A ship self-propulsion simulation analysis method based on Fluent

Abstract

The invention discloses a ship self-propulsion simulation analysis method based on Fluent. The method creates a whole ship calculation model including a hull and a propeller, adopts a RANS equation SST k‑ω model to simulate the flow field, adopts a multiphase flow VOF model to simulate the water-air interface, adopts an overlapping grid model, a dynamic grid model, an SDOF model and cooperates with a UDF open source setting to simulate the flow field of the hull and the propeller when they are actually working, and applies the thrust of several propellers to the hull to promote the movement of the hull. At the same time, by improving the boundary condition setting and optimizing the force transmission process, the calculation time is shortened, the calculation efficiency is improved, the autonomous navigation of the hull under the action of several propellers is simulated quickly and realistically, the mutual interference between the ship and the propeller is truly reflected, and the ship self-propulsion parameters are predicted.

Description

Technical Field

[0001] The present invention relates to the technical field of ship simulation, and in particular to a ship self-propulsion simulation analysis method based on Fluent. Background Art

[0002] In recent years, with the rapid development of computer technology, computational fluid dynamics (CFD) has attracted more and more attention by simulating the movement and flow phenomena and processes of various objects in the engineering field through computer numerical simulation to obtain realistic physical effects. It has the advantages of good simulation effect and low calculation cost, and has a broad application market. It has been widely used in the field of numerical simulation of hydrodynamics of ship motion.

[0003] During the navigation of a ship, the hull and the propeller interact with each other. The traditional method is to comprehensively predict the self-propulsion parameters of the ship through calculations of the ship's resistance, the propeller's water-clearing performance, and the self-propulsion factor based on the mutual interference between the ship and the propeller. However, in the actual process, the fluid load of the external flow field and the ship's motion state are coupled and conditional on each other. The operation of the propeller will affect the external flow field and the motion state of the ship, and cannot truly reflect the mutual interference between the ship and the propeller. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a ship self-propulsion simulation analysis method based on Fluent which can truly reflect the mutual interference between the ship and the propeller.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A ship self-propulsion simulation analysis method based on Fluent includes the following steps:

[0007] Model creation: Establish a three-dimensional model of the ship and create an external flow field calculation domain, which includes a background calculation domain and a component calculation domain. The component calculation domain includes a hull calculation domain and several propeller calculation domains;

[0008] Computational domain mesh generation: Use background mesh to create a computational pool on the background computational domain, use overlapping mesh to simulate the movement of the ship on the hull computational domain, use dynamic mesh to simulate the rotation of the propeller on the propeller computational domain, and verify the mesh independence and stability of the numerical method after the mesh generation is completed. If the verification is passed, output the final computational domain mesh;

[0009] Solution model settings: The RANS equation SST k-ω turbulence model is used to simulate the flow field, the multiphase flow VOF model is used to simulate the water-air interface, the overlapping grid model, the dynamic grid model, the SDOF model and the UDF open source settings are used to simulate the flow field of the hull and propeller when they are actually working, and the thrust of several propellers is applied to the hull to drive the hull to move;

[0010] Boundary conditions and initial conditions setting: Boundary conditions and initial conditions are set according to the operating conditions. Boundary conditions include the inlet and outlet boundaries of the calculation area, the wall boundaries of the calculation domain, the ship boundary, the propeller boundary, the dynamic mesh boundary and the overlapping mesh boundary. The initial conditions include reference pressure, gravitational acceleration, fluid viscosity, density, turbulence intensity and flow field velocity.

[0011] Discrete mode setting: The basic control equation is discretized using the finite volume method, the velocity-pressure coupling solver is solved using the PISO algorithm, and the sub-relaxation factor is set according to the mesh quality;

[0012] Solution report and visualization definition: set the parameters of solution report output and the content of visualization window output;

[0013] Flow field initialization and solution parameter setting: Set the flow field initialization conditions according to the calculation conditions, including setting the flow field initialization method, free surface initialization type, initialization reference position and turbulence parameters; set the time step according to the CFL conditions, and select the appropriate number of iterations per step and calculation time according to the calculation conditions;

[0014] Solution and post-processing: Start the calculation and solution, monitor and output the thrust and torque of each propeller and the overall resistance of the hull.

