Propeller surface cavitation area evaluation method and system under oblique flow working condition

Abstract

The invention belongs to the technical field of propeller surface cavitation area evaluation, and particularly relates to a propeller surface cavitation area evaluation method and system under an oblique flow working condition. According to the method, on the basis of key parameters of a propeller, numerical simulation is carried out on the cavitation performance of the propeller surface under the oblique flow working condition by adopting a grid division and encryption method and a numerical simulation method, and the area of the surface cavitation is evaluated by adopting a specific calculation method. The system comprises a key parameter acquisition module, a mesh generation module, a boundary processing module, a model establishment module and a propeller surface cavitation area evaluation module which are connected in sequence. The method can quickly and accurately forecast the cavitation area of the propeller surface under the oblique flow condition.

Description

Technical Field

[0001] The present invention belongs to the technical field of propeller surface cavitation area evaluation, and in particular relates to a propeller surface cavitation area evaluation method and system under oblique flow conditions. Background Art

[0002] In recent years, as ships have become larger and faster, people have been increasingly demanding ship speeds and main engine performance, but the propeller size cannot be increased at will due to various factors, resulting in a significant increase in propeller load. In this case, propeller cavitation is often inevitable. When propeller cavitation occurs, many adverse effects will occur, such as a decrease in the propeller's hydrodynamic performance, erosion damage to the blade surface, and often causing strong vibration of the stern hull.

[0003] Common propeller cavitation includes surface cavitation, tip vortex cavitation and hub vortex cavitation. Due to the complexity of propeller cavitation problems, the research on propeller cavitation performance still mainly relies on model test methods. It is necessary to process propeller blade models and conduct model tests in cavitation water tunnel laboratories. The test results are obtained by photographing the cavitation morphology of the propeller blades with a high-speed camera, and then measuring the cavitation area on the propeller surface by hand. The model test method not only has a long test cycle, but also has a very high test cost, and cannot obtain flow details.

[0004] In recent years, with the rapid development of computer performance and CFD technology, numerical simulation methods are becoming an important means to study propeller cavitation problems. Compared with model tests, numerical simulation methods have many advantages, such as short cycle, low cost, and the ability to obtain richer flow field details. Summary of the invention

[0005] The present invention solves the problems of long cycle, high cost and inability to obtain flow details in the current propeller surface cavitation area evaluation under oblique flow conditions, and provides a propeller surface cavitation area evaluation method and system under oblique flow conditions, which adopts a specific calculation method to evaluate the propeller surface cavitation area, and can quickly and accurately predict the propeller surface cavitation area under oblique flow conditions.

[0006] The technical solution claimed in the present invention is as follows:

[0007] A method for evaluating the cavitation area of ​​a propeller surface under oblique flow conditions comprises the following steps:

[0008] S1: Obtain the key parameters of the propeller under oblique flow conditions, and establish a complete numerical model of the propeller based on the key parameters. Assemble the established numerical model into a closed geometric body, and then divide the geometric body into two regions, namely the rotating domain where the propeller is located and the external static domain;

[0009] S2: Mesh the two regions of the geometric body obtained in S1 respectively, and divide them in order from large to small grids, and encrypt the divided grids to generate multiple sets of grids, and verify the grid independence of each set of generated grids;

[0010] S3: setting boundary conditions for the geometry obtained by S2, specifically: the inlet of the computational domain is a velocity inlet, the outlet is a pressure outlet, and the data transmission between the internal rotating domain and the external large domain adopts an interface;

[0011] S4: The RANS method is used in combination with the turbulence model method to numerically simulate the cavitation area of ​​the geometric body obtained in S3 under the oblique flow condition, and finally the SST K-Omega model is established;

[0012] S5: The SST K-Omega model is used for calculation. After the calculation of the propeller cavitation performance under the oblique flow condition converges, the cavitation information of the propeller blade surfaces at different time phases is extracted, and the propeller blade cavitation area is calculated, and the proportion of the propeller surface cavitation area under the oblique flow condition is calculated.

[0013] Preferably, the key parameters in S1 include: diameter, chord length, disk ratio, and pitch ratio.

