Simulation and suppression method for fluid-structure interaction vibration of water-jet propeller

Abstract

The invention provides a fluid-structure interaction vibration simulation and suppression method for a water-jet propeller, and belongs to the field of numerical simulation of computational fluid mechanics. The method comprises the following steps: establishing a finite element model of a water-jet propeller impeller-shafting coupling system; performing dry mode and wet mode comparative analysis on the finite element model by utilizing a finite element method, and verifying the effectiveness of the model; dividing finite element grids of a structural domain and a flow field domain; establishing a fluid-solid coupling model; on the basis of bidirectional fluid-solid coupling numerical calculation, the flow field speed and acting force are solved through CFD, the structure movement speed and displacement are updated, a flow field grid is further updated, flow numerical calculation continues to be conducted, and iteration is conducted till the simulation time ends; flow-induced vibration characteristics are analyzed through post-processing, and the vibration amplitude is restrained by adjusting the mass center position of the impeller airfoil. According to the method, the bidirectional fluid-solid coupling model of the impeller-shafting system of the water-jet propeller is constructed, structural deformation and transient flow field information are directly associated, and the hydrodynamic performance is improved.

Description

Technical Field

[0001] The present invention belongs to the field of computational fluid dynamics numerical simulation, and in particular relates to a method for simulating and suppressing fluid-solid coupling vibration of a water jet propulsion system. Background Art

[0002] Currently, there are two main types of ship propulsion: propeller propulsion and waterjet propulsion. Waterjet propulsion utilizes the reaction force generated by the water pump's suction and ejection of water through a nozzle to propel the ship forward. Compared to propeller propulsion, waterjet propulsion offers unparalleled advantages, including high propulsion efficiency at high speeds, low vibration and noise, excellent maneuverability, minimal shallow water effects, superior cavitation performance, and strong adaptability to variable operating conditions. Waterjet propulsion is widely used in quiet vessels, vessels with high dynamic positioning requirements, high-performance ships, heavy-load large transport vessels, and shallow-draft vessels.

[0003] Structurally, a waterjet propulsion system uses the jet portion of its propulsion mechanism, which is submerged in water, to propel the vessel forward using the reaction force generated by the jetting water. The jet nozzle can be used to change the direction of the water jet to achieve ship maneuverability, providing excellent maneuverability and adaptability, particularly in shallow waterways with muddy bottoms. Waterjets also enable rapid steering, using steering and reversing mechanisms consistent with the principles of a jet engine, utilizing diversion nozzles to achieve vessel deceleration, steering, and reversing.

[0004] With the iterative updates of ultra-large, ultra-powerful waterjets, the traditional design method of treating the impeller as a rigid body will no longer be applicable, and the elastic vibration of the impeller needs to be considered in the design. However, research on fluid-structure interaction of waterjets is still very scarce. Some domestic universities only calculate the surface forces and strength characteristics of the water inlet flow channel grid, and rarely conduct fluid-structure interaction analysis of the waterjets as a whole. There is also a lack of a reasonable explanation of the inherent logical relationship between the wet modal characteristics of waterjets and the occurrence of flow-induced vibration. Summary of the Invention

[0005] Technical issues to be solved:

[0006] To overcome the shortcomings of existing technologies, this paper provides a method for simulating and suppressing fluid-structure coupling vibrations in waterjets. By combining CFD with FEM, this method analyzes the influence of water on the modal characteristics of waterjets (e.g., reduction in natural frequency), revealing the inherent connection between wet modes and flow-induced vibrations. Furthermore, it proposes suppressing vibrations by adjusting the center of mass position of the impeller airfoil (e.g., shifting it forward), significantly reducing the amplitude. By constructing a bidirectional fluid-structure coupling model of the waterjets' impeller-shaft system, this paper directly links structural deformation with transient flow field information, thereby improving hydrodynamic performance.

[0007] The technical solution of the present invention is: a method for simulating and suppressing fluid-structure coupling vibration of a water jet thruster, the specific steps of which are as follows:

[0008] Establish a finite element model of the waterjet impeller-shaft coupling system;

[0009] The finite element method is used to conduct a comparative analysis of the dry and wet modes of the finite element model to identify the influence of the water medium on the natural frequency and mode shape of the water jet propulsion and verify the effectiveness of the model;

[0010] Divide the finite element mesh of the structural domain and the flow field domain. The impeller and guide vanes use structured meshes and the area around the blades and the rim gap is locally encrypted. The rest of the area uses unstructured meshes, and a boundary layer mesh is set at the interface of the external flow field.

[0011] Establish a fluid-structure coupling model, including the flow field control equations and the structural field control equations;

[0012] Based on bidirectional fluid-structure coupling numerical calculation, CFD is used to solve the flow field velocity and force, and the structure motion velocity and displacement are updated. The flow field grid is further updated to continue the process numerical calculation, and iterate until the simulation time ends;

[0013] Post-processing analyzes the flow-induced vibration characteristics, extracts the displacement, velocity response and flow field information of the impeller, and suppresses the vibration amplitude by adjusting the center of mass position of the impeller airfoil. The optimization scheme of moving the center of mass forward reduces the amplitude by 8.9% to 25%.

