CFD (computational fluid dynamics)-based LNG (liquefied natural gas) ship C-type liquid tank fluid sloshing load calculation method

Abstract

The invention discloses a CFD-based ship multi-C-type liquid tank fluid sloshing load calculation method, and belongs to the technical field of ship and ocean engineering. Aiming at the problem of fluid sloshing load calculation of a multi-C-shaped liquid tank arranged on an upper deck, based on the STAR-CCM + simulation technology, when a ship sails in waves close to the real sea condition, the coupling effect of ship motion and multi-liquid-tank fluid sloshing is analyzed. The magnitude of the fluid load borne by the wall surface of the liquid tank under different parameters is output through the simulation result, and then the safety of the liquid tank structure is analyzed.

Description

Technical Field

[0001] The present invention relates to the technical field of ship and ocean engineering, and in particular to a CFD-based method for calculating fluid sloshing loads in multiple C-type liquid tanks of an LNG ship. Background Art

[0002] With the growing global demand for clean energy, LNG is becoming widely used as a main engine fuel in the marine sector. During the voyage of LNG vessels, fuel consumption can lead to partial filling of C-type tanks. The liquid within the tanks sloshes due to the vessel's motion, causing impact loads on the tank hull. Because bunkering vessels need to carry more fuel, their C-type tanks are often larger, resulting in higher sloshing impact loads. Tank sloshing not only reduces vessel stability but can also cause severe fatigue damage to the tank structure and even lead to major safety incidents such as tank rupture and leakage.

[0003] Some LNG carriers have multiple Type C tanks arranged on the upper deck, but current research on tank sloshing mainly focuses on the impact of a single tank on the ship's motion performance, and most of the tanks discussed are located in the lower middle part of the hull. Regarding the actual engineering application scenario of arranging multiple tanks on the upper deck, relevant research is still insufficient. The CFD method is a technology that studies fluid flow phenomena through numerical calculations and image display. It can simulate complex fluid motion and has significant advantages in dealing with multiphase flow problems such as tank sloshing. Through the CFD method, information such as the flow state and pressure distribution of the fluid in the tank can be intuitively obtained. It is of great significance to conduct in-depth research on the CFD-based calculation method of fluid sloshing loads in Type C tanks of LNG ships. Accurate calculation of sloshing loads helps to optimize the design of tanks, improve the safety and reliability of ship structures, and reduce operational risks. Summary of the Invention

[0004] The main purpose of this invention is to overcome the shortcomings and deficiencies of the existing technology and propose a CFD-based method for calculating the fluid sloshing loads in multiple C-type liquid tanks of LNG ships. The method can quickly and accurately calculate the loads generated by the sloshing of multiple C-type liquid tanks during LNG ship transportation, thereby determining whether there is any danger.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions

[0006] 1) Geometric model establishment: Import the hull model with multiple tanks and saddles assembled into STAR-CCM+; use the Create Volume module to create the overall background domain, overlapping mesh volume, free surface, and tank densification volume;

[0007] 2) Setting boundary conditions: Cut the surface of the geometric model into different areas and set the corresponding boundary types in turn;

[0008] 3) Grid division: When gridding, use the cutting body grid unit generator and the prismatic layer grid generator; use the low Reynolds number turbulence model SST Model, when dividing the grid, the total thickness of the boundary layer is calculated according to the parameters, and then the basic size of the grid is determined as the basis for division;

[0009] 4) Continuum, Flow Field Properties, and Region Settings: A multi-tank physical continuum is established and its corresponding properties are set. Incorporating the multiphase characteristics of the fluids within the tanks, the VOF method is used to model interfacial fluctuations. 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) Solve and output the results: During the numerical simulation process, STAR-CCM+ constructs a numerical simulation model based on the finite volume method; uses the VOF method to capture the deformation of multiple tanks and the free surface of the water body; uses the Reynolds-averaged Stokes method with The turbulence model is used to calculate the multiphase flow field; the overlapping grid and grid deformation techniques are used to handle the movement and deformation of the grid; and finally, the sloshing characteristics of the C-type liquid tank during movement and the curve of the liquid sloshing load borne by the liquid tank are obtained.

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

[0012] Furthermore, the background domain is segmented, and the boundary types of the bow, top, and bottom surfaces are assigned as velocity inlets, the stern surface is set as a pressure outlet, 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 body from the hull model is set as an overlapping mesh.

