Integrated drag reduction design method for underwater vehicle with flat porous medium surface

Abstract

The invention relates to a porous medium surface flat type underwater vehicle integrated resistance reduction design method, which comprises the following steps: S1, establishing a porous medium surface-flat type underwater vehicle resistance analysis and evaluation model: setting a surface porous medium area, dividing a grid by adopting a non-structural grid, performing grid independence verification, and establishing a resistance analysis and evaluation model of a porous medium surface-flat type underwater vehicle resistance analysis and evaluation model; the total resistance borne by the underwater navigation body under the condition of different grid numbers is calculated; s2, establishing resistance evaluation reference data; s3, determining an objective function and constraint conditions of the airfoil profile of the underwater vehicle to be optimized; and S4, according to the objective function, determining an iterative optimization process, determining related operation parameters of an iterative algorithm, performing selection and crossing repeatedly and iteratively through the iterative algorithm, improving the fitness of the iterative algorithm, gradually approaching an optimal solution, and finally outputting a global optimal solution when an iteration exit condition is met to obtain an optimal resistance reduction optimization variable parameter. According to the method, the traditional design process is optimized, double breakthrough is achieved in structure and performance, and a new solving path is developed for the drag reduction problem of the underwater navigation body.

Description

Technical Field

[0001] The present invention relates to the field of ship and ocean engineering, and more specifically, to an integrated drag reduction design method for a flat underwater vehicle with a porous medium surface. Background Art

[0002] In an underwater environment, since there is a lack of oxygen, fuels that rely on oxygen cannot be used as a power source. Currently, most underwater vehicles are driven by electricity. However, due to the small volume and limited number of power sources that can be carried, the speed and endurance of underwater vehicles are severely restricted. The structure of an underwater vehicle is as Figure 3 shown. The energy consumption of an underwater vehicle is related to its speed and the resistance it encounters during navigation. At the same battery level, to increase speed and endurance, reducing the resistance suffered by the underwater vehicle is a feasible solution. The frictional resistance suffered by an underwater vehicle during underwater navigation accounts for more than sixty percent of the total resistance. Therefore, finding a new method to reduce frictional resistance is very necessary for improving the navigation speed and distance of an underwater vehicle.

[0003] Optimizing the external shape design is one of the cores of traditional drag reduction research and development for underwater vehicles. Currently, the hull form design has matured, and the drag optimization design has reached its limit. Further improving the drag reduction performance cannot be achieved by changing the hull form design. Since the wetted surface area of a flat underwater vehicle is very large, it will encounter a large frictional resistance during navigation. Researchers innovatively applied new superhydrophobic surface technology and porous medium super-slippery surface treatment on the basis of a mature hull form design, aiming to change the flow field characteristics around the underwater vehicle. Using the superhydrophobic surface drag reduction technology can significantly affect the surface velocity change rate, and thus change the magnitude of the frictional resistance received by the surface. Various parameters such as the thickness and porosity of the porous medium material are important factors affecting its slip characteristics. Currently, the thickness of the airfoil of some underwater vehicles can reach 2 - 14 mm (Xu Shixun, Liu Yuhong, Zhu Yaqiang, et al. Analysis of the Influence of Airfoil on the Performance of Underwater Vehicles [J]. China Mechanical Engineering, 2017, 28(03): 286 - 293.). The order of magnitude of the porous medium coating is the same as that of the airfoil thickness. After adding the coating, the size will change, and the shape of the trailing edge of the airfoil will change significantly. For example, in Figure 5 the A area, the original airfoil is damaged, thus affecting the drag reduction effect, resulting in an unsatisfactory drag reduction effect, with a large difference from the theoretical preset, and a large difference between the actual drag reduction effect and the expectation. An overly thick coating will change the original mature low-drag hull form, which may generate additional flow resistance or change the local flow regime, resulting in the hydrodynamic behavior deviating from the prediction of the ideal model. Therefore, requirements need to be imposed on the design of using a porous medium surface to reduce the resistance of an underwater vehicle.