[0014] A further technical solution of the present invention is that the finite element model of the water jet propulsion impeller-shaft system coupling system includes accurate modeling of the geometric parameters of the impeller, guide vanes and shaft system; and takes into account the fluid added mass effect and damping effect.

[0015] A further technical solution of the present invention is that the wet modal analysis adopts an acoustic-solid coupling model, simplifies the fluid into an inviscid and incompressible medium, and solves the acoustic wave equation and the structural dynamics equation simultaneously. The acoustic wave equation is expressed as:

[0016]

[0017] Where: c is the sound velocity of the fluid medium, k is the bulk modulus of the fluid, ρ0 is the density of the fluid; P is the acoustic pressure;

[0018] Discretize the acoustic wave equation and get

[0019]

[0020] The dynamic equation of the structure itself is

[0021]

[0022] The simultaneous equations are:

[0023]

[0024] Where s represents the structure, f represents the fluid; R is the coupling matrix of the fluid-solid coupling interface, which represents the effective area at each node on the fluid-solid interface. It converts the fluid pressure on the interface into the load of the structure. The square brackets represent the matrix.

[0025] A further technical solution of the present invention is that the structured grid is divided using TurboGrid software, the unstructured grid is divided using ANSYS Workbench platform, and the thickness of the boundary layer grid is 0.1% to 0.5% of the impeller diameter.

[0026] A further technical solution of the present invention is: the flow field control equation includes:

[0027] Continuity equation:

[0028]

[0029] Where ρ is the fluid density, j = 1, 2, 3;

[0030] Momentum conservation equation:

[0031]

[0032] Where p is the pressure value; μ is the fluid dynamic viscosity; S i is the source term, i = 1, 2, 3, j = 1, 2, 3;

[0033] Energy conservation equation:

[0034]

[0035] Where T is temperature; c is specific heat capacity; k is heat transfer coefficient; S T is the source item.

[0036] A further technical solution of the present invention is: the structural field control equation includes:

[0037] Balance equation: According to Newton's second law, the force balance equation inside a solid is:

[0038]

[0039] Where, ρ s is the solid density, d is the displacement vector, σ is the Cauchy stress tensor, F b is the body force (such as gravity).

[0040] Constitutive equation:

[0041] Linear elastic materials:

[0042] σ=C:∈

[0043] Where C is the elasticity tensor and ∈ is the strain tensor (defined by the displacement gradient).

[0044] Geometric equations:

[0045] Under small deformation, the relationship between strain and displacement is:

[0046]

[0047] Where, Represents the gradient.

[0048] A further technical solution of the present invention is: the specific method of the bidirectional fluid-solid coupling numerical calculation is:

[0049] The flow field inlet adopts uniform velocity condition, the outlet is a pressure outlet, and the wall is a non-slip, smooth wall boundary; the flow field area boundary adopts a symmetrical boundary;

[0050] The bearing stiffness in the structural field is set to 5×10 7 N / m, and constrain axial displacement;

[0051] The Newton-Raphson method is used to solve the coupled equations, and the total simulation time is an integer multiple of the impeller rotation period.

[0052] A further technical solution of the present invention is: the fluid-solid coupling equation is:

[0053]

[0054] Where M is the mass matrix, C is the damping matrix, and K is the stiffness matrix; Represent the fluid added mass, damping, and stiffness matrix respectively; F f The water dynamics caused by the incoming flow.

[0055] A further technical solution of the present invention is: when the impeller airfoil is selected from NACA6309, NACA6508, and NACA65006 airfoils, the ratio of the distance from the center of mass to the center of the elastic axis to the chord length is 36.44%, 14.36%, and 1.73%, respectively, and the amplitude suppression rate is increased to 25% by moving the center of mass forward.

[0056] A waterjet fluid-structure coupling vibration and suppression system includes the following modules:

[0057] Modeling module, used to establish the finite element model of the waterjet impeller-shaft coupling system, including accurate modeling of the geometric parameters of the impeller, guide vanes and shaft system, and integrating the fluid added mass effect and damping effect;

[0058] A modal analysis module performs dry modal and wet modal comparative analysis on the model based on the finite element method, identifies the influence of the water medium on the natural frequency and mode shape, and outputs wet modal mode shape and frequency matching data;

[0059] A meshing module is configured to mesh the structure domain and the flow field domain. The impeller and guide vanes use structured meshes and the area around the blades and the rim gap is encrypted. A boundary layer mesh is set at the interface of the external flow field, and unstructured meshes are used for the rest of the area.

[0060] Fluid-solid coupling calculation module, including flow field solving unit, structure solving unit, and bidirectional coupling unit;

[0061] The post-processing module extracts the displacement and velocity response curve of the impeller and the pressure distribution of the flow field, and analyzes the flow-induced vibration characteristics;

[0062] The vibration suppression module suppresses vibration by adjusting the center of mass position of the impeller airfoil.

[0063] Beneficial effects

[0064] The beneficial effects of the present invention are as follows: through wet modal mechanism analysis, high-precision bidirectional coupling simulation and center of mass optimization technology, the present invention achieves effective suppression of flow-induced vibration of water jet thrusters and performance improvement, and has both theoretical innovation and engineering practicality, providing an efficient and reliable solution for the design of ship propulsion systems.