[0014] Preferably, S2 is specifically: in order of grids from large to small, the propeller blade surface, the hub surface, the key area surface in the stationary domain, and the outer boundaries of the rotating domain and the external stationary domain are divided into surface grids with the same grid scale size, and the surface grids with the same scale size are encrypted to generate multiple sets of grids, and then the grid independence of each set of generated grids is verified, and each set of grids whose number of grids is within a preset number threshold range is retained; the key area in the stationary domain includes the inclined propeller shaft and the cylinder wall of the cavitation water tunnel working section.

[0015] In the above method, the SST K-Omega model described in S4 includes two equations, namely the continuity equation and the momentum equation, which are as follows:

[0016]

[0017] ρ=ρ l α l +ρ v (1-α l )=ρ l α l +ρ v α v

[0018] μ=μ l α l +μ v αv

[0019] Where: i, j = 1, 2, 3; subscripts l and v refer to the liquid phase and gas phase respectively; ρ is the fluid density of the mixed flow; ρ l ,ρ v represent the liquid density and gas density respectively; μ is the dynamic viscosity coefficient of the mixed flow; μ l ,μ v Respectively represent the dynamic viscosity coefficient of liquid and gas; i ,u j is the velocity component; x i ,x j is the spatial coordinate point; τ ij is the Reynolds stress, f i is the volume force per unit mass; α l ,α v Represent the volume fractions of liquid and others respectively; t represents the time term.

[0020] The calculation formula of the propeller blade cavitation area in S5 is:

[0021]

[0022] The formula for the proportion of cavitation area on the propeller surface in S5 is:

[0023] A C / A E

[0024] Where: S i is the area of ​​the grid unit where cavitation occurs; A C is the cavitation coverage area of ​​a single propeller blade; A E is the back area of ​​a single propeller blade; n is the number of surface units where cavitation occurs.

[0025] The present invention also provides a propeller surface cavitation area assessment system under oblique flow conditions, comprising a key parameter acquisition module, a grid division module, a boundary processing module, a model establishment module and a propeller surface cavitation area assessment module connected in sequence;

[0026] The key parameter acquisition module acquires the key parameters of the propeller under the oblique flow condition, and establishes a complete numerical model of the propeller according to the key parameters, assembles the established numerical model into a closed geometric body, and then divides the geometric body into two regions, namely, a rotating domain where the propeller is located and an external static domain;

[0027] The mesh division module performs mesh division on the two regions of the geometric body obtained by the key parameter acquisition module, and divides the meshes in order from large to small, and encrypts the divided meshes to generate multiple sets of meshes, and verifies the mesh independence of each set of generated meshes;

[0028] The boundary processing module sets boundary conditions for the geometric body processed by the meshing module, specifically: the calculation domain inlet is the velocity inlet, the outlet is the pressure outlet, and the data transmission between the internal rotating domain and the external large domain adopts the interface;

[0029] The model building module adopts the RANS method and combines the turbulence model method to numerically simulate the cavitation area of ​​the geometric body obtained by S3 under the oblique flow condition, and finally establishes the SST K-Omega model;

[0030] The propeller surface cavitation area evaluation module uses the SST K-Omega model obtained by the model building module for calculation. After the propeller cavitation performance calculation under the oblique flow condition converges, the surface cavitation information of each propeller blade surface at different time phases is extracted, and the propeller blade cavitation area is calculated, and the propeller surface cavitation area ratio under the oblique flow condition is calculated.

[0031] Preferably, in the above system, the key parameters include: diameter, chord length, disk ratio, and pitch ratio.

[0032] Preferably, the specific process of meshing by the meshing module is: in order from large to small, the propeller blade surface, the hub surface, the key area surface in the stationary domain, and the outer boundary of the rotating domain and the external stationary domain are divided into surface meshes with the same mesh scale, and the surface meshes with the same scale are encrypted to generate multiple sets of meshes, and then the generated sets of meshes are verified for mesh independence, and the sets of meshes whose number of meshes is within a preset number threshold are retained; the key areas in the stationary domain include the inclined propeller shaft and the wall of the working section of the cavitation water tunnel.

