Method and system for calculating low-frequency exciting force of underwater vehicle propeller

Abstract

The invention belongs to the technical field of propeller noise control, and discloses a method and a system for calculating low-frequency exciting force of an underwater vehicle propeller. A system model of an underwater vehicle is constructed, the system model comprises a three-dimensional model of a boat body and a propeller, a fluid calculation domain is established and divided into a static domain and a rotating domain, the rotating domain is a cylinder comprising a rotating part of the propeller, the static domain is a calculation domain outside the rotating domain, and the rotating domain is a cylinder comprising a rotating part of the propeller. A polyhedral grid is used for carrying out grid division on a rotation domain, and a cutting body grid is used for carrying out grid division on a stationary domain; performing simulation operation of the system model to obtain time domain data of the axial pulsation thrust of the rotating part of the propeller; and converting the time domain data into frequency domain data to obtain low-frequency exciting force information of the propeller. The calculation scheme of the low-frequency exciting force of the propeller is high in universality and high in precision.

Description

Technical Field

[0001] The present invention belongs to the technical field of propeller noise control, and more specifically, relates to a method and system for calculating the low-frequency excitation force of an underwater vehicle propeller. Background Art

[0002] The propeller of an underwater vehicle operates in the wake field of the vehicle. Due to the anisotropic and non-uniform characteristics of the flow velocity in the wake field, the forces and bending moments acting on the propeller blades in the wake field will exhibit a periodic pulsating form in the time domain. When transformed to the frequency domain, pulsation peaks will appear at the blade frequency and its multiples, etc. Such pulsations will be transmitted along the propeller drive shaft to the hull, and excite the hull to generate strong vibration noise. Since the rotational speed of the underwater vehicle propeller is relatively low, the frequency of the above-mentioned pulsating forces is also relatively low, so it is called the low-frequency excitation force of the propeller. A large number of studies have shown that the vibration noise caused by the low-frequency excitation force of the propeller accounts for an important proportion in the noise of the underwater vehicle. Therefore, the accurate calculation of the low-frequency excitation force of the propeller is of great significance for the evaluation and control of the noise of the underwater vehicle.

[0003] The existing methods for calculating the low-frequency excitation force of a propeller can be mainly summarized into two categories: The first category is to use empirical formulas for estimation. This method is summarized based on a large number of test results for a specific type of propeller. Its advantage is fast prediction speed, but the disadvantages are obvious, that is, the applicable range is very limited. When the propeller form changes, its prediction accuracy will be greatly reduced. At the same time, this type of method also considers insufficiently the changes in the wake field. When the hull form changes resulting in changes in the wake field, the low-frequency excitation force of the propeller will also change significantly, but the influence of these changes cannot be reflected in the empirical formula. The second category is to use numerical simulation based on the finite volume method for calculation. This type of method is based on the concept of control volume, and divides the calculation area into many non-overlapping control volumes (usually grid cells). Physical quantities (such as velocity, pressure, temperature, etc.) are stored and calculated on these control volumes. By applying conservation laws (such as mass conservation, momentum conservation, energy conservation, etc.) to the control volumes, the partial differential form of the conservation equation is transformed into a discrete form of algebraic equations, and then the approximate values of the physical quantities on each control volume are solved. The advantages of the above numerical calculation method are good applicability and can fully consider different combinations of hulls and propellers. The disadvantage is that the calculation accuracy is greatly affected by the grid quality. If the grid division is not reasonable, it will lead to large calculation errors and difficult convergence.

[0004] At present, there are some public reports on the research of calculating the low-frequency excitation force of an underwater vehicle's propeller through numerical simulation. However, most of them mainly focus on qualitative analysis, with very limited quantitative analysis, and discussions on the calculation accuracy of the low-frequency excitation force are even rarer. In some studies, the low-frequency excitation force obtained from simulation is compared with the measured value in experiments, but no reproducible and general method has been summarized. Summary of the Invention

[0005] The present invention provides a method and system for calculating the low-frequency excitation force of an underwater vehicle's propeller, aiming to solve the problem of poor generality or low accuracy in the existing calculation schemes for the low-frequency excitation force of an underwater vehicle's propeller.

