Numerical prediction method of cavitation noise of a vaned pump-jet propeller

Abstract

This invention provides a numerical prediction method for cavitation noise in bladed pump-jet propulsion. It conducts steady and unsteady numerical simulations of the internal flow of the bladed pump-jet propulsion, analyzing its hydrodynamic performance and internal flow field. Secondly, it performs numerical simulations of the propulsion's cavitation flow, comparing and analyzing the propulsion's hydrodynamic performance under non-cavitation and cavitation states. It also analyzes the cavitation flow field of the propulsion at different speeds and cavitation numbers, exploring the cavitation evolution law and its dynamic characteristics. Finally, based on the unsteady calculation results of the internal flow of the bladed pump-jet propulsion, combined with acoustic finite element method calculations and spherical cavitation radiation theory, it numerically simulates the noise induced by the cavitation flow inside the propulsion, analyzing the relationship between the propulsion's cavitation flow and its induced cavitation noise. This aims to provide a reasonable and effective prediction method for the numerical prediction of cavitation noise in bladed pump-jet propulsion, possessing significant theoretical and engineering value.

Description

Technical Field

[0001] This invention belongs to the field of underwater propulsion equipment technology, and in particular relates to a numerical prediction method for cavitation noise of a bladed pump-jet propulsion device. Background Technology

[0002] With the further development of various underwater vehicles (submarines, torpedoes, unmanned probes, etc.), higher requirements have been placed on the comprehensive performance (noise, efficiency, and cavitation) of propulsion systems. Since the 1980s, pump-jet propulsion technology has gradually attracted widespread attention from scholars at home and abroad. It has good stealth performance, high propulsion performance, and anti-cavitation performance, and has been widely used in the propulsion systems of various marine equipment.

[0003] Cavitation is a common phenomenon in rotating machinery and other hydraulic machinery, and it is particularly prone to occur inside high-speed rotating bladed pump-jet propulsion systems. Cavitation not only leads to a sharp decline in the hydrodynamic performance of bladed pump-jet propulsion systems, affecting the stability and lifespan of the rotor blades, but also results in a significant increase in radiated noise, directly impacting the stealth capabilities of underwater vehicles. Therefore, studying the generation mechanism and dynamic characteristics of cavitation noise in bladed pump-jet propulsion systems is of great significance for improving the cavitation and acoustic performance of underwater vehicles. Summary of the Invention

[0004] To address the above technical problems, this invention provides a numerical prediction method for cavitation noise inside a bladed pump-jet propulsion unit based on computational fluid dynamics software, so as to accurately and efficiently study the cavitation noise inside the bladed pump-jet propulsion unit under different operating conditions.

[0005] To achieve the above objectives, the following plan is adopted: A numerical prediction method for cavitation noise in a bladed pump-jet propulsion system, characterized by comprising the following steps: S1: Establish a 3D model of the bladed pump-jet propulsion system and perform mesh generation; S2: Set boundary conditions and perform steady numerical simulation of the internal flow of a bladed pump-jet propulsion unit under non-cavitation conditions; S3: Load the ZGB cavitation model to perform numerical simulation of the cavitation flow of the bladed pump-jet propulsion system, and obtain the hydrodynamic performance of the propulsion system under cavitation and the hydrodynamic parameters under different cavitation numbers; S4: Perform unsteady cavitation flow numerical simulation and compare the hydrodynamic efficiency of the thruster with and without cavitation at different speeds; the hydrodynamic efficiency includes thrust coefficient, torque coefficient and hydrodynamic efficiency. S5: Monitoring points are set inside the thruster flow channel to extract the pressure pulsation of the internal flow channel under different cavitation numbers, and the pressure pulsation information and frequency distribution inside the thruster under different cavitation numbers are analyzed to study the changes in the degree of cavitation development inside the thruster. S6: Based on the numerical simulation of the internal flow of the bladed pump-jet propulsion unit in steps S2 and S3, numerical simulation of the non-cavitation flow field noise and cavitation noise performance of the bladed pump-jet propulsion unit is performed to obtain the sound pressure level distribution, axial and radial sound pressure variation law, total sound pressure level attenuation law, and axial plane sound pressure data of the cavitation volume pulsation radiation noise under cavitation and non-cavitation conditions.

[0006] Furthermore, the specific steps of S1 are as follows: S1.1 Unigraphics NX is used to perform 3D modeling of the geometry of the vane pump-jet propulsion unit with a rear stator, and the model file with the suffix .stp is output. S1.2 Import the 3D model established in S1.1 into ICEM, mesh the fluid domain, including: duct, hub, rotor blades and stator blades. The mesh form adopts a hexahedral structure mesh, and a boundary layer is added to the structure mesh to make the average y+ value of each component mesh of the bladed pump jet propulsion device about 4.5. Check the mesh quality, select 11.18 million meshes to ensure calculation accuracy, perform mesh independence analysis, and output a mesh file with the suffix .cfx.

[0007] Furthermore, the specific steps of S2 are as follows: S2.1 Import the mesh file obtained in S1.2 into ANSYS-CFX, select Steady calculation mode, and set the solver parameters; Set the working fluid properties: T=25℃ clean water, and set the density and viscosity coefficient of water at this temperature; the computational domain is divided into the external flow field, rotor domain, and stator domain; The simulated external flow field is enclosed by a cylinder coaxial with the pump-jet propulsion unit; the inlet and outlet boundary conditions of the external flow field are set as follows: velocity inlet, pressure outlet, velocity magnitude v = 7 m / s, turbulence intensity I = 5%; pressure outlet value is 0 Pa; Select the isothermal heat transfer model and the SST k-ω turbulence model; change the advance coefficient of the bladed pump-jet propulsion by setting different pump-jet rotor speeds; The walls of the external flow field are free-slip walls, while all other walls are non-slip walls. The bladed pump-jet propulsion unit is located 4D from the inlet of the external flow field. Based on the location of the rotor and stator, the internal flow field of the propulsion unit is divided into the rotor domain and the stator domain. The inlet and outlet of the three computational domains are set as interfaces. Set the rotation axis to be aligned with the impeller rotation direction, set the relative velocity of the wall to 0, and use the rotating coordinate system; set the static computational domain wall to a no-slip wall and set it to a global static coordinate system. S2.2 Based on S2.1, perform steady numerical simulation calculations of the internal flow of a bladed pump-jet propulsion unit under non-cavitation conditions, and output numerical calculation result files with the suffix "res".