[0004] To improve the performance of underwater gliders, the research team used computational fluid dynamics methods to deeply analyze the influence of airfoils on gliding efficiency and stability. In this study, the researchers found that a specific airfoil could significantly improve gliding economy but had little impact on the static stability of the glider. At the same time, the bending amplitude and direction of the airfoil had a significant impact on the gliding economy and static stability of the asymmetric airfoil. However, this article did not study airfoils with porous media coatings. (Xu Shixun, Liu Yuhong, Zhu Yaqiang, et al. Analysis of the Influence of Airfoils on the Gliding Performance of Underwater Gliders [J]. China Mechanical Engineering, 2017, 28(03): 286-293.)

[0005] Researchers from the Shenyang Institute of Automation, Chinese Academy of Sciences proposed a method of applying porous media materials to the surface of the pressure hull to improve the navigation efficiency of autonomous underwater vehicles. In this study, the researchers analyzed the mechanical properties and drag reduction mechanism of porous media materials when used as the surface of the pressure hull. They used the Renus100 AUV and the Suboff body as experimental objects for testing. The results showed that the navigation resistance of the underwater vehicle was affected by the thickness and viscosity coefficient of the porous media materials. Under specific conditions, coating with porous media materials could reduce the navigation resistance of the underwater vehicle, thereby improving its navigation efficiency. Although this study emphasized the application of porous media materials, it did not conduct research on flat underwater vehicles. (Meng L, Yang L, Su T, et al. Study on the influence of porous material on underwater vehicle's hydrodynamic characteristics [J]. Ocean Engineering, 2019, 191106528-106528.)

[0006] At present, the profile design and coating application of underwater vehicles are separate. In the traditional design method, the profile is usually determined first, and then a porous medium coating is applied on this basis. However, when the airfoil thickness is between 2.0 and 14.0 mm, covering the surface with about 1.5 mm of porous medium coating (Long Hao. Numerical study on the heat transfer characteristics enhanced by composite porous media [D]. University of Chinese Academy of Sciences (Institute of Engineering Thermophysics, Chinese Academy of Sciences), 2020.) may significantly affect the original profile, and at the same time reduce the effect of the porous medium. For underwater vehicles with a flat design, simply modifying the original profile often destroys the drag profile during design. Although the porous medium surface can improve the flow field distribution, increase the surface slip velocity, reduce the normal velocity gradient and frictional drag, the separate design cannot fully utilize the advantages of both, thus weakening the drag reduction effect and even causing a significant increase in drag in some cases. Problems may arise if only the shape design is concerned without considering the material thickness and its influence on the shape change. To effectively overcome these limitations, porous medium materials with corresponding parameters need to be used according to different shapes to adapt to the flow field characteristics of specific regions. In addition, in the traditional design process, designers first determine the basic shape of the airfoil according to the principles of fluid mechanics, and then adjust the coating parameters according to experimental data or experience. This process requires first designing and optimizing the shape of the airfoil, and then designing the surface coating, and cannot consider the synergistic effect of both at the same time, resulting in a longer overall design cycle. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide an integrated drag reduction design method for a flat underwater vehicle with a porous medium surface, which can organically combine the geometric shape design and thickness design of the airfoil, systematically adjust the airfoil and coating parameters to maximize the drag reduction performance of the airfoil, break the traditional sequential relationship between the airfoil shape and coating design, and fully consider the interaction between the airfoil shape of the underwater vehicle and the slip coating.

[0008] The technical solution adopted by the present invention to solve its technical problems is to construct an integrated drag reduction design method for a flat underwater vehicle with a porous medium surface, including the following steps:

[0009] S1. Establish a resistance analysis and evaluation model for a porous medium surface - flat underwater vehicle: Set the surface porous medium area, use unstructured grids to divide the grids, conduct grid independence verification, and calculate the total resistance suffered by the underwater vehicle under different grid numbers;

[0010] S2. Establish a resistance evaluation reference data;

[0011] S3. Determine the objective function and constraint conditions that need to be optimized for the airfoil of the underwater vehicle;

[0012] S4. Determine the iterative optimization process according to the objective function, determine the relevant operating parameters of the iterative algorithm, repeatedly perform selection and crossover through the iterative algorithm to improve the fitness of the iterative algorithm, gradually approach the optimal solution, and finally output the global optimal solution when the iterative exit condition is met to obtain the optimal drag reduction optimization variable parameters.