[0015] The beneficial technical effect of the present invention is that the above-mentioned Fluent-based ship self-propulsion simulation and analysis method creates a whole ship calculation model including a hull and a propeller, adopts the RANS equation SST k-ω model to simulate the flow field, adopts the multiphase flow VOF model to simulate the water-air interface, adopts an overlapping grid model, a dynamic grid model, an SDOF model and cooperates with the UDF open source setting to simulate the flow field when the hull and the propeller are actually working, and applies the thrust of several propellers to the hull to promote the movement of the hull, truly simulates the autonomous navigation of the hull under the action of several propellers, truly reflects the mutual interference between the ship and the propeller, and predicts the ship's self-propulsion parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a flow chart of the ship self-propulsion simulation analysis method based on Fluent of the present invention;

[0017] Figure 2 This is a schematic diagram of the structure of a three-pontoon sightseeing boat driven by four propellers. DETAILED DESCRIPTION

[0018] In order to enable those skilled in the art to more clearly understand the objectives, technical solutions and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0019] The invention provides a ship self-propulsion simulation analysis method based on Fluent.

[0020] like Figure 1 As shown, in one embodiment of the present invention, the ship self-propulsion simulation analysis method based on Fluent includes steps S1-S8:

[0021] S1. Model creation: Establish a three-dimensional model of the ship and create an external flow field calculation domain.

[0022] In specific implementation, the model data size is determined according to the geometric parameters of the ship itself, and a three-dimensional model of the ship is established. Figure 2 As shown, in this embodiment, a three-float sightseeing boat driven by four propellers is used as a prototype to establish a three-dimensional model of the ship. Since the speed of the sightseeing boat is low and the wind resistance is small, in order to reduce the amount of calculation, only structures such as pontoons and propellers are retained. The mass attributes of the ship model are determined by CAD software modeling and calculation, for example, the commonly used CAD software Solidworks modeling software is used for modeling and calculation.

[0023] The external flow field calculation domain includes a background calculation domain and a component calculation domain, and the component calculation domain includes a hull calculation domain and a plurality of propeller calculation domains. Among them, a rectangular area with a length of L, a width of W, and a height of H is used as the background calculation domain, wherein L≥8*L1, W≥3*L1, and H≥2*L1, and L1 represents the waterline length of the ship. For example, the waterline length of the sightseeing boat in this embodiment is 5m, so a rectangular calculation domain with a length of 40m, a width of 15m, and a depth of 10m is created as the background calculation domain; the length, width, and height of the circumscribed rectangular parallelepiped of the ship (the rectangular parallelepiped that wraps the hull area) are enlarged by 50% respectively as the hull calculation domain; at the same time, four cylinders are nested at the propeller position to generate the propeller calculation domain, the cylinder has the propeller hub end face as the bottom surface, the cylinder has the propeller hub axis as the central axis, the cylinder has the propeller diameter enlarged by 25% as the diameter, the cylinder has the propeller hub length increased by 25% as the height, and the cylinder completely covers the blade area.

[0024] S2. Grid division of computational domain: Based on the computational domain created in step S1, a computational pool is created on the background computational domain using a background grid, the motion of the ship is simulated on the hull computational domain using an overlapping grid, and the rotation of the propeller is simulated on the propeller computational domain using a dynamic grid.

[0025] Among them, the background grid uses a regular hexahedral structured grid, the overlapping grid and the dynamic grid use a tetrahedral grid, and a triangular prism grid is created in the wall area to capture the boundary layer flow. In order to improve the calculation accuracy, the grid encryption method is used to encrypt the overlapping grid and the dynamic grid, and the ship motion path above and below the waterline is encrypted, thereby improving the accuracy of data exchange between the background grid and the overlapping grid.

[0026] After the mesh generation is completed, the mesh independence and stability of the numerical method are verified. The calculation is performed by dividing the meshes into different densities to compare whether the calculation results are similar. If the calculation results are similar, it means that the verification is passed and the final computational domain mesh is output for solver calculation.

