An airplane air water-throwing simulation analysis method and device based on VOF-DPM

Abstract

This invention relates to the field of simulation and computation technology, specifically to a simulation analysis method and apparatus for aircraft in-flight water drop based on VOF-DPM. It addresses the problems of actual flight tests being significantly affected by weather, incurring high manpower and material costs, and difficulties in measuring local dynamic physical quantities, as well as the time-consuming nature and scale-effect errors in scaled-down model tests. The method includes setting up a flow field domain, dividing it into an air domain and a water domain; spatially discretizing the flow field domain based on a Cartesian octree grid; effectively capturing the free interface of the gas-liquid two-phase flow using a VOF-DPM combined h-type refined adaptive grid; and progressively refining the local grid of the Cartesian octree grid, with a grid refinement standard of 2 in each direction. x The physical parameters of the flow field are initialized; the position and shape of the phase interface are captured using the VOF method based on grid resolution; and the free-moving droplets are simulated to realize the simulation analysis of airborne water droplets from aircraft.

Description

Technical Field

[0001] This invention relates to the field of simulation and calculation technology, and in particular to a simulation analysis method and apparatus for aircraft aerial water drop based on VOF-DPM. Background Technology

[0002] In forest fire prevention, efficient firefighting measures can minimize losses of manpower and resources. Among these, aerial firefighting, characterized by rapid response, high efficiency, minimal damage, and low casualties, has become an effective means of controlling forest fires in modern times. Aerial firefighting utilizes two types of vehicles: helicopters and fixed-wing aircraft. Compared to helicopters, fixed-wing aircraft can perform long-range, large-scale firefighting missions and are currently the most efficient firefighting tools in use. However, actual flight tests are significantly affected by weather, have high manpower and material costs, and are difficult to measure local dynamic physical quantities. Scaled-down model tests, on the other hand, suffer from time-consuming processes and errors caused by scale effects. Summary of the Invention

[0003] In view of this, the purpose of this invention is to propose a simulation analysis method and device for aircraft in-flight water drop based on VOF-DPM, so as to solve the problems that actual flight tests are greatly affected by weather, have high manpower and material costs, and are difficult to measure local dynamic physical quantities, while scaled-down model tests are time-consuming and have errors caused by scale effects.

[0004] To achieve the above objectives, this invention provides a simulation analysis method for aircraft in-flight water drop based on VOF-DPM, including...

[0005] Set up a flow field domain, and divide the flow field domain into two regions: the air domain and the water domain;

[0006] The flow field is spatially discretized based on a Cartesian octree grid; the VOF-DPM combined h-type refined adaptive grid is used to effectively capture the free interface of the gas-liquid two-phase flow.

[0007] The local mesh of the Cartesian-octree mesh is refined layer by layer, with a mesh refinement standard of 2 in each direction. x ;

[0008] Initialize the physical parameters of the flow field;

[0009] The position and shape of the phase interface are captured using a grid-resolution-based VOF method;

[0010] The simulation method simulates freely moving droplets. When the size of the simulated droplets is within the range that the VOF method can capture, the VOF method is used to capture them. If the size of the simulated droplets further decreases after they break up, the method is converted to a discrete phase DPM model to capture them, thereby realizing the simulation analysis of aircraft dropping water in the air.

[0011] As a further improvement to this application, the method of using a grid-resolution-based VOF method to capture the position and shape of the phase-intersection interface includes...

[0012] By calculating the volume fraction a of the local phase i To determine the phase distribution in space and the location of the interface, the volume fraction within the element is:

[0013]

[0014] In the formula V i Let V represent the volume occupied by the i-th phase in the cell, and let V represent the total volume of the cell. For any mesh, the sum of the volume fractions of all phases is 1.

[0015]

[0016] when a i When a = 0, the current unit does not contain the i-th phase; when a i When a = 1, the current unit contains only the i-th phase; when 0 < a i At μ1, there is an interface between different phases in the current cell.