[0013] According to the above solution, in the step S1, use CFD simulation to simulate the surface water resistance during the underwater vehicle's submerged navigation, and use ANSYS-Fluent software to establish the evaluation model.

[0014] According to the above solution, in the step S1, in the mesh generation stage, calculate the y+ value to determine the height of the first layer of the mesh, gradually refine the mesh around the underwater vehicle, and determine the symmetric boundary; where y+ is a dimensionless parameter related to the distance between the fluid and the solid boundary during the mesh generation process, that is, the wall distance; select the SST k-ω turbulence model and use the water resistance as an index of calculation accuracy, set the velocity inlet boundary, inlet velocity, pressure outlet boundary, and porous medium parameters on the airfoil surface, and the porous medium parameters on the airfoil surface include porosity, permeability, pore structure parameters, porosity, saturation, and permeability equation parameters.

[0015] According to the above solution, in the step S1, regard the porous medium surface as a porous jump boundary condition, and the drag coefficient during the stable movement of the underwater vehicle is determined by the following formula:

[0016]

[0017] where, ρ sw is the fluid density, determined by the fluid in the flow field; C D is the drag coefficient; A s is the cross-sectional area of the underwater vehicle; U is the navigation speed of the underwater vehicle;

[0018] Cross-sectional area of the underwater vehicle:

[0019] A s = N(t,Δt)

[0020] where t and Δt represent the internal height of the airfoil and the height of the porous medium respectively.

[0021] According to the above solution, in the step S2, the method for establishing the drag evaluation reference data includes the following steps:

[0022] S201. Through numerical simulation of the designed hydrofoil profile, numerically simulate the corresponding relationship between the profile and the drag reduction effect on the hydrofoil surface, establish a drag analysis and evaluation model for the porous medium surface - flat underwater vehicle, and obtain the best hydrofoil profile for movement in the flow field by continuously changing parameters;

[0023] S202. With the thickness of the porous medium coating as the design variable, a porous medium coating with a thickness of Δt is added to the wing surface. Set the airfoil height as T, the thickness of the outer porous medium as Δt, and the height inside the airfoil as t. At this time, T = t + 2Δt;

[0024] S203. The outer surface of the porous medium inherits the original external shape data of the vehicle body. A certain thickness is taken from the outside to the inside as the porous area, and the inner surface of the porous medium adheres tightly to the vehicle body shell: Set the airfoil height as t, keep the thickness Δt of the porous medium coating unchanged. At this time, the height inside the airfoil covered by the porous medium is t - 2Δt. Select the same boundary conditions and flow field parameters to analyze the pressure distribution, drag, and lift of the airfoil covered by the porous medium.

[0025] According to the above scheme, the step S202 further includes: Conducting numerical simulation on the airfoil covered with a porous medium coating of Δt, evaluating the influence of the increase in the thickness of the porous medium coating on relevant performance, and conducting a detailed analysis of the simulated data. The data includes the differences in pressure distribution, drag, and lift between the wing with the porous medium coating and the airfoil without the coating. By comparing the numerical simulation results under the same working conditions, analyze the changes in the performance indicators of the pressure distribution, drag, and lift on the airfoil surface as the thickness of the porous medium coating increases;

[0026] According to the above scheme, in the step S3,

[0027] The determined constraint conditions include:

[0028] [ε min ≤ [ε] ≤ [ε max ,

[0029] where ε is the porosity, ε min and ε max are the minimum and maximum values that the porosity of the porous medium can take under the working conditions;

[0030] Δt min ≤ Δt ≤ Δt max

[0031] where Δt is the thickness of the porous medium, Δt min and Δt max are the minimum and maximum values that the thickness of the porous medium can take under the working conditions;

[0032] t min ≤ t ≤ t max

[0033] where t is the height inside the airfoil, Δt min and Δt maxThe minimum and maximum values that the internal height of the airfoil can take under the working conditions.

[0034] According to the above solution, in the step S4, when the airfoil height T = t + 2Δt group has an excellent drag reduction effect, then the optimal porous medium thickness is selected between T = t + 0 and T = t + 2Δt.