[0065] (1) A large number of existing studies have used numerical simulation methods to explore the design theory, internal flow characteristics and cavitation characteristics of water jet propulsion, but the research on its fluid-structure coupling and flow-induced vibration response is still insufficient. Some domestic universities only calculate the surface force and strength characteristics of the water inlet flow channel grid, and do not conduct fluid-structure coupling analysis of the entire machine. Therefore, the present invention will take the water jet propulsion as the object, use CFD methods, structural dynamics theory and fluid-structure coupling methods, and establish a high-fidelity simulation model of the flow-induced vibration of the water jet propulsion as a whole. Compared with the existing technology, the present invention conducts dynamic modeling and vibration characteristic research on the wet mode of the water jet propulsion from a system perspective, and fully reveals the inherent logical relationship between its wet mode frequency and the vibration amplitude of the flow-induced vibration.

[0066] (2) The present invention provides a method for simulating and suppressing the fluid-solid coupling vibration of a water jet thruster. The method examines the flow-induced vibration performance of the water jet thruster after changing the center of mass position. The method is used to study the excitation mechanism and formation mechanism of the flow-induced vibration of the impeller under the center of mass position of the impeller hydrofoil. The simulation results show that the forward movement of the impeller center of mass position can effectively suppress the deformation of the impeller blades in all directions.

[0067] (3) The present invention establishes a high-fidelity simulation model of the flow-induced vibration of the water jet propulsion system, embeds the FEM-based water jet propulsion system structural dynamics model into the water jet propulsion system structural CFD model, realizes bidirectional fluid-solid coupling simulation calculation, and can directly use numerical methods to obtain structural deformation and transient flow field information, and can also greatly reduce the number of experiments and save experimental costs.

[0068] (4) This invention precisely quantifies the effect of water on the natural frequency of a waterjet propulsion system through comparative analysis of wet and dry modalities, revealing the inherent relationship between the added mass effect of the fluid and flow-induced vibration, and providing a theoretical basis for vibration suppression. The spatial consistency of the wet modal shape and the deformation distribution of flow-induced vibration directly guides the direction of structural optimization, avoiding the blindness of traditional empirical design.

[0069] (5) The present invention adopts structured and unstructured hybrid grid division (the impeller gap grid is encrypted to 0.1% to 0.5% of the impeller diameter), combined with boundary layer grid technology, which significantly improves the calculation accuracy of flow field and structure coupling, and the fluid force prediction error is less than 5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 It is a schematic diagram of the basic flow in an embodiment of the present invention.

[0071] Figure 2 Schematic diagram of a simplified water jet model in an embodiment of the present invention.

[0072] Figure 3 Schematic diagram of the modal calculation bearing and constraint setting in an embodiment of the present invention.

[0073] Figure 4 4 is a schematic diagram of the dry mode finite element mesh of the water jet thruster in an embodiment of the present invention.

[0074] Figure 5 It is the wet mode water body and cross-section finite element mesh of the water jet propulsion device in the embodiment of the present invention.

[0075] Figure 6 1 is the vibration shape diagram of the first eight wet modes of the water jet propulsion device in the embodiment of the present invention.

[0076] Figure 7 Schematic diagram of the overall calculation domain of the water jet propulsion system in an embodiment of the present invention.

[0077] Figure 8 Schematic diagram of the calculation domain of each part of the water jet propulsion system in an embodiment of the present invention.

[0078] Figure 9 Schematic diagram of water jet grid division in an embodiment of the present invention.

[0079] Figure 10 This is a general cloud diagram of the deformation of the water jet impeller blades in an embodiment of the present invention.

[0080] Figure 11 This is a total cloud diagram of radial deformation of the water jet impeller blades in an embodiment of the present invention.

[0081] Figure 12 It is a total cloud diagram of the circumferential deformation of the water jet impeller blades in an embodiment of the present invention.

[0082] Figure 13 Schematic diagram of the centroid and elastic axis positions of different airfoil shapes of the waterjet impeller in an embodiment of the present invention.

[0083] Figure 14 1 is a nephogram of the rotor blade deformation at different mass center positions of the water jet impeller in an embodiment of the present invention. DETAILED DESCRIPTION

[0084] The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention.

[0085] Due to the current lack of research on the fluid-solid coupling of water jets, some domestic universities only calculate the surface forces and strength characteristics of the water inlet flow channel grille, and rarely conduct fluid-solid coupling analysis on the entire water jet. At the same time, there is also a lack of a reasonable explanation of the inherent logical relationship between the wet modal characteristics of the water jet and the flow-induced vibration. Therefore, it has become an important topic to analyze the wet modal characteristics of the water jet and to study the flow-induced vibration characteristics of the impeller as an elastic body. The impeller of the water jet is composed of hydrofoil airfoils. Since the blades of the impeller can be regarded as rotating hydrofoils, the research methods and results of the fluid-solid coupling vibration problems of the hydrofoils can provide a reference for the study of the flow-solid coupling vibration of the impeller. The present invention suppresses the flow-induced vibration of the impeller by studying the influence of the center of mass position of the impeller hydrofoil on the flow-induced vibration response of the impeller. Therefore, a fluid-solid coupling vibration simulation method and suppression method of a water jet are proposed. The specific steps are as follows:

[0086] A waterjet fluid-structure coupling vibration simulation method and suppression method, characterized by comprising the following steps:

[0087] Step 1: Establish a finite element model of the waterjet impeller-shaft coupling system; determine the structural composition of the waterjet, simplify the structure, retain the main dynamic characteristics while reducing the computational complexity, and establish a physical model;

[0088] Step 2: Use the finite element method to perform modal analysis on the finite element model of the waterjet impeller-shaft coupling system. The process is as follows:

[0089] Set boundary conditions and initial conditions to simulate the behavior of the waterjet in the actual working environment;

[0090] Identify the modal characteristics of the system in water, specifically the natural frequencies and mode shapes;

[0091] Analyze the effect of water on the modal characteristics of the waterjet and conduct a comparative analysis of the natural frequencies of the wet and dry modes of the waterjet. If the natural frequency of the wet mode is lower than that of the dry mode, the model is established. Store the identified vibration modes for subsequent verification (step 6).

[0092] Step 3: Perform finite element meshing on the structural domain and flow field domain respectively; since the impeller blades of the axial-flow waterjet propulsion pump are severely twisted and the impeller rim gap is small, the minimum angle and mesh quality are difficult to adjust, so the impeller and guide vanes are automatically structured meshed, and the walls around the blades and the rim gap area are locally encrypted; unstructured meshes are divided for the water inlet channel and other parts; unstructured meshes are divided for the external flow field, and a boundary layer is added at the interface between the external flow field and the waterjet propulsion pump.

[0093] Step 4: Establish a fluid-structure coupling model of the waterjet propulsion system;

[0094] Step 5: Fluid-structure coupling calculation;

[0095] First, determine the initial conditions and boundary conditions. The velocity inlet is used as the flow field inlet, and the given fluid flow velocity is uniform. The pressure outlet is selected as the outlet, and the average static pressure is standard atmospheric pressure. The fluid domain surface has no-slip, smooth wall boundary conditions. The flow field area boundary adopts a symmetric boundary. Then, in the CFD solver, solve the fluid control equation to obtain the flow field velocity and the force acting on the jet pump. Substitute the force generated by the fluid on the jet pump to solve the displacement and velocity at the next moment. Then, use the jet pump's motion velocity and displacement to update the flow field grid, and obtain a new flow field calculation grid for the next time step of the flow field numerical calculation, realizing the fluid-structure coupling numerical calculation of the water jet thruster.

[0096] Step 6: Calculate the arrival simulation time, post-process the calculated data, extract the displacement and velocity response curves of the waterjet impeller, and the flow field structure information of the flow field, and obtain the flow-induced vibration characteristics of the waterjet.

[0097] Then, the impeller airfoil is changed and the same calculation method as in step five is used to study the effect of different center of mass positions on flow-induced vibration. The impeller amplitude is suppressed by moving the center of mass forward.

[0098] Specifically, the establishment of the finite element model described in step 1 also includes the following details:

[0099] The geometric parameters of each waterjet component are precisely measured and modeled to ensure the geometric accuracy of the model. The interaction between the fluid and the structure, including the added mass effect and damping effect of the fluid on the structure, is considered. The boundary conditions and initial conditions set in step five are used to simulate the behavior of the waterjet in the actual working environment. The finite element model is calibrated and optimized through comparative analysis with experimental data to improve the model's prediction accuracy.

[0100] Specifically, in step 2, the wet modes of the waterjet propulsion system are calculated using the acoustic-solid coupling model. However, when the waterjet propulsion system impeller is located in a marine environment, the influence of the water medium on the impeller vibration characteristics needs to be considered. Therefore, the wet mode method must be used to analyze its dynamic characteristics. In the acoustic-solid coupling model, water is an inviscid and incompressible fluid, and through the Kinsler assumption, the momentum equation (Navier-Stokes) and continuity equation of the fluid are simplified to the acoustic wave equation:

[0101]

[0102] Where: c is the sound velocity of the fluid medium, k is the bulk modulus of the fluid, ρ0 is the density of the fluid; P is the acoustic pressure.

[0103] Discretize the acoustic wave equation to obtain

[0104]

[0105] The dynamic equation of the structure itself is

[0106]

[0107] The simultaneous equations are:

[0108]

[0109] Where s represents the structure, f represents the fluid, and R is the coupling matrix of the fluid-structure interface. It represents the effective area at each node on the fluid-structure interface and converts the fluid pressure on the interface into a load on the structure. The square brackets represent the matrix.

[0110] Specifically, the finite element meshes of the impeller and guide vanes in step 3 are structured meshes, and the rest are unstructured meshes. The outer flow field near the junction of the water jet propulsion pump is a boundary layer mesh.

[0111] Specifically, the specific method for establishing the calculation model in step 4 is:

[0112] The flow-induced vibration model of waterjet propulsion includes the flow field control equations and the structure field control equations;

[0113] In the flow field, CFD methods are used to solve the flow field. The three basic governing equations of fluid dynamics include the mass conservation equation (continuity equation), the momentum conservation equation, and the energy conservation equation. These three conservation equations constitute the most important Navier-Stokes equations (NS equations) in fluid dynamics.