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

[0014]

[0015] Where, and They are Volume fraction and volume of the phase, Represents the volume of the unit; the volume fractions of all phases in a unit need to satisfy the sum of one, that is,

[0016]

[0017] For the gas-liquid two-phase flow problem, if the liquid phase volume fraction is defined as , then the volume fraction of the gas phase ; The density of the mixed fluid and dynamic viscosity Expressed as:

[0018]

[0019]

[0020] Where, and represent the density of liquid and gas phase respectively, and Represent the dynamic viscosity of the two phases respectively.

[0021] Furthermore, in step 5), overlapping grids refer to meshing the hull model and then embedding it into the background grid of the overall flow field; by interpolating the overlapping parts between the two groups of grids with the background grid, data transmission and simulation calculations are performed on the movement of the object and the interaction between the fluid.

[0022] Specifically, the sloshing load calculation method for a Type C LNG tank involves importing a hull model with multiple tanks and saddles assembled into STAR-CCM+. Using the Create Volume module, a global background domain, overlapping mesh volumes, free surfaces, and tank infill volumes are created. The geometric model surface is segmented into different regions and corresponding boundary types are assigned, such as velocity inlet, pressure outlet, symmetry plane, free stream boundary, wall, and overlapping mesh. Boundary damping is also applied at the pressure outlet to eliminate boundary reflections caused by the ship's waves and prevent them from affecting computational convergence. Meshing is performed using the Cut Volume Mesh Generator and the Prismatic Layer Mesh Generator to better conform to the model boundaries, simulate boundary layer flows, and improve computational accuracy. Meshing is performed to enhance simulation accuracy in areas with complex bow and stern contours and on the free surface. Dimensional transitions must be maintained at overlapping mesh boundaries to meet interpolation accuracy requirements. A physical continuum is established and its properties are assigned. For each tank, the fluid phases and VOF waves are created, matching the corresponding physical volumes and field functions for volume fraction, velocity, and pressure in each region. After completing the settings, numerical simulation was performed, and finally the curve of the liquid sloshing load borne by the C-type liquid tank during movement was obtained.

[0023] The present invention provides a method for calculating fluid sloshing loads in multiple C-type tanks on LNG ships. Based on STAR-CCM+, it calculates the sloshing loads generated by the fluid acting on the tank walls when the ship's motion is coupled with the sloshing of fluids within multiple tanks. This method is more realistic, and the resulting data is accurate with minimal error. It also improves the safety of C-type tanks on LNG ships and provides reliable data support for the design of structures such as tanks and saddles. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1This is a flow chart of the entire process simulation analysis method for calculating fluid sloshing loads in multiple C-type tanks of LNG ships based on CFD provided by the present invention.

[0025] Figure 2 Diagram of the geometric model of a real ship with two tanks and saddles

[0026] Figure 3 Cross-sectional view of fluid meshing for hydrodynamic analysis of a real ship.

[0027] Figure 4 This is the x-component curve of the sloshing load on the wall of the LNG tank of a real ship.

[0028] Figure 5 This is the y-component curve of the sloshing load on the wall of the LNG tank on a real ship.

[0029] Figure 6 This is the z-component curve of the sloshing load on the LNG tank wall of a real ship. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments, but the embodiments of the present invention are not limited thereto.

[0031] It should also be noted here that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions of the present invention are shown in the drawings, while other details that are not closely related to the present invention are omitted.

[0032] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0033] This paper uses STAR-CCM+ to calculate the sloshing loads on multiple C-type tanks on LNG vessels. When solving the flow field using STAR-CCM+, the fluid is assumed to be Newtonian, incompressible, and impermeable. The fluid flow satisfies the fundamental governing equations for conservation of mass, momentum, and energy.

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

[0035]

[0036] Where, represents the fluid density, Indicates time, Represents coordinates, The continuity equation mathematically accurately expresses that "the sum of the rate of change of fluid density per unit time and the net mass flux of the fluid through the surface of the control volume is zero", reflecting the basic physical law of conservation of mass.

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

[0038]

[0039] Where, is the flow field pressure, For source items customized according to actual conditions, are coordinates, is the corresponding velocity component, represents the viscous shear stress. Dynamic viscosity ,coordinate and Kronecker symbols The specific relationship is as follows:

[0040]

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

[0042]

[0043] Where, represents the specific heat capacity, Represents energy, represents thermal conductivity, and represent temperature and pulsating temperature respectively, is the pulsation speed.

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

[0045] 1) Geometric model establishment: Import the hull model with many tanks and saddles assembled into STAR-CCM+. The assembly model is as follows: Figure 2 . Create the overall background domain, overlapping mesh volume, free liquid surface and liquid tank density volume through the Create Volume module.