[0017] As a further improvement of this application, fluids existing in the same interface mesh element are considered as a mixture:

[0018] ρ=∑ i ρ i a i

[0019] μ=Σ i μ i a i

[0020]

[0021] In the formula ρ i μ i , (C p ) i , respectively, represent the density, dynamic viscosity, and specific heat of the i-th phase.

[0022] As a further improvement to this application, the simulated free-moving droplet includes the introduction of discrete-phase DPM particles defined in a Lagrange coordinate system for simulating the free-moving droplet.

[0023] As a further improvement to this application, the equation of motion for a collisionless single particle follows Newton's second law:

[0024]

[0025]

[0026] In the formula x p V represents the spatial position of the particle. p Let m be the volume of the particle. p g is the particle's gravity, V p ρg is the buoyancy of the particle, F fp For the liquid interaction force of the particles and F bp This refers to the gas-bubble interaction force.

[0027] As a further improvement to this application, the gas-bubble interaction force F fp Including resistance F D Additional mass force F AM and Basset force F BA :

[0028] F fp =F D +F AM +F BA

[0029] Among them, resistance F D for:

[0030]

[0031] In the formula, A is the cross-sectional area along the incident direction of the particle, and C′ D The effective drag coefficient;

[0032] The effective drag coefficient is obtained by multiplying the drag coefficient of a single particle by a correction factor:

[0033]

[0034] In the formula C D Re is a function of the Reynolds number of a single droplet p The added mass force originates from the fluid with the same acceleration as the particle. For a spherical DPM particle, the added mass is equal to the particle volume V. p Half of:

[0035]

[0036] Basset force is:

[0037]

[0038]

[0039] f H (Re) = 0.75 + 0.105Re

[0040] In the formula, υ is the dynamic viscosity of the fluid, and U is the average flow velocity.

[0041] As a further improvement to this application, the flow field is spatially discretized based on a Cartesian-octree grid; and an effective capture of the free interface of the gas-liquid two-phase flow is achieved using a VOF-DPM combined h-type refined adaptive grid.

[0042] The mesh is automatically coarsened when the volume fraction of the second phase within a cell is less than 0.01.

[0043] The mesh is automatically refined when the volume fraction of the second phase within a cell is greater than 0.05.

[0044] An aircraft in-flight water drop simulation analysis device, including

[0045] The setting module is used to set the flow field domain, dividing the flow field domain into two regions: the air domain and the water domain;

[0046] The discrete module is used to spatially discretize the flow field based on a Cartesian-octree grid; it uses a VOF-DPM combined h-type refined adaptive grid to effectively capture the free interface of the gas-liquid two-phase flow.

[0047] The encryption module is used to perform layer-by-layer encryption on the local mesh of the Cartesian-octree mesh, with a mesh encryption standard of 2 in each direction. x ;

[0048] The initialization module is used to initialize the physical parameters of the flow field.

[0049] The capture module is used to capture the position and shape of phase interfaces using the VOF method based on grid resolution;

[0050] The simulation module is used to simulate freely moving droplets. When the size of the simulated droplets is within the range that the VOF method can capture, the VOF method is used to capture them. If the size of the simulated droplets further shrinks after they break apart, the module is converted to a discrete phase DPM model for capture, thereby realizing the simulation analysis of aircraft dropping water in the air.

[0051] The beneficial effects of this invention are: using numerical simulation methods can effectively reduce research costs and development cycles. It makes it relatively easy to calculate the interactions between discrete phases, defines a phase transition mechanism based on the VOF method, allows for the study of more multiphase flow types, and shortens simulation time. This invention provides a numerical simulation method for aerial water-dropping fire suppression from fixed-wing aircraft, which has advantages such as high computational efficiency, accurate numerical simulation, and strong applicability. It can be used to predict the fire suppression effect under various operating conditions, including different flight altitudes, flight speeds, and different fire extinguishing agent material parameters. Attached Figure Description

[0052] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0053] Figure 1 This is a schematic diagram of the water flow field at a scale according to an embodiment of the present invention;

[0054] Figure 2 This is a schematic diagram of a proportional water tank according to an embodiment of the present invention;

[0055] Figure 3 This is a mesh diagram for CFD calculation of the flow field domain in an embodiment of the present invention;

