Flexible Extendable Nozzle Profile Development Prediction Method Based on Dynamic and Overlapping Meshes

Abstract

The present invention discloses a method for predicting the profile expansion of a flexible extension nozzle based on a dynamic grid and an overlapping grid, which belongs to the field of flexible extension nozzles for rocket engines. In view of the dynamic motion characteristics of the profile expansion of the flexible extension nozzle, the present invention sets the nozzle extension section and the external flow field area as a dynamic calculation domain, and the remaining areas as a static calculation domain; based on the FLUENT dynamic grid, the dynamic simulation of the profile expansion movement of the flexible extension nozzle is realized; based on the overlapping grid coupling setting, the data transmission between the dynamic and static calculation domains is realized, and the unsteady dynamic flow prediction of the flexible extension nozzle is carried out based on the coupling of the dynamic grid technology and the overlapping grid technology, so as to realize the performance estimation of the flexible extension nozzle of the rocket engine under supersonic gas flow. The present invention can analyze and construct the supersonic flow law of the flexible extension nozzle, facilitate the optimization of the structure of the flexible extension nozzle, realize the on-the-fly control and adjustment of the profile expansion of the flexible extension nozzle of the rocket engine, and solve the engineering application problems of the flexible extension nozzle.

Description

Technical Field

[0001] The present invention relates to a method for predicting the profile expansion of a flexible extension nozzle based on a dynamic grid and an overlapping grid, and in particular to a method for predicting the profile expansion of a flexible extension nozzle of a rocket engine based on the dynamic grid and the overlapping grid technology, belonging to the field of flexible extension nozzles of rocket engines. Background Art

[0002] The nozzle is a crucial component of a rocket engine, converting the thermal energy of propellant combustion products into the rocket's kinetic energy. Losses in solid rocket engines are primarily concentrated in the nozzle. These losses primarily include two-phase flow losses, expansion losses, viscous losses, heat dissipation losses, chemical non-equilibrium losses, and nozzle fluid flow non-adaptive losses. Nozzle non-adaptive losses refer to the inability of the fluid flow within the nozzle to adapt to changes in the external environment. Environmental changes, particularly those at operating altitude, can cause variations in atmospheric parameters such as temperature, pressure, and density. Non-adaptive losses account for the largest proportion of all nozzle losses. For strategic weapons or large launch vehicles operating between the atmosphere and orbit, altitude fluctuations have a significant impact on engine performance. Therefore, reducing nozzle fluid flow non-adaptive losses is a highly effective approach to improving rocket engine performance.

[0003] The use of altitude-compensating nozzles in rocket engines allows for altitude compensation, thereby reducing non-adaptive losses. The flexible extendable nozzle is a new type of extendable nozzle, consisting primarily of a base nozzle and a profile deployment system. The base nozzle is made of rigid material, and its profile remains stationary during operation. The profile deployment system comprises a rigid extension rod and a flexible deployment skirt, forming the nozzle profile deployment component. The extension rod is controlled and adjusted by a drive device, continuously extending outward as the operating altitude changes. During this process, the flexible deployment skirt gradually stretches and deforms circumferentially, forming the nozzle profile. The inner side of the deployment skirt is coated with a memory alloy and a thermal insulation layer. The former enhances profile rigidity and maintains profile integrity, while the latter provides thermal protection against the high-temperature combustion gases during nozzle operation. The flexible extendable nozzle offers the advantages of a continuously variable area ratio, high mass ratio, simple structure, and excellent sealing, offering broad research potential.

[0004] At present, there is no relatively accurate analysis means and numerical prediction method for the dynamic flow of supersonic gas in the flexible extension nozzle of a rocket engine, especially the numerical prediction method for the surface expansion process of the relative motion between the basic nozzle section and the extension section of the flexible extension nozzle. Summary of the Invention

[0005] The main purpose of the present invention is to provide a flexible extension nozzle profile deployment prediction method based on dynamic grids and overlapping grids, simulate the profile deployment motion process of the flexible extension nozzle component of a rocket engine under supersonic gas flow conditions, especially simulate the relative motion between the base nozzle and the extension section, and perform unsteady dynamic flow prediction of the flexible extension nozzle of a rocket engine. The present invention realizes dynamic simulation of the profile deployment motion of the flexible extension nozzle based on the FLUENT dynamic grid; realizes data transfer between the dynamic and static computational domains based on the overlapping grid coupling setting to perform unsteady dynamic flow prediction of the flexible extension nozzle under supersonic gas flow conditions, and further accurately simulates and analyzes the unsteady flow characteristics of the flexible extension nozzle of a rocket engine under supersonic gas flow, and realizes performance prediction. The present invention can analyze and construct the supersonic flow law of the flexible extension nozzle, facilitate further optimization of the structure of the flexible extension nozzle, realize the on-the-fly control and adjustment of the profile deployment of the flexible extension nozzle of a rocket engine, and solve the practical engineering application problems of the flexible extension nozzle.