[0008] Furthermore, S2.2 also includes a simulation optimization process: by analyzing the characteristics of the flow field pressure distribution of the propeller at different speeds, and based on the characteristics of the flow field pressure distribution, the steady-state numerical simulation is optimized by correcting the turbulence model, open-water performance parameters, and external flow field length.

[0009] Furthermore, in S3: the unsteady flow adopts the DES turbulence model, and when performing cavitation calculations, the time step ∆t for the unsteady cavitation flow calculation is set to 1.33 × 10⁻⁶. -4 The total computation time was 0.48 seconds.

[0010] Furthermore, the monitoring point settings in S5 are specifically as follows: Five flow channels are arranged radially inside the thruster, denoted as streamlines 1, 2, 3, ..., 5. The thruster is also divided into five axial planes, representing the thruster inlet plane M, the rotor inlet plane N, the rotor-stator interface plane O, the stator outlet plane P, and the duct outlet plane Q. Based on these, 25 monitoring points are selected from the five flow channels for analysis.

[0011] Furthermore, the noise performance numerical simulation in step S6 is based on the Lighthill theory combined with Actran acoustic software to simulate and analyze the internal flow noise under cavitation and non-cavitation conditions, using the unsteady flow field simulation results of the bladed pump-jet propulsion device. The cavitation noise of the bladed pump-jet propulsion device is analyzed based on the spherical cavitation radiation theory.

[0012] Furthermore, the specific steps of step S6 are as follows: S6.1 uses a structural space of the same size as the CFD computation domain constructed by S1. The acoustic computation domain model of the thruster is constructed using ICEM mesh generation software. Unstructured mesh generation is used, and local refinement is performed near the thruster. S6.2 sets the interface between the external flow field region and the stator and rotor parts as a surface sound source, and sets the outer side of the volume sound source as a non-reflective interface. The maximum grid size of the acoustic calculation domain is 0.03m. S6.3 Under the acoustic calculation domain settings in steps S6.1 and S6.2, use Ansys CFX-SolverManager to perform unsteady numerical simulation of the propeller flow field, solve for the velocity field, pressure field, and density field information, and export the flow field data in .ensightgold format as the input file for noise calculation in Actran acoustic software; S6.4 Create an acoustic model, import the flow field data into the Actran acoustic software, set the corresponding physical parameters and solver type, perform acoustic calculations, and insert the obtained sound sources into the acoustic mesh using integral interpolation. S6.5 sets the sound field sampling point and sound field detection point, and collects data on the change of cavitation volume over time. The cavitation volume is: In the formula, Vc is the total volume of the cavitation bubble, in meters. 3 N is the total number of control entities; α i To control the volume fraction of vacuoles within the body; V i This represents the volume of each control unit, in meters. 3 ; Substitute the cavitation volume at different times into the formula: This allows us to obtain the sound field detection and sound pressure values ​​at various sampling points at different times; where the radiated noise is at time [equation missing]. R is the cavitation radius, r is the distance between the sampling point or sound field detection point and the center of the virtual cavitation sphere; and the sound power is calculated. The time-domain sound pressure data is converted into frequency-domain sound pressure data through inverse Fourier transform, thereby obtaining the sound pressure level distribution of cavitation volume pulsation radiation noise. The axial and radial sound pressure variation law of the bladed pump-jet propulsion is analyzed, and the noise performance of the propulsion under non-cavitation and cavitation conditions is compared and analyzed, as well as the axial plane sound pressure comparison under non-cavitation and cavitation conditions in various frequency bands. S6.6 Based on the sound pressure level distribution data obtained in S6.5, and according to the total sound pressure level (OSPL) in the spherical cavitation radiation theory, calculate the total sound pressure level (OSPL) at each sampling point and sound field detection point. In the formula, f min and f max These represent the lower and upper limits of the frequency, respectively, in Hz; Δf represents the frequency resolution, in Hz, p. e The effective sound pressure level is expressed in Pa; p ref Using the reference sound pressure level (Pa), the attenuation law of the total sound pressure level is obtained. S6.7 After the unsteady flow of the bladed pump-jet propulsion unit exhibits periodic changes, the cavitation volume is calculated according to the processes in S6.5 and S6.6 for the process from cavitation generation to breakup under different cavitation numbers. The time-domain and frequency-domain results of the noise are obtained and analyzed.

[0013] Furthermore, in step S6.5, the sound field detection point is set at a distance of 2m from the rotor of the bladed pump-jet propeller, and the sound field sampling points are distributed at equal intervals along the axial and radial directions of the bladed pump-jet propeller.

[0014] Furthermore, in step S6.5, the formula for calculating sound power is: In the formula, α is a constant, usually taken as α = 0.1; c represents the speed of sound in water, taken as c = 1500 m / s; l represents the turbulence scale; u represents the turbulence velocity.

[0015] This invention enables numerical simulation studies of unsteady cavitation flow in a bladed pump-jet propulsion system, analyzing the internal flow characteristics and cavitation development degree. Simultaneously, based on unsteady calculations of the internal flow under cavitation and non-cavitation conditions, the pressure pulsation under cavitation development is investigated. Furthermore, the flow-induced noise of the bladed pump-jet propulsion system is numerically simulated and analyzed using Actran acoustic software, studying the characteristic frequency changes of cavitation volume pulsation radiated noise. Combining these research results, a numerical study of the internal flow, flow cavitation, and its induced noise mechanism in a bladed pump-jet propulsion system is achieved. Attached Figure Description

[0016] Figure 1 This is a flowchart of the numerical prediction method for cavitation noise of the bladed pump-jet propulsion system described in this invention.

[0017] Figure 2 This is a three-dimensional geometric model of the blade-type pump-jet propulsion system in an embodiment of the present invention.

[0018] Figure 3 The computational domain structure mesh of the bladed pump-jet propulsion system in this embodiment of the invention is shown in (a) for the pump-jet computational domain size, (b) for the external flow field mesh of the pump-jet system, and (c) for the pump-jet wall mesh.