[0035] According to the above solution, the method for selecting the optimal porous medium thickness between T = t + 0 and T = t + 2Δt includes:

[0036] S401. Taking the porous medium coating thickness as the design variable and the minimum drag on the airfoil as the optimization goal, establish a mathematical model:

[0037] Declare the value range of the porous medium thickness and the drag calculation formula:

[0038] D = f(Δt);

[0039] ε min = 0;

[0040] ε max = Δt;

[0041] Among them, ε min is the minimum value of the porous medium thickness value range, and ε max is the maximum value of the porous medium thickness value range;

[0042] S402. Define the binary coding length of the independent variable as V = 10 bits, the population size is defined as N = 30 individuals, the maximum number of iterations is defined as maxiter = 200, and the offspring ratio is defined as c = 0.8;

[0043] S403. Calculate the number of offspring: the offspring size S = (N * c / 2) * 2, and the result is taken as an integer;

[0044] S404. Define the mutation probability as L = 0.05;

[0045] S405. Define three structures to store the binary coding, decimal coding, and fitness value of each individual respectively;

[0046] S406. Define an array to record the optimal individual fitness value of each iteration, corresponding to the individual with the minimum drag;

[0047] S407. Create an array of size N to store the information of the parent population;

[0048] S408. Loop to generate each individual in the population;

[0049] S409. Generate a random binary code of length equal to the independent variable length for the i-th individual, then convert it into a decimal value and map it to [ε min , ε max ;

[0050]

[0051] S410. Then calculate the fitness value of this individual;

[0052] S411. Perform iteration, with the number of times being maxiter;

[0053] S412. Store the offspring population results in an array;

[0054] S413. Create an array of size S to store the offspring population information;

[0055] S414. Select individuals from the parent population to generate two offspring individuals;

[0056] S415. Then perform a mutation operation on the binary encoding of each offspring individual, convert it to decimal, and calculate the fitness value of the mutated individual;

[0057] S416. Merge the populations, merging the parent population and the offspring population into a new population;

[0058] S417. Sort the new population in descending order according to the fitness value, and arrange the new population according to the sorted indices;

[0059] S418. Record the individual with the highest fitness in the current iteration, and select the top N individuals from the sorted new population as the parent population for the next round;

[0060] S419. Record the number of iterations in the current iteration and the optimal individual. When the difference in drag reduction effect is less than 1% during the iteration process, the optimal porous medium thickness is considered to be selected.

[0061] According to the above scheme, in the step S4, when the internal airfoil thickness is t - 2Δt and the porous medium thickness is Δt with a better drag reduction effect, the optimal porous medium thickness is selected directly between T = t - 2Δt + 0 and T = t.

[0062] Implementing the porous medium surface flat underwater vehicle integrated drag reduction design method of the present invention has the following beneficial effects:

[0063] The present invention aims to improve the maneuverability of underwater vehicles, with a focus on the integrated design of airfoils with porous media surfaces. It has successfully achieved the perfect combination of porous media surfaces and airfoil structures, abandoning the practice of first independently designing airfoils and then considering coatings. Instead, a method of simultaneously considering the distribution of porous media surfaces and airfoil design is adopted, which not only optimizes the traditional design process but also achieves double breakthroughs in structure and performance, opening up a new solution path for the drag reduction problem of underwater vehicles. Brief Description of the Drawings

[0064] The present invention will be further described below in conjunction with the drawings and embodiments. In the drawings:

[0065] Figure 1 is a flowchart of the integrated drag reduction design method for a flat underwater vehicle with a porous media surface according to the present invention;

[0066] Figure 2 is a schematic diagram of the computational domain of the integrated drag reduction design method for a flat underwater vehicle with a porous media surface according to the present invention;

[0067] Figure 3 is a schematic diagram of the underwater vehicle according to the present invention;

[0068] Figure 4 is the airfoil cross-section of the underwater vehicle according to the present invention;

[0069] Figure 5 is a schematic diagram of the coating layer covering the hydrofoil surface of the underwater vehicle according to the present invention;

[0070] Figure 6 is a schematic diagram of the hydrofoil of the underwater vehicle extending inward by Δt according to the present invention;

[0071] In the figure: 1. Symmetric boundary, 2. Velocity inlet boundary, 3. Pressure outlet boundary, 4. Underwater vehicle, 5. Flow field fluid, 6. Porous media, 7. Inside the airfoil. Detailed Embodiment

[0072] For a clearer understanding of the technical features, objectives, and effects of the present invention, the detailed embodiments of the present invention will now be described in detail with reference to the drawings.