[0006] The purpose of the present invention is achieved through the following technical solutions.

[0007] The present invention discloses a method for predicting the profile expansion of a flexible extension nozzle based on dynamic grids and overlapping grids. An axisymmetric model is established based on the flexible extension nozzle of a rocket engine. According to the dynamic motion characteristics of the profile expansion of the flexible extension nozzle, the nozzle extension section and the external flow field area are set as dynamic calculation domains, and the remaining areas are set as static calculation domains, wherein: the dynamic calculation domain is used to improve the prediction accuracy of the unsteady flow characteristics of the flexible extension nozzle of the rocket engine under supersonic gas flow, and the static calculation domain is used to improve the prediction efficiency under the premise of ensuring the prediction accuracy of the unsteady flow characteristics of the flexible extension nozzle of the rocket engine under supersonic gas flow; the dynamic and static calculations are realized based on the overlapping grid coupling setting. Data transfer between computational domains, while the mesh of the dynamic computational domain is updated, keeps the mesh of the static computational domain unchanged; based on the dynamic mesh technology, dynamic simulation of the flexible extended nozzle profile expansion movement is realized in the dynamic computational domain, and the mesh nodes are only updated on both sides of the dynamic computational domain, which can maintain the boundary layer mesh quality near the nozzle extension section unchanged, ensuring accurate prediction of the unsteady flow characteristics of the rocket engine flexible extended nozzle under supersonic gas flow; based on the coupling of dynamic mesh technology and overlapping mesh technology, the unsteady dynamic flow prediction of the flexible extended nozzle under supersonic gas flow conditions is carried out, realizing the performance estimation of the rocket engine flexible extended nozzle under supersonic gas flow. The present invention can analyze and construct the supersonic dynamic flow law of the flexible extended nozzle, facilitate further optimization of the flexible extended nozzle structure, realize the on-the-fly control and adjustment of the rocket engine flexible extended nozzle profile expansion, and solve the practical engineering application problems of the flexible extended nozzle.

[0008] The present invention discloses a method for predicting the profile development of a flexible extension nozzle based on a dynamic grid and an overlapping grid, comprising the following steps:

[0009] Step 1: To predict the surface deployment motion of a rocket engine's flexible extension nozzle under supersonic gas flow conditions, the relative motion between the base nozzle and the extension section must first be simulated. The flexible extension nozzle's surface deployment is achieved by the continuous outward extension of the nozzle extension section and the gradual stretching of the gas expansion skirt. The memory alloy on the inside of the gas expansion skirt maintains the integrity of the nozzle's internal surface while enhancing the surface rigidity. To address the aforementioned characteristics of the rocket engine's flexible extension nozzle's surface deployment motion, the rocket engine's flexible extension nozzle is modeled as an axisymmetric model. The relative motion between the base nozzle and the extension section is simplified to the relative parallel motion of two stacked rigid bodies, and the geometric parameters of the axisymmetric model are determined.

[0010] Preferably, the rocket engine flexible extension nozzle is modeled as an axisymmetric model using CAD software.

[0011] Step 2: Establish dynamic and static fluid calculation domains for the geometric model of the flexible extension nozzle of the rocket engine established in step 1, and mesh the fluid calculation domain in blocks. In view of the characteristics of the surface expansion movement process of the flexible extension nozzle of the rocket engine, the nozzle extension section and the external flow field area are set as the dynamic calculation domain, and the remaining areas are set as the static calculation domain, wherein: the dynamic calculation domain is used to improve the prediction accuracy of the unsteady flow characteristics of the flexible extension nozzle of the rocket engine under supersonic gas flow, and the static calculation domain is used to improve the prediction efficiency while ensuring the prediction accuracy of the unsteady flow characteristics of the flexible extension nozzle of the rocket engine under supersonic gas flow. Mesh the dynamic and static fluid calculation domains to obtain the calculation grid of the flexible extension nozzle of the rocket engine. In the calculation grid, the boundary layer grid of the nozzle extension section area of ​​the dynamic calculation domain is encrypted so that the boundary layer grid satisfies y + The grid is refined at the nozzle inner wall, throat and nozzle exit areas in the static calculation domain to improve the calculation accuracy and better capture the nozzle near-wall parameters.

