Coupling analysis method for aircraft water forced landing and application of coupling analysis method

Abstract

The invention relates to the technical field of aircraft water surface impact, in particular to an aircraft water forced landing coupling analysis method and application thereof, and the method comprises the following steps: establishing an aircraft structure finite element model, constructing a three-dimensional model, dividing grids, defining material attributes, and carrying out convergence analysis; initial conditions and loads are set, the forced landing posture is adjusted, and the initial speed and gravity loads are applied; constructing a fluid particle pool model, establishing a fluid dynamic model, and defining initial parameters and wave conditions of fluid particles; solving and analyzing fluid-structure interaction, setting an FEM-SPH bidirectional contact mode, and calculating fluid dynamic load, motion response and structural deformation damage results. According to the FEM-SPH bidirectional fluid-solid coupling method, grid distortion is effectively avoided, the calculation precision and stability are improved, and the calculation process is simplified; the method is suitable for evaluating the crashworthiness of multiple aircrafts in water landing and covers multiple initial load conditions and complex water conditions; the water surface impact crashworthiness response of the structure is accurately predicted, and the engineering analysis requirement is met.

Description

Technical Field

[0001] The present invention relates to the technical field of aircraft water surface impact, and in particular to a coupling analysis method for aircraft forced landing on water and an application thereof. Background Art

[0002] Water landing performance is a key factor that must be considered in the structural design of aircraft, and it is also one of the core indicators for civil aircraft to obtain airworthiness certification. The absorption of energy during water impact mainly depends on the bending characteristics of the fuselage skin and the plastic collapse effect of the attached structures. The load transfer mechanism between aircraft structures during the impact is more complex, posing a major challenge to the fuselage structure and skin. In previous studies, water landing tests of full-scale aircraft were expensive and risky, and theoretical models had limitations in describing the highly nonlinear fluid dynamic characteristics of aircraft and the real fluid-solid coupling mechanism in failure behavior. With the improvement of computing power and the development of simulation tools, numerical modeling technology has become an effective means to predict the nonlinear hydrodynamic response of aircraft during sea landing.

[0003] The forced landing process of an aircraft can be divided into four stages: approach, impact, landing, and floating. The impact stage is the key to the success of the forced landing. During this stage, the aircraft is subjected to extreme overloads and the belly of the fuselage suffers a huge impact, resulting in structural deformation or damage. Numerous accident cases have shown that when an aircraft makes an emergency landing at sea, the rapid sinking of the fuselage caused by skin rupture is a key factor affecting the survival rate of passengers. In addition, the relevant certification standards for aircraft forced landings on water require that the aircraft should land on the water as safely as possible during an emergency landing to avoid affecting its buoyancy due to excessive deformation and rupture of the fuselage, ensuring that all passengers can evacuate safely.

[0004] Given the importance of accurately, efficiently, and economically predicting an aircraft's ditching performance, analytical methods for studying the overall loads, motion response, structural deformation, and damage of the aircraft are essential. This not only ensures the safety of passengers and crew, but also provides important guidance for the design of next-generation aircraft. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the present invention discloses a coupling analysis method for aircraft ditching and its application, which are used to solve the above problems.

[0006] The present invention is achieved through the following technical solutions:

[0007] The present invention provides a coupled analysis method for an aircraft ditching, comprising the following steps:

[0008] Establishing a finite element model of the aircraft structure: Build a 3D model based on actual structural parameters, divide the fuselage frame solid element mesh and the skin and floor shell element mesh, define material properties, and perform convergence analysis;

[0009] Set initial conditions and loads: adjust the initial attitude for forced landing, apply initial X / Y / Z axis speed and gravity load;

[0010] Construct a fluid particle pool model: Build an enhanced fluid dynamics model based on the aircraft size and initial velocity, and define the initial motion parameters and wave conditions of the fluid particles;

[0011] Fluid-structure coupling solution and analysis: Set the FEM-SPH bidirectional contact mode and parameters, and use the solver to calculate and output the fluid dynamic loads, motion response, and structural deformation and damage results.