[0056] Figure 4 This is a schematic diagram of the VOF-DPM model conversion method according to an embodiment of the present invention;

[0057] Figure 5 This is a schematic diagram of mesh refinement near the interface of two-phase flow according to an embodiment of the present invention;

[0058] Figure 6 This is a CFD calculation diagram of water diffusion at 0.028s according to an embodiment of the present invention;

[0059] Figure 7 This is a water diffusion test diagram for 0.028s according to an embodiment of the present invention;

[0060] Figure 8 This is a graph showing the results of gravity-based water injection at a speed of 46.26 m / s in Embodiment 4 of the present invention.

[0061] Figure 9 This is a map showing the distribution of water bodies on the ground under gravity discharge at 46.26 m / s according to Embodiment 4 of the present invention.

[0062] Figure 10 This is a distribution diagram of the pressurized water surface released at a speed of 80.96 m / s according to an embodiment of the present invention. Detailed Implementation

[0063] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0064] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0065] like Figures 1-5 As shown, a simulation analysis method for aircraft in-flight water drop based on VOF-DPM includes...

[0066] Set up a flow field domain, and divide the flow field domain into two regions: the air domain and the water domain;

[0067] The flow field is spatially discretized based on a Cartesian octree grid; the VOF-DPM combined h-type refined adaptive grid is used to effectively capture the free interface of the gas-liquid two-phase flow.

[0068] The mesh is automatically coarsened when the volume fraction of the second phase within a cell is less than 0.01.

[0069] The mesh is automatically refined when the volume fraction of the second phase within a cell is greater than 0.05.

[0070] The local mesh of the Cartesian-octree mesh is refined layer by layer, with a mesh refinement standard of 2 in each direction. x ;

[0071] Initialize the physical parameters of the flow field;

[0072] The location and shape of phase interfaces are captured using a grid-resolution-based VOF method; including...

[0073] By calculating the volume fraction a of the local phase i To determine the phase distribution in space and the location of the interface, the volume fraction within the element is:

[0074]

[0075] In the formula V i Let V represent the volume occupied by the i-th phase in the cell, and let V represent the total volume of the cell. For any mesh, the sum of the volume fractions of all phases is 1.

[0076]

[0077] when a i When a = 0, the current unit does not contain the i-th phase; when a i When a = 1, the current unit contains only the i-th phase; when 0 < a i When <1, there is an interface between different phases in the current cell.

[0078] Fluids existing within the same interface mesh cell are considered a mixture:

[0079] ρ=Σ i ρ i a i

[0080] μ=∑ i μ i a i

[0081]

[0082] In the formula ρ i μ i , (C p ) i , respectively, represent the density, dynamic viscosity, and specific heat of the i-th phase.

[0083] To simulate freely moving droplets, discrete-phase (DPM) particles defined in a Lagrange coordinate system are introduced. When the simulated droplet size is within the capture range of the VOF (Volatile Variable Forecasting) method, VOF is used. If the simulated droplet size further decreases after breakup, the model is switched to a discrete-phase DPM model for capture, enabling simulation analysis of aerial water droplets from an aircraft. The motion field of the DPM particles is established in a Lagrange coordinate system, with the origin at the center of the moving particles. For particles that do not collide with each other, translation and rotation are used to define their motion. Since the particle size is much smaller than the mesh size, rotational acceleration caused by collisions can be ignored.

[0084] During simulation, boundary conditions for the CFD numerical simulation flow field need to be set. Velocity inlet and pressure outlet boundary conditions are set at the inlet and outlet boundaries, respectively. Symmetrical boundary conditions and velocity inlet boundary conditions are set on the sides, and wall boundary conditions are applied to the bottom surface. Furthermore, the volume fraction of water within the tank domain is initialized to 1, while the volume fraction of water in other parts of the flow field domain is 0.