[0012] In order to improve the accuracy and efficiency of numerical calculations, as a preference, structured grids are used for both the dynamic and static regions.

[0013] y described in step 2 + is an indicator to measure the grid accuracy, where y is the thickness of the first layer of grid, μ is the molecular viscosity, ρ is the fluid density, and τ w is the wall shear stress; the reasonable range for the nozzle supersonic gas flow should be 0.1≤y + ≤2.

[0014]

[0015] The near-wall parameters described in step 2 include wall pressure and friction.

[0016] Step 3: Based on the computational grid for the rocket engine's flexible extended nozzle created in Step 2, an overlapping mesh technique is used to couple the dynamic and static computational domains. This overlapping mesh coupling setting enables data transfer between the dynamic and static computational domains, improving the efficiency of predicting the unsteady dynamic flow of the flexible extended nozzle while maintaining sufficient prediction accuracy.

[0017] The data transfer between the dynamic and static computational domains described in step three includes the transfer of mass, momentum, and energy.

[0018] In order to ensure the rationality of the calculation model and improve the calculation accuracy and efficiency, as a preferred method, the Matching method is used to couple the grids at the interface between the fluid domain and the fluid domain, and the Coupled method is used to couple the grids at the interface between the fluid domain and the solid domain.

[0019] Step 4: Use dynamic mesh technology to dynamically simulate the expansion movement of the flexible extension nozzle profile of the rocket engine flexible extension nozzle calculation model. The dynamic simulation of the nozzle profile expansion movement is realized based on the dynamic mesh technology of FLUENT. The dynamic mesh technology is used in the dynamic calculation domain to realize the movement of the grid nodes, and the movement of the grid is realized by the movement and update of the grid nodes, thereby realizing the dynamic movement of the extension section of the flexible extension nozzle. The movement of the grid nodes is carried out in the entire dynamic calculation domain, while the update of the grid nodes is only carried out on both sides of the dynamic calculation domain. In this way, the boundary layer mesh quality near the nozzle extension section can be kept unchanged, ensuring the accurate prediction of the unsteady flow characteristics of the rocket engine flexible extension nozzle under supersonic gas flow, thereby realizing the performance estimation of the flexible extension nozzle under supersonic gas flow.

[0020] In order to improve the accuracy of mesh node motion, it is preferred to use UDF to compile the mesh motion conditions and link them to the FLUENT solver.

[0021] Step 5: Based on Steps 1 to 4, perform unsteady numerical calculations of the flexible extended nozzle under supersonic gas flow conditions. This involves using dynamic and overlapping meshes to achieve efficient and high-precision predictions of the flexible extended nozzle's profile. Accurately simulating the unsteady flow characteristics of the flexible extended nozzle under supersonic gas flow requires the use of appropriate turbulence models, reasonable and accurate boundary conditions, and a high-precision solution method.

[0022] Preferably, FLUENT is used to perform unsteady numerical prediction of the flexible extended nozzle under supersonic gas flow conditions.

[0023] In order to ensure the rationality of the calculation model and improve the prediction accuracy and efficiency, the SST k-ω model is used as the preferred turbulence model, the pressure inlet and pressure outlet conditions are used as the boundary conditions, and the second-order upwind double-precision solver is used as the solution method.

[0024] It also includes step six: based on the unsteady flow characteristics of the flexible extended nozzle of the rocket engine under the supersonic gas flow obtained by simulation in step five, the supersonic gas flow process of the flexible extended nozzle is numerically simulated and analyzed and post-processed to obtain the Mach number cloud map, temperature cloud map, density cloud map of the supersonic gas flow of the flexible extended nozzle of the rocket engine, as well as the distribution curve map of the nozzle thrust changing with time, the distribution curve map of the nozzle specific impulse and various specific impulse losses changing with time, analyze and construct the supersonic flow law of the flexible extended nozzle, facilitate further optimization of the structure of the flexible extended nozzle, realize the on-the-fly control and adjustment of the profile deployment of the flexible extended nozzle of the rocket engine, and be able to solve the actual engineering application problems of the flexible extended nozzle.

[0025] Because the calculation model of the flexible extendable nozzle is different from that of the traditional fixed nozzle, in order to accurately obtain the thrust, specific impulse, and various specific impulse losses of the flexible extendable nozzle, the calculation formulas of the nozzle thrust, nozzle specific impulse, and various losses need to be modified. The nozzle thrust, nozzle specific impulse, and various losses described in step 6 are calculated using the following formula:

[0026] The formula for calculating the rocket nozzle thrust is:

[0027]

[0028] in, is the mass flow rate, v e is the exhaust velocity at the nozzle outlet, P e is the nozzle outlet pressure, P a is the external pressure, A e is the nozzle outlet cross-sectional area.