[0114] The flow field governing equations include the continuity equation, momentum conservation equation, and energy conservation equation:

[0115] Among them, the continuity equation is:

[0116]

[0117] Where ρ is the fluid density, j = 1, 2, 3.

[0118] Momentum conservation equation:

[0119]

[0120] Where p is the pressure value; μ is the fluid dynamic viscosity; S i is the source term. i=1, 2, 3, j=1, 2, 3, energy conservation equation:

[0121]

[0122] Where T is temperature; c is specific heat capacity; k is heat transfer coefficient; S T is the source item.

[0123] The structural field governing equations are used to describe the deformation and stress distribution of solids, including:

[0124] Balance equation (conservation of momentum)

[0125] According to Newton's second law, the force balance equation inside a solid is:

[0126]

[0127] Among them, ρ s is the solid density, d is the displacement vector, σ is the Cauchy stress tensor, F b is the body force (such as gravity).

[0128] Constitutive equation (stress-strain relationship)

[0129] Linear elastic materials (Hooke's law):

[0130] σ=C:∈

[0131] Where C is the elasticity tensor and ∈ is the strain tensor (defined by the displacement gradient).

[0132] Geometric equation (strain-displacement relationship)

[0133] Under small deformation, the relationship between strain and displacement is:

[0134]

[0135] Specifically, the specific method for fluid-structure coupling calculation in steps 5 and 6 is:

[0136] CFD is used to calculate the flow-induced vibration characteristics of the three-dimensional whole machine system. The dynamic equation is as follows:

[0137]

[0138] Where: M is the mass matrix, C is the damping matrix, and K is the stiffness matrix. are the displacement, velocity, and acceleration of the node. The equation is constructed based on full-Lagrangian integration and solved using the Newton-Raphson method. Where F is the fluid force acting on the riser surface, calculated using the DES-based computational fluid dynamics method. F changes with time, and according to the specific physical action, F(x, y, z, t) = F f +F FSI , F f The hydrodynamic force caused by the incoming flow; F FSI is the radiation force caused by the vibration acceleration and velocity of the shaft system-impeller, and the restoring force caused by the vibration displacement of the shaft system-impeller; F FSI It can be written as:

[0139]

[0140] Where, denote the fluid added mass, damping and stiffness matrices respectively. Therefore, the fluid-structure coupling equation can be written as:

[0141]

[0142] The present invention provides a waterjet propulsion fluid-solid coupling vibration and suppression system, comprising the following modules:

[0143] Modeling module, used to establish the finite element model of the waterjet impeller-shaft coupling system, including accurate modeling of the geometric parameters of the impeller, guide vanes and shaft system, and integrating the fluid added mass effect and damping effect;

[0144] A modal analysis module performs dry modal and wet modal comparative analysis on the model based on the finite element method, identifies the influence of the water medium on the natural frequency and mode shape, and outputs wet modal mode shape and frequency matching data;

[0145] A meshing module is configured to mesh the structure domain and the flow field domain. The impeller and guide vanes use structured meshes and the area around the blades and the rim gap is encrypted. A boundary layer mesh is set at the interface of the external flow field, and unstructured meshes are used for the rest of the area.

[0146] Fluid-solid coupling calculation module, including flow field solving unit, structure solving unit, and bidirectional coupling unit;

[0147] The post-processing module extracts the displacement and velocity response curve of the impeller and the pressure distribution of the flow field, and analyzes the flow-induced vibration characteristics;

[0148] The vibration suppression module suppresses vibration by adjusting the center of mass position of the impeller airfoil.

[0149] The above technical solution is further described below with reference to examples and drawings:

[0150] In one embodiment, an axial flow water jet propulsion system is used, in which the number of guide vane blades N is 9 and the number of impeller blades N is 6, the diameter of the guide vane and the impeller D is 550 mm, and the design operating conditions are a rotation speed of 1300 RPM and a ship speed of 42 knots.

[0151] Reference Figure 1 As shown, this embodiment provides a method for simulating and suppressing fluid-structure coupling vibration of a water jet thruster, and the specific steps are as follows:

[0152] Step 1: Establish a finite element model of the waterjet impeller-shaft coupling system; determine the structural composition of the waterjet, simplify the structure, retain the main dynamic characteristics while reducing the computational complexity, and establish a physical model, such as Figure 2 As shown; combined with the fluid-structure coupling theory, the coupling relationship between the shaft system and the impeller is established, and the effect of fluid dynamics on the structure and its feedback mechanism are clarified; the dynamic method is used to construct the kinematic equations and dynamic equations of the system.

[0153] Step 2: Using the Modal and ModalAcoustic modules in Ansys Workbench, perform modal analysis on the waterjet using the finite element method. During the modal calculation, the rotor and stator components are both constructed of structural steel. The material parameters for the structural steel are as follows: density, Young's modulus, and Poisson's ratio. In an actual waterjet installation, the hub shaft connects to the rotor blades. Bearings are used at both ends of the hub shaft to drive the rotor blades. The bearing stiffness is 5×10 7 N / m, while constraining the axial displacement of both ends of the pump shaft, such as Figure 3 As shown, the dry modal finite element mesh is as follows Figure 4 In wet modal analysis, it is necessary to divide the solid finite element mesh of the water jet propulsion system and the water body mesh wrapped around it at the same time, as shown in Figure 5 As shown in Figure 2, the bearing and constraint settings are the same as those in the dry modal analysis. The modal characteristics of the first eight wet modes of the waterjet rotor blade system are shown in Figure 2. Figure 6 The calculation results of dry mode and wet mode natural frequencies are shown in Table 1 and Table 2.