[0033] In the above system, the SST K-Omega model includes two equations, namely the continuity equation and the momentum equation, which are as follows:

[0034]

[0035] ρ=ρ l α l +ρ v (1-α l )=ρ l α l +ρ v α v

[0036] μ=μ l α l +μ v α v

[0037] Where: i, j = 1, 2, 3; subscripts l and v refer to the liquid phase and gas phase respectively; ρ is the fluid density of the mixed flow; ρ l ,ρ v represent the liquid density and gas density respectively; μ is the dynamic viscosity coefficient of the mixed flow; μ l ,μ v Respectively represent the dynamic viscosity coefficient of liquid and gas; i ,u j is the velocity component; x i ,x j is the spatial coordinate point; τ ij is the Reynolds stress, f i is the volume force per unit mass; α l ,α v Represent the volume fractions of liquid and others respectively; t represents the time term.

[0038] In the above system, the calculation formula of the propeller blade cavitation area is:

[0039]

[0040] The propeller surface cavitation area ratio formula is:

[0041] A C / A E

[0042] Where: S i is the area of ​​the grid unit where cavitation occurs; A C is the cavitation coverage area of ​​a single propeller blade; A E is the back area of ​​a single propeller blade; n is the number of surface units where cavitation occurs.

[0043] Beneficial effects:

[0044] The present invention provides a method for evaluating the cavitation area of ​​a propeller surface under an oblique flow condition, which obtains key parameters of the propeller, and establishes a complete numerical model of the propeller according to the key parameters, assembles the established numerical model into a closed geometric body, and then divides the geometric body into two regions, namely a rotating domain and an external static domain where the propeller is located; the complete geometric body under the oblique flow condition is almost the same as the model test conditions of the working section in the cavitation water tunnel, and the external boundary and size of the calculation domain are consistent with the actual working conditions of the working section of the cavitation water tunnel, which can accurately simulate the propeller under the oblique flow condition and improve the accuracy of model prediction; the two regions of the obtained geometric body are grid-divided respectively, and the grids are divided in order from large to small, and the grids after division are encrypted to generate multiple sets of grids, and the grid independence of each set of generated grids is verified to ensure the reliability and stability of subsequent calculations; the RANS method is adopted, and the turbulence model method is combined to perform numerical simulation on the cavitation area of ​​the geometric body obtained by S3 under the oblique flow condition to obtain SST K-Omega model, in order to more quickly and accurately predict the propeller cavitation performance under oblique flow conditions, and then accurately evaluate the propeller blade cavitation area, the present invention has conducted a comprehensive exploration and research on the subsequent calculation strategy. After comprehensively considering the computing resources and calculation cycle, and comparing the large eddy simulation and the RANS method, it is found that for the numerical simulation of the complex system of cavitation water tunnel, although the large eddy simulation is more refined, the calculation cost is much higher than the RANS method, and the calculation result is not necessarily more accurate than the RANS method. Therefore, the RANS method is combined with the turbulence model method to numerically simulate the propeller blade cavitation area under oblique flow conditions. Finally, the turbulence model selects the two-equation SST K-Omega model to solve the current problem of long cycle and high cost in the propeller surface cavitation area evaluation under oblique flow conditions. In addition, the method provided by the present invention can obtain more flow details, such as the velocity distribution near the propeller and the pressure distribution on the propeller blade surface, solving the problem that the flow details cannot be obtained in the propeller surface cavitation area evaluation under oblique flow conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a flow chart of a method for evaluating the cavitation area on a propeller surface under oblique flow conditions according to an embodiment of the present invention.

[0046] Figure 2 Schematic diagram of a propeller surface cavitation area evaluation system under oblique flow conditions according to an embodiment of the present invention. DETAILED DESCRIPTION

[0047] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0048] First Group of Examples: Method for Estimating Cavitation Area on Propeller Surface under Oblique Flow Conditions

[0049] This group of embodiments provides a method for evaluating the cavitation area on the propeller surface under oblique flow conditions, such as Figure 1 As shown, the following steps are included:

[0050] S1: Obtain the key parameters of the propeller under oblique flow conditions, and establish a complete numerical model of the propeller based on the key parameters, assemble the established numerical model into a closed geometric body, and then divide the geometric body into two regions, namely the rotating domain where the propeller is located and the external static domain; the key parameters include: diameter, chord length, disk ratio, and pitch ratio.