[0019] Figure 4 This describes the computational domain division and boundary conditions for the bladed pump-jet propulsion system in this embodiment of the invention.

[0020] Figure 5 This paper analyzes the hydrodynamic performance of the bladed pump-jet propulsion unit under non-cavitation conditions in an embodiment of the present invention.

[0021] Figure 6 This is a comparison of the cavitation efficiency of the bladed pump-jet propulsion system before and after cavitation at different speeds in an example of the present invention.

[0022] Figure 7 These are the hydrodynamic parameters of the bladed pump-jet propulsion system under different cavitation numbers.

[0023] Figure 8The image shows the volume fraction distribution of cavitation on the surface of the rotor blades of a vane-type pump-jet propulsion unit at different rotational speeds with a cavitation number of σ=2.73.

[0024] Figure 9 The isosurface analysis of cavitation volume fraction of bladed pump-jet propulsion under different cavitation numbers is given, where (a) σ=3.544; (b) σ=3.136; (c) σ=2.73; (d) σ=2.32; (e) σ=1.94.

[0025] Figure 10 This is a schematic diagram showing the distribution of monitoring points inside the flow channel of a bladed pump-jet propulsion system.

[0026] Figure 11 Time-domain analysis of a bladed pump-jet propulsion system with a cavitation number of σ=1.94: (a) M section; (b) N section; (c) O section; (d) P section; (e) Q section.

[0027] Figure 12 The time-domain distribution of the cavitation number of the bladed pump-jet propulsion system with σ=3.544 is shown in the following sections: (a) M section; (b) N section; (c) O section; (d) P section; (e) Q section.

[0028] Figure 13 The diagram shows the global mesh entity of the acoustic computational domain of the bladed pump-jet propulsion system, where (a) is a schematic diagram of the global acoustic computational domain mesh and (b) is the mesh distribution in the yz plane.

[0029] Figure 14 The calculation model for the pulsating noise of the rotor cavitation volume of the bladed pump-jet propulsion unit and the location of the sound field monitoring points are set.

[0030] Figure 15 This is a schematic diagram showing the location of the sound field monitoring points for a bladed pump-jet propulsion system.

[0031] Figure 16 The sound pressure curves of the bladed pump-jet propulsion system at various monitoring points are shown in the non-cavitation and cavitation states. (a) represents the axial direction without cavitation, (b) represents the radial direction without cavitation, (c) represents the axial direction with cavitation, and (d) represents the radial direction with cavitation.

[0032] Figure 17 This is a comparison chart of the total sound pressure level attenuation characteristics of a bladed pump-jet propulsion system at various monitoring points.

[0033] Figure 18 The image shows a comparison of axial plane sound pressure distribution of a bladed pump-jet propulsion unit under cavitation and non-cavitation conditions at various frequency bands. It includes the sound pressure distribution at 9 blade frequencies, (a) non-cavitation and (b) cavitation.

[0034] Figure 19The figures show the time-domain and frequency-domain results of rotor cavitation volume variation in a bladed pump-jet propulsion unit under different cavitation numbers: (a) σ=3.5, (b) σ=2.73, (c) σ=2.32, and (d) σ=1.94. The red curve in the upper left corner of the figure represents the time-domain result of cavitation volume versus time, while the main figure shows the frequency-domain distribution after Fourier transform of the cavitation volume fluctuations.

[0035] Figure 20 The spectrum of cavitation volume pulsation radiation noise of a bladed pump-jet propulsion unit under different cavitation numbers.

[0036] Explanation of reference numerals in the attached drawings: 1. Conduit, 2. Hub, 3. Rotor, 4. Stator, 5. Stator region, 6. Rotor region Detailed Implementation

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0038] This embodiment takes a bladed pump-jet propulsion device as an example, including a duct, hub, rotor, and stator. Its main design parameters are: the working environment medium is clean water, the number of rotor blades is 7, the number of stator blades is 9, the cross-sectional shape of the duct is airfoil NACA66, the maximum rotor diameter D=349.2mm, the rotor blade tip clearance is 1mm, and the rotor blade spacing is 18.15mm.

[0039] The numerical prediction method for cavitation noise of a bladed pump-jet propulsion system described in this invention studies the causes of cavitation flow and its induced noise mechanism in bladed pump-jet propulsion systems. This method is used to investigate the cavitation and induced noise phenomena that occur during the operation of the bladed pump-jet propulsion system. The process is as follows: Figure 1 As shown, the specific steps include:

[0040] Step S1: 3D model creation and mesh generation: The geometry of a vane pump-jet propulsion system with a rear-mounted stator was modeled in 3D using Unigraphics NX, and the output model file had the .stp extension. Figure 2 This involves creating a 3D model of a bladed pump-jet propulsion system. The fluid domain is meshed, and the mesh quality is checked, ensuring all mesh quality values ​​are greater than 0.4. The average y+ value of the mesh for each water component of the bladed pump-jet propulsion system is approximately 4.5. The total number of meshes is 11.18 million. The output mesh file has the .cfx extension. Figure 3 As shown.

[0041] Step S2 sets boundary conditions and performs steady numerical simulation of the internal flow of the bladed pump-jet propulsion unit under non-cavitation conditions, analyzing the characteristics of the flow field pressure distribution of the propulsion unit at different rotational speeds.

[0042] Specifically, in step S2.1, the mesh file obtained in step S1 is first imported into ANSYS-CFX, and the Steady calculation mode is selected. Pre-calculation settings are then configured as follows:

[0043] Setting the working fluid properties: The working fluid is set to T=25℃ clean water, and the density and viscosity coefficient of water at this temperature are set. The computational domain is divided into an external flow field, a rotor domain, and a stator domain. The simulated external flow field is enclosed by a cylinder coaxial with the bladed pump-jet propeller. The inlet and outlet boundary conditions of the external flow field are set as follows: velocity inlet, pressure outlet, velocity magnitude v=7m / s, turbulence intensity I=5%. The pressure outlet value is 0Pa. The isothermal heat transfer model and the SST k-ω turbulence model are selected. The walls of the external flow field are free-slip walls, and all other walls are non-slip walls. The bladed pump-jet propeller is located 4D from the external flow field inlet. Based on the location of the rotor and stator, the internal flow field of the propeller is divided into a rotor domain and a stator domain. The inlets and outlets of the three computational domains are set as interfaces, such as... Figure 4 As shown. The rotation axis is set to be aligned with the impeller rotation direction, the relative velocity of the wall is set to 0, and a rotating coordinate system is used; the static computational domain wall is set to a no-slip wall, and a global static coordinate system is used; the interface between the dynamic and static computational domains uses the MRF multireference system, the transformation coordinate system is set to FrozenRotor, and the mesh connection method is set to GGI; the interface connection model between the static and static computational domains is set to normal connection, and the mesh connection method is set to GGI; the convection term, turbulence numerical term, solution steps, and computational convergence condition in the solver parameters are all set to 1e. -4 Complete the setup for steady numerical calculations of internal flow under various operating conditions, and output the numerical calculation file as .def.