[0027] S3. Solution model setting: The shear stress transport (SST) k-ω turbulence model in the Reynolds Average Navier-Stockes (RANS) equation is used to simulate the flow field, and the multiphase flow VOF (Volume of Fluent) model is used to simulate the water-air interface. The overlapping grid model, dynamic grid model, SDOF (Six-Degrees of Freedom, SDOF) model and UDF (Users Defined Function, UDF) open source settings are used to simulate the flow field of the hull and propeller when they are actually working, and the thrust of several propellers is applied to the hull to drive the hull to move.

[0028] In the specific implementation, the relative motion state between the hull and several propellers is controlled by UDF, and the thrust and torque generated by the propellers are applied to the hull and the additional friction resistance correction force, and the degrees of freedom for ship simulation calculation are set through the SDOF model. In the hull coordinate system, the propeller is set to rotate relative to the hull through the UDF macro DEFINE_CG_MOTION provided by Fluent, and the mass properties of the hull are defined through the DEFINE_SDOF_PROPERTIES macro, opening the three degrees of freedom of sway, heave, and pitch, and obtaining the force conditions of the propeller and the hull at the end of each time step in the solver, so that it acts on the hull and the additional friction resistance correction force.

[0029] The additional friction resistance correction force caused by surface roughness is converted according to the three-dimensional method, and the specific calculation is as follows:

[0030] ;

[0031] In the formula, is the roughness compensation coefficient; It is the apparent height of roughness. If there is no special requirement, it can be , unit: m; is the waterline length of the ship, in m; is the additional friction resistance correction force, is the water density, is the forward speed of the hull, and the average speed of the hull relative to the water flow is taken according to the calculation results of the solver. is the wet surface area of ​​the hull.

[0032] In this embodiment, the calculation results of the solver settings are directly obtained through the Get_Report_Definition macro to obtain the force conditions of the propeller and the hull at the end of each time step in the solver. In order to speed up the stabilization speed, the propeller thrust obtained from the solver can be limited based on the propeller's water-clearing performance and the thrust corresponding to a single propeller at a specific speed as a reference value. For example, the speed of a single propeller at 80% of the design speed The corresponding thrust As a reference value, the reference value is increased by 50% as the upper limit, and the reference value is reduced by 50% as the lower limit to obtain a limit range. For the acquired propeller thrust, if it is less than the lower limit of the limit range, the lower limit of the limit range is used to replace the propeller thrust; if it is greater than the upper limit of the limit range, the upper limit of the limit range is used to replace the propeller thrust.

[0033] In other embodiments, the rotation speed of a single propeller at the design speed may be The corresponding thrust As a reference value, the speed of a single propeller at 120% of the design speed can also be used. The corresponding thrust As a reference value. The reference value can be increased by 30% as the upper limit, and the reference value can be reduced by 30% as the lower limit to obtain a limit range for limiting the propeller thrust obtained from the solver; the reference value can also be increased by 70% as the upper limit, and the reference value can be reduced by 70% as the lower limit to obtain a limit range for limiting the propeller thrust obtained from the solver.

[0034] S4. Setting boundary conditions and initial conditions: Boundary conditions and initial conditions are set according to the operating conditions. Boundary conditions include the inlet and outlet boundaries of the calculation domain, the wall boundaries of the calculation domain, the ship boundary, the propeller boundary, the dynamic mesh boundary and the overlapping mesh boundary; initial conditions include reference pressure, gravitational acceleration, fluid viscosity, density, turbulence intensity and flow field velocity.

[0035] In the specific implementation process, when setting the initial state, the gas-liquid free interface of the VOF model in the calculation domain coincides with the waterline of the hull; in order to reduce the excessive amplitude of the ship's longitudinal swing during self-propulsion calculation, the lack of convergence causes the ship to rush out of the calculation domain, and at the same time reduce the amount of calculation, the external flow field can be given a speed less than the estimated value of the ship speed according to the estimated value of the ship speed, and at the same time, an initial velocity in the forward direction of the hull is given according to the difference between the estimated value of the ship speed and the speed of the external flow field. After that, the propeller speed is set through UDF, and the hull performs autonomous self-propulsion motion under the push of four propellers.

[0036] S5. Discrete mode setting: The basic control equations are discretized using the finite volume method, and the velocity-pressure coupling solver uses the Pressure-Implicit with Splitting of Operators (PISO algorithm) to solve, and the sub-relaxation factor is set according to the mesh quality. It is recommended to use a sub-relaxation factor of 1.0 or close to 1.0 for all equations; if the mesh is highly distorted, it is recommended to use PISO tilt correction and set the sum of the momentum and pressure sub-relaxation factors to 1.0.

