Cross-medium navigation body shell thickness gradient distribution optimization design method

Abstract

The invention discloses a cross-medium navigation body shell thickness gradient distribution optimization design method, which comprises the following steps: establishing a navigation body geometric model and a computational domain external flow field model, carrying out grid division on the navigation body geometric model and the computational domain external flow field model, setting material attributes and a deformation calculation method in a solid domain, and setting boundary conditions in a fluid domain. After the solid and fluid domains are set, starting to carry out data exchange and iterative solution, evaluating a calculated underwater impact deformation result of the navigation body, sequentially dividing a plastic deformation area of the navigation body into a plurality of girdle areas from the center to the outside, carrying out shell thickness distribution stepped optimization on the navigation body according to the divided girdle areas, and carrying out shell thickness distribution stepped optimization on the navigation body. And correcting the shell thickness of the undeformed part on the rear side. The prediction method disclosed by the invention is relatively high in accuracy and small in calculation burden.

Description

Technical Field

[0001] The present invention relates to the fields of impact dynamics and structural optimization, and in particular to a method for optimizing the gradient distribution of shell thickness of a cross-medium navigation body. Background Art

[0002] Cross-media vehicles usually need to maintain extremely high speeds at the moment of water entry, which means that the weapon will face the risk of structural deformation caused by the impact of entering the water. When the shape of the vehicle changes, the fluid resistance and motion characteristics it is subject to will also change. It usually faces the risk of reduced range and damage to internal components. In severe cases, it may even experience motion instability or direct failure, so it needs to be studied in detail. Although the use of experimental methods to study the high-speed entry of cross-media vehicles into the water is more accurate, it is costly and difficult to measure data. Therefore, it is necessary to obtain a calculation method for the deformation of the vehicle due to water entry through numerical simulation before the test. At present, the numerical research on cross-media vehicles in engineering mostly uses rigid body dynamics methods, which makes it difficult to intuitively evaluate the deformation at the moment of impact.

[0003] Furthermore, after completing the numerical calculations for the trans-medium vehicle's water entry, if severe deformation occurs, load reduction design is required. Current mainstream load reduction solutions include reverse jet injection, airbag cushioning, and parachute deployment. While these methods can effectively reduce the impact load on the vehicle upon water entry, they also reduce its entry speed, affecting its original maneuverability. Summary of the Invention

[0004] The purpose of the present invention is to provide a real-time numerical prediction method for the impact deformation of a cross-medium navigation body entering water to assist experimental research, and on this basis, to propose a passive load reduction scheme with gradient shell thickness distribution optimization.

[0005] The technical solution to the purpose of the present invention is:

[0006] A shell thickness gradient distribution optimization design method for a cross-medium vehicle is proposed. The plastic deformation area of ​​the vehicle is divided into multiple annular zones from the center outward. The shell thickness distribution of the vehicle is optimized stepwise according to the divided annular zones:

[0007]

[0008] where h i0 、h i are the original shell thickness and optimized shell thickness of the annular zone i of the vehicle, η i is the energy absorption rate of annular zone i, are the equivalent plastic strain and stress of the annular zone i in the deformation region, respectively; v0 and v1 are the velocities of the navigation body before and after the impact of water entry, respectively;

[0009] The shell thickness of the undeformed rear portion is corrected using the following formula:

[0010]

[0011] Where n refers to the number of plastic deformation rings, S i is the area of ​​the different plastic deformation annular zones, S0 is the total outer surface area of ​​the navigation body, h0 is the initial thickness of the navigation body before optimization, and h e 、S e are the shell thickness and area of ​​the undeformed region, respectively.

[0012] Compared with the prior art, the present invention has the following significant advantages:

[0013] (1) A weak bidirectional fluid-solid coupling method is used to calculate the deformation of a cross-medium vehicle upon entry into water. Numerical solutions are obtained in both the fluid and solid domains, and real-time data exchange is performed through data channels to achieve real-time deformation prediction. The calculation of the solid domain uses a shell element model and explicit dynamics methods, which reduces the computational burden and increases computational efficiency while remaining close to engineering practice.

[0014] (2) The Mises yield criterion and equivalent plastic strain are used to evaluate the degree of plastic deformation of the vehicle at the position subjected to the impact load. Considering that the stress wave is centrally symmetrical during the transmission of the structure, and the main structure of the cross-medium vehicle is mostly a rotating body, the plastic deformation area is divided into multiple annular areas according to different deformation degrees. The damage risk of different positions of the vehicle is intuitively obtained, which facilitates structural optimization of different positions.

