cause major safety accidents such as liquid tank rupture and leakage.

[0003] Some LNG ships arrange multiple C-type liquid tanks on the upper deck, but the current research on liquid tank sloshing mainly focuses on the influence of single liquid tank on the motion performance of the ship, and most of the discussed liquid tanks are located in the middle and lower parts of the ship body. For the actual engineering application scene of arranging multiple liquid tanks on the upper deck, relevant research is still insufficient. The CFD method is a technology for studying fluid flow phenomena through numerical calculation and image display. It can simulate complex fluid motion and has a significant advantage in dealing with multi-phase flow problems such as liquid tank sloshing. Through the CFD method, the flow state, pressure distribution and other information of the fluid in the liquid tank can be obtained intuitively. It is of great significance to deeply study the CFD-based LNG ship C-type liquid tank fluid sloshing load calculation method. Accurate calculation of the sloshing load helps to optimize the design of the liquid tank, improve the safety and reliability of the ship structure, and reduce the operation risk. SUMMARY

[0004] The main purpose of the present application is to overcome the shortcomings and deficiencies of the prior art, and to provide a CFD-based LNG ship multi-C-type liquid tank fluid sloshing load calculation method, which can quickly and accurately calculate the load generated by the sloshing of multiple C-type liquid tanks during the transportation of the LNG ship, so as to determine whether there is danger.

[0005] To achieve the above purpose, the following technical scheme is adopted in the present application

[0006] 1) Geometric model establishment: import the ship body model with multiple liquid tanks and saddles assembled into STAR-CCM+; create the overall background domain, overlapping grid body, free liquid surface and liquid tank encryption body by creating a body module;

[0007] 2) Set boundary conditions: cut the surface of the geometric model according to different regions and set the corresponding boundary types in turn;

[0008] 3) Mesh generation: A cut-body mesh generator and a prism layer mesh generator are used for mesh generation; a low Reynolds number turbulence model (SST) is employed. The model calculates the total thickness of the boundary layer based on parameters when dividing the mesh, and then determines the basic mesh size as the basis for dividing the mesh;

[0009] 4) Continuum, Flow Field Properties, and Region Settings: A multi-tank physical continuum is established and corresponding properties are set. Considering the multiphase characteristics of the fluids within the tanks, the VOF method is used to model the interface fluctuation behavior. For each tank region, different fluid phases are defined, and initial and boundary conditions for volume fraction, velocity field, and pressure field are established.

[0010] 5) Solving and outputting results: During the numerical simulation, STAR-CCM+ constructs a numerical simulation model based on the finite volume method; the VOF method is used to capture the deformation of multiple liquid tanks and the free surface of the water body; the Reynolds-averaged Stokes method is used in conjunction with... A turbulence model was used to calculate the multiphase flow field; overlapping meshes and mesh deformation techniques were employed to handle the motion and deformation of the meshes; finally, the sloshing characteristics of the C-shaped liquid tank during the motion process and the curves of the liquid sloshing load borne by the liquid tank were obtained.

[0011] Furthermore, the boundary types include velocity inlet, pressure outlet, symmetry plane, free flow boundary, and wall.

[0012] Furthermore, the background domain is segmented, and the boundary types of the surface, top surface, and bottom surface in the bow direction are assigned as velocity inlets, the stern surface is set as pressure outlets, the two sides of the ship are set as symmetry planes, and the outer surface boundary type of the body obtained by subtracting the overlapping mesh from the hull model is set as an overlapping mesh.

[0013] Furthermore, in step 5), the volume fraction of the simulated phase in the VOF method is defined as follows:

[0014]

[0015] In the formula, and They are respectively Phase volume fraction and volume, This represents the volume of a cell; the volume fractions of all phases in a cell must sum to one, i.e.

[0016]

[0017] For gas-liquid two-phase flow problems, if the liquid phase volume fraction is defined as... Then the phase volume fraction of the gas The density of the mixed fluid and dynamic viscosity denotes:

[0018]

[0019]

[0020] wherein, and denote the density of the liquid and gas phase, respectively, and denote the dynamic viscosity of the two phases, respectively.

[0021] Further, in step 5), the overlapping grid refers to embedding the hull model into the background grid of the overall flow field after meshing the hull model; by interpolating the overlapping part between the two groups of grids and the background grid, the action between the movement of the object and the fluid is data-transferred and simulated.