[0012] Furthermore, the enhanced fluid dynamics formula momentum equation used in the method to establish the enhanced fluid dynamics model is as follows:

[0013]

[0014] in, At position x i At , the rate of change of the velocity of the i-th particle in the α direction with time, N is the total number of adjacent particles, m j is the mass of the jth particle, σ α,β (x i ) is the stress tensor at position x, α and β are different directions in space, ρ i and ρ j are the densities of the i-th and j-th particles, respectively, A ij is the interaction area vector from the i-th particle to the j-th particle, A ji is the interaction area vector from the jth particle to the i-th particle;

[0015] The mass equation for the enhanced fluid dynamics formula is:

[0016]

[0017] in, is the rate of change of the density of the i-th particle with time, where D / Dt represents the material derivative, N is the total number of neighboring particles; m j is the mass of the jth particle, and are the velocity components of the i-th and j-th particles in the β direction, W ij is the kernel function between the i-th and j-th particles, used to calculate the interaction between particles, W ij The partial derivative of the position of the i-th particle in the β direction is used to describe the gradient of the interaction between particles.

[0018] Furthermore, the enhanced fluid dynamics formula energy equation used in the method to establish the enhanced fluid dynamics model is:

[0019]

[0020]

[0021]

[0022] in, is the material derivative of the energy of the i-th particle, which represents the rate of change of energy with the movement of fluid particles, p i and p j is the pressure of the i-th and j-th particles, ρ i and ρ j is the density of the i-th and j-th particles, Π ij is the artificial viscosity term between the i-th and j-th particles, used to simulate the viscosity effect, The strain rate tensor of the i-th particle, δ αβ is the Kronecker function, which is 1 when α = β and 0 otherwise.

[0023] Furthermore, in the method, the particle parameters of the wave type being a forward wave are:

[0024] Waveform η:

[0025] Horizontal speed u:

[0026] Vertical speed w:

[0027] Pressure p: p=-ρgz+ρgηK p (z)

[0028] Where: h is the wave height, ν is the frequency, ω is the angular frequency, g is the acceleration of gravity, H is the average water depth, ρ is the density, and t is the period.

[0029] Furthermore, in the method, the particle parameters of the wave type being a standing wave are:

[0030] Waveform η:

[0031] Horizontal speed u:

[0032] Vertical speed w:

[0033] Pressure p: p=-ρgz+ρgηK p (z).

[0034] Furthermore, when the FEM-SPH bidirectional contact mode and parameters are set in the method, the flight structure material model of the FEM-SPH bidirectional fluid-structure coupling model adopts the Piecewise Linear IsotropiePlasticity model that takes into account the strain rate effect, and its deviation stress satisfies the yield function:

[0035] in

[0036] Where: s ij is the deviatoric stress tensor, σ y is the yield strength, σ0 is the initial yield strength, β is the hardening parameter, is the hardening function, is the effective plastic strain.

[0037] Furthermore, in the method, the yield stress and plastic strain curve of the added material is used as the effective strain rate. When implementing the material model, the deviatoric stress is dynamically updated and the yield function is checked. If the deviatoric stress is satisfied, it is accepted. If not, the increment of the effective plastic strain is calculated:

[0038]

[0039] Where: G is the shear modulus, E p is the current plastic hardening modulus.

[0040] Furthermore, in the method, the water state equation of the FEM-SPH bidirectional fluid-solid coupling model adopts a Linear Polynomial model, and for the fluid material, its pressure is:

[0041] p=C0+C1μ+C2μ 2 +C3μ 3 +(C4+C5μ+C6μ 2 )E,

[0042] Where: E is the internal energy per unit initial volume, C i is the material constant.

[0043] Furthermore, the method establishes contact between the fluid and the solid, using a node segmentation algorithm to calculate the collision surface contact. At each time step, the slave surface nodes are detected and the nearest master node is located. If a slave node penetrates the master surface, a resistance is applied in the direction of the master surface normal to prevent penetration. The magnitude of the resistance is determined by the amount of penetration and the properties of the elements on both sides of the contact surface. When a slave node penetrates the master segment, it is subjected to a restoring force proportional to the penetration, calculated according to the following formula:

[0044] F M→S =-k(z S -zM )

[0045] Where: z S -z M is the penetration distance, F M→S Used to remove penetration, k is the penalty factor;

[0046]

[0047] Where: α is the scaling factor, K is the bulk modulus, A is the contact area, and V is the solid unit volume.