[0006] The present invention provides a method for calculating the low-frequency excitation force of an underwater vehicle's propeller, comprising the following steps:

[0007] Construct a system model of the underwater vehicle, where the system model includes three-dimensional models of the hull and the propeller; establish a fluid calculation domain, divide the fluid calculation domain, and perform mesh generation.

[0008] Among them, the fluid calculation domain is divided into a stationary domain and a rotating domain. The rotating domain is a cylinder containing the rotating components of the propeller, and the stationary domain is the calculation domain outside the rotating domain; use polyhedral meshes to perform mesh generation on the rotating domain, and use cut-cell meshes to perform mesh generation on the stationary domain.

[0009] Conduct a simulation run of the system model to obtain the time-domain data of the axial pulsating thrust of the rotating components of the propeller; convert the time-domain data into frequency-domain data to obtain the low-frequency excitation force information of the propeller.

[0010] Preferably, the fluid calculation domain is a cuboid. Taking the head of the underwater vehicle as the origin, the distance from the upstream boundary of the fluid calculation domain to the origin is greater than or equal to the length L of the underwater vehicle, and the distance from the downstream boundary of the fluid calculation domain to the origin is greater than or equal to 4L.

[0011] The diameter of the rotating components of the propeller is R, and the axial dimension is H; the radius of the rotating domain is set to 1.2R to 1.5R, and the axial dimension of the rotating domain is set to 1.2H to 1.5H.

[0012] Preferably, the basic size Hm of the mesh of the stationary domain satisfies: Hm ≤ 0.01L.

[0013] Locally refine the mesh of the stationary domain, including the hull refinement area, the tail refinement area, and the wake refinement area.

[0014] The mesh size Hm1 in the hull refinement area satisfies: Hm1 ≤ 0.5Hm.

[0015] The grid size Hm2 in the tail encryption area satisfies: Hm2 ≤ 0.5Hm1;

[0016] The grid size Hm3 in the wake encryption area satisfies: Hm3 ≤ 0.5Hm2;

[0017] The basic size of the grid in the rotating domain is the same as the grid size Hm3 in the wake encryption area;

[0018] Surface grid encryption is performed on the surface of the rotating component of the thruster, and the encrypted grid size Hm4 satisfies: Hm4 ≤ 0.5Hm3.

[0019] Preferably, the hull encryption area is a cuboid including the entire hull. The distance from the upstream boundary of the hull encryption area to the origin is 0.4L to 0.6L, the distance from the downstream boundary of the hull encryption area to the origin is 1.4L to 1.6L, and the distances from the remaining boundaries to the origin are 0.2L to 0.3L;

[0020] The tail encryption area is cylindrical. The tail encryption area starts from the hull length at a distance of 0.7L to 0.8L from the origin and ends at the hull length at a distance of 1.05L to 1.15L from the origin. The radius of the tail encryption area is 1.02 times to 1.08 times the radius of the hull;

[0021] The wake encryption area is cylindrical. The wake encryption area starts from the hull length at a distance of 0.85L to 0.95L from the origin and ends at the hull length at a distance of 1.05L to 1.15L from the origin. The radius of the wake encryption area is 1.3 times to 1.7 times the radius of the hull.

[0022] Preferably, when performing grid division, it further includes: adding boundary layer grids on the surfaces of the hull and the thruster, and dividing the boundary layer grids.

[0023] Preferably, the boundary layer grids adopt prismatic grids, and the number of layers ≥ 15; the first layer of the boundary layer is the layer in contact with the wall surface, and the last layer of the boundary layer is the layer in contact with the environmental fluid; the grid size of the first layer of the boundary layer satisfies the dimensionless wall distance Y+ < 1, and the grid size of the last layer of the boundary layer is the same as the grid size of the area where it is located.