[0154] Table 1 Rotor dry modal natural frequency

[0155]

[0156] Table 2 Rotor wet modal natural frequency

[0157]

[0158] Step 3: Perform finite element mesh division on the structural domain and flow field domain respectively;

[0159] To obtain simulation values closer to reality and reduce the influence of the model inlet and outlet on the calculation results, an extension section was added to the waterjet outlet. Because the waterjet is affected by the ship's speed and the boundary layer at the bottom of the hull, a fluid control body was added below the water inlet. The length, width, and height of the control body were 30, 10, and 4 times the propulsion pump inlet diameter, respectively.

[0160] Therefore, the calculation domain of the water jet propulsion system includes five parts: flow field control body, water inlet channel, impeller, guide vane body and outlet extension section, such as Figure 7 、 Figure 8 As shown in the figure. Since the impeller blades of the axial-flow water jet propulsion pump are severely twisted and the impeller rim gap is small, the minimum angle and mesh quality are difficult to adjust. TurboGrid software is used to automatically divide the impeller and guide vanes into structured meshes, and the local mesh is encrypted on the wall surface around the blade and the rim gap area. Workbench mesh is used to divide the water inlet channel and other parts into unstructured meshes; unstructured meshes are divided for the external flow field, and a boundary layer is added at the interface between the external flow field and the water jet propulsion pump, as shown in the figure. Figure 9 As shown;

[0161] Step 4: Establish a flow-induced vibration model of the waterjet propulsion system;

[0162] Step 5: Fluid-structure coupling calculation;

[0163] Based on the Ansys Transient Structure module and CFX module, a two-way fluid-solid coupling calculation of the water jet propulsion is performed. First, the initial conditions and boundary conditions are determined. The velocity inlet is used as the flow field inlet, and the fluid flow velocity is given as a uniform velocity; the pressure outlet is selected as the outlet, and the average static pressure is the standard atmospheric pressure; the fluid domain surface is a no-slip, smooth wall boundary condition; the flow field area boundary uses a symmetric boundary; then, in the CFD solver, the fluid control equation is solved to obtain the flow field velocity and the force acting on the jet pump, and the force generated by the fluid on the jet pump is substituted into the solution for the displacement and velocity at the next moment; then, the jet pump's motion velocity and displacement are used to update the flow field grid, and a new flow field calculation grid is obtained for the numerical calculation of the flow field in the next time step, thereby realizing the fluid-solid coupling numerical calculation of the water jet propulsion;

[0164] Step 6: Calculate the arrival simulation time, post-process the calculated data, extract the displacement of the waterjet impeller, velocity response curve and flow field structure information of the flow field, and obtain the flow-induced vibration characteristics of the waterjet propulsion; then change the impeller airfoil, use the same calculation method as in step 5, study the influence of different center of mass positions on flow-induced vibration, and suppress the impeller amplitude by moving the center of mass forward. In this example, the calculation results of the bidirectional fluid-structure coupling of the waterjet propulsion are as follows: Figure 10 As shown. It can be seen that the maximum deformation of the blade occurs at the impeller inlet rim, and the degree of deformation gradually increases in the radial direction from the hub to the rim. At the inlet of the impeller, there is a relatively large pressure difference between the pressure side and the suction side of the blade, while on the outlet side of the impeller, the pressure difference between the pressure side and the suction side of the impeller is relatively small. Therefore, the pressure difference between the pressure side and the suction side at the inlet will cause a greater torque effect on the inlet of the hub than on the outlet, which is the reason for the maximum deformation near the blade inlet rim. It can also be seen that the deformation on the blade inlet side close to the hub side is the smallest.

[0165] The wet modal analysis in step 2 reveals the significant effect of the added mass of the fluid on the natural frequency of the structure, and the occurrence of flow-induced vibration is essentially the result of the coupling between the fluid excitation frequency and the natural frequency of the wet mode. The high amplitude of the blade tip in the wet mode vibration mode (such as Figure 6 a) The maximum deformation in flow-induced vibration is located at the impeller inlet rim ( Figure 10 ) indicates that the fluid excitation force amplifies the vibration response of a specific mode through the added mass effect. Therefore, wet modal analysis provides a direct explanation for the frequency matching mechanism of flow-induced vibration.

[0166] like Figure 11As shown in the figure, the radial deformation of the impeller blades under the action of various loads is much smaller than the total deformation of the impeller blades. The radial deformation is not the main reason for the deformation of the blades. Therefore, it can be determined that during the operation of the water jet propulsion system, there will be no interference between the impeller and the outer wall due to excessive radial deformation.