[0051] In a specific embodiment of the present invention, in order to simulate the cavitation performance of the propeller under oblique flow conditions and evaluate the cavitation area on the propeller surface, it is necessary to construct a calculation domain. According to conventional processing methods, the calculation domain is divided into a rotating domain where the propeller is located and an external stationary large domain.

[0052] The complete propeller geometry under oblique flow conditions is almost the same as the model test conditions of the working section in the cavitation water tunnel, and the external boundary and size of the calculation domain are consistent with the actual working conditions of the working section of the cavitation water tunnel.

[0053] S2: Mesh the two regions (rotating domain and external stationary domain) of the geometric body obtained in S1 respectively, and divide them in order from large to small grids, encrypt the divided grids, generate multiple sets of grids, and verify the grid independence of each set of generated grids; specifically: divide the propeller blade surface, hub surface, key area surface in the stationary domain, and external boundaries of the rotating domain and the external stationary domain into surface grids of the same grid scale in the order from large to small grids, and encrypt the surface grids of the same scale to generate multiple sets of grids, and then verify the grid independence of each set of generated grids, and retain each set of grids whose number of grids is within the preset number threshold range; the key areas in the stationary domain include the inclined propeller shaft and the wall of the working section of the cavitation water tunnel.

[0054] In a specific embodiment of the present invention, in the rotating domain, since the propeller is in an oblique flow condition, there is a certain angle between the propeller flow direction and the propeller disk surface. Therefore, the grid division of this area needs to pay attention to the grid direction in the rotating domain, so as to more accurately capture the flow characteristics near the propeller. In order to more accurately simulate the flow field characteristics near the propeller, the surface of the propeller blade is divided into boundary layer grids, and the surface grids are encrypted to generate multiple sets of grids; in order to ensure the reliability and stability of the numerical calculation method, a grid independence verification study is carried out, and the number of grids of each set of grids is compared with the preset number threshold range. If the number of grids of a set of grids is within the preset number threshold range, then The set of grids is retained; for example, a grid independence verification study is conducted on 5 sets of grids. The number of grids in Case 1-Case 5 varies from 2 million to 12 million. The 5 sets of grids use the same layout, and only the grid scale changes. Through analysis and comparison, it can be seen that if the number of grids in a set of grids is around 6.5 million, the propeller thrust and torque show convergence characteristics, then the set of grids is retained as the numerical calculation strategy for the cavitation area of ​​the propeller oblique flow condition surface; if it is within 4.5 million, it will be discarded because the calculation result is greatly affected by the number of grids; if the number of grids is greater than 7.8 million, the calculation task cannot be completed due to limited calculation time and resources. Therefore, after comprehensive consideration, a set of numerical calculation methods with a moderate number of grids, high numerical calculation accuracy and stability is finally selected, that is, about 6.5 million grids (case 3). The number of grids for grid independence verification is shown in Table 1.

[0055] Table 1. Grid quantities for grid independence verification

[0056] Calculation example Grid quantity (10,000) Case 1 200 Case 2 450 Case 3 650 Case 4 780 Case 5 1200

[0057] S3: setting boundary conditions for the geometry obtained by S2, specifically: the inlet of the computational domain is a velocity inlet, the outlet is a pressure outlet, and the data transmission between the internal rotating domain and the external large domain adopts an interface;

[0058] S4: The RANS method is used in combination with the turbulence model method to numerically simulate the geometric cavitation area obtained in S3 under oblique flow conditions. Since the SSTk-ω turbulence model combines the advantages of the k-ω model in near-wall calculations and the k-ε model in far-field calculations, and considers the lateral dissipation derivative term, the transport process of turbulent shear stress is considered in the definition of the model's turbulent viscosity, and has a wider range of applicability, the SST K-Omega model is finally established;

[0059] In a specific embodiment of the present invention, in order to more quickly and accurately predict the propeller cavitation performance under oblique flow conditions, and then accurately evaluate the propeller blade cavitation area, the subsequent calculation strategy is comprehensively explored and studied. After comprehensively considering the computing resources and computing cycle, and comparing the large eddy simulation and the RANS method, for the numerical simulation of the complex system of the cavitation water tunnel, although the large eddy simulation is more refined, the calculation cost is much higher than the RANS method, and the calculation result is not necessarily more accurate than the RANS method. Therefore, the RANS method is combined with the turbulence model method to perform numerical simulation of the propeller blade cavitation area under oblique flow conditions, and finally the SST K-Omega model of the turbulence model selection two equations is established; the two equations of the SST K-Omega model are the continuity equation and the momentum equation, which are as follows:

[0060]

[0061] ρ=ρ l α l +ρ v (1-α l )=ρ l α l +ρ v α v

[0062] μ=μ l α l +μ v α v

[0063] Where: i, j = 1, 2, 3; subscripts l and v refer to the liquid phase and gas phase respectively; ρ is the fluid density of the mixed flow; ρ l ,ρ v represent the liquid density and gas density respectively; μ is the dynamic viscosity coefficient of the mixed flow; μ l ,μ v Respectively represent the dynamic viscosity coefficient of liquid and gas; i ,u j is the velocity component; x i ,x j is the spatial coordinate point; τ ij is the Reynolds stress, f i is the volume force per unit mass; α l ,α v Represent the volume fractions of liquid and others respectively; t represents the time term.

[0064] S5: The SST K-Omega model is used for calculation. After the calculation of propeller cavitation performance under oblique flow conditions converges, the cavitation information of each propeller blade surface at different time phases is extracted, and the propeller blade cavitation area is calculated, and the proportion of propeller surface cavitation area under oblique flow conditions is calculated;

[0065] In a specific embodiment of the present invention, the calculation formula of the propeller blade cavitation area is:

[0066]

[0067] The propeller surface cavitation area ratio formula is:

[0068] A C / A E

[0069] Where: S i is the area of ​​the grid unit where cavitation occurs; A C is the cavitation coverage area of ​​a single propeller blade; A E is the entire back area of ​​a single propeller blade; n represents the number of surface units where cavitation occurs.

[0070] Second group of embodiments: Propeller surface cavitation area evaluation system under oblique flow conditions

[0071] This group of embodiments provides a propeller surface cavitation area evaluation system under oblique flow conditions, such as Figure 2 As shown, it includes a key parameter acquisition module, a meshing module, a boundary processing module, a model building module and a propeller surface cavitation area evaluation module connected in sequence;

[0072] The key parameter acquisition module acquires the key parameters of the propeller under the oblique flow condition, and establishes a complete numerical model of the propeller according to the key parameters, assembles the established numerical model into a closed geometric body, and then divides the geometric body into two regions, namely, a rotating domain where the propeller is located and an external static domain;

[0073] The mesh division module meshes the two regions of the geometric body obtained by the key parameter acquisition module respectively, and divides them in sequence from large to small grids, and encrypts the divided grids to generate multiple sets of grids, and verifies the mesh independence of each generated set of grids; specifically: in order from large to small grids, the propeller blade surface, the hub surface, the key area surface in the stationary domain, and the outer boundary of the rotating domain and the external stationary domain are respectively divided into surface grids of the same grid scale size, and the surface grids of the same scale size are meshed and encrypted to generate multiple sets of grids, and then the grid independence of each generated set of grids is verified, and each set of grids whose number of grids is within a preset number threshold range is retained; the key areas in the stationary domain include the inclined propeller shaft and the cylinder wall of the working section of the cavitation water tunnel.

[0074] The boundary processing module sets boundary conditions for the geometric body processed by the meshing module, specifically: the calculation domain inlet is the velocity inlet, the outlet is the pressure outlet, and the data transmission between the internal rotating domain and the external large domain adopts the interface;

[0075] The model building module adopts the RANS method and combines the turbulence model method to numerically simulate the cavitation area of ​​the geometric body obtained by S3 under the oblique flow condition, and finally establishes the SST K-Omega model;

[0076] The propeller surface cavitation area evaluation module uses the SST K-Omega model obtained by the model building module for calculation. After the propeller cavitation performance calculation under the oblique flow condition converges, the surface cavitation information of each propeller blade surface at different time phases is extracted, and the propeller blade cavitation area is calculated, and the propeller surface cavitation area ratio under the oblique flow condition is calculated.

[0077] The key parameters include: diameter, chord length, disk ratio, and pitch ratio.