[0048] The present invention also provides an application of a coupling analysis method for aircraft forced landing on water, which is applied to simulation verification of forced landing conditions in aircraft airworthiness certification.

[0049] The beneficial effects of the present invention are:

[0050] 1. The pool model of this invention uses an enhanced fluid particle dynamics formula to accurately simulate the dynamic behavior of the fluid, effectively improving the calculation accuracy of fluid-structure interaction problems and significantly reducing the instability caused by pressure fluctuations in numerical simulations. In particular, it improves the calculation accuracy and stability of fluid-structure interaction in the simulation of complex flow fields.

[0051] 2. Regarding the method for establishing a numerical model of an aircraft, this invention, based on theoretical tools such as continuum mechanics and material mechanics, uses simplified and equivalent methods to simulate structures and materials with complex surfaces and mechanical properties, while ensuring the accuracy of numerical simulations. This significantly reduces the time required for calculations and greatly improves the efficiency of forced landing calculations. By using a material model that considers effective plastic strain, the plastic strain behavior of the aircraft structure can be predicted, enabling accurate prediction of the crashworthiness response of the aircraft structure.

[0052] 3. This invention supports the ditching crashworthiness assessment of various aircraft types, including fixed-wing aircraft (such as passenger aircraft, cargo aircraft, fighter jets, and military transport aircraft), helicopters, rotorcraft, airships, drones, space shuttles, rockets, and manned rocket re-entry capsules. Its analysis capabilities cover a variety of complex flight conditions, such as the effects of different initial speeds, pitch angles, approach angles, and various water conditions. Its broad applicability and multi-scenario analysis capabilities provide comprehensive support for design optimization and emergency response assessments.

[0053] 4. Compared to traditional fluid-structure interaction analysis methods, the bidirectional fluid-structure interaction method proposed in this invention can directly generate structural deformation and damage data while calculating the overall load and motion response of the aircraft. This eliminates complex post-processing steps, significantly simplifies the calculation process, and improves overall calculation efficiency and accuracy. By reducing the demand for computing resources and reliance on experiments, this invention has excellent environmental protection characteristics. At the same time, the optimized calculation process reduces R&D costs and improves the efficiency of design verification, thus achieving significant economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0055] Figure 1 The FEM-SPH coupling analysis method flow for aircraft ditching in water of the present invention;

[0056] Figure 2 To implement the numerical model diagram of the aircraft floor and pool;

[0057] Figure 3 Schematic diagram of sensor positions in an embodiment;

[0058] Figure 4 Schematic diagram of the fluid particle motion response;

[0059] Figure 5 Comparison results between numerical model and test data at a speed of 3 m / s;

[0060] Figure 6 Comparison results between numerical model and test data at a speed of 8 m / s;

[0061] Figure 7 Comparison results between the numerical model and the experimental data at a speed of 10 m / s;

[0062] The numbers in the figure represent:

[0063] 1-base plate skin, 2-base plate frame, 3-fluid particle pool. DETAILED DESCRIPTION

[0064] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0065] In one embodiment, this embodiment provides a coupled analysis method for an aircraft ditching, comprising the following steps:

[0066] Establishing a finite element model of the aircraft structure: Build a 3D model based on actual structural parameters, divide the fuselage frame solid element mesh and the skin and floor shell element mesh, define material properties, and perform convergence analysis;

[0067] Set initial conditions and loads: adjust the initial attitude for forced landing, apply initial X / Y / Z axis speed and gravity load;

[0068] Construct a fluid particle pool model: Build an enhanced fluid dynamics model based on the aircraft size and initial velocity, and define the initial motion parameters and wave conditions of the fluid particles;

[0069] Fluid-structure coupling solution and analysis: Set the FEM-SPH bidirectional contact mode and parameters, and use the solver to calculate and output the fluid dynamic loads, motion response, and structural deformation and damage results.