[0167] Then analyze the deformation in the circumferential direction, such as Figure 12 As shown, the left side is the pressure surface and the right side is the suction surface. It should be noted that since the circumferential deformation needs to be displayed, all the deformation values here are expressed in a cylindrical coordinate system. As can be seen from the figure, the maximum deformation value of the circumferential deformation of the impeller blades also appears at the rim on the inlet side of the impeller, and its change law is the same as the total deformation law of the impeller blades. At the same time, it can be clearly seen that the circumferential deformation value and the total deformation value of the blade are very close, and the maximum deformation value and the minimum deformation value are also almost the same, which shows that the circumferential deformation of the blade is the main cause of the deformation of the blade, and the reaction force of the fluid on the structure is the most fundamental cause of the deformation of the structure.

[0168] In this example, NACA6309, NACA6508, and NACA65006 airfoils are selected to adjust the center of mass position. The distances between the three centers of mass (G) of the airfoils and the center of the elastic axis (E) are set to be like Figure 13 As shown. The distance between the center of mass of the NACA6309 airfoil and the center of the elastic axis is The ratio of the airfoil chord length c is The distance between the center of mass of the NACA6508 airfoil and the center of the elastic axis The ratio of the airfoil chord length c is The distance between the center of mass of the NACA65006 airfoil and the center of the elastic axis The ratio of the airfoil chord length c is

[0169] Here we will analyze the deformation cloud diagrams of the rotor blades at three different center of mass positions, where the cloud diagram on the left is the pressure surface and the cloud diagram on the right is the suction surface, as shown in the figure below. Figure 14As shown. It can be seen that the change trends of the three airfoil blades are roughly the same, but the deformation amplitude of the blade corresponding to the NACA6309 airfoil is smaller, at 1.8569mm. The color gradient in the cloud map is more uniform, and the deformation is concentrated in the area near the end, but the overall amplitude is controllable. When the center of mass is located forward, the center of gravity of the blade structure is closer to the support root (installation point), and the torque effect is smaller, so the fluid force is more evenly distributed on the blade; while the deformation amplitude of the blade corresponding to the NACA6508 airfoil increases to 2.0388mm, and the amplitude increases by about 8.9% compared to the forward center of mass. In particular, the deformation is greater in the area near the end of the blade. The more pronounced color gradient of the cloud map indicates that stress and deformation are concentrated in the trailing edge region, and the center of mass position shifts rearward, resulting in changes in the blade force distribution, especially at the outer edge of the blade, which produces a higher pressure concentration effect. The overall deformation of the blade corresponding to the NACA65006 airfoil reaches a maximum value of 2.478 mm. Compared with the forward center of mass, the amplitude increases by approximately 25%, and the color gradient is the most dramatic. A significant high deformation area appears at the trailing edge and blade tip. The center of mass shifts further rearward, causing the blade's force point and center of gravity to deviate from the support root, significantly enhancing the torque effect. The blades corresponding to the NACA6309, NACA6508, and NACA65006 airfoils are named Model 1, Model 2, and Model 3, respectively. The calculation results for these three airfoils are compared in Table 3. A comparison of the open-water propulsion performance of the three airfoils, considering both elasticity and rigidity, is shown in Tables 4 to 6.

[0170] Table 3 Model comparison

[0171]

[0172] Table 4 Comparison of elastic and rigid hydrodynamic performance of model 1

[0173]

[0174] Table 5 Comparison of elastic and rigid hydrodynamic performance of model 2

[0175]

[0176] Table 6 Comparison of elastic and rigid hydrodynamic performance of model 3

[0177]

[0178] From the above analysis and data, it can be seen that under rigid conditions, Model 1 exhibits the highest hydrodynamic coefficient (thrust coefficient K T =0.990, torque coefficient K Q=0.330, open water efficiency is 86.41%). According to theoretical expectations, this should lead to the largest vibration amplitude. However, the calculation results show that the vibration amplitude of model 1 is the smallest (1.8569mm). In contrast, model 3 shows the lowest hydrodynamic coefficient (thrust coefficient K) under rigid conditions. T =0.785, torque coefficient K Q =0.271, and the open water efficiency is 83.65%). This result shows that the forward shift of the center of mass in model 1 not only significantly reduces the vibration amplitude but also improves the hydrodynamic performance.

[0179] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and purpose of the present invention.

Claims

1. A method for simulating and suppressing fluid-structure coupling vibration of a water jet propulsion system, characterized in that The specific steps are as follows: Establish a finite element model of the waterjet impeller-shaft coupling system; The finite element method is used to conduct a comparative analysis of the dry and wet modes of the finite element model to identify the influence of the water medium on the natural frequency and mode shape of the water jet propulsion and verify the effectiveness of the model; Divide the finite element mesh of the structural domain and the flow field domain. The impeller and guide vanes use structured meshes and the area around the blades and the rim gap is locally encrypted. The rest of the area uses unstructured meshes, and a boundary layer mesh is set at the interface of the external flow field. Establish a fluid-structure coupling model, including the flow field control equations and the structural field control equations; Based on bidirectional fluid-structure coupling numerical calculation, CFD is used to solve the flow field velocity and force, and the structure motion velocity and displacement are updated. The flow field grid is further updated to continue the process numerical calculation, and iterate until the simulation time ends; Post-processing analyzes the flow-induced vibration characteristics, extracts the displacement, velocity response and flow field information of the impeller, and suppresses the vibration amplitude by adjusting the center of mass position of the impeller airfoil. The optimization scheme of moving the center of mass forward reduces the amplitude by 8.9% to 25%.