[0044] After setting the boundary conditions in S2.2, a steady numerical simulation of the internal flow of the bladed pump-jet propeller under non-cavitation conditions was performed. The pump-jet advance coefficient was varied by setting different propeller rotor speeds. The simulation output a numerical calculation result file with the suffix "res," and the characteristics of the flow field pressure distribution at different propeller speeds were analyzed. Based on the characteristics of the flow field pressure distribution, the turbulence model, open-water performance parameters, and external flow field length were modified. Steady numerical simulations of the internal flow of the bladed pump-jet propeller under non-cavitation conditions were then performed, optimized, and verified until a prediction accuracy of 3% was achieved.

[0045] Based on the steady numerical calculation results of the internal flow of the optimized bladed pump-jet propeller in step S2.2, a fixed inflow velocity of 7 m / s was selected. The pump-jet advance coefficient was changed by altering the pump-jet rotor speed. Numerical simulations were performed with pump-jet rotor speeds n = 1150 r / min, 1250 r / min, 1350 r / min, 1450 r / min, 1550 r / min, 1650 r / min, 1750 r / min, 850 r / min, and 950 r / min to obtain the thrust coefficient, torque coefficient, and hydrodynamic efficiency of the pump-jet under different inflow coefficient conditions. Figure 5 As shown. From Figure 5 As can be seen, with the increase of the advance coefficient, the thrust coefficient and torque coefficient gradually decrease, while the pump-jet efficiency first increases and then decreases, reaching the highest hydrodynamic efficiency under the condition of inflow coefficient J=0.884.

[0046] Step S3, based on the optimized steady-state calculation model of the internal flow of the bladed pump-jet propeller, numerical simulations of the cavitation flow of the bladed pump-jet propeller are performed under different speeds and the same cavitation number by loading the ZGB cavitation model. This obtains the propeller's hydrodynamic performance under cavitation and the hydrodynamic parameters under different cavitation numbers. The cavitation models for the selected bladed pump-jet propeller at different speeds use the steady-state calculation results of the cavitation flow as initial conditions, and the DES turbulence model is used for unsteady cavitation flow numerical simulation. During the cavitation calculation, the time step ∆t for the unsteady cavitation flow calculation is set to 1.33 × 10⁻⁶. -4 The total computation time was 0.48 seconds.

[0047] Step S4 involves performing unsteady cavitation flow numerical simulations to compare the hydrodynamic efficiency of the thruster with and without cavitation at different rotational speeds; the hydrodynamic efficiency includes thrust coefficient, torque coefficient, and hydrodynamic efficiency.

[0048] The hydrodynamic efficiency of a bladed pump-jet propeller under non-cavitation and cavitation conditions at different speeds is compared. The differences in hydrodynamic efficiency values ​​of the bladed pump-jet propeller under non-cavitation and cavitation conditions at different cavitation numbers and speeds are analyzed to establish a comparison of the propeller's hydrodynamic efficiency under non-cavitation and cavitation conditions at different speeds. Simultaneously, the hydrodynamic performance of the propeller under different cavitation numbers is studied, and the influence of different cavitation degrees on the propeller's hydrodynamic performance parameters (such as thrust coefficient, torque coefficient, and hydrodynamic efficiency) is analyzed.

[0049] Using the steady numerical calculation results of the bladed pump-jet propulsion engine under the determined optimal inlet coefficient J=0.884, and taking the steady cavitation numerical simulation results as the initial conditions, unsteady cavitation flow numerical simulation was carried out.

[0050] Comparing the hydrodynamic efficiency of the vane pump-jet propulsion unit before and after cavitation at different speeds (n=1150 r / min, n=1250 r / min, n=1350 r / min, n=1450 r / min, n=1550 r / min, n=1650 r / min, n=1750 r / min, n=1850 r / min, n=1950 r / min), such as... Figure 6 As shown, at low speeds, no cavitation occurs in the thruster, and the hydrodynamic efficiencies obtained from the two simulations are almost identical. At a speed of 1150 r / min, cavitation begins to occur in the cavitation-steady numerical simulation, and the difference in hydrodynamic efficiency between the non-cavitation and cavitation conditions gradually increases. With further increases in speed, the hydrodynamic efficiency of the thruster under cavitation conditions reaches its maximum at 1250 r / min, and from this speed onwards, the hydrodynamic efficiency decreases significantly with increasing speed. Under non-cavitation conditions, the hydrodynamic efficiency reaches its maximum at 1450 r / min, and from this speed onwards, the decrease in hydrodynamic efficiency with increasing speed is relatively small.

[0051] Numerical simulations were conducted based on the optimal efficiency operating condition with an inflow coefficient J=0.884 and a rotational speed of 1250 r / min. The hydrodynamic parameters of the bladed pump-jet propulsion unit under this condition at different cavitation numbers σ were obtained, such as... Figure 7 As shown in the figure, the thrust coefficient, torque coefficient, and efficiency of the thruster gradually decrease as the cavitation number decreases. This indicates that cavitation directly affects the hydrodynamic performance of the thruster, leading to a decrease in its operating efficiency and thrust performance. After cavitation occurs, the thrust coefficient drops significantly, while the torque coefficient changes relatively little.