[0073] As Figure 1-6 shown, the integrated drag reduction design method for a flat underwater vehicle with a porous media surface of the invention includes a non-coated drag analysis and evaluation model and an iterative optimization method; the specific step flow is as follows:

[0074] S1. Establish a drag analysis and evaluation model for a porous media surface - flat underwater vehicle

[0075] The adopted evaluation model scheme is to simulate the surface water resistance of an underwater vehicle during submerged navigation using CFD simulation. The evaluation model is established using ANSYS-Fluent software. Set the surface porous medium region, and the thickness range of the porous medium surface is 0 - Δt. To reduce the influence of the flow field boundary on the simulation results, the distance between the fluid inlet boundary and one end of the underwater vehicle is set to 3.5 times the length of the underwater vehicle, the distance between the outlet boundary and the other end of the underwater vehicle is set to 6 times the length of the underwater vehicle, and the water area radius is set to 60 times the turning radius of the underwater vehicle. Given the complex shape of the underwater vehicle, a non-structured grid with good adaptability is used to divide the grid. The computational domain is as Figure 2 shown.

[0076] To ensure the accuracy of the simulation, grid independence verification is carried out, and the total resistance received by the underwater vehicle is calculated for different numbers of grids. In the grid division stage, first calculate the y+ value to determine the height of the first layer of grids, then gradually refine the grids around the underwater vehicle, and determine boundary 1 as the symmetric boundary. Take the water resistance as the index of computational accuracy, select the SST k-ω turbulence model, and set boundary 2 as the velocity inlet boundary with an inlet velocity of U, and set boundary 3 as the pressure outlet boundary. The velocity refers to the actual submerged navigation velocity of the underwater vehicle at 4, for example, 1.5 m / s. At the same time, set the porous medium parameters on the airfoil surface, including porosity, permeability, pore structure parameters, porosity, saturation, and permeability equation parameters, and regard the porous medium surface as the porous jump boundary condition. The relative error gradually decreases as the number of grids increases. When the relative error is less than one percent, it is considered stable, and the grid accuracy at this time is used for subsequent calculations. The drag coefficient when the underwater vehicle moves stably is determined by the following formula:

[0077]

[0078] where ρ sw is the fluid density, determined by the fluid in the flow field at 5; C D is the drag coefficient; A s is the cross-sectional area of the underwater vehicle; U is the navigation speed of the underwater vehicle.

[0079] Cross-sectional area of the underwater vehicle:

[0080] A s = N(t,Δt)

[0081] where t and Δt represent the internal height of the airfoil and the height of the porous medium respectively.

[0082] S2. Establish the reference data for drag evaluation

[0083] S201: The original optimized airfoil design scheme

[0084] To evaluate the drag reduction effect on the hydrofoil surface of an aqueous vehicle, the numerical simulation of the designed hydrofoil profile is carried out, and the corresponding relationship between the profile and the drag reduction effect on the hydrofoil surface is obtained through numerical simulation. A resistance analysis and evaluation model of a porous medium surface-flat underwater vehicle is established, and by continuously changing the parameters, the optimal hydrofoil profile for movement in the flow field is obtained. As Figure 4 shown.

[0085] S202: Directly coat the porous medium structure on the original outer shape

[0086] In this part, with the thickness of the porous medium coating as the design variable, a porous medium coating with a thickness of Δt is added to the wing surface 6. The wing profile height is set as T, the thickness of the outer porous medium is Δt, and the height inside the wing profile is t. At this time, T = t + 2Δt. As Figure 4 shown. Numerical simulation is carried out on the wing profile covered with a Δt porous medium coating to evaluate the influence of the increase in the thickness of the porous medium coating on the relevant performance. In this process, the wing without the porous medium coating is used as a control. The data obtained from the simulation is analyzed in detail. Mainly focus on the differences in pressure distribution, drag, lift, etc. between the wing with the porous medium coating and the uncoated wing profile. By comparing the numerical simulation results of the two under the same working conditions, analyze how the performance indicators such as the pressure distribution, drag, and lift on the wing profile surface change with the increase in the thickness of the porous medium coating.