[0070] In this example, a piecewise linear isotropic plastic material model is used for the aircraft structure, and the effective plastic strain of the material is considered. By adjusting the initial velocity of the aircraft in the X, Y, and Z axes, a constant velocity can be set to study the crashworthiness response under constant velocity impact.

[0071] Another preferred improvement of this embodiment is that a large-scale water pool model can be constructed to simulate the forced landing process of a high-speed aircraft. While ensuring calculation accuracy, the calculation efficiency and calculation time are optimized by adopting appropriate particle spacing.

[0072] A further preferred improvement of this embodiment is that a soft-constraint contact mode based on a penalty method is adopted between the finite element unit and the fluid particles, which effectively reduces the particle penetration phenomenon and thus significantly improves the calculation accuracy.

[0073] Another preferred improvement of this embodiment is that the FEM-SPH bidirectional fluid-structure coupling method is used, which can not only calculate the overall load and motion response of the aircraft, but also obtain structural deformation and damage data.

[0074] In one embodiment, please refer to Figure 1As shown, in practical application, this embodiment provides a coupling analysis method for aircraft ditching and its application, including the following steps:

[0075] S1. Based on the actual structural parameters of the aircraft, a three-dimensional finite element model of the aircraft's main structure is established, including the fuselage frame, skin, wings, floor, and floor beams, and the connection methods between the structures are set; the aircraft structure is meshed with finite elements, the frame is meshed with solid elements, and the skin and floor are meshed with shell elements; the material model of the aircraft structure is selected, and the material properties are defined, including density, elastic modulus, Poisson's ratio, yield strength, tensile shear modulus, and failure strain;

[0076] S2, adjusting the initial posture of the model established in step S1 at the time of forced landing, applying initial velocities and gravity loads on the X-axis, Y-axis, and Z-axis;

[0077] S3, establishing a matching fluid particle pool model based on the aircraft size established in step S1 and the initial velocity applied in step S2;

[0078] S4, defining the initial motion parameters and wave conditions of the fluid particles established in step S3;

[0079] S5. Set the FEM-SPH bidirectional fluid-solid coupling contact mode and contact parameters;

[0080] S6. The fluid dynamic load, motion response, structural deformation and damage results of the aircraft are calculated by the solver.

[0081] This embodiment further establishes a helicopter base plate model based on the actual helicopter base plate structure, performs finite element meshing, verifies mesh convergence, and generates a fluid particle pool model of corresponding size under the aircraft base plate, such as Figure 2 shown.

[0082] The material model of this embodiment adopts the Piecewise Linear IsotropiePlasticity model that takes into account the strain rate effect, and its deviation stress satisfies the yield function:

[0083] in

[0084] Where: s ij is the deviatoric stress tensor, σ y is the yield strength, σ0 is the initial yield strength, β is the hardening parameter, is the hardening function, is the effective plastic strain.

[0085] As a preferred embodiment of this invention, the yield stress and plastic strain curve of the added material is the effective strain rate. When implementing the material model, the deviatoric stress is dynamically updated and the yield function is checked. If the deviatoric stress is satisfied, it is accepted. If not, the increment of the effective plastic strain is calculated:

[0086]

[0087] Where: G is the shear modulus, E p is the current plastic hardening modulus. The specific material mechanical properties parameters are shown in Table 1:

[0088] Table 1 Mechanical properties of aircraft base materials

[0089] <![CDATA[Density ρ (g·cm -3 )]]> 7.85 Elastic modulus E(GPa) 205 Poisson's ratio PR 0.3 <![CDATA[Initial yield stress (σ0)]]> 0.2985 Tangent modulus G (GPa) 7 Skin thickness T(mm) 2

[0090] As a preferred feature of this embodiment, meticulous setup measures were taken to ensure that the initial attitude of the aircraft baseboard remained precisely level with the horizontal plane. First, the aircraft baseboard was carefully adjusted to ensure it was completely parallel to the horizontal plane to achieve the required horizontal initial attitude. Next, different initial vertical velocities were set for the aircraft to facilitate subsequent testing and analysis. These initial vertical velocities were set to 3 meters per second, 8 meters per second, and 10 meters per second, respectively, to cover different testing requirements.