[0052] pass Figure 7 It can be seen that the cavitation number basically stabilizes at 2.73, with a very small difference compared to subsequent values, and is also the closest to the stable condition. The cavitation degree on the rotor blade surface is analyzed at different speeds with a cavitation number of σ=2.73, and the cavitation evolution law of the bladed pump-jet propulsion system at different speeds is studied using the cavitation volume fraction. Numerical simulations are performed under this cavitation number condition to obtain... Figure 8 The image shows the cavitation volume fraction distribution on the surface of the rotor blades of a vane-type pump-jet propeller at different speeds with a cavitation number of σ=2.73. From... Figure 8As can be seen, at a propeller speed of 1150 r / min, the cavitation volume fraction on the blade surface is 0, and no significant cavitation is observed. At a speed of 1250 r / min, areas with a cavitation volume fraction greater than 0 appear on the rotor blades, indicating the onset of cavitation in the propeller, initially occurring at the leading edge of the rotor blades. With increasing speed, cavitation first develops towards the root of the rotor blades, and the cavitation volume fraction also extends towards the trailing edge. When the speed reaches 1650 r / min, the cavitation area exceeds nearly half of the blade surface. After the speed further reaches 1750 r / min, cavitation occurs on almost the entire surface of the blades.

[0053] Figure 9 The isosurface analysis of cavitation volume fraction in a bladed pump-jet propeller under different cavitation numbers at the same rotational speed (n=1250 r / min) is shown in the figure, where (a) σ=3.544; (b) σ=3.136; (c) σ=2.73; (d) σ=2.32; and (e) σ=1.94. As can be seen from the figure, cavitation inside the propeller becomes more severe as the cavitation number decreases. When the cavitation number σ=2.32, cavitation begins to appear in the propeller, but the degree of cavitation is small, and it first appears at the blade tip clearance. As the cavitation number decreases, the cavitation region gradually expands, not only further expanding the cavitation region at the blade tip clearance, but also beginning to appear on the surface of the propeller rotor; and the cavitation region extends backward along the blade rotation direction, forming tip vortex cavitation. The main reason for cavitation in the bladed pump-jet propeller is that under high-speed rotation of the rotor blades, the pressure on the suction surface is lower than the saturated vapor pressure under local environmental conditions. As the cavitation number decreases, the degree of cavitation intensifies. Cavitation directly affects the stability of the internal flow field and hydrodynamic performance of the thruster, and can lead to more severe cavitation-induced noise caused by cavitation pulsation, thus affecting the stealth performance of the thruster propulsion system.

[0054] Step S5: Based on the numerical simulation of cavitation flow in the S3 bladed pump-jet propulsion system, monitoring points were set up inside the propulsion channel to extract pressure pulsations within the propulsion channel under different cavitation numbers. The pressure pulsation information and frequency distribution inside the propulsion under different cavitation numbers were analyzed, along with the cavitation development process inside the propulsion under different cavitation numbers.

[0055] The locations of the monitoring points are as follows: Figure 10 As shown, the internal pipes of the thruster are radially distributed and arranged into 5 flow channels, which are represented by streamlines 1, 2, 3, ..., 5. At the same time, the axial distribution position of the thruster is divided into 5 axial planes, representing the thruster inlet plane M, the rotor inlet plane N, the rotor-stator interface plane O, the stator outlet plane P, and the duct outlet plane Q. Based on this, the position points on the 5 flow channels are selected as monitoring points for analysis, for a total of 25 monitoring points.

[0056] The influence of cavitation development degree on the internal pressure pulsation of the propeller under different cavitation numbers was studied by comparing and analyzing the pressure pulsation. Pressure pulsation analysis was conducted under different cavitation numbers at the same propeller speed of 1250 r / min. The pressure pulsation was analyzed under the conditions of most severe cavitation (cavitation number σ = 1.94) and the initial stage of cavitation (cavitation number σ = 3.554). Specifically, time-domain analysis was performed on the bladed pump-jet propeller with cavitation numbers of σ = 1.94 and σ = 3.544. Figure 11 , Figure 12 As shown, (a) is section M, (b) is section N, (c) is section O, (d) is section P, and (e) is section Q.

[0057] By comparing and analyzing the internal pressure pulsation information of the propeller under different cavitation numbers, such as frequency, amplitude, and rotor rotation frequency, the study investigates the changes in the degree of cavitation development inside a bladed pump-jet propeller. To analyze the frequency distribution of pressure pulsations in the internal flow field of the propeller, a Fast Fourier Transform (FFT) was performed on the pressure monitoring data. Figure 11 , Figure 12 Data analysis shows that cavitation is severe when the cavitation number σ = 1.94. Section M, located at the propeller inlet, is less affected by the internal rotor and cavitation level. Compared to other planes, the pressure pulsation patterns of the propeller under both cavitation numbers are similar, with the dominant frequency close to the rotor rotation frequency. Section N is close to the rotor blade inlet of the propeller, and the pressure coefficient at this monitoring point is negative. However, under different cavitation numbers, a significant difference in pressure fluctuation range is observed. The pressure coefficient at section N after cavitation generally falls below -0.3, while the pressure coefficient pulsation in areas without cavitation is all below -0.3. Section O, located between the rotor outlet and stator inlet, is affected by cavitation; when cavitation is severe, the pressure change is larger than during initial cavitation. Sections P and Q are mainly affected by the stator. Due to the longer stator blade roots, the impact is greatest, and the fluctuation frequency is significant. The stator blades near the duct are shorter, resulting in more chaotic fluctuations. Because both sections are close to the external flow field, the fluctuation range is relatively smaller compared to the pressure pulsation within the propeller's internal flow field. The pressure pulsations before and after cavitation clearly show the peaks and troughs of the pulsations. The pressure pulsations under the initial cavitation conditions have smaller amplitudes and more complex frequency components. This indicates that the intensity of cavitation affects the intensity of pressure fluctuations at the thruster exit, but the degree of influence is attenuating.

[0058] Step S6, based on the numerical simulation of the internal flow of the bladed pump-jet propulsion unit in steps S2 and S3, numerically simulates the non-cavitation flow field noise and cavitation noise performance of the bladed pump-jet propulsion unit, and obtains the axial and radial sound pressure variation law, the total sound pressure level attenuation law, and the axial plane sound pressure distribution under cavitation and non-cavitation conditions.