[0024] Preferably, when performing the simulation operation of the system model, the upstream boundary of the fluid calculation domain is used as the velocity inlet, the downstream boundary of the fluid calculation domain is used as the pressure outlet, the surfaces of the hull and the thruster located within the fluid calculation domain are set as non-slip wall surfaces, and simulation parameters are set. The simulation parameters include the density, kinematic viscosity, and flow velocity of the fluid.

[0025] Preferably, under given simulation parameters, a steady calculation is carried out using a turbulence model, and the calculation duration T0 of the flow field initialization satisfies: T0≥4L / v, where v is the flow velocity under the set working condition;

[0026] After the flow field initialization is completed, an unsteady calculation is carried out using a detached eddy simulation (DES) turbulence model. The time step t of the unsteady calculation satisfies: t≤w / 21600, where w is the rotational speed of the thruster; the frequency resolution of the unsteady calculation is higher than 0.25 Hz, and the calculation duration is greater than 4 s.

[0027] Preferably, the low-frequency excitation force information of the thruster includes the pulsating pressure amplitude and the pulsating pressure phase angle.

[0028] On the other hand, the present invention provides a calculation system for the low-frequency excitation force of an underwater vehicle thruster, including:

[0029] A construction unit for constructing a system model of the underwater vehicle, establishing a fluid calculation domain, dividing the fluid calculation domain, and performing mesh division;

[0030] A simulation and data processing unit for performing a simulation run of the system model to obtain time-domain data of the axial pulsating thrust of the rotating component of the thruster, and for converting the time-domain data into frequency-domain data to obtain low-frequency excitation force information of the thruster;

[0031] The calculation system for the low-frequency excitation force of the underwater vehicle thruster is used to execute the steps in the above-mentioned calculation method for the low-frequency excitation force of the underwater vehicle thruster.

[0032] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0033] The present invention is for calculating the low-frequency excitation force of an underwater vehicle's thruster. First, a system model of the underwater vehicle is constructed (the system model includes the three-dimensional models of the hull and the thruster), a fluid computational domain is established, and the fluid computational domain is divided into a stationary domain and a rotating domain (the rotating domain is a cylinder containing the rotating components of the thruster, and the stationary domain is the computational domain outside the rotating domain), and mesh generation is carried out (polyhedral meshes are used for mesh generation of the rotating domain, and cut cell meshes are used for mesh generation of the stationary domain); then, the system model is simulated to obtain the time-domain data of the axial pulsating thrust of the rotating components of the thruster; finally, the time-domain data is transformed into frequency-domain data to obtain the low-frequency excitation force information of the thruster. In view of the different characteristics of the stationary domain and the rotating domain, the present invention adopts different mesh generation methods, which can accurately fit, and can take into account the computational efficiency while ensuring the computational accuracy. The present invention gives a general division scheme for the stationary domain and the rotating domain using relative values. Compared with the commonly used absolute value division scheme in the prior art, the present invention has strong generality and is no longer limited to a certain model only. Moreover, the computational accuracy can be further improved by zoning and densification in the present invention. In addition, the settings of the time step and the computational duration in the present invention are also helpful for improving the computational accuracy. In summary, the present invention proposes a low-frequency excitation force calculation scheme with high accuracy and strong generality, which has important guiding significance for the research and control of thruster noise. Brief Description of the Drawings

[0034] Figure 1 It is a schematic diagram of the fluid computational domain established in a method for calculating the low-frequency excitation force of an underwater vehicle's thruster provided in Embodiment 1 of the present invention;

[0035] Figure 2 They are the simulation results and experimental results obtained under two different working conditions. Detailed Embodiments

[0036] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings of the specification and specific embodiments.

[0037] Embodiment 1:

[0038] Embodiment 1 provides a method for calculating the low-frequency excitation force of an underwater vehicle's thruster, including the following steps:

[0039] Step 1: Construct a system model of the underwater vehicle, establish a fluid computational domain, divide the fluid computational domain, and carry out mesh generation.