[0078] The SST K-Omega model includes two equations, namely the continuity equation and the momentum equation, which are as follows:

[0079]

[0080] ρ=ρ l α l +ρ v (1-α l )=ρ l α l +ρ v α v

[0081] μ=μ l α l +μ v αv

[0082] Where: i, j = 1, 2, 3; subscripts l and v refer to the liquid phase and gas phase respectively; ρ is the fluid density of the mixed flow; ρ l ,ρ v represent the liquid density and gas density respectively; μ is the dynamic viscosity coefficient of the mixed flow; μ l ,μ v Respectively represent the dynamic viscosity coefficient of liquid and gas; i ,u j is the velocity component; x i ,x j is the spatial coordinate point; τ ij is the Reynolds stress, f i is the volume force per unit mass; α l ,α v Represent the volume fractions of liquid and others respectively; t represents the time term.

[0083] The calculation formula of the propeller blade cavitation area is:

[0084]

[0085] The propeller surface cavitation area ratio formula is:

[0086] A C / A E

[0087] Where: S i is the area of ​​the grid unit where cavitation occurs; A C is the cavitation coverage area of ​​a single propeller blade; A E is the entire back area of ​​a single propeller blade; n represents the number of surface units where cavitation occurs.

[0088] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the present invention. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made in form and details without departing from the scope defined by the claims of the present invention.

Claims

1. A method for evaluating the cavitation area of ​​a propeller surface under oblique flow conditions, characterized in that: The steps include: S1: Obtain the key parameters of the propeller under oblique flow conditions, and establish a complete numerical model of the propeller based on the key parameters. Assemble the established numerical model into a closed geometric body, and then divide the geometric body into two regions, namely the rotating domain where the propeller is located and the external static domain; S2: Mesh the two regions of the geometric body obtained in S1 respectively, and divide them in order from large to small grids, and encrypt the divided grids to generate multiple sets of grids, and verify the grid independence of each set of generated grids; S3: setting boundary conditions for the geometry obtained by S2, specifically: the inlet of the computational domain is a velocity inlet, the outlet is a pressure outlet, and the data transmission between the internal rotating domain and the external large domain adopts an interface; S4: The RANS method is used in combination with the turbulence model method to numerically simulate the cavitation area of ​​the geometric body obtained in S3 under the oblique flow condition, and finally the SST K-Omega model is established; S5: The SST K-Omega model obtained in S4 is used for calculation. After the calculation of the propeller cavitation performance under the oblique flow condition converges, the cavitation information of the propeller blade surfaces at different time phases is extracted, and the propeller blade cavitation area is calculated, and the proportion of the propeller surface cavitation area under the oblique flow condition is calculated.

2. The method for evaluating the propeller surface cavitation area under oblique flow conditions according to claim 1, characterized in that: The key parameters described in S1 include: diameter, chord length, disk ratio, and pitch ratio.

3. The method for evaluating the propeller surface cavitation area under oblique flow conditions according to claim 1, characterized in that: S2 is specifically as follows: in descending order of the grids, the propeller blade surface, the hub surface, the key area surface in the stationary domain, and the outer boundaries of the rotating domain and the external stationary domain are divided into surface grids of the same grid scale, and the surface grids of the same scale are encrypted to generate multiple sets of grids, and then the grid independence of each set of generated grids is verified, and each set of grids whose number of grids is within the preset number threshold is retained; the key areas in the stationary domain include the inclined propeller shaft and the wall of the working section of the cavitation water tunnel.

4. The method for evaluating the propeller surface cavitation area under oblique flow conditions according to claim 1, characterized in that: The SST K-Omega model described in S4 includes two equations, namely the continuity equation and the momentum equation, as follows: p=p l a l +r v (1-a l )=ρ l a l +r v a v μ=μ l a l +m v a v Where: i, j = 1, 2, 3; subscripts l and v refer to the liquid phase and gas phase respectively; ρ is the fluid density of the mixed flow; ρ l ,ρ v represent the liquid density and gas density respectively; μ is the dynamic viscosity coefficient of the mixed flow; μ l ,μ v Respectively represent the dynamic viscosity coefficient of liquid and gas; i ,u j is the velocity component; x i ,x j is the spatial coordinate point; τ ij is the Reynolds stress, f i is the unit mass volume force; α l ,α v Represent the volume fractions of liquid and others respectively; t represents the time term.