[0059] The numerical simulation of the noise performance uses Lighthill theory combined with Actran acoustic software to simulate and analyze the internal flow noise under cavitation and non-cavitation conditions based on the unsteady flow field simulation results of the bladed pump-jet propulsion system. The cavitation noise of the bladed pump-jet propulsion system is analyzed based on the spherical cavitation radiation theory.

[0060] Step S6.1 Since the acoustic calculation method used requires noise simulation based on the computational domain, a propeller acoustic computational domain model was constructed using ICEM meshing software based on a structural space of the same size as the CFD computational domain in S1. Figure 13 As shown. Figure 13 Image (a) is a global mesh entity diagram of the propeller acoustic computational domain, which uses an unstructured mesh. The internal mesh distribution of the computational domain is shown below. Figure 13 As shown in (b), it can be seen that the grid distribution is uniform within the computational domain, while local refinement is performed near the thruster.

[0061] S6.2 sets the interface between the external flow field region and the stator and rotor parts as a surface sound source, sets the outer side of the volume sound source as a non-reflective interface, and uses an acoustic computation domain with a maximum grid size of 0.03m.

[0062] S6.3 Under the acoustic calculation domain settings in steps S6.1 and S6.2, use Ansys CFX-SolverManager to perform unsteady numerical simulation of the propeller flow field, solve for flow field information such as velocity field, pressure field, and density field, and export the flow field data in .ensightgold format as the input file for noise calculation in Actran acoustic software.

[0063] S6.4 creates an acoustic model, imports the flow field data obtained in S6.3 into the Actran acoustic software, sets the corresponding physical parameters and solver type, and performs acoustic calculations. The calculated sound sources are then interpolated into the acoustic mesh using integral interpolation.

[0064] S6.5 sets sound field detection points and sound field sampling points in the acoustic model, collects data on the change of cavitation volume over time, obtains the time-domain data of sound pressure value and sound power at the sound field detection points and sound field sampling points, and converts them into frequency-domain data through inverse Fourier transform, thereby obtaining the sound pressure level distribution of cavitation volume pulsating radiation noise, and analyzing the axial and radial sound pressure variation law of the bladed pump-jet propulsion device.

[0065] Specifically, the calculation model for the cavitation volume pulsation radiation noise of the rotor of the bladed pump-jet propeller and the location of the sound field detection points are as follows: Figure 14As shown in Figure S4, based on the analysis of the cavitation volume distribution inside the propeller, cavitation occurs at the suction surface of the rotor blades. Therefore, the center of the virtual cavitation bubble is set at the rotor's rotation center, and the sound field detection point is set 2m away from the rotor of the bladed pump-jet propeller, as shown by the red dot in the figure. A total of 16 sound field sampling points are set in the noise simulation, namely 8 monitoring points in the axial direction and 8 in the radial direction of the bladed pump-jet propeller. Figure 15 This is a schematic diagram of the location of the sound field monitoring points. The blue area is the acoustic calculation domain grid area. The first monitoring point is 1D away from the center of the bladed pump-jet propulsion unit. A monitoring point is added every 1D. B1 to B8 are axial monitoring points, and B'1 to B'8 are radial monitoring points.

[0066] The formula for calculating the volume of a cavitation bubble is as follows:

[0067] In the formula, V c Let m be the total volume of the cavitation bubble. 3 N is the total number of control entities; α i To control the volume fraction of vacuoles within the body; V i This represents the volume of each control unit, in meters. 3 .

[0068] Substitute the cavitation volume at different times into the following formula:

[0069] This allows us to obtain the sound pressure values ​​at any sound field monitoring point and sampling point. In the formula, the radiated noise time is... R is the cavitation radius, and r is the distance between the sampling point or sound field detection point and the center of the virtual cavitation sphere.

[0070] The physical meaning of sound power is the total sound energy radiated by a sound source into a space per unit time, and its unit is W. The formula for calculating sound power is: In the formula, α is a constant, typically taken as α = 0.1; c represents the speed of sound in water, typically taken as c = 1500 m / s; l represents the turbulence scale; and u represents the turbulence velocity. The sound power at the sampling points and detection points of the sound field is obtained using the sound power calculation formula.

[0071] The time-domain sound pressure data is converted to frequency-domain sound pressure data through inverse Fourier transform, thereby obtaining the sound pressure level distribution of cavitation volume pulsation radiation noise. The axial and radial sound pressure variation patterns of the bladed pump-jet propulsion unit are analyzed, along with the comparison of axial plane sound pressure under cavitation and non-cavitation conditions in various frequency bands. S6.6 uses the total sound pressure level (OSPL) in spherical cavitation radiation theory to represent the total sound intensity across the entire frequency range. Based on the sound pressure level distribution data obtained in S6.5, the total sound pressure level (OSPL) at each sampling point and sound field detection point is calculated according to the total sound pressure level (OSPL) in spherical cavitation radiation theory.

[0072]

[0073] In the formula, f min and f max These represent the lower and upper limits of the frequency, respectively, in Hz; Δf represents the frequency resolution, in Hz, p. e The effective sound pressure level is expressed in Pa; p ref Using the reference sound pressure level (Pa), the attenuation law of the total sound pressure level is obtained.

[0074] Based on the simulation results of hydrodynamic numerical thrust coefficient, torque coefficient, and efficiency, this embodiment simulates and monitors the cavitation noise of a bladed pump-jet propeller under the conditions of no cavitation and cavitation number σ=2.32 at a speed of n=1250r / min. The axial and radial sound pressure variation law of the bladed pump-jet propeller is analyzed, and the noise performance of the propeller under non-cavitation and cavitation conditions is compared and analyzed to obtain the attenuation law of the total sound pressure level at each monitoring point.