[0091] In this embodiment, an acceleration sensor module and a pressure sensor module are added and fixed on the base plate model to monitor the motion state of the aircraft and the changes in ambient pressure under different conditions in real time. The specific position of each sensor has been Figure 3 The ACC stands for acceleration sensor, and the P stands for pressure sensor. These precise settings and layouts enable accurate data to be obtained, providing strong support for aircraft performance evaluation and optimization.

[0092] This embodiment further defines the parameters of the fluid particle pool model. The fluid particle pool model is an enhanced fluid dynamics formula that significantly improves calculation accuracy and numerical stability. The momentum equation of the enhanced fluid dynamics formula is:

[0093]

[0094] in, At position x i At , the rate of change of the velocity of the i-th particle in the α direction with time, N is the total number of adjacent particles, m j is the mass of the jth particle, σ α,β (x i ) is the stress tensor at position x, α and β are different directions in space, ρ i and ρ jare the densities of the i-th and j-th particles, respectively, A ij is the interaction area vector from the i-th particle to the j-th particle, A ji is the interaction area vector from the jth particle to the i-th particle.

[0095] Mass equation:

[0096] in, is the rate of change of the density of the i-th particle with time, where D / Dt represents the material derivative, that is, the movement of the fluid particle, and N is the total number of neighboring particles. j is the mass of the jth particle, and are the velocity components of the i-th and j-th particles in the β direction, W ij is the kernel function between the i-th and j-th particles, used to calculate the interaction between particles, W ij The partial derivative of the position of the i-th particle in the β direction, which describes the gradient of the interaction between particles.

[0097] Energy equation:

[0098]

[0099] in is the material derivative of the energy of the i-th particle, which represents the rate of change of energy with the movement of fluid particles, p i and p j is the pressure of the i-th and j-th particles, ρ i and ρ j is the density of the i-th and j-th particles, Π ij is the artificial viscosity term between the i-th and j-th particles, used to simulate the viscosity effect, The strain rate tensor of the i-th particle, δ αβ is the Kronecker delta function, which is 1 when α=β and 0 otherwise.

[0100] The water state equation of the FEM-SPH bidirectional fluid-solid coupling model adopts a linear polynomial model. For the fluid material, its pressure is:

[0101] p=C0+C1μ+C2μ 2 +C3μ 3 +(C4+C5μ+C6μ 2 )E (8)

[0102] Where: E is the internal energy per unit initial volume, C iare material constants. In this model, C1=2.723GPa, C2=7.727GPa, C3=14.66GPa, C0=C4=C5=C6=0.

[0103] Then, the wave type of the particle fluid domain is deployed according to the required sea conditions. The wave forms include forward waves and standing waves. The particle parameters for the forward wave are:

[0104] Waveform η:

[0105] Horizontal speed u:

[0106] Vertical speed w:

[0107] Pressure p: p=-ρgz+ρgηK p (z) (12)

[0108] Where: h is the wave height, ν is the frequency, ω is the angular frequency, g is the acceleration of gravity, H is the average water depth, ρ is the density, and t is the period.

[0109] The particle parameters for a standing wave are:

[0110] Waveform η:

[0111] Horizontal speed u:

[0112] Vertical speed w:

[0113] Pressure p: p=-ρgz+ρgηK p (z) (16)

[0114] Where: h is the wave height, ν is the frequency, ω is the angular frequency, g is the acceleration of gravity, H is the average water depth, ρ is the density, and t is the period.