[0037] S6. Solution report and visualization definition: Set the parameters for solution report output, including propeller torque, propeller thrust, hull resistance, hull lift, hull speed, and hull posture information; set the content of visualization window output, including pressure cloud maps of hull and propeller surface, gas-liquid distribution cloud map in calculation domain, and velocity distribution cloud map of free liquid surface.

[0038] S7, flow field initialization and solution parameter setting: Set the flow field initialization conditions according to the calculation conditions, including setting the flow field initialization method, free surface initialization type, initialization reference position and turbulence parameters, etc., among which, the flow field initialization method adopts standard initialization or mixed initialization, the initialization iteration number is not less than 10 steps, the free surface initialization type selects the plane mode, the initialization reference position selects the pressure inlet, and the turbulence parameters remain the default. Set the time step according to the CFL condition (Courant-Friedrichs-Lewy Condition), and select the appropriate number of iterations per step and calculation time according to the calculation conditions. In this embodiment, the number of iterations per step is set to 30, so that each step can obtain a more realistic result, and the calculation time is recommended to be set to about 10s.

[0039] S8, solution and post-processing: Start calculation and solution, monitor and output the thrust and torque of each propeller and the overall resistance of the hull.

[0040] The ship self-propulsion simulation analysis method based on Fluent of the present invention creates a whole ship calculation model including a hull and propellers, adopts the RANS equation SST k-ω model to simulate the flow field, adopts the multiphase flow VOF model to simulate the water-air interface, adopts the overlapping grid model, the dynamic grid model, the SDOF model and cooperates with the UDF open source setting to simulate the flow field when the hull and the propeller are actually working, and the thrust of several propellers is applied to the hull to promote the movement of the hull; at the same time, by improving the boundary condition setting and optimizing the force transmission process, the calculation time is shortened, the calculation efficiency is improved, the autonomous navigation of the hull under the action of several propellers is simulated quickly and realistically, the mutual interference between the ship and the propeller is truly reflected, and the ship self-propulsion parameters are predicted.

[0041] The above description is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Those skilled in the art can make various equivalent changes and improvements based on the above embodiments, and all equivalent changes or modifications made within the scope of the claims should fall within the protection scope of the present invention.

Claims

1. A ship self-propulsion simulation analysis method based on Fluent, characterized in that: The steps include: S1. Model creation: Establish a three-dimensional model of the ship and create an external flow field calculation domain, wherein the external flow field calculation domain includes a background calculation domain and a component calculation domain, and the component calculation domain includes a hull calculation domain and several propeller calculation domains; S2. Grid generation of computational domain: Use background grid to create computational pool on the background computational domain, use overlapping grid to simulate the motion of the ship on the hull computational domain, use dynamic grid to simulate the rotation of the propeller on the propeller computational domain, and verify the grid independence and stability of the numerical method after grid generation. If the verification is passed, output the final computational domain grid; S3. Solution model setting: Use the RANS equation SST k-ω turbulence model to simulate the flow field, use the multiphase flow VOF model to simulate the water-air interface, use the overlapping grid model, dynamic grid model, SDOF model and cooperate with the UDF open source setting to simulate the flow field of the hull and propeller when they are actually working, and apply the thrust of several propellers to the hull to drive the hull to move; S4, boundary conditions and initial conditions setting: Boundary conditions and initial conditions are set according to the operating conditions. Boundary conditions include the inlet and outlet boundaries of the calculation area, the wall boundary of the calculation domain, the ship boundary, the propeller boundary, the dynamic grid boundary and the overlapping grid boundary. The initial conditions include reference pressure, gravitational acceleration, fluid viscosity, density, turbulence intensity and flow field velocity. S5. Discrete mode setting: The basic control equation is discretized using the finite volume method, the velocity-pressure coupling solver is solved using the PISO algorithm, and the sub-relaxation factor is set according to the mesh quality; S6. Solution report and visualization definition: set the parameters of solution report output and the content of visualization window output; S7, flow field initialization and solution parameter setting: Set the flow field initialization conditions according to the calculation conditions, including setting the flow field initialization method, free surface initialization type, initialization reference position and turbulence parameters; set the time step according to the CFL conditions, and select the appropriate number of iterations per step and calculation time according to the calculation conditions; S8, solution and post-processing: start calculation and solution, monitor and output the thrust and torque of each propeller and the overall resistance of the hull; In step S3, the relative motion state between the hull and the propellers is controlled by UDF, and the thrust and torque generated by the propellers are applied to the hull and the additional friction resistance correction force, and the ship simulation calculation degrees of freedom are set by the SDOF model: In the hull coordinate system, the propeller is set to rotate relative to the hull through Fluent's own UDF macro DEFINE_CG_MOTION, and the mass properties of the hull are defined through the DEFINE_SDOF_PROPERTIES macro. The three degrees of freedom of surge, heave and pitch are opened, and the force conditions of the propeller and the hull at the end of each time step in the solver are obtained, so that they act on the hull, and the friction resistance correction force is added; The additional friction resistance correction force caused by surface roughness is converted according to the three-dimensional method, and the specific calculation is as follows: ； ； In the formula, is the roughness compensation coefficient, is the apparent height of roughness, is the waterline length of the ship, is the additional friction resistance correction force, is the water density, is the forward speed of the ship, is the wet surface area of ​​the hull.