[0015] (3) The shell thickness of the navigation body is optimized using empirical formulas according to actual engineering conditions, and the shell thickness at the undeformed position is appropriately reduced. This ensures sufficient structural strength without increasing the mass burden of the structure entering the water. Compared with the current mainstream load reduction technology, it is more feasible and does not affect the maneuverability of the navigation body. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Flowchart of the present invention

[0017] Figure 2 Schematic diagram of the flow field outside the navigation body and the calculation domain in an example of the present invention

[0018] Figure 3 The stress, equivalent plastic strain and area division of the navigation body after entering the water in the example

[0019] Figure 4 Schematic diagram of the structure after optimizing the gradient distribution of the shell thickness of the vehicle in the example

[0020] Figure 5 Comparison of deformation of the vehicle before and after optimization in the example

[0021] Figure 6 Comparison curves of impact stress and plastic strain of the navigation body before and after optimization in the example DETAILED DESCRIPTION

[0022] The present invention will be described in further detail below with reference to embodiments and accompanying drawings.

[0023] The cross-medium vehicle in this example is a spherical aluminum rotating shell with a diameter of 30 mm and an initial thickness of 1.5 mm, which enters the water vertically at a speed of 300 m / s. Figure 1 As shown in the example, a numerical solution is first performed in the fluid and solid domains, and data is exchanged between the two domains. Iterative calculations are then performed to predict the impact deformation of the vehicle during water entry. The calculated results are then used to assess the degree of plastic deformation of the vehicle. Furthermore, based on the assessed degree of plastic deformation, a step-by-step optimization of the shell thickness distribution is performed to achieve load reduction and anti-deformation effects.

[0024] S1: Establish the vehicle geometry model and the flow field model outside the computational domain. First, establish the vehicle geometry model in the solid domain, and then construct the flow field model outside the computational domain in the fluid domain based on this model. To exchange data between the two domains, ensure that the surface of the vehicle model fits the inner wall of the flow field model outside the computational domain, and set the two surfaces as coupling boundary conditions. The vehicle geometry model and the flow field model are set as follows: Figure 2 shown.

[0025] S2: Mesh the vehicle geometry model and external flow field set in S1. Considering that this example is a vertical entry condition with good central symmetry, the solid domain model is first cut into basic geometric bodies to divide the structural grid, and the head is locally encrypted. The grid unit uses the hourglass-controlled reduced integration unit. In the fluid domain, overlapping grids are used to realize the following motion of the external flow field. A cylindrical area with a length twice the length of the rotating body and a diameter twice the diameter of the rotating body is used as the overlapping area to wrap the vehicle. The overlapping area is set to follow the motion according to the displacement vector difference of the boundary node of the vehicle. The external flow field in the fluid domain is meshed. In order to ensure the symmetry of the numerical solution, hexahedral grid units are selected. In order to accurately capture the evolution of the flow field and ensure the calculation accuracy, multi-layer grid encryption is performed at key positions such as the liquid surface, overlapping area and vehicle surface. In addition, a boundary layer grid is set on the vehicle surface to capture the near-wall flow. Considering the vehicle speed in this example, the first layer grid height is set to 2×10 -7 m.

[0026] S3: Set the material properties and deformation calculation method in the solid domain. In the solid domain, the material properties of the aluminum alloy of the vehicle are given by the Johnson-Cook constitutive model:

[0027]

[0028] Where, σ s is the material yield stress, ε and are material strain and strain rate respectively, and T m With T r are the melting point and room temperature of the material respectively, A is the yield stress under quasi-static conditions, B is the strain hardening constant, C is the strain rate correlation coefficient, m0 is the temperature correlation coefficient, n0 is the strain hardening exponent, and T is the temperature variable of the material itself. This model can accurately simulate the nonlinearity of the material deformation process. The cross section of the vehicle is set as a shell unit, and the load in the shell thickness direction is ignored, which is close to engineering practice while reducing the computational burden. Considering that the speed of the vehicle in this example is relatively high and the deformation caused by entering the water is completed instantaneously, the explicit dynamics method is used for the calculation. This method is suitable for calculating instantaneous impact problems. It uses current data to predict parameter changes at the next moment. There is no need to consider single-step iteration errors and there is no convergence problem. Its expression is:

[0029]

[0030] Where x, and are the displacement, velocity and acceleration vectors of the unit respectively. The change in the unit displacement of the vehicle reflects the overall deformation. t is time. β and γ are the parameters of the differential equation, which are taken as 0.5 and 0 respectively in the display dynamics. Δt is the time difference of each iteration.