[0075] Figure 16 The image shows the sound pressure level spectrum of a bladed pump-jet propulsion unit under both non-cavitation and cavitation conditions. As can be seen from the image, the noise of the bladed pump-jet propulsion unit exhibits a distinct blade frequency characteristic, with significant amplitude fluctuations occurring at the octaves of the blade frequency, and the maximum sound pressure level around the first octave. Figure 16 (a) and Figure 16 (b) shows the sound pressure level results at various monitoring points when the thruster is in a non-cavitation state. The figure shows that the axial noise is higher than the radial noise, which is because the axial monitoring points are closer to the rotor's motion area and are affected by the rotor's movement. At the monitoring points B1 and B'1, closest to the thruster, the low-frequency noise is significantly higher than at other monitoring points, with sound pressure levels exceeding 130 dB. As the monitoring points move further away from the thruster, the sound pressure levels at all monitoring points generally decrease, while the low-frequency sound pressure level drops rapidly to around 90 dB. Figure 16 (c) and Figure 16(d) shows the sound pressure distribution curves at various monitoring points of the propeller under cavitation conditions. Both axial and radial sound pressure decrease with frequency, with the highest sound pressure level in the low-frequency band. As the distance of the monitoring point from the center of the bladed pump-jet propeller increases, the sound pressure value at the same frequency shows a decreasing trend. Furthermore, as the frequency increases, the sound pressure level gradually decreases, from over 180 dB at the blade frequency to approximately 120-130 dB at 2000 Hz. Comparing the sound pressure levels of the cavitation flow field and the non-cavitation flow field, it can be seen that the sound pressure level of the cavitation flow field is higher than that of the non-cavitation flow field at all frequency bands. Cavitation directly leads to an increase in flow field noise of approximately 50 dB.

[0076] Figure 17 The figure shows the total sound pressure level (SPL) results of the thruster at various monitoring points. SPL characterizes the sound intensity across the entire frequency range at the monitoring point. As can be seen from the figure, the SPL gradually decreases as the monitoring point moves further away from the pump. The initial attenuation rate is relatively fast, but decreases with increasing distance. At a distance of 1D from the thruster, the difference between the axial and radial SPL levels is less than 5dB. With increasing distance, the difference between the axial and radial SPL levels gradually decreases, and at a distance of 4D from the thruster, the axial and radial SPL levels are almost equal. Comparing the noise results with and without cavitation reveals that cavitation increases the noise level at the same monitoring point by approximately 50dB.

[0077] Figure 18 This is a comparative contour map of the axial plane sound pressure of the thruster under cavitation and non-cavitation conditions at various frequency bands, including the sound pressure distribution at nine blade frequencies. The map shows that the sound pressure is greater in cavitation than in non-cavitation at every frequency band. At 1-2 times the blade frequency, the sound pressure at the inlet and outlet of both the cavitation and non-cavitation flow fields is greater than the sound pressure of the surrounding flow field. At 3-8 times the blade frequency, the sound pressure at the thruster inlet under non-cavitation conditions is almost equivalent to that of the surrounding flow field, while the inlet of the cavitation flow field still shows a significantly higher sound pressure distribution than the surrounding flow field. Furthermore, the sound pressure at the thruster outlet gradually decreases with increasing frequency.

[0078] S6.7 After the unsteady flow of the bladed pump-jet propulsion unit exhibits periodic changes (cavitation stabilization), the cavitation volume is calculated according to the processes in S6.5 and S6.6 for the process from cavitation generation to breakup under different cavitation numbers. The time-domain and frequency-domain results of the noise are obtained and analyzed.

[0079] Figure 19This is a time-domain and frequency-domain graph showing the rotor cavitation volume change under different cavitation numbers. The calculation results can be divided into three stages based on the degree of cavitation: cavitation initiation, development, and maturity. From the cavitation initiation stage to the cavitation development stage and then to cavitation maturity, the magnitude and amplitude of the propeller's cavitation volume pulsation increase accordingly. The cavitation shedding frequency increases from twice the blade frequency in the initiation stage to 3.8 times the blade frequency as the cavitation gradually develops, with a significant increase in fluctuation frequency. The cavitation is in a high-frequency generation and collapse process. Upon reaching the cavitation maturity stage, the cavitation frequency remains essentially unchanged, maintaining around one blade frequency.

[0080] Figure 20 The noise spectrum of cavitation volume pulsation radiation of a bladed pump-jet propulsion unit under different cavitation numbers includes the noise frequency domain results under four different cavitation conditions.

Claims

1. A numerical prediction method for cavitation noise in a bladed pump-jet propulsion system, characterized in that, Includes the following steps: S1: Establish a 3D model of the bladed pump-jet propulsion system and perform mesh generation; S2: Set boundary conditions and perform steady numerical simulation of the internal flow of a bladed pump-jet propulsion unit under non-cavitation conditions; S3: Load the ZGB cavitation model to perform numerical simulation of the cavitation flow of the bladed pump-jet propulsion system, and obtain the hydrodynamic performance of the propulsion system under cavitation and the hydrodynamic parameters under different cavitation numbers; S4: Perform unsteady cavitation flow numerical simulation and compare the hydrodynamic efficiency of the thruster with and without cavitation at different speeds; the hydrodynamic efficiency includes thrust coefficient, torque coefficient and hydrodynamic efficiency. S5: Monitoring points are set inside the thruster flow channel to extract the pressure pulsation of the internal flow channel under different cavitation numbers, and the pressure pulsation information and frequency distribution inside the thruster under different cavitation numbers are analyzed to study the changes in the degree of cavitation development inside the thruster. S6: Based on the numerical simulation of the internal flow of the bladed pump-jet propulsion unit in steps S2 and S3, numerical simulation of the non-cavitation flow field noise and cavitation noise performance of the bladed pump-jet propulsion unit is performed to obtain the sound pressure level distribution, axial and radial sound pressure variation law, total sound pressure level attenuation law, and axial plane sound pressure data of the cavitation volume pulsation radiation noise under cavitation and non-cavitation conditions.

2. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 1, characterized in that, The specific steps of S1 are as follows: S1.1 Unigraphics NX is used to perform 3D modeling of the geometry of the vane pump-jet propulsion unit with a rear stator, and the model file with the suffix .stp is output. S1.2 Import the 3D model established in S1.1 into ICEM, mesh the fluid domain, including: duct, hub, rotor blades and stator blades. The mesh form adopts a hexahedral structure mesh, and a boundary layer is added to the structure mesh to make the average y+ value of each component mesh of the bladed pump jet propulsion device about 4.

5. Check the mesh quality, select 11.18 million meshes to ensure calculation accuracy, perform mesh independence analysis, and output a mesh file with the suffix .cfx.

3. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 1, characterized in that, The specific steps of S2 are as follows: S2.1 Import the mesh file obtained in S1.2 into ANSYS-CFX, select Steady calculation mode, and set the solver parameters; Set the working fluid properties: T=25℃ clean water, and set the density and viscosity coefficient of water at this temperature; the computational domain is divided into the external flow field, the rotor domain, and the stator domain; The simulated external flow field is enclosed by a cylinder coaxial with the pump-jet propulsion unit; Set the inlet and outlet boundary conditions for the external flow field: velocity inlet, pressure outlet, velocity magnitude v=7m / s, turbulence intensity I=5%; pressure outlet value is 0Pa; Select the isothermal heat transfer model and the SST k-ω turbulence model; change the advance coefficient of the bladed pump-jet propulsion by setting different pump-jet rotor speeds; The walls of the external flow field are free-slip walls, while all other walls are non-slip walls. The bladed pump-jet propulsion unit is located 4D from the inlet of the external flow field. Based on the location of the rotor and stator, the internal flow field of the propulsion unit is divided into the rotor domain and the stator domain. The inlet and outlet of the three computational domains are set as interfaces. Set the rotation axis to be aligned with the impeller rotation direction, set the relative velocity of the wall to 0, and use the rotating coordinate system; set the static computational domain wall to a no-slip wall and set it to a global static coordinate system. S2.2 Based on S2.1, perform steady numerical simulation calculations of the internal flow of a bladed pump-jet propulsion unit under non-cavitation conditions, and output the numerical calculation result file with the suffix "res".

4. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 1, characterized in that, S2.2 also includes a simulation optimization process: by analyzing the characteristics of the flow field pressure distribution of the propeller at different speeds, and based on the characteristics of the flow field pressure distribution, the steady-state numerical simulation is optimized by correcting the turbulence model, open-water performance parameters, and external flow field length.

5. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 1 or 4, characterized in that, In S3: the unsteady flow adopts the DES turbulence model, and the time step ∆t for cavitation calculation is set to 1.33 × 10⁻⁶ when performing cavitation calculations. -4 The total computation time was 0.48 seconds.

6. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 1, characterized in that, The monitoring point settings in S5 are specifically as follows: Five flow channels are arranged radially inside the thruster, denoted as streamlines 1, 2, 3, ..., 5. The thruster is also divided into five axial planes, representing the thruster inlet plane M, the rotor inlet plane N, the rotor-stator interface plane O, the stator outlet plane P, and the duct outlet plane Q. Based on these, 25 monitoring points are selected from the five flow channels for analysis.

7. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 1, characterized in that, The noise performance numerical simulation in step S6 is based on Lighthill theory combined with Actran acoustic software. The simulation results of unsteady flow field of bladed pump-jet propulsion are used to simulate and analyze the internal flow noise under cavitation and non-cavitation conditions. The cavitation noise of bladed pump-jet propulsion is analyzed based on the spherical cavitation radiation theory.

8. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 1 or 7, characterized in that, The specific steps of step S6 are as follows: S6.1 uses a structural space of the same size as the CFD computation domain constructed by S1. The acoustic computation domain model of the thruster is constructed using ICEM mesh generation software. Unstructured mesh generation is used, and local refinement is performed near the thruster. S6.2 sets the interface between the external flow field region and the stator and rotor parts as a surface sound source, sets the outer side of the volume sound source as a non-reflective interface, and uses an acoustic computation domain with a maximum grid size of 0.03m. S6.3 Under the acoustic calculation domain settings in steps S6.1 and S6.2, use Ansys CFX-Solver Manager to perform unsteady numerical simulation of the propeller flow field, solve for the velocity field, pressure field, and density field information, and export the flow field data in .ensightgold format as the input file for noise calculation in Actran acoustic software; S6.4 Create an acoustic model, import the flow field data into the Actran acoustic software, set the corresponding physical parameters and solver type, perform acoustic calculations, and insert the obtained sound sources into the acoustic mesh using integral interpolation. S6.5 sets the sound field sampling point and sound field detection point, and collects data on the change of cavitation volume over time. The cavitation volume is: In the formula, Vc is the total volume of the cavitation bubble, in meters. 3 N is the total number of control entities; α i To control the volume fraction of vacuoles within the body; V i The volume of each control unit is represented in m. 3 ; Substitute the cavitation volume at different times into the formula: This allows us to obtain the sound field detection and sound pressure values ​​at various sampling points at different times; where the radiated noise is at time [equation missing]. R is the cavitation radius, r is the distance between the sampling point or sound field detection point and the center of the virtual cavitation sphere; and the sound power is calculated. The time-domain sound pressure data is converted into frequency-domain sound pressure data through inverse Fourier transform, thereby obtaining the sound pressure level distribution of cavitation volume pulsation radiation noise. The axial and radial sound pressure variation law of the bladed pump-jet propulsion is analyzed, and the noise performance of the propulsion under non-cavitation and cavitation conditions is compared and analyzed, as well as the axial plane sound pressure comparison under non-cavitation and cavitation conditions in various frequency bands. S6.6 Based on the sound pressure level distribution data obtained in S6.5, and according to the total sound pressure level (OSPL) in the spherical cavitation radiation theory, calculate the total sound pressure level (OSPL) at each sampling point and sound field detection point. In the formula, f min and f max These represent the lower and upper limits of the frequency, respectively, in Hz; Δf represents the frequency resolution, in Hz, p. e The effective sound pressure level is expressed in Pa; p ref Using the reference sound pressure level (Pa), the attenuation law of the total sound pressure level is obtained. S6.7 After the unsteady flow of the bladed pump-jet propulsion unit exhibits periodic changes, the cavitation volume is calculated according to the processes in S6.5 and S6.6 for the process from cavitation generation to breakup under different cavitation numbers. The time-domain and frequency-domain results of the noise are obtained and analyzed.

9. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 8, characterized in that, In step S6.5, the sound field detection point is set at a distance of 2m from the rotor of the blade pump-jet propeller, and the sound field sampling points are distributed at equal intervals in the axial and radial directions of the blade pump-jet propeller.

10. The numerical prediction method for cavitation noise of a bladed pump-jet propulsion unit according to claim 8, characterized in that, In step S6.5, the formula for calculating sound power is: In the formula, α is a constant, usually taken as α=0.1; c represents the speed of sound in water, taken as c=1500m / s; l represents the turbulence scale; u represents the turbulent velocity.

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