[0022] Specifically, the calculation method of the sloshing load of the C-type LNG liquid tank is as follows: the hull model assembled with multiple liquid tanks and saddles is introduced into STAR-CCM+. The overall background domain, overlapping grid body, free liquid surface and liquid tank encryption body are created by creating a body module. The surface of the geometric model is cut according to different regions and the corresponding boundary types are set in sequence, such as velocity inlet, pressure outlet, symmetry plane, free flow boundary, wall, overlapping grid and the like, and the boundary damping is set at the pressure outlet to eliminate the boundary reflection of ship waves and prevent affecting the calculation convergence. The mesh is divided, the cutting body mesh unit generator and the prism layer mesh generator are selected, so that the mesh can better fit the model boundary, simulate the flow of the boundary layer and improve the calculation precision. The simulation precision is improved by encrypting the grid at the complex lines at the bow and stern of the hull and on the free liquid surface. The size transition must be well done at the overlapping grid interface to meet the interpolation accuracy requirements. The physical continuum is established and the corresponding properties are set, the VOF wave of each liquid tank is created, the corresponding physical body and the volume fraction, velocity and pressure field functions in each region are matched. After the setting is completed, numerical simulation is carried out, and finally the curve of the liquid sloshing load borne by the C-type liquid tank in the movement process is obtained.

[0023] The LNG ship multi-C-type liquid tank fluid sloshing load calculation method provided by the application is based on STAR-CCM+ to calculate the sloshing load generated by the fluid acting on the liquid tank wall when the hull movement and the coupling action of the fluid sloshing in multiple liquid tanks. The calculation method is closer to the actual situation, and the calculated data is accurate and has small error, which improves the safety of the C-type liquid tank of the LNG ship and provides reliable data support for the design of the liquid tank, saddle and other structures. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1A flowchart of a simulation analysis method for the whole process of LNG ship multi-C type liquid tank fluid sloshing load calculation based on CFD provided by the application is shown.

[0025] Figure 2 A geometric model diagram of an actual ship equipped with two liquid tanks and saddles

[0026] Figure 3 A profile view of fluid mesh division for hydrodynamic analysis of an actual ship.

[0027] Figure 4 A curve diagram of the x-direction component of the sloshing load on the LNG liquid tank wall of an actual ship.

[0028] Figure 5 A curve diagram of the y-direction component of the sloshing load on the LNG liquid tank wall of an actual ship.

[0029] Figure 6 A curve diagram of the z-direction component of the sloshing load on the LNG liquid tank wall of an actual ship. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the application clearer, the application will be described in detail below with reference to the drawings and specific embodiments, but the embodiments of the application are not limited thereto.

[0031] Here, it also needs to be noted that, in order to avoid obscuring the application due to unnecessary details, only the structures and / or processing steps closely related to the solutions of the application are shown in the drawings, and other details not closely related to the application are omitted.

[0032] In addition, it also needs to be noted that the term “comprises”, “includes” or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such a process, method, article or equipment.

[0033] The application adopts STAR-CCM+ to calculate the fluid sloshing load of the LNG ship multi-C type liquid tank. When solving the flow field using STAR-CCM+, the fluid is assumed to be a Newtonian fluid, which is incompressible and impermeable. The fluid flow satisfies the basic control equations of mass, momentum and energy conservation.

[0034] The continuity equation is a specific form of the mass conservation equation in fluid mechanics. Its mathematical expression is:

[0035]

[0036] In the formula, represents the fluid density, represents time, represents the coordinate, is the corresponding velocity component. The continuity equation mathematically accurately expresses that "the rate of change of fluid density per unit time and the sum of the net mass flux of the fluid through the control surface is zero", which embodies the basic physical law of mass conservation.

[0037] The momentum conservation equation describes the variation law of fluid momentum. The specific expression is:

[0038]

[0039] In the formula, is the pressure of the flow field, is a source term defined according to the actual situation, is the coordinate, is the corresponding velocity component, represents the viscous shear stress. It can also be represented by the dynamic viscosity , the coordinate and the Kronecker symbol , and the specific relationship is as follows:

[0040]

[0041] The energy conservation equation is used to describe the transfer and conversion of fluid energy. Its expression is as follows:

[0042]

[0043] In the formula, represents the specific heat capacity, represents energy, represents thermal conductivity, and represent temperature and fluctuating temperature respectively, is the fluctuating velocity.

[0044] As shown in Figure 1 , the present application provides a LNG ship C type liquid tank fluid sloshing load calculation method, and the calculation method of the C type LNG liquid tank fluid sloshing load is studied based on STAR-CCM+. The specific analysis method is as follows:

[0045] 1) Geometric model establishment: import the ship model assembled with multiple liquid tanks and saddles into STAR-CCM+, and the assembly model is as shown in Figure 2 . Create the overall background domain, overlapping grid body, free liquid surface and liquid tank encryption body by creating the body module.