[0046] Overlapping meshes, often referred to as "nested" meshes, refer to meshing the ship model and then embedding it within the overall flow field background mesh. By interpolating the overlapping portions of the two meshes with the background mesh, data transfer and simulation can be better integrated into the object's motion and fluid interactions. The core advantage of the overlapping mesh method lies in its ability to discretize computational domains with multiple, arbitrarily overlapping meshes, accurately simulating flow field motion, when dealing with complex problems such as tank sloshing. 2) Setting boundary conditions: The surface of the geometric model is segmented into different regions and the corresponding boundary types, such as velocity inlets, pressure outlets, symmetry planes, free stream boundaries, and walls, are assigned to ensure a more realistic and accurate flow field. In this calculation, the background domain is segmented, and the bow, top, and bottom surfaces are assigned the velocity inlet boundary type. The stern is designated as the pressure outlet, and the two sides of the ship are designated as symmetry planes. The outer surface boundary type of the volume obtained by subtracting the overlapping mesh from the hull model is set to overlapping mesh. At the same time, boundary damping is set at the pressure outlet to eliminate boundary reflections of ship-generated waves and prevent them from affecting the convergence of the calculation.

[0047] 3) Grid division: Since the geometric model used in this calculation is a 1:1 real-scale model and the overall size of the model is large, it is necessary to reasonably adjust the grid size through local encryption so that the calculation can obtain more accurate simulation results with higher computational efficiency. Considering the local details on the hull model will not only have higher requirements for grid accuracy, but also have almost no effect on the calculation results in CFD simulation, so the hull should be reasonably simplified during the modeling stage. When dividing the grid, the cutting body grid unit generator and the prismatic layer grid generator are selected so that the grid can better fit the model boundary, simulate the flow of the boundary layer, and improve the calculation accuracy. Since the fluid will form a boundary layer near the wall, the physical quantities such as velocity therein change dramatically. At the same time, the first layer of grid can directly contact the wall. Therefore, accurately capturing the flow details in the boundary layer, such as velocity gradient, shear stress, etc., is crucial for accurately simulating the interaction between the fluid and the wall. This calculation uses the low Reynolds number turbulence model SST When dividing the mesh, the total thickness of the boundary layer can be calculated based on the parameters, and the basic mesh size can be determined as the basis for division. The lines of the bow and stern of the hull are relatively complex, so a denser mesh is required to improve the simulation accuracy. The free liquid surface has wave-making changes in the flow field. At the same time, according to the specifications, the mesh size here should be less than one-tenth of the wave height, so it is also necessary to use a denser mesh to better reflect the actual situation. In addition, the size transition must be done at the junction of overlapping meshes so that the size of the background mesh is similar to the overlapping mesh to meet the requirements for interpolation accuracy. Figure 3The figure shows the meshing of the numerical tank in STAR-CCM+ and a local view of the mesh in the hull and tank areas.

[0048] 4) Continuum, Flow Field Properties, and Region Settings: In CFD simulations, treating fluids as continua and using continuous functions to describe their physical quantities transforms complex microscopic molecular behavior into macroscopic changes in physical quantities, greatly simplifying the description and analysis of fluid flow. This allows macroscopic mathematical equations to characterize fluid motion, facilitating numerical calculations and simulations. In this calculation, a physical continuum was established based on the position and geometric characteristics of each tank, and corresponding properties were set. Fluids and VOF waves were created for each phase, matching the corresponding physical bodies and field functions for volume fraction, velocity, and pressure in each region.

[0049] The VOF (Volume of Fluid) multiphase flow model was selected to simulate the wave-generating phenomena in the flow field of the LNG hull model and the overall flow field within the LNG tank. The VOF (Volume of Fluid) model is a numerical method used in CFD to solve multiphase flow problems. It is particularly suitable for simulating the flow and interface evolution between multiple immiscible fluids on a numerical grid, and can meet the requirements of this invention for simulating the flow field within the tank.

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

[0051]

[0052] Where, and They are Volume fraction and volume of the phase, Represents the volume of the unit. The volume fractions of all phases in a unit must sum to one, i.e.

[0053]

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

[0055]

[0056]

[0057] Where, and represent the density of liquid and gas phase respectively, and Represent the dynamic viscosity of the two phases respectively.

[0058] The DFBI (Dynamic Fluid Body Interaction) model is used in this paper to simulate the interaction between the ship model and the flow field. It calculates the forces and moments acting on the ship model in the flow field and solves the governing equations accordingly, accurately simulating the multi-degree-of-freedom motion of the ship as it interacts with the complex flow field.