[0087] S203: The outer surface of the porous medium inherits the original outer shape data of the underwater vehicle, and a certain thickness is taken from the outside to the inside as the porous area, and the inner surface of the porous medium adheres tightly to the vehicle shell.

[0088] Set the wing profile height as t, keep the thickness Δt of the porous medium coating unchanged. At this time, the height inside the wing profile covered by the porous medium is t - 2Δt. Select the same boundary conditions and flow field parameters, and analyze the pressure distribution, drag, and lift of this wing profile.

[0089] S3. Determine the optimal thickness of the underwater vehicle wing profile and the surface porous medium

[0090] (1) Determine the constraint conditions as follows:

[0091] a) [ε min ≤ [ε] ≤ [ε max ,

[0092] where ε is the porosity, and ε m i n and ε max are the minimum and maximum values that the porosity of the porous medium can take under the working conditions.

[0093] b) Δt min ≤ Δt ≤ Δt max

[0094] where Δt is the thickness of the porous medium, and Δt min and Δt max are the minimum and maximum values that the thickness of the porous medium can take under the working conditions.

[0095] c) t min ≤t≤t max

[0096] where t is the internal height of the airfoil, and Δt min and Δt max are the minimum and maximum values that the internal height of the airfoil can take under the working conditions.

[0097] (2) Compare the case where the internal height of the airfoil is t - 2Δt and the surface porous medium thickness is Δt with the case where the internal height of the airfoil is t and the surface porous medium thickness is Δt, analyze the drag force on the airfoil under these two settings of the porous medium coating, and select the group with the minimum drag force; t is the internal height of the airfoil determined in (1), and Δt is the thickness of the porous medium determined in (1).

[0098] S4. Writing of the rent reduction optimization software and iterative output of the optimal solution: Determine the iterative optimization process according to the objective function, determine the relevant operating parameters of the iterative algorithm, and then repeatedly perform selection and crossover through the iterative algorithm to improve the fitness of the iterative algorithm, gradually approach the optimal solution, and finally output the global optimal solution when the iterative exit condition is met to obtain the optimal variable parameters for drag reduction optimization.

[0099] Assume that the group with better drag reduction effect is the airfoil height T = t + 2Δt group. Then, select the optimal porous medium thickness between T = t + 0 and T = t + 2Δt. The specific process is as follows:

[0100] a) Take the thickness of the porous medium coating as the design variable. Take the minimum drag force on the airfoil as the optimization objective,

[0101] and establish a mathematical model:

[0102] Declare the value range of the porous medium thickness and the drag force calculation formula:

[0103] D = f(Δt);

[0104] ε min = 0;

[0105] ε max = Δt;

[0106] where ε min is the minimum value of the value range of the porous medium thickness, and ε max is the maximum value of the value range of the porous medium thickness.

[0107] b) Define the binary encoding length of the independent variable as V = 10 bits, the population size as N = 30 individuals, the maximum number of iterations as maxiter = 200, and the offspring ratio as c = 0.8.

[0108] c) Calculate the number of offspring: The offspring size S = (N * c / 2) * 2 (round the result to an integer)

[0109] d) Define the mutation probability as L = 0.05.

[0110] e) Define three structures to store the binary encoding, decimal encoding, and fitness value of each individual respectively.

[0111] f) Define an array to record the fitness value of the optimal individual in each iteration (i.e., the individual corresponding to the minimum resistance).

[0112] g) Create an array of size N to store the information of the parent population.

[0113] h) Generate each individual in the population in a loop

[0114] i) Generate a random binary code of length equal to the length of the independent variable for the i-th individual, then convert it to a decimal value and map it to [ε min , ε max .

[0115] For example:

[0116] j) Then calculate the fitness value of this individual. (Assume a defined fitness function)

[0117] k) Conduct iterations for maxiter times.