[0040] Among them, the system model includes the three-dimensional models of the hull and the thruster. In addition, according to application requirements, the system model may also include the three-dimensional models of other appendages of the underwater vehicle.

[0041] The fluid computational domain is divided into a stationary domain and a rotating domain. The rotating domain is a cylinder containing the rotating components of the thruster, and the rotating components are propellers, rotors, etc. The stationary domain is the computational domain outside the rotating domain, that is, the stationary domain is the remaining flow domain outside the rotating domain.

[0042] The rotating domain is meshed using polyhedral meshes. Since the rotating components of the thruster located within the rotating domain have three-dimensional small curvature characteristics, the present invention preferably uses polyhedral meshes for discretization within the rotating domain, which can accurately fit the geometry of the thruster. The stationary domain is meshed using cut-cell meshes, which can take into account the computational efficiency while ensuring the computational accuracy.

[0043] Step 2: Simulate and run the system model to obtain the time-domain data of the axial pulsating thrust of the rotating components of the thruster; convert the time-domain data into frequency-domain data to obtain the low-frequency excitation force information of the thruster.

[0044] The following specifically describes Example 1.

[0045] The fluid computational domain is a cuboid. Taking the head of the underwater vehicle as the origin, the distance from the upstream boundary of the fluid computational domain to the origin is greater than or equal to the length L of the underwater vehicle, and the distance from the downstream boundary of the fluid computational domain to the origin is greater than or equal to 4L.

[0046] For example, referring to Figure 1 , set the distance from the upstream boundary of the fluid computational domain to the origin to be L, and the distance from the downstream boundary of the fluid computational domain to the origin to be 4L. That is, the distance from the downstream boundary of the fluid computational domain to the end of the underwater vehicle is 3L, and the overall length of the fluid computational domain is 5L. The width and height of the fluid computational domain can be set to 0.5L.

[0047] Let the diameter of the rotating components of the thruster be R and the axial dimension be H. Then the radius of the rotating domain is set to 1.2R to 1.5R, and the axial dimension of the rotating domain is set to 1.2H to 1.5H.

[0048] The basic size Hm of the mesh of the stationary domain satisfies: Hm ≤ 0.01L.

[0049] The mesh of the stationary domain is locally refined, including the hull refinement area, the tail refinement area, and the wake refinement area.

[0050] Among them, the encrypted area of the hull is a cuboid that includes the entire hull. The distance from the upstream boundary of the encrypted area of the hull to the origin is 0.4L to 0.6L, the distance from the downstream boundary of the encrypted area of the hull to the origin is 1.4L to 1.6L, and the distances from the remaining boundaries to the origin are 0.2L to 0.3L. That is, the upstream boundary of the encrypted area of the hull extends 0.4L to 0.6L beyond the head of the underwater vehicle, and the downstream boundary of the encrypted area of the hull extends 0.4L to 0.6L beyond the tail of the underwater vehicle. The grid size Hm1 within the encrypted area of the hull satisfies: Hm1 ≤ 0.5Hm.

[0051] Among them, the encrypted area of the tail is cylindrical. The encrypted area of the tail starts from the hull length at a distance of 0.7L to 0.8L from the origin and ends at the hull length at a distance of 1.05L to 1.15L from the origin. For example, in the positive direction along the hull length, the encrypted area of the tail covers the hull length from 0.75L to 1.1L from the origin. The radius of the encrypted area of the tail is 1.02 times to 1.08 times the radius of the hull. The grid size Hm2 within the encrypted area of the tail satisfies: Hm2 ≤ 0.5Hm1.