[0115] In this embodiment, by establishing contact between the fluid and the solid, the node segmentation contact algorithm is used to calculate the contact between the collision structure and the water surface. In each time step, each node on the surface is checked and the main node closest to it is determined. If it is found that the node penetrates the main node, a force will be applied in the direction perpendicular to the surface of the main node to prevent further penetration. The strength of this force is related to the degree of penetration and the properties of the elements on both sides of the contact surface. This is the so-called penalty method. The present invention adopts a soft constraint penalty mode and sets the force scaling factor in the soft constraint option to 0.5. When applying the penalty method, the contact conditions are moderately relaxed, allowing the node to penetrate the main segment. When the node penetrates the main segment, it will be subject to a restoring force proportional to the degree of penetration. The specific calculation formula is as follows:

[0116] F M→S =-k(z S -z M ) (17)

[0117] Where: z S -z M is the penetration distance, F M→S Used to remove penetration, k is the penalty factor;

[0118]

[0119] Where: α is the scaling factor, K is the bulk modulus, A is the contact area, and V is the solid unit volume.

[0120] Friction acts on each surface, and the magnitude of the force during sliding is equal to the normal force multiplied by the coefficient of friction using the standard Coulomb's law of friction. The direction of the friction force is opposite to the direction of relative motion of the surfaces.

[0121] This embodiment submits the calculation in the solver, performs post-processing and exports the required data. Figure 4 is the fluid particle motion response, Figure 5-7 The acceleration and pressure are compared with the experimental results at speeds of 3m / s, 8m / s and 10m / s respectively. The simulation results show that the acceleration and pressure change trends are highly consistent with the experimental data. The peak difference between the two is small and the change trends are consistent, which shows that the simulation method has extremely high accuracy under various speed conditions.

[0122] In summary, the present invention relates to the field of aircraft structure impact on water surface and discloses a coupled analysis method for aircraft forced landing on water and its application. Based on the actual structural parameters of the aircraft, a finite element model is established, and the velocities and pitch angles of the model during forced landing are set; a fluid particle pool model of matching size is constructed, and the motion parameters and wave conditions of the initial fluid particles are defined; an FEM-SPH bidirectional fluid-solid coupling contact mode is set, and the program is run to solve and derive the deformation and damage results of the aircraft structure.

[0123] Compared with commonly used traditional methods, the FEM-SPH bidirectional fluid-solid coupling method of the present invention effectively avoids the grid distortion problem caused by large deformation, and adopts enhanced fluid particle dynamics to greatly improve the calculation accuracy and stability of the fluid-solid coupling problem, while significantly simplifying the calculation process; its wide applicability supports the water landing crashworthiness assessment of various aircraft, covering a variety of initial load conditions and complex water conditions; by introducing a material model that considers effective plastic strain, the present invention can accurately predict the water surface impact crashworthiness response of the structure to meet the needs of engineering analysis; it has an efficient calculation process, reduces resource consumption and experimental dependence, and has extremely high economic benefits. In summary, the present invention plays an important role in protecting the lives of aircraft passengers and crew members, and can provide effective guidance for the design of a new generation of aircraft.

[0124] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A coupling analysis method for aircraft ditching, characterized in that: The following steps are involved: Establish a finite element model of the aircraft structure: Build a 3D model based on actual structural parameters, divide the fuselage frame solid element mesh and the skin and floor shell element mesh, define material properties, and perform convergence analysis; Set initial conditions and loads: adjust the initial attitude for forced landing, apply initial X / Y / Z axis speed and gravity load; Construct a fluid particle pool model: Build an enhanced fluid dynamics model based on the aircraft size and initial velocity, and define the initial motion parameters and wave conditions of the fluid particles; Fluid-structure coupling solution and analysis: Set the FEM-SPH bidirectional contact mode and parameters, and use the solver to calculate and output the fluid dynamic loads, motion response, and structural deformation and damage results.

2. The coupled analysis method for aircraft ditching according to claim 1, characterized in that: The enhanced fluid dynamics formula momentum equation used in the method to establish the enhanced fluid dynamics model is as follows: in, At position x i At , the rate of change of the velocity of the i-th particle in the α direction with time, N is the total number of adjacent particles, m j is the mass of the jth particle, σ α,β (x i ) is the stress tensor at position x, α and β are different directions in space, ρ i and ρ j are the densities of the i-th and j-th particles, respectively, A ij is the interaction area vector from the i-th particle to the j-th particle, A ji is the interaction area vector from the jth particle to the i-th particle; The mass equation for the enhanced fluid dynamics formulation is: in, is the rate of change of the density of the i-th particle with time, where D / Dt represents the material derivative, N is the total number of neighboring particles; m j is the mass of the jth particle, and are the velocity components of the i-th and j-th particles in the β direction, W ij is the kernel function between the i-th and j-th particles, used to calculate the interaction between particles, W ij The partial derivative of the position of the i-th particle in the β direction is used to describe the gradient of the interaction between particles.