[0031] S4: Set the boundary conditions in the fluid domain. The Reynolds number (Re) in this example is approximately 1×10 7 Therefore, an unsteady turbulence model was used for numerical simulation, and the SSTk-ω model was selected as the turbulence model. Because the process involves the interaction and transformation of multiple phases, including water, air, and water vapor, the VOF multiphase flow model and the Schnerr-Sauer cavitation model were used simultaneously. These models excel in bubble dynamics and accurately capture multiphase interfaces. The fluid computational domain is a hexahedron, with the top set as the velocity inlet boundary condition and the remaining five faces as the pressure outlet boundary conditions to simulate water bodies of infinite size and depth.

[0032] S5: After the solid and fluid domains are set up, data exchange and iterative solution begin. Data exchange is performed according to the coupled equations:

[0033] p f ·e=p s ·e

[0034] u f =u s

[0035] Where p sWith p f They represent the structural surface load in the solid domain and the fluid pressure in the fluid domain, e is the normal vector of the vehicle surface, u s with u f Represents the displacements in the solid and fluid domains. Because the explicit dynamics algorithm and explicit coupling method are selected, the solutions in the solid and fluid domains can be performed simultaneously, ensuring consistent time step settings for the explicit dynamics method iterations. In this example, the iterations continue until the vehicle reaches a distance equal to one projectile length after crossing the water surface. The calculated data, representing the deformation results of the vehicle upon entering the water, are saved.

[0036] S6: Evaluate the calculated deformation results of the vehicle entering water. Use the Mises yield condition and equivalent plastic strain (PEEQ) to evaluate the deformation of the vehicle head after the impact. The two expressions are:

[0037] (σ x -σ y ) 2 +(σ y -σ z ) 2 +(σ z -σ x ) 2 =2σ s 2

[0038]

[0039] Establish a body coordinate system, with its origin o as the center of mass of the vehicle. The ox axis is the central axis of the vehicle shell, pointing to the head; the oy axis is located in the longitudinal symmetry plane of the vehicle (the radial direction of the shell), and the oy axis is perpendicular to the ox axis; the oz axis is perpendicular to the longitudinal symmetry plane of the vehicle, and its direction is determined by the right-hand rule. x , σ y , σ z are the principal stresses in the x, y, and z directions in the projectile coordinate system, σ s is the stress yield limit, t is the time parameter, ε P is the equivalent plastic strain, Δε P Plastic strain increment, specifically:

[0040]

[0041] Where Δλ is the plasticity multiplier, Characterize the direction of plastic flow after the plastic enters the plastic stage, mark the part where plastic deformation occurs, and obtain the elastic-plastic deformation boundary on the vehicle. f is the resultant force per unit volume, and σ is the stress per unit volume. In this example, plastic deformation mainly occurs at the head of the vehicle. Within the plastic deformation area of ​​the vehicle's head, a series of annular zones are divided according to different degrees of deformation, with the maximum deformation position as the center. The largest circular zone ensures coverage of the elastic-plastic deformation boundary. In this example, four annular zones are divided, marked as 1, 2, 3, and 4 from the center outward. Figure 3 As shown in Figure 3, the average equivalent plastic strains in each region are approximately 0.035, 0.056, 0.03, and 0.015, respectively.

[0042] S7: Perform a stepwise optimization of the shell thickness distribution of the vehicle according to the divided annular zones. According to the degree of plastic deformation, change the shell thickness distribution of the vehicle, and use a semi-empirical formula to calculate the maximum and minimum shell thickness ratios before and after optimization. Calculate the energy absorption rate during the water impact by using the approximate equivalent strain energy and the energy loss of the vehicle during water entry:

[0043]

[0044] Where η i is the energy absorption rate of annular zone i, They are the equivalent plastic strain and stress of the annular zone i in the deformation region, which can be obtained from x in step S3 and p in step S5. s We can obtain v0 and v1 respectively, where v0 and v1 are the velocities of the vehicle before and after entering the water, V is the volume of the deformation area of ​​the vehicle, m is the mass of the deformation area of ​​the vehicle, ρ is the material density of the vehicle, and h is the volume of the deformation area of ​​the vehicle. i0 、h i They are the original shell thickness and the optimized shell thickness of the annular zone i of the vehicle, respectively.