[0052] Among them, the encrypted area of the wake is cylindrical. The encrypted area of the wake starts from the hull length at a distance of 0.85L to 0.95L from the origin and ends at the hull length at a distance of 1.05L to 1.15L from the origin. For example, in the positive direction along the hull length, the encrypted area of the wake covers the hull length from 0.9L to 1.1L from the origin. The radius of the encrypted area of the wake is 1.3 times to 1.7 times the radius of the hull. The grid size Hm3 within the encrypted area of the wake satisfies: Hm3 ≤ 0.5Hm2.

[0053] The basic size of the grid in the rotating domain is the same as the grid size Hm3 in the encrypted area of the wake.

[0054] The surface of the rotating component of the thruster is encrypted with surface grids, and the encrypted grid size Hm4 satisfies: Hm4 ≤ 0.5Hm3.

[0055] In addition, when performing grid division, it may also include: adding boundary layer grids to the surfaces of the hull and the thruster, and dividing the boundary layer grids. The boundary layer grids are prismatic grids, and the number of layers ≥ 15; the first boundary layer is the layer in contact with the wall surface, and the last boundary layer is the layer in contact with the environmental fluid; the grid size of the first boundary layer satisfies the dimensionless wall distance Y+ < 1, and the grid size of the last boundary layer is the same as the grid size of the area where it is located.

[0056] Among them, Y+ is calculated using a generally recognized calculation formula in the art. Since there is a flow velocity term in the formula, the grid size of the first-layer boundary layer needs to be determined according to the calculation conditions. At the same time, due to the different local flow velocities in different regions, the local Y+ is also different. Therefore, the grid sizes of the first-layer boundary layers in different regions are different, but all should satisfy Y+ < 1.

[0057] When performing the simulation operation of the system model, the upstream boundary of the fluid calculation domain is used as the velocity inlet, the downstream boundary of the fluid calculation domain is used as the pressure outlet, and the remaining external boundaries are symmetric boundaries. The surfaces of the hull and the propeller located within the fluid calculation domain are set as non-slip wall surfaces. Based on the principles of computational fluid dynamics, simulation parameters are set, and the simulation parameters include the density, kinematic viscosity, and flow velocity of the fluid, etc.

[0058] Under the given simulation parameters, a steady calculation is carried out using a turbulence model (such as the SST-kw turbulence model). The calculation duration T0 of the flow field initialization satisfies: T0 ≥ 4L / v, where v is the flow velocity under the set conditions.

[0059] After completing the flow field initialization, an unsteady calculation is carried out using a detached eddy simulation turbulence model. The time step t of the unsteady calculation satisfies: t ≤ w / 21600, where w is the rotational speed of the propeller. Since w is the rotational speed with the unit of revolutions per minute, it is first divided by 60 to convert to revolutions per second, and then one revolution is divided into 360°, and dividing by 360° gives the number of seconds for turning 1°. To accurately capture the blade frequency characteristics, the frequency resolution of the unsteady calculation is higher than 0.25 Hz, and the calculation duration is greater than 4 s.

[0060] The settings of the time step and the calculation duration in the present invention are obtained based on the following considerations: First, the formation of the transient wake flow field needs to experience a certain calculation duration to gradually converge and stabilize. Usually, it is required that the fluid within the calculation domain experiences at least one process from the inlet flow to the outlet. Therefore, the present invention limits the initialization duration T0. After experiencing the calculation time of T0, it can be considered that the wake flow field reaches a stable state. This is a prerequisite for meeting the calculation accuracy of the exciting force. Second, the bandwidth of the low-frequency exciting force of the propeller is very narrow, so a high frequency resolution is required to restore it. It can be imagined that when the frequency resolution is too large, such as 10 Hz, the amplitude of each frequency point on the spectrum is the superposition of all signal energies within the 10 Hz bandwidth, which includes both the real exciting force energy and other non-exciting force signal energies. This will lead to an overprediction of the amplitude and cannot truly reflect the actual magnitude of the exciting force. Based on research and experiments, the present invention finds that when the frequency resolution is higher than 0.25 Hz (i.e., the bandwidth is less than 0.25 Hz), the consistency between the simulation results and the test results is better. And the frequency resolution is related to the calculation duration, satisfying the relationship of △f = 1 / T. Therefore, after completing the initialization calculation of T0, at least 4 s or more of calculation is required.