[0118] l) Store the results of the offspring population in the array

[0119] m) Create an array of size S to store the information of the offspring population.

[0120] n) Select individuals from the parent population, such as ε1, ε2, and perform crossover operations on ε1 and ε2 to generate two offspring individuals.

[0121] o) Then perform mutation operations on the binary encoding of each offspring individual, convert it to decimal, and calculate the fitness value of the mutated individual.

[0122] p) Merge the populations, merging the parent population and the offspring population into a new population.

[0123] q) Sort the new population in descending order according to the fitness value, and arrange the new population according to the sorted indices.

[0124] r) Record the individual with the highest fitness in the current iteration, and select the top N individuals from the sorted new population as the parental population for the next round.

[0125] s) Record the number of current iterations and the optimal individual. When the difference in drag reduction effect during the iteration process is less than 1%, it is considered that the optimal porous medium thickness is selected.

[0126] t) If the internal airfoil thickness is t - 2Δt and the porous medium thickness is Δt with a better drag reduction effect, then the selection of the optimal porous medium thickness is directly carried out between T = t - 2Δt + 0 and T = t, and the process is similar to the above process, as Figure 6 shown.

[0127] The embodiments of the present invention have been described above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the purpose of the present invention and the scope protected by the claims. All of these fall within the protection scope of the present invention.

Claims

1. A method for integrated drag reduction design of a flat underwater vehicle with a porous medium surface, characterized in that: The following steps are involved: S1. Establish a porous medium surface-flat underwater vehicle resistance analysis and evaluation model: set the surface porous medium area, use unstructured grids to divide the grid, verify the grid independence, and calculate the total resistance of the underwater vehicle under different grid numbers; S2. Establishing resistance assessment baseline data; S3, determine the objective function and constraints that need to be optimized for the underwater vehicle airfoil; S4. Determine the iterative optimization process according to the objective function, determine the relevant operating parameters of the iterative algorithm, repeatedly perform selection and crossover through the iterative algorithm, improve the fitness of the iterative algorithm, gradually approach the optimal solution, and finally output the global optimal solution when the iterative exit condition is met, and obtain the optimal drag reduction optimization variable parameters.

2. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 1 is characterized in that: In the step S1, CFD simulation is used to simulate the surface water resistance of the underwater vehicle during diving, and ANSYS-Fluent software is used to establish an evaluation model.

3. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 2 is characterized in that: In the step S1, during the grid division stage, the y+ value is calculated to determine the height of the first layer of grids, the grids around the underwater vehicle are encrypted layer by layer, and the symmetric boundaries are determined; wherein y+ is a dimensionless parameter related to the distance between the fluid and the solid boundary during the grid division process; the SST k-ω turbulence model is selected to take water resistance as an indicator of calculation accuracy, and the velocity inlet boundary, inlet velocity, pressure outlet boundary, and porous medium parameters of the airfoil surface are set, and the porous medium parameters of the airfoil surface include porosity, permeability, pore structure parameters, porosity, saturation, and permeability equation parameters.

4. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 3 is characterized in that: In step S1, the porous medium surface is regarded as a porous step boundary condition, and the drag coefficient of the underwater vehicle during stable motion is determined by the following formula: Among them, ρ sw is the fluid density, which is determined by the flow field fluid; C D is the drag coefficient; A s is the cross-sectional area of ​​the underwater vehicle; U is the navigation speed of the underwater vehicle; Cross-sectional area of ​​underwater vehicle: A s =N(t,Δt) Where t and Δt represent the internal height of the airfoil and the height of the porous medium, respectively.

5. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 4 is characterized in that: In step S2, the method for establishing resistance evaluation benchmark data includes the following steps: S201. By numerically simulating the designed hydrofoil profile, the numerical simulation obtains the corresponding relationship between the profile and the drag reduction effect of the hydrofoil surface, and establishes a porous medium surface-flat underwater vehicle resistance analysis and evaluation model. By continuously changing parameters, the hydrofoil profile that moves best in the flow field is obtained; S202, taking the thickness of the porous medium coating as the design variable, adding a porous medium coating with a thickness of Δt to the wing surface, setting the airfoil height to T, the thickness of the outer porous medium to Δt, and the airfoil inner height to t, at which point T=t+2Δt; S203. The outer surface of the porous medium inherits the original shape data of the vehicle body, and a certain thickness is taken from the outer layer to the inside as the porous area. The inner surface of the porous medium is tightly adhered to the shell of the vehicle body: the airfoil height is set to t, and the thickness of the porous medium coating is kept unchanged. At this time, the inner height of the airfoil covered by the porous medium is t-2Δt. The same boundary conditions and flow field parameters are selected to analyze the pressure distribution, drag, and lift of the airfoil covered by the porous medium.

6. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 5 is characterized in that: The step S202 also includes: performing numerical simulation on the airfoil covered with Δt porous medium coating, evaluating the influence of thickening of the porous medium coating on relevant performance, and performing detailed analysis on the simulated data, wherein the data include the differences in pressure distribution, drag and lift between the wing with the porous medium coating and the uncoated airfoil, and by comparing the numerical simulation results under the same working conditions, analyzing the changes in the performance indicators of pressure distribution, drag and lift on the airfoil surface as the thickness of the porous medium coating increases.

7. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 6 is characterized in that: In step S3, Determine the constraints including: [e min ]≤[e]≤[e max ], Where ε is the porosity, ε min and ε max is the minimum and maximum value that the porosity of the porous medium can take under the working condition; Δt min ≤Δt≤Δt max Where Δt is the thickness of the porous medium, Δt min and Δt max are the minimum and maximum values ​​that the porous medium thickness can take under the working conditions; t min ≤t≤t max Where t is the internal height of the airfoil, Δt min and Δt max are the minimum and maximum values ​​that the internal height of the airfoil can take under the working conditions.

8. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 7 is characterized in that: In step S4, when the airfoil height group T=t+2Δt is the group with the best drag reduction effect, the optimal porous medium thickness is selected between T=t+0 and T=t+2Δt.

9. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 8, characterized in that: The method for selecting the optimal porous medium thickness between T=t+0 and T=t+2Δt includes: S401. Taking the thickness of the porous medium coating as the design variable and the minimum resistance of the airfoil as the optimization goal, a mathematical model is established: Declare the value range of porous medium thickness and resistance calculation formula: D = f(Δt); e min =0; e max =Δt; Among them, ε min is the minimum value of the porous medium thickness range, ε max is the maximum value of the porous medium thickness range; S402, define the binary code length of the independent variable as V = 10 bits, define the population size as N = 30 individuals, define the maximum number of iterations as maxiter = 200, and define the offspring ratio as c = 0.8; S403, calculate the number of offspring: offspring size S = (N*c / 2)*2, the result is an integer; S404, define the mutation probability as L=0.05; S405, define three structures to store the binary code, decimal code and fitness value of each individual respectively; S406, define an array to record the optimal individual fitness value of each iteration, corresponding to the individual with the least resistance; S407, create an array of size N to store information of the parent population; S408, cyclically generate each individual in the population; S409, generate a random binary code for the i-th individual with a length equal to the length of the independent variable, convert it into a decimal value, and map it to [ε min ,ε max ]middle; S410, then calculating the fitness value of the individual; S411, iterate for maxiter times; S412, storing the offspring population result in an array; S413, create an array of size S for storing offspring population information; S414, select individuals from the parent population to generate two offspring individuals; S415, then performing a mutation operation on the binary code of each offspring individual, converting it into decimal, and calculating the fitness value of the individual after the mutation; S416, merging the populations, merging the parent population and the child population into a new population; S417, sorting the new population in descending order according to the fitness value, and arranging the new population according to the sorted index center; S418, record the individual with the highest fitness in the current iteration, and select the first N individuals from the sorted new population as the parent population for the next round; S419, recording the number of current iterations and the best individual, when the difference in drag reduction effect is less than 1% during the iteration process, it is considered that the best porous medium thickness is selected.

10. The integrated drag reduction design method for a flat underwater vehicle with a porous medium surface according to claim 7, characterized in that: In step S4, when the thickness of the internal airfoil is t-2Δt and the porous medium thickness is Δt, the drag reduction effect is better, and the optimal porous medium thickness is directly selected at T=t-2Δt+0 and T=t.

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