5. The method for evaluating the propeller surface cavitation area under oblique flow conditions according to claim 1, characterized in that: The calculation formula of the propeller blade cavitation area in S5 is: The formula for the proportion of cavitation area on the propeller surface in S5 is: A C / A E Where: S i is the area of ​​the grid unit where cavitation occurs; A C is the cavitation coverage area of ​​a single propeller blade; A E is the back area of ​​a single propeller blade; n is the number of surface units where cavitation occurs.

6. A propeller surface cavitation area evaluation system under oblique flow conditions, characterized in that: It includes a key parameter acquisition module, a meshing module, a boundary processing module, a model building module and a propeller surface cavitation area evaluation module connected in sequence; The key parameter acquisition module acquires the key parameters of the propeller under the oblique flow condition, and establishes a complete numerical model of the propeller according to the key parameters, assembles the established numerical model into a closed geometric body, and then divides the geometric body into two regions, namely, a rotating domain where the propeller is located and an external static domain; The mesh division module performs mesh division on the two regions of the geometric body obtained by the key parameter acquisition module, and divides the meshes in order from large to small, and encrypts the divided meshes to generate multiple sets of meshes, and verifies the mesh independence of each set of generated meshes; The boundary processing module sets boundary conditions for the geometric body processed by the meshing module, specifically: the calculation domain inlet is the velocity inlet, the outlet is the pressure outlet, and the data transmission between the internal rotating domain and the external large domain adopts the interface; The model building module adopts the RANS method and combines the turbulence model method to numerically simulate the cavitation area of ​​the geometric body obtained by S3 under the oblique flow condition, and finally establishes the SST K-Omega model; The propeller surface cavitation area evaluation module uses the SST K-Omega model obtained by the model building module for calculation. After the propeller cavitation performance calculation under the oblique flow condition converges, the surface cavitation information of each propeller blade surface at different time phases is extracted, and the propeller blade cavitation area is calculated, and the propeller surface cavitation area ratio under the oblique flow condition is calculated.

7. The propeller surface cavitation area evaluation system under oblique flow conditions according to claim 6, characterized in that: The key parameters include: diameter, chord length, disk ratio, and pitch ratio.

8. The propeller surface cavitation area evaluation system under oblique flow conditions according to claim 6, characterized in that: The specific process of meshing by the meshing module is as follows: in descending order of the meshes, the propeller blade surface, the hub surface, the key area surface in the stationary domain, and the outer boundary of the rotating domain and the external stationary domain are respectively divided into surface meshes of the same mesh scale, and the surface meshes of the same mesh scale are respectively meshed to generate multiple sets of meshes, and then the mesh independence of each set of generated meshes is verified, and each set of meshes whose number of meshes is within a preset number threshold is retained; the key areas in the stationary domain include the inclined propeller shaft and the cylinder wall of the working section of the cavitation water tunnel.

9. The propeller surface cavitation area evaluation system under oblique flow conditions according to claim 6, characterized in that: The SST K-Omega model includes two equations, namely the continuity equation and the momentum equation, which are as follows: p=p l a l +r v (1-a l )=ρ l a l +r v a v μ=μ l a l +m v a v Where: i, j = 1, 2, 3; subscripts l and v refer to the liquid phase and gas phase respectively; ρ is the fluid density of the mixed flow; ρ l ,ρ v represent the liquid density and gas density respectively; μ is the dynamic viscosity coefficient of the mixed flow; μ l ,μ v Respectively represent the dynamic viscosity coefficient of liquid and gas; i ,u j is the velocity component; x i ,x j is the spatial coordinate point; τ ij is the Reynolds stress, f i is the unit mass volume force; α l ,α v Represent the volume fractions of liquid and others respectively; t represents the time term.

10. The propeller surface cavitation area evaluation system under oblique flow conditions according to claim 6, characterized in that: The calculation formula of the propeller blade cavitation area is: The propeller surface cavitation area ratio formula is: A C / A E Where: S i is the area of ​​the grid unit where cavitation occurs; A C is the cavitation coverage area of ​​a single propeller blade; A E is the back area of ​​a single propeller blade; n is the number of surface units where cavitation occurs.

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