3. The coupled analysis method for aircraft ditching according to claim 1, characterized in that: The enhanced fluid dynamics formula energy equation used in the method to establish the enhanced fluid dynamics model is: in, is the material derivative of the energy of the i-th particle, which represents the rate of change of energy with the movement of fluid particles, p i and p j is the pressure of the i-th and j-th particles, ρ i and ρ j is the density of the i-th and j-th particles, Π ij is the artificial viscosity term between the i-th and j-th particles, used to simulate the viscosity effect, The strain rate tensor of the i-th particle, δ αβ is the Kronecker function, which is 1 when α = β and 0 otherwise.

4. The coupled analysis method for aircraft ditching according to claim 1, characterized in that: In the method, the particle parameters of the forward-moving wave type are: Waveform η: Horizontal speed u: Vertical speed w: Pressure p: p=-ρgz+ρgηK p (z) Where: h is the wave height, ν is the frequency, ω is the angular frequency, g is the acceleration of gravity, H is the average water depth, ρ is the density, and t is the period.

5. The coupled analysis method for aircraft ditching according to claim 1, characterized in that: In the method, the particle parameters of the wave type being a standing wave are: Waveform η: Horizontal speed u: Vertical speed w: Pressure p: p=-ρgz+ρgηK p (z).

6. The coupled analysis method for aircraft ditching according to claim 1, characterized in that: When setting the FEM-SPH bidirectional contact mode and parameters, the flight structure material model of the FEM-SPH bidirectional fluid-structure coupling model adopts the Piecewise Linear Isotropie Plasticity model that takes into account the strain rate effect, and its deviation stress satisfies the yield function: in Where: s ij is the deviatoric stress tensor, σ y is the yield strength, σ0 is the initial yield strength, β is the hardening parameter, is the hardening function, is the effective plastic strain.

7. The coupled analysis method for aircraft ditching according to claim 6, characterized in that: In the method, the yield stress and plastic strain curve of the added material is the effective strain rate. When implementing the material model, the deviatoric stress is dynamically updated and the yield function is checked. If the deviatoric stress is satisfied, it is accepted. If not, the increment of the effective plastic strain is calculated: Where: G is the shear modulus, E p is the current plastic hardening modulus.

8. The coupled analysis method for aircraft ditching according to claim 6, characterized in that: In the method, the water state equation of the FEM-SPH bidirectional fluid-solid coupling model adopts the Linear Polynomial model. For the fluid material, its pressure is: p=C0+C1μ+C2μ 2 +C3μ 3 +(C4+C5μ+C6μ 2 )E, Where: E is the internal energy per unit initial volume, C i is the material constant.

9. The coupled analysis method for aircraft ditching according to claim 1, characterized in that: The method establishes contact between a fluid and a solid, and uses a node segmentation algorithm to calculate collision surface contact. In each time step, slave surface nodes are detected and the nearest master node is located. If a slave node penetrates the master surface, a resistance is applied in the normal direction of the master surface to prevent penetration. The resistance is determined by the penetration amount and the properties of the elements on both sides of the contact surface. When the slave node penetrates the main segment, it is subjected to a restoring force proportional to the penetration, which is calculated according to the following formula: F M→S =-k(z S -With M ) Where: z S -z M is the penetration distance, F M→S Used to remove penetration, k is the penalty factor; Where: α is the scaling factor, K is the bulk modulus, A is the contact area, and V is the solid unit volume.

10. An application of a coupled analysis method for aircraft ditching, characterized in that: Applied to the simulation verification of forced landing conditions in aircraft airworthiness certification.

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