[0029] The calculation of thrust using (1.2) requires obtaining the aerodynamic parameters of the nozzle outlet cross section. The position of the extension section of the flexible extension nozzle is different at each moment, and the nozzle outlet cross section changes in real time, so it is not convenient to use this formula for calculation. For the flexible extension nozzle model, the thrust calculation formula should be

[0030]

[0031] Among them, P n is the cross-sectional pressure of the nozzle fixed expansion section, A n is the cross-sectional area of ​​the fixed expansion section, P i is the inner wall pressure of the nozzle expansion section, and A is the cross-sectional area of ​​the expansion section.

[0032] In (1.3), The term characterizes the thrust of the fixed section nozzle, The term characterizes the thrust provided by the moving expansion section. This term is directly obtained by the numerical software, so it is easier to obtain the thrust of the flexible extension nozzle at each moment.

[0033] The formula for calculating the specific impulse of a rocket engine is:

[0034]

[0035] (1.4) where F is the nozzle thrust, is the mass flow rate.

[0036] The expansion loss calculation formula is:

[0037] Fx=2π∫r(P+ρU x 2 )dr (1.5)

[0038]

[0039] (1.5) where P is the pressure, ρ is the gas density, Ux is the axial velocity, and r is the radial distance from the axis;

[0040] (1.6) U y = radial velocity;

[0041] Expansion loss is the difference between the total thrust and the axial thrust and is expressed in terms of specific impulse as

[0042]

[0043] As the propellant gas passes through the nozzle, friction is generated between the gas flow and the nozzle wall, and the resistance is equal to the impulse loss of the gas. This lost impulse cannot generate nozzle thrust and is defined as surface friction loss (viscous loss).

[0044] The shear stress at the wall is defined as

[0045]

[0046] (1.8) where μ EFF is the effective eddy viscosity.

[0047] The axial resistance caused by surface friction is

[0048] F drag =∫0LT ω cosθd(A s ) (1.9)

[0049] (1.9) where θ is the angle of the wall, A s is the wall surface area, and the viscous loss is expressed in terms of specific impulse as

[0050]

[0051] In summary, the loss caused by the extension section of the flexible extension nozzle is

[0052] I(sys)=I0-Isp-I(div)-I(vis) (1.11)

[0053] Where I0 is the ideal specific impulse calculated from the one-dimensional isentropic flow, Isp is the calculated specific impulse of the nozzle, I(div) is the nozzle expansion loss, and I(vis) is the viscous loss in the fixed section of the nozzle.

[0054] Beneficial results:

[0055] 1. The present invention discloses a method for predicting the profile deployment of a flexible extended nozzle based on dynamic grids and overlapping grids. The method realizes the simulation of the profile deployment of a flexible extended nozzle of a rocket engine based on the FLUENT dynamic grid and overlapping grid technology. The dynamic grid technology is used to realize the dynamic simulation of the nozzle profile deployment process of the flexible extended nozzle under supersonic gas flow conditions, providing a solution for the profile motion simulation of the flexible extended nozzle and an effective prediction approach and simulation means for the unsteady numerical calculation of the flexible extended nozzle.

[0056] 2. The flexible extension nozzle profile expansion prediction method based on dynamic grid and overlapping grid disclosed in the present invention adopts a numerical algorithm that couples overlapping grid technology with dynamic grid technology to realize data transmission between the dynamic and static calculation domains of the flexible extension nozzle, while ensuring the prediction accuracy of the dynamic calculation domain, while improving the prediction accuracy and prediction efficiency of the static calculation domain, providing an efficient prediction method for the numerical calculation of the flexible extension nozzle.

[0057] 3. The flexible extended nozzle profile deployment prediction method based on dynamic grids and overlapping grids disclosed by the present invention can accurately predict the unsteady flow characteristics of the flexible extended nozzle under supersonic gas flow conditions on the basis of achieving beneficial effects 1 and 2, and can analyze and construct the supersonic dynamic flow law of the flexible extended nozzle, so as to facilitate further optimization of the structure of the flexible extended nozzle, realize the on-the-fly control and adjustment of the profile deployment of the flexible extended nozzle of the rocket engine, and solve the actual engineering application problems of the flexible extended nozzle. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 The present invention provides a flow chart of a method for simulating the profile expansion of a flexible extension nozzle of a rocket engine based on FLUENT dynamic grid and overlapping grid technology.