[0061] In Embodiment 1, based on simulation runs, the extraction of the time-domain excitation force and the frequency-domain excitation force is carried out. Among them, the extraction of the time-domain excitation force includes: while performing the time-domain calculation of the flow field, monitoring the axial component of the surface force integral value of the rotating components of the thruster to obtain the axial time-domain excitation force of the thruster. The extraction of the frequency-domain excitation force includes: converting the axial time-domain excitation force of the thruster into frequency-domain data through fast Fourier transform, where the peak value corresponding to the blade frequency is the required low-frequency excitation force of the thruster, and its amplitude and phase angle can be obtained. That is, the low-frequency excitation force information of the thruster finally obtained in Embodiment 1 includes the pulsating pressure amplitude and the pulsating pressure phase angle.

[0062] To verify the effect of the present invention, the simulation results (i.e., the calculation results) and the experimental results of the blade frequency peak values of the thruster excitation force obtained under two different working conditions by using a calculation method for the low-frequency excitation force of an underwater vehicle thruster provided in Embodiment 1 are given below. See Figure 2 , the incoming flow velocity in Working Condition 1 is set to 1.6 m / s, and the rotational speed of the rotating components is set to 8 rps; the incoming flow velocity in Working Condition 2 is set to 2.1 m / s, and the rotational speed of the rotating components is set to 10.2 rps. It can be seen that the errors under both working conditions are within 7%, indicating that the calculation method provided by the present invention has high calculation accuracy.

[0063] Embodiment 2:

[0064] Embodiment 2 provides a calculation system for the low-frequency excitation force of an underwater vehicle thruster, including:

[0065] A construction unit for constructing a system model of the underwater vehicle, establishing a fluid calculation domain, dividing the fluid calculation domain, and performing mesh division;

[0066] A simulation and data processing unit for performing simulation runs of the system model to obtain time-domain data of the axial pulsating thrust of the rotating components of the thruster, and for converting the time-domain data into frequency-domain data to obtain low-frequency excitation force information of the thruster.

[0067] The calculation system for the low-frequency excitation force of the underwater vehicle thruster provided in Embodiment 2 is used to execute the steps in the calculation method for the low-frequency excitation force of the underwater vehicle thruster as described in Embodiment 1.

[0068] Since the functions of the units in the calculation system provided in Embodiment 2 correspond to the steps in the calculation method provided in Embodiment 1, understanding can be made by referring to the description of Embodiment 1, and details will not be repeated here.

[0069] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for calculating the low-frequency excitation force of an underwater vehicle thruster, characterized in that: The following steps are involved: Constructing a system model of the underwater vehicle, the system model including a three-dimensional model of the hull and the propeller; establishing a fluid calculation domain, dividing the fluid calculation domain, and performing grid division; The fluid calculation domain is divided into a stationary domain and a rotating domain, the rotating domain is a cylinder containing the rotating parts of the propeller, and the stationary domain is a calculation domain outside the rotating domain; the rotating domain is meshed using a polyhedral mesh, and the stationary domain is meshed using a cutting body mesh; The system model is simulated and run to obtain time domain data of the axial pulsating thrust of the rotating parts of the propeller; the time domain data is converted into frequency domain data to obtain low-frequency excitation force information of the propeller.

2. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 1, characterized in that: The fluid calculation domain is a cuboid, with the bow of the underwater vehicle as the origin, the distance between the upstream boundary of the fluid calculation domain and the origin is greater than or equal to the length L of the underwater vehicle, and the distance between the downstream boundary of the fluid calculation domain and the origin is greater than or equal to 4L; The diameter of the rotating part of the propeller is R, and the axial dimension is H; the radius of the rotating domain is set to 1.2R to 1.5R, and the axial dimension of the rotating domain is set to 1.2H to 1.5H.

3. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 2, characterized in that: The basic size Hm of the grid of the stationary domain satisfies: Hm≤0.01L; Locally encrypting the grid of the static domain, including a hull encryption area, a tail encryption area, and a wake encryption area; The grid size Hm1 in the hull densification area satisfies: Hm1≤0.5Hm; The grid size Hm2 in the tail encrypted area satisfies: Hm2≤0.5Hm1; The grid size Hm3 in the wake density area satisfies: Hm3≤0.5Hm2; The basic size of the grid in the rotation domain is the same as the grid size Hm3 in the wake density area; The surface mesh of the rotating part of the propeller is encrypted, and the encrypted mesh size Hm4 satisfies: Hm4≤0.5Hm3.

4. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 3, characterized in that: The hull densification area is a cuboid containing the entire hull, the upstream boundary of the hull densification area is 0.4L to 0.6L away from the origin, the downstream boundary of the hull densification area is 1.4L to 1.6L away from the origin, and the distances of the remaining boundaries from the origin are 0.2L to 0.3L; The tail densification area is cylindrical, starting from a hull length of 0.7L to 0.8L from the origin and ending at a hull length of 1.05L to 1.15L from the origin, and the radius of the tail densification area is 1.02 to 1.08 times the radius of the hull; The wake densification zone is cylindrical, starting from a hull length of 0.85L to 0.95L from the origin and ending at a hull length of 1.05L to 1.15L from the origin, and the radius of the wake densification zone is 1.3 to 1.7 times the radius of the hull.

5. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 1, characterized in that: When performing mesh division, the method further includes: adding boundary layer meshes on the surfaces of the hull and the propeller, and dividing the boundary layer meshes.

6. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 5, characterized in that: The boundary layer grid adopts a prismatic grid with a layer number of ≥15; the first boundary layer is the layer connected to the wall, and the last boundary layer is the layer connected to the ambient fluid; the grid size of the first boundary layer satisfies the dimensionless wall distance Y+<1, and the grid size of the last boundary layer is the same as the grid size of the area where it is located.

7. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 2, characterized in that: When the system model is simulated, the upstream boundary of the fluid calculation domain is used as the velocity inlet, the downstream boundary of the fluid calculation domain is used as the pressure outlet, the surfaces of the hull and the propeller located in the fluid calculation domain are set as no-slip walls, and simulation parameters are set, which include density, kinematic viscosity and flow rate of the fluid.

8. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 7, characterized in that: Under the given simulation parameters, the turbulence model is used to carry out steady calculations, and the calculation time T0 of the flow field initialization satisfies: T0 ≥ 4L / v, where v is the flow velocity under the set working conditions; After completing the flow field initialization, the detached vortex turbulence model is used to carry out unsteady calculations. The time step t of the unsteady calculations satisfies: t≤w / 21600, w is the rotation speed of the propeller; the frequency resolution of the unsteady calculations is higher than 0.25 Hz, and the calculation time is greater than 4 s.

9. The method for calculating the low-frequency excitation force of an underwater vehicle thruster according to claim 1, characterized in that: The thruster low-frequency excitation force information includes a pulsating pressure amplitude and a pulsating pressure phase angle.

10. A calculation system for low-frequency excitation force of underwater vehicle propeller, characterized in that: include: A construction unit is used to construct a system model of the underwater vehicle, establish a fluid calculation domain, divide the fluid calculation domain, and perform grid division; A simulation and data processing unit, used for performing simulation operation of the system model to obtain time domain data of the axial pulsating thrust of the rotating parts of the propeller, and for converting the time domain data into frequency domain data to obtain low-frequency excitation force information of the propeller; The system for calculating the low-frequency excitation force of an underwater vehicle thruster is used to execute the steps in the method for calculating the low-frequency excitation force of an underwater vehicle thruster according to any one of claims 1 to 9.

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