[0085] The CFD numerical calculations employed a k-ω turbulence model to solve the Reynolds-mean stress term in the momentum equation. Pressure was discretized using the PRESTO! scheme, and time and space were solved using a second-order implicit scheme. Convection and dissipation terms were discretized using second-order upwind and second-order central difference schemes, respectively. The VOF (Volatile Air-Water) two-phase interface was captured using the HRIC scheme, and the pressure-velocity coupling was solved using the SIMPLE scheme. An adaptive time step was set, with a maximum time step size of 1×10⁻⁶. -4 Minimum time step 1×10 -8 Initial time step 1×10 -5 The maximum inner iteration is 20.

[0086] The equation of motion for a collisionless single particle follows Newton's second law:

[0087]

[0088]

[0089] In the formula x p V represents the spatial position of the particle. p Let m be the volume of the particle. p g is the particle's gravity, V p ρg is the buoyancy of the particle, F fp For the liquid interaction force of the particles and F bp This refers to the gas-bubble interaction force.

[0090] The gas-bubble interaction force F fp Including resistance F D Additional mass force F AM and Basset force F BA :

[0091] F fp =F D +F AM +F BA

[0092] Among them, resistance F D for:

[0093]

[0094] In the formula, A is the cross-sectional area along the incident direction of the particle, and C′ D The effective drag coefficient;

[0095] The effective drag coefficient is obtained by multiplying the drag coefficient of a single particle by a correction factor:

[0096]

[0097] In the formula CD Re is a function of the Reynolds number of a single droplet p The added mass force originates from the fluid with the same acceleration as the particle. For a spherical DPM particle, the added mass is equal to the particle volume V. p Half of:

[0098]

[0099] Basset force is:

[0100]

[0101]

[0102] f H (Re) = 0.75 + 0.105Re

[0103] In the formula, υ is the dynamic viscosity of the fluid, and U is the average flow velocity.

[0104] Specifically, a water tank model with equal-scale water injection was scaled down to a 1 / 16 scale for numerical simulation. The initial diameter of the DPM particles was 10 mm, and the lateral inflow velocity was set to 11.3 m / s. The monitoring point was located 1.8 m away from the water injection point. Figures 6 and 7 show the comparison between the CFD numerical simulation results and the experimental results of water diffusion at 0.028 s.

[0105] Numerical simulations were performed at a scale equivalent to those of gravity-based water delivery at a flight speed of 46.26 m / s and pressurized water delivery at a flight speed of 80.96 m / s, with a total water volume of 12 t. The captured water volume fraction after complete landing was greater than 0.01% of the total water area. Figure 8 shows the gravity-based water delivery results at 46.26 m / s. Figure 9 , 10 The distribution of surface water under different release speeds for the two release methods is shown.

[0106] An aircraft in-flight water drop simulation analysis device, including

[0107] The setting module is used to set the flow field domain, dividing the flow field domain into two regions: the air domain and the water domain;

[0108] The discrete module is used to spatially discretize the flow field based on a Cartesian-octree grid; it uses a VOF-DPM combined h-type refined adaptive grid to effectively capture the free interface of the gas-liquid two-phase flow.

[0109] The encryption module is used to perform layer-by-layer encryption on the local mesh of the Cartesian-octree mesh, with a mesh encryption standard of 2 in each direction. x ;

[0110] The initialization module is used to initialize the physical parameters of the flow field.

[0111] The capture module is used to capture the position and shape of phase interfaces using the VOF method based on grid resolution;

[0112] The simulation module is used to simulate freely moving droplets. When the size of the simulated droplets is within the range that the VOF method can capture, the VOF method is used to capture them. If the size of the simulated droplets further shrinks after they break apart, the module is converted to a discrete phase DPM model for capture, thereby realizing the simulation analysis of aircraft dropping water in the air.

[0113] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in the details for the sake of brevity.

[0114] This invention is intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A simulation analysis method for aircraft in-flight water drop based on VOF-DPM, characterized in that, include Set up a flow field domain, and divide the flow field domain into two regions: the air domain and the water domain; Spatial discretization of the flow field is based on a Cartesian octree grid. Effective capture of the free interface of gas-liquid two-phase flow is achieved by using a combination of VOF-DPM and h-type refined adaptive meshes, including: The mesh is automatically coarsened when the volume fraction of the second phase within a cell is less than 0.