[0046] The overlapping grid is also commonly referred to as a "nested" grid, and in the present application refers to embedding the hull model grid into the background grid of the overall flow field after the hull model grid is generated. By interpolating the overlapping part between the two sets of grids and the background grid, the interaction between the object motion and the fluid can be better transferred and simulated. The core advantage of the overlapping grid method is that when dealing with complex problems such as tank sloshing, it can discretely process a calculation domain with multiple different grids that overlap in an arbitrary manner, thereby accurately simulating the motion of the flow field.

[0047] 3) Dividing the grid: Since the geometric model used in this calculation is a 1:1 full-scale model, the overall size of the model is large, so it is necessary to adjust the grid size by local encryption to make the calculation more accurate and efficient. Considering the local details on the hull model not only have higher requirements for grid accuracy, but also have little effect on the calculation results in CFD simulation, the hull should be reasonably simplified during modeling. When dividing the grid, the cutting body grid generator and the prism layer grid generator are selected to make the grid better fit the model boundary, simulate the flow of the boundary layer, and improve the calculation accuracy. Since the fluid near the wall will form a boundary layer, the velocity and other physical quantities in the boundary layer change sharply, and the first layer of grid can directly contact the wall, so it is important to accurately capture the flow details in the boundary layer, such as velocity gradient and shear stress, for accurate simulation of the interaction between the fluid and the wall. The low Reynolds number turbulence model SST is used in this calculation, which can calculate the total thickness of the boundary layer according to the parameters when dividing the grid, and then determine the basic size of the grid as the basis for division. The lines at the bow and stern of the hull are complex, so more dense grids are needed to improve the simulation accuracy. The free surface has wave changes in the flow field, and according to the specification, the grid size at this point should be less than one-tenth of the wave height, so encryption of the grid is also needed to better reflect the true situation. In addition, the size transition at the intersection of the overlapping grid must be done well, so that the size of the background grid is similar to that of the overlapping grid, in order to meet the requirements of interpolation accuracy. Figure 3The numerical tank meshing in STAR-CCM+ and the mesh local view of the hull and tank region are shown.

[0048] 4) Continuum, flow field properties and region settings: In CFD simulation, fluid is regarded as a continuum, and the physical quantity of fluid is described by continuous function, which can convert complex microscopic molecular behavior into macroscopic physical quantity change, greatly simplifies the description and analysis of fluid flow, so as to be able to use macroscopic mathematical equation to depict the motion of fluid, and facilitate numerical calculation and simulation. In this calculation, according to the position and geometric characteristics of each tank, the physical continuum is established and the corresponding properties are set, the phase fluid and VOF wave are created, and the volume fraction, velocity and pressure field functions in each region are matched.

[0049] The VOF multiphase flow model is selected to simulate the wave making phenomenon of the LNG ship model and the overall flow field in the LNG tank. The VOF (Volume of Fluid) model is a numerical method for processing multiphase flow problems in CFD, especially suitable for simulating the flow conditions and interface evolution between multiple immiscible fluids on a numerical grid, which can meet the requirements of the invention for simulating the flow field in the tank.

[0050] In the present invention, the definition of the simulation phase volume fraction is as follows:

[0051]

[0052] In the formula, and are the volume fraction and volume of the phase, respectively, indicates the volume of the unit. The volume fractions of all phases in a unit need to satisfy the total sum of one, that is,

[0053]

[0054] For gas-liquid two-phase flow problems, if the phase volume fraction of the liquid is defined as , the phase volume fraction of the gas is . The density and dynamic viscosity of the mixed fluid can be represented as:

[0055]

[0056]

[0057] In the formula, and represent the density of the liquid phase and the gas phase, respectively, and​ These represent the dynamic viscosity of the two phases, respectively.

[0058] DFBI (Dynamic Fluid Body Interaction) is primarily used in this invention to simulate the interaction between a ship hull model and the flow field. It can calculate the forces and moments acting on the ship hull model under the influence of the flow field, and solve the governing equations accordingly, thereby accurately simulating the multi-degree-of-freedom motion of the ship hull interacting with complex flow fields.