[0045] In this example, the equivalent plastic strain and stress in the four annular deformation regions are respectively substituted into the above empirical formula to obtain the optimized shell thickness ratio of the two annular regions, namely:

[0046] When the vehicle enters the water vertically, the head hits the water surface and forms a bubble instantly. After that, only the head contacts the water, and the rest of the body remains untouched. The impact load is transmitted from the head to the rear of the vehicle in the form of a stress wave, and gradually decreases during the transmission process. Therefore, there is no risk of deformation or damage to the rear structure of the vehicle. The shell mass is represented by calculating the product of the shell thickness and its corresponding area, such as Figure 4 As shown in the figure, a cross-sectional view of the annular zone 4 is shown. The shell mass in this zone can be expressed as: h4·S4. To ensure the consistency of mass before and after structural optimization, no additional water load is added. This balances the lightweight and impact deformation resistance of the navigation body. The shell thickness of the rear undeformed portion is corrected using the following formula:

[0047]

[0048] Where n refers to the number of plastic deformation rings, i ranges from 1 to 4, S i is the area of ​​the different plastic deformation annular zones, S0 is the total outer surface area of ​​the navigation body, h0 is the initial thickness of the navigation body before optimization, and h e 、S e are the shell thickness and area of ​​the undeformed region, respectively.

[0049] S8: Use the same water entry conditions to simulate and predict the deformation of the hull after shell thickness optimization, and analyze the optimization effect. Figure 4 The figure shows the deformation of the vehicle head observed from a side perspective. It can be seen that the structure after the shell thickness distribution is optimized has no obvious plastic deformation and has not formed an approximately planar structure after the impact.

[0050] Figure 4 The figure shows the comparison of the instantaneous maximum stress of the water entry impact before and after the optimization of the shell thickness distribution of the navigation body. The load reduction rate reaches 270%, and the stress fluctuation phenomenon after the impact is weakened. Figure 5 The figure shows the strain comparison before and after optimization. The maximum equivalent plastic strain on the navigation body is reduced from 0.056 to 0.0002. It can be seen that the load reduction and anti-deformation effects of the optimized navigation body have met expectations.

[0051] Obviously, the above embodiments of the present invention are only examples to clearly illustrate the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for optimizing the thickness gradient distribution of a cross-medium vehicle shell, characterized in that: The plastic deformation area of ​​the vehicle is divided into multiple annular zones from the center outwards, and the shell thickness distribution of the vehicle is optimized stepwise according to the divided annular zones: where h i0 、h i are the original shell thickness and optimized shell thickness of the annular zone i of the vehicle, η i is the energy absorption rate of annular zone i, are the equivalent plastic strain and stress of the annular zone i in the deformation region, respectively; v0 and v1 are the velocities of the navigation body before and after the impact of water entry, respectively; The shell thickness of the undeformed part of the rear side is corrected using the following formula Where n refers to the number of plastic deformation rings, S i is the area of ​​the different plastic deformation annular zones, S0 is the total outer surface area of ​​the navigation body, h0 is the initial thickness of the navigation body before optimization, and h e 、S e are the shell thickness and area of ​​the undeformed region, respectively.

2. The cross-medium vehicle shell thickness gradient distribution optimization design method according to claim 1 is characterized in that: By setting the cross section of the vehicle body to shell elements, the explicit dynamics method is used to obtain: Where x, and are the displacement, velocity and acceleration vectors of the shell element respectively, t is time, β and γ are the difference equation parameters, Δt is the time difference of each iteration, and solving x can get 3. The method for optimizing the shell thickness gradient distribution of a cross-medium vehicle according to claim 1, characterized in that: p s It is obtained by the following coupled equations: p f ·e=p s ·And in f =in s Where p s With p f They represent the structural surface load in the solid domain and the fluid pressure in the fluid domain in the geometric model of the vehicle, e is the normal vector of the vehicle surface, u s with u f represents the displacements in the solid and fluid domains.

4. The method for optimizing the shell thickness gradient distribution of a cross-medium vehicle according to claim 3, characterized in that: The iterative calculation of the coupling equation continues until the distance moved by the vehicle after crossing the water surface reaches 1 times the length of the projectile, and the calculation data, i.e., the deformation result of the vehicle entering the water, is saved.

5. The method for optimizing the shell thickness gradient distribution of a cross-medium vehicle according to claim 3, characterized in that: The fluid domain is set as a hexahedron structure, the top of which is set as the velocity inlet boundary condition, and the remaining five faces are set as the pressure outlet boundary conditions.

6. The cross-medium vehicle shell thickness gradient distribution optimization design method according to claim 3 is characterized in that: The material properties in the solid domain are given by the Johnson-Cook constitutive model.

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