01. The mesh is automatically refined when the volume fraction of the second phase within a cell is greater than 0.

05. The local mesh of the Cartesian-octree mesh is refined layer by layer, and the mesh refinement standard in each direction is as follows: ; Initialize the physical parameters of the flow field; The position and shape of the phase interface are captured using a grid-resolution-based VOF method; The simulation of freely moving droplets is performed. When the size of the simulated droplets is within the range that the VOF method can capture, the VOF method is used to capture them. If the size of the simulated droplets further decreases after they break up, the simulation is converted to a discrete phase DPM model to capture them, thereby realizing the simulation analysis of aircraft dropping water in the air. The equation of motion for a single, collisionless particle follows Newton's second law: ； In the formula For particle mass, For the spatial position of the particle, It is the velocity of the particle. For the gravity of the particles, Let be the volume of the particle. For the density of the liquid, For the buoyancy of particles, For the liquid interaction forces of particles and Gas-bubble interaction force; gas-bubble interaction force Including resistance Additional mass force and Basset force : ； Among them, resistance for: ； In the formula, A is the cross-sectional area along the incident direction of the particle. The effective drag coefficient; The effective drag coefficient is obtained by multiplying the drag coefficient of a single particle by a correction factor: ； In the formula Reynolds number of a single droplet The function, This is a correction coefficient function; The added mass force originates from the fluid with the same acceleration as the particle; for a spherical DPM particle, the added mass equals the particle volume. Half of: ； Basset force is: ; In the formula, It is the dynamic viscosity of the fluid. It is the diameter of the particle. Representing the current moment, It is the proposed integration variable. This is a kernel function used to describe the influence of past motion on current motion. For fluid velocity, Let be the relative acceleration between the particle and the fluid. The formula for the kernel function is: ； In the formula, The kinematic viscosity of the fluid. Let U be the radius of the particle, and U be the average velocity of the fluid. This is the Reynolds number correction factor.

2. The simulation analysis method for aircraft in-flight water drop based on VOF-DPM according to claim 1, characterized in that, The method of using a grid-resolution-based VOF method to capture the position and shape of phase interfaces includes... By calculating the volume fraction of the local phase To determine the phase distribution in space and the location of the interface, the volume fraction within the element is: ; In the formula This represents the volume occupied by the i-th phase in the cell. This represents the total volume of the element. For any mesh, the sum of the volume fractions of all phases is 1. ； when When, the current unit does not contain the i-th phase; when When, the current unit contains only the i-th phase; when At that time, there are interfaces between different phases in the current unit.

3. The simulation analysis method for aircraft in-flight water dropping based on VOF-DPM according to claim 2, characterized in that, Fluids existing within the same interface mesh cell are considered a mixture: ； In the formula, , respectively, represent the density, dynamic viscosity, and specific heat of the i-th phase.

4. The simulation analysis method for aircraft in-flight water drop based on VOF-DPM according to claim 1, characterized in that, The simulated freely moving droplets include Discrete-phase DPM particles, defined in Lagrange coordinates, are introduced to simulate freely moving droplets.

5. A simulation and analysis device for aircraft in-flight water dropping, characterized in that, For performing the method according to any one of claims 1 to 4, comprising: The setting module is used to set the flow field domain, dividing the flow field domain into two regions: the air domain and the water domain; The discrete module is used to spatially discretize the flow field based on a Cartesian-octree grid; it uses a VOF-DPM combined h-type refined adaptive grid to effectively capture the free interface of the gas-liquid two-phase flow. The encryption module is used to perform layer-by-layer encryption of the local mesh of the Cartesian-octree mesh, with the following encryption standard for each direction: ; The initialization module is used to initialize the physical parameters of the flow field. The capture module is used to capture the position and shape of phase interfaces using the VOF method based on grid resolution; The simulation module is used to simulate freely moving droplets. When the size of the simulated droplets is within the range that the VOF method can capture, the VOF method is used to capture them. If the size of the simulated droplets further shrinks after they break apart, the module is converted to a discrete phase DPM model for capture, thereby realizing the simulation analysis of aircraft dropping water in the air.

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