[0059] 5) Solve and output the results: During the numerical simulation process, STAR-CCM+ builds a numerical simulation model based on the finite volume method (FVM). In order to capture the deformation of the free liquid surface, the VOF method is used. For the calculation of the multiphase flow field, the Reynolds-averaged Stokes method is used with This is achieved through a turbulence model. The mesh's motion and deformation are handled using overlapping meshes and mesh deformation techniques. Ultimately, the characteristics of fluid sloshing within the C-type tank during motion are determined, along with a curve depicting the liquid sloshing loads borne by the tank. Figure 4 、 5 Figures 6 and 7 show examples of the calculated x-, y-, and z-components of the sloshing loads on the port and starboard LNG tanks, respectively, under the same operating conditions. The red dashed line in the figure above represents the sloshing load on the port C-type tank of the LNG carrier, while the black solid line represents the sloshing load on the starboard C-type tank.

[0060] In summary, this paper proposes a method for calculating fluid sloshing loads in multiple C-type tanks on LNG carriers. This method utilizes STAR-CCM+ software to simulate the coupled effects of ship motion and fluid sloshing within the tanks, accurately calculating the sloshing loads exerted by the fluid on the tank walls. This method is more responsive to actual operating conditions, resulting in highly accurate and minimal errors. This significantly improves the safety of C-type tanks on LNG carriers and provides reliable data support for the design of key structures such as 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 the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention may be modified or replaced by equivalents 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 multiple C-type tanks of LNG ships, characterized by: The following steps are involved: 1) Geometric model establishment: Import the hull model with multiple tanks and saddles assembled into STAR-CCM+; use the Create Volume module to create the overall background domain, overlapping mesh volume, free surface, and tank densification volume; 2) Setting boundary conditions: Cut the surface of the geometric model into different areas and set the corresponding boundary types in turn; 3) Grid division: When gridding, use the cutting body grid unit generator and the prismatic layer grid generator; use the low Reynolds number turbulence model SST Model, when dividing the grid, the total thickness of the boundary layer is calculated according to the parameters, and then the basic size of the grid is determined as the basis for division; 4) Continuum, flow field properties, and region settings: Establish a multi-tank physical continuum and set corresponding properties. Combined with the multiphase characteristics of the fluid inside the tank, the VOF method is used to model the interface fluctuation behavior. For each tank area, different fluid phases are defined, and initial and boundary conditions for volume fraction, velocity field, and pressure field are established; 5) Solve and output the results: During the numerical simulation process, STAR-CCM+ constructs a numerical simulation model based on the finite volume method; uses the VOF method to capture the deformation of each tank and the free surface of the water body; and uses the Reynolds-averaged Stokes method with The turbulence model is used to calculate the multiphase flow field; the overlapping grid and grid deformation techniques are used to handle the movement and deformation of the grid; and finally, the sloshing characteristics of the C-type liquid tank during movement and the curve of the liquid sloshing load borne by the liquid tank are obtained.

2. The CFD-based method for calculating fluid sloshing loads in multiple C-type tanks of an LNG ship according to claim 1, characterized in that: The boundary types include velocity inlet, pressure outlet, symmetry plane, free stream boundary, and wall.

3. The CFD-based method for calculating fluid sloshing loads in multiple C-type tanks of an LNG ship according to claim 2, characterized in that: The background domain is segmented, and the boundary types of the bow, top, and bottom surfaces are assigned as velocity inlets, the stern surface is set as a pressure outlet, and the two sides of the ship are set as symmetry planes. The outer surface boundary type of the body obtained by subtracting the overlapping mesh body from the hull model is set as an overlapping mesh.

4. The CFD-based method for calculating fluid sloshing loads in multiple C-type tanks of an LNG ship according to claim 1, characterized in that: In step 5), in the VOF method, the volume fraction of the simulated phase is defined as follows: ； Where, and They are Volume fraction and volume of the phase, Represents the volume of the unit; the volume fractions of all phases in a unit need to satisfy the sum of one, that is, ； For the gas-liquid two-phase flow problem, if the liquid phase volume fraction is defined as , then the volume fraction of the gas phase ; The density of the mixed fluid and dynamic viscosity Expressed as: ； ； Where, and represent the density of liquid and gas phase respectively, and Represent the dynamic viscosity of the two phases respectively.

5. The CFD-based method for calculating fluid sloshing loads in multiple C-type tanks of an LNG ship according to claim 1, characterized in that: In step 5), overlapping grids refer to meshing the hull model and then embedding it into the background grid of the overall flow field; by interpolating the overlapping parts between the two groups of grids with the background grid, data transmission and simulation calculations are performed on the movement of the object and the interaction between the fluid.

Images