2. The ship self-propulsion simulation analysis method based on Fluent according to claim 1, characterized in that: In the step S1, a rectangular area with a length of L, a width of W, and a height of H is used as the background calculation domain, wherein L≥8*L1, W≥3*L1, and H≥2*L1, and L1 represents the waterline length of the ship; the length, width, and height of the circumscribed rectangular parallelepiped of the ship are enlarged by 50% respectively as the hull calculation domain; a cylinder is nested at the propeller position to generate the propeller calculation domain, the cylinder has the propeller hub end face as the bottom surface, the cylinder has the propeller hub axis as the center axis, the cylinder has the propeller diameter enlarged by 25% as the diameter, and the cylinder has the propeller hub length increased by 25% as the height.

3. The ship self-propulsion simulation analysis method based on Fluent according to claim 1, characterized in that: In step S2, the background grid adopts a regular hexahedral structured grid, the overlapping grid and the dynamic grid adopt a tetrahedral grid, and triangular prism grids are created at the ship boundary and the propeller boundary to capture the boundary layer flow.

4. The ship self-propulsion simulation analysis method based on Fluent as claimed in claim 3, characterized in that: The grid encryption method is used to encrypt the overlapping grids and the inside of the dynamic grid, and to encrypt the ship's motion path above and below the waterline.

5. The ship self-propulsion simulation analysis method based on Fluent according to claim 1, characterized in that: In step S3, the calculation results of the solver settings are directly obtained through the Get_Report_Definition macro to obtain the force conditions of the propeller and the hull at the end of each time step in the solver, and according to the propeller water-clearing performance, the thrust corresponding to a single propeller at a specific speed is used as a reference value to limit the propeller thrust obtained from the solver.

6. The ship self-propulsion simulation analysis method based on Fluent according to claim 1, characterized in that: In the step S4, when the initial state is set, the gas-liquid free interface of the multiphase flow VOF model in the computational domain coincides with the waterline of the hull; a speed less than the estimated value of the ship speed is given to the external flow field according to the estimated value of the ship speed, and at the same time, an initial speed in the forward direction is given to the hull according to the difference between the estimated value of the ship speed and the speed of the external flow field, and then the rotation speed of the propeller is set through the UDF, and the hull performs autonomous self-propulsion motion under the push of a number of propellers.

7. The ship self-propulsion simulation analysis method based on Fluent according to claim 1, characterized in that: In step S6, the parameters output by the solution report include propeller torque, propeller thrust, hull resistance, hull lift, hull speed and hull posture information; the content output by the visualization window includes pressure cloud maps of the hull and propeller surfaces, calculation domain gas-liquid distribution cloud maps and free liquid surface velocity distribution cloud maps.

8. The ship self-propulsion simulation analysis method based on Fluent according to claim 1, characterized in that: In step S7, the initialization adopts standard initialization or mixed initialization.

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