2. The method for simulating and suppressing fluid-structure interaction vibration of a water jet according to claim 1, characterized in that: The finite element model of the water jet propulsion impeller-shaft system coupling system includes accurate modeling of geometric parameters of the impeller, guide vanes and shaft system; and takes into account the fluid added mass effect and damping effect.

3. The method for simulating and suppressing fluid-structure interaction vibration of a water jet according to claim 2, characterized in that: The wet modal analysis adopts the acoustic-solid coupling model, simplifies the fluid into an inviscid and incompressible medium, and solves the acoustic wave equation and the structural dynamics equation simultaneously. The acoustic wave equation is expressed as: Where: c is the sound velocity of the fluid medium, k is the bulk modulus of the fluid, ρ0 is the density of the fluid; P is the acoustic pressure; Discretize the acoustic wave equation and get The dynamic equation of the structure itself is The simultaneous equations are: Where: s represents structure, f represents fluid; R is the coupling matrix of the fluid-solid coupling interface, which represents the effective area at each node on the fluid-solid interface. It converts the fluid pressure on the interface into the load on the structure. The square brackets represent the matrix.

4. The method for simulating and suppressing fluid-structure coupling vibration of a water jet according to claim 1, characterized in that: The structured grid is divided by TurboGrid software, and the unstructured grid is divided by ANSYS Workbench platform, the boundary layer mesh thickness is 0.1% to 0.5% of the impeller diameter.

5. The method for simulating and suppressing fluid-structure interaction vibration of a water jet according to claim 3, characterized in that: The flow field control equations include: Continuity equation: Where ρ is the fluid density, j = 1, 2, 3; Momentum conservation equation: Where p is the pressure value; μ is the fluid dynamic viscosity; S i is the source term, i = 1, 2, 3, j = 1, 2, 3; Energy conservation equation: Where T is temperature; c is specific heat capacity; k is heat transfer coefficient; S T is the source item.

6. The method for simulating and suppressing fluid-structure interaction vibration of a water jet according to claim 5, characterized in that: The structural field control equations include: Balance equation: According to Newton's second law, the force balance equation inside a solid is: Where, ρ s is the solid density, d is the displacement vector, σ is the Cauchy stress tensor, F b is the body force (such as gravity). Constitutive equation: Linear elastic materials: σ=C:∈ Where C is the elasticity tensor and ∈ is the strain tensor (defined by the displacement gradient). Geometric equations: Under small deformation, the relationship between strain and displacement is: Where, Represents the gradient.

7. The method for simulating and suppressing fluid-structure interaction vibration of a water jet according to claim 6, characterized in that: The specific method of the bidirectional fluid-solid coupling numerical calculation is: The flow field inlet adopts uniform velocity condition, the outlet is a pressure outlet, and the wall is a non-slip, smooth wall boundary; the flow field area boundary adopts a symmetrical boundary; The bearing stiffness in the structural field is set to 5×10 7 N / m, and constrain axial displacement; The Newton-Raphson method is used to solve the coupled equations, and the total simulation time is an integer multiple of the impeller rotation period.

8. The method for simulating and suppressing fluid-structure interaction vibration of a water jet according to claim 7, characterized in that: The fluid-solid coupling equation is: Where M is the mass matrix, C is the damping matrix, and K is the stiffness matrix; Represent the fluid added mass, damping, and stiffness matrix respectively; F f The water dynamics caused by the incoming flow.

9. The method for simulating and suppressing fluid-structure interaction vibration of a water jet according to claim 1, characterized in that: When the impeller airfoil is selected from NACA6309, NACA6508, and NACA65006, the ratio of the distance from the center of mass to the center of elastic axis to the chord length is 36.44%, 14.36%, and 1.73%, respectively. The amplitude suppression rate is increased to 25% by moving the center of mass forward.

10. A water jet propulsion fluid-structure coupling vibration and suppression system, used to implement the water jet propulsion fluid-structure coupling vibration simulation and suppression method according to any one of claims 1 to 9; characterized in that: Includes the following modules: Modeling module, used to establish the finite element model of the waterjet impeller-shaft coupling system, including accurate modeling of the geometric parameters of the impeller, guide vanes and shaft system, and integrating the fluid added mass effect and damping effect; A modal analysis module performs dry modal and wet modal comparative analysis on the model based on the finite element method, identifies the influence of the water medium on the natural frequency and mode shape, and outputs wet modal mode shape and frequency matching data; A meshing module is configured to mesh the structure domain and the flow field domain. The impeller and guide vanes use structured meshes and the area around the blades and the rim gap is encrypted. A boundary layer mesh is set at the interface of the external flow field, and unstructured meshes are used for the rest of the area. Fluid-solid coupling calculation module, including flow field solving unit, structure solving unit, and bidirectional coupling unit; The post-processing module extracts the displacement and velocity response curve of the impeller and the pressure distribution of the flow field, and analyzes the flow-induced vibration characteristics; The vibration suppression module suppresses vibration by adjusting the center of mass position of the impeller airfoil.

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