[0059] 5) Solving and Outputting Results: During the numerical simulation, STAR-CCM+ constructed a numerical simulation model based on the Finite Volume Method (FVM). The VOF method was used to capture the deformation of the free surface. For the calculation of the multiphase flow field, the Reynolds-averaged Stokes method was employed in conjunction with... The turbulence model is used to achieve this. Overlapping meshes and mesh deformation techniques are employed to handle the motion and deformation of the mesh, ultimately yielding the characteristics of fluid sloshing within the C-shaped liquid tank during motion and the curves of the liquid sloshing load borne by the tank. Figure 4 , 5 Figures 6 and 7 respectively show examples of the calculated x, y, and z components of the sway load on the port and starboard LNG tanks under the same operating condition. The red dashed line in the figure above represents the sway load on the port C-type tank of the LNG ship, and the black solid line represents the sway load on the starboard C-type tank.

[0060] In summary, this invention proposes a method for calculating fluid sloshing loads in multi-C-type liquid tanks of LNG ships. This method utilizes STAR-CCM+ software to simulate the coupling effect between hull motion and fluid sloshing within the liquid tanks, accurately calculating the sloshing loads generated by the fluid on the tank walls. This calculation method more closely reflects actual operating conditions, yielding high accuracy and minimal error, significantly improving the safety of C-type liquid tanks in LNG ships, and providing reliable data support for the design of key structures such as liquid tanks and saddles.

[0061] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A CFD-based method for calculating fluid sloshing loads in multi-C-type liquid tanks of LNG ships, characterized in that, Includes the following steps: 1) Geometric model creation: Import the hull model of multiple C-type liquid tanks and saddles already assembled on the upper deck into STAR-CCM+; create the overall background domain, overlapping mesh, free surface and liquid tank densified body through the volume creation module; 2) Set boundary conditions: Cut the surface of the geometric model into different regions and set the corresponding boundary types in sequence; the boundary types include velocity inlet, pressure outlet, symmetry plane, free flow boundary, and wall. The background domain is segmented, and the boundary types of the surface, top surface and bottom surface in the bow direction are assigned as velocity inlets, the stern surface is set as pressure outlets, the two sides of the ship are set as symmetry planes, and the outer surface boundary type of the body obtained by subtracting the overlapping mesh from the hull model is set as an overlapping mesh. 3) Meshing: During mesh generation, a cut-body mesh generator and a prism layer mesh generator are used to ensure the mesh fits the model boundary and simulates boundary layer flow. A boundary layer forms near the wall, and the first mesh layer directly contacts the wall, capturing flow details within the boundary layer: velocity gradient, shear stress, and the interaction between the simulated fluid and the wall. A low Reynolds number turbulence model is employed. The model calculates the total boundary layer thickness based on parameters when dividing the mesh, and then determines the basic mesh size as the basis for meshing; dense meshes are used at the bow and stern of the hull to improve simulation accuracy. Wave-making changes of the flow field are present on the free liquid surface, and the mesh size here should be less than one-tenth of the wave height. 4) Continuum, flow field properties and region settings: Establish a physical continuum of multiple liquid tanks and set the corresponding properties. Combine the multiphase characteristics of the fluid inside the liquid tanks and use the VOF method to model the interface fluctuation behavior. For each liquid tank region, different fluid phases are defined, and initial and boundary conditions for volume fraction, velocity field, and pressure field are established. 5) Solving and outputting results: During the numerical simulation, STAR-CCM+ constructs a numerical simulation model based on the finite volume method; the VOF method is used to capture the deformation of each liquid tank and the free surface of the water body; the Reynolds-averaged Stokes method is used in conjunction with... A turbulence model is used to calculate multiphase flow fields; overlapping meshes and mesh deformation techniques are employed to handle mesh motion and deformation. Finally, the sloshing characteristics of the C-type liquid tank during the motion and the curves of the liquid sloshing load borne by the liquid tank were obtained. The calculation results of the x, y, and z components of the sway load on the port and starboard LNG tanks were obtained. Overlapping meshes refer to the process of meshing a ship hull model and then embedding it into the background mesh of the overall flow field. By interpolating the overlapping parts between the two sets of meshes with the background mesh, the motion of the object and the interaction between the fluid can be transferred and simulated.

2. The method for calculating fluid sloshing loads in multi-C-type liquid tanks of LNG ships based on CFD, as described in claim 1, is characterized in that: In step 5), the volume fraction of the simulated phase in the VOF method is defined as follows: ; In the formula, and They are respectively Phase volume fraction and volume, This represents the volume of a cell; the volume fractions of all phases in a cell must sum to one, i.e. ; For gas-liquid two-phase flow problems, if the liquid phase volume fraction is defined as... Then the phase volume fraction of the gas The density of the mixed fluid and dynamic viscosity Represented as: ； ； In the formula, and These represent the densities of the liquid and gas phases, respectively. and These represent the dynamic viscosity of the two phases, respectively.

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