[0059] Figure 2 Schematic diagram of a flexible extension nozzle of a rocket engine according to an embodiment of the present invention, wherein: Figure 2 .1 is a schematic diagram of the geometric model of the flexible extension nozzle. Figure 2 .2 is a schematic diagram of the computational geometry model of the flexible extension nozzle.

[0060] Figure 3 Schematic diagram of grid division in an embodiment of the present invention.

[0061] Figure 4 Schematic diagram of boundary conditions in an embodiment of the present invention.

[0062] Figure 5 Mach number cloud diagram of the numerical simulation results at different times in an embodiment of the present invention.

[0063] Figure 6 Temperature cloud diagram of the numerical simulation results at 0.08s in an embodiment of the present invention, where: Figure 6 .1 is the local temperature cloud map of the step, Figure 6 .2 is the local density cloud map of the step.

[0064] Figure 7 A distribution curve of nozzle thrust changing with time in an embodiment of the present invention.

[0065] Figure 8 Distribution curve of nozzle specific impulse and various specific impulse losses over time in an embodiment of the present invention. DETAILED DESCRIPTION

[0066] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0067] Example 1

[0068] like Figure 1 As shown in FIG. 1 , the surface deployment simulation method of the flexible extension nozzle of a rocket engine disclosed in this embodiment is shown. The specific implementation steps are as follows:

[0069] Step 1: Establish an axisymmetric model of the rocket engine flexible extension nozzle.

[0070] The flexible extension nozzle of the rocket engine is used as the verification object to determine the geometric parameters of the axisymmetric model of the flexible extension nozzle of the rocket engine. The geometric modeling is carried out using CAD software, such as Figure 2 , which is a schematic diagram of the geometric model of the flexible extension nozzle of the rocket engine used in an embodiment of the present invention.

[0071] Step 2: Establish dynamic and static fluid calculation domains and perform structural mesh division.

[0072] The specific implementation method of step 2 includes the following steps:

[0073] Step 2.1: Import the geometric model created in step 1 into ICEM. Set up a dynamic computational domain to simulate the parallel movement of the nozzle extension, and a static computational domain to simulate the flow of the nozzle supersonic gas flow. The interface between the two computational domains is always aligned, and an interface is set at the interface between the two computational domains. During the simulation process, data is transferred through the interface pairs set between the dynamic and static computational domains.

[0074] Step 2.2: Divide the flow domain into blocks and structure the grid. Considering the wall effect, the grid density should be increased in the throat, inner wall and nozzle outlet of the rocket engine flexible extension nozzle. At the same time, the wall y + Within a reasonable range.

[0075] Step 2.3: Verify the grid independence. Take the nozzle thrust index as the verification standard, take multiple groups of flow domains with different grid numbers to perform calculations under the same working conditions, and compare the nozzle thrusts calculated by flow domains with different grid numbers. When the difference between the nozzle thrust calculated by the flow domain with a certain grid number and the nozzle thrust calculated by the flow domain with the maximum grid number is ≤0.1%, the optimal grid number is obtained. Figure 3 FIG. 1 is a schematic diagram of grid division in an embodiment of the present invention.

[0076] Step 3: Use overlapping grid technology to couple the dynamic and static computational domains.

[0077] The interface between the dynamic computational domain grid and the static computational domain grid is coupled and set. The Matching type interface is selected at the interface between the fluid domain and the fluid domain, and the Coupled Wall & Matching type interface is selected at the interface between the fluid domain and the solid domain.

[0078] Step 4: Use dynamic mesh technology to perform dynamic simulation of the flexible extended nozzle profile expansion process.

[0079] Use the Layering type dynamic mesh update method, select the Constant Height update option, select 0.4 for SplitFactors, and 0.2 for Collapse Factor. Compile the UDF file, specify Fluid-moving, Wall-moving, and Interface-wall as rigid bodies, perform parallel movement, and specify interface-left and interface-right as stationary. Since FLUENT prioritizes the movement of stationary areas over unit areas, the nodes of interface-left and interface-right remain stationary during the mesh change process, resulting in the mesh being updated in these two locations. The advantage of this approach is that the mesh can be maintained unchanged near the nozzle extension section, ensuring that the mesh near the wall has a more stable y + value, and avoid frequent interpolation in areas with large gradients, thereby improving calculation accuracy,

[0080] Step 5: Carry out numerical calculation of the unsteady flow of the flexible extended nozzle under supersonic gas flow conditions.

[0081] The specific implementation method of step 5 includes the following steps:

[0082] Step 5.1: Set up the computational model.

[0083] The control equations are the NS equations based on Reynolds average, including mass, momentum, energy and component transport equations; the turbulence model is the SST k-ω model.

[0084] Step 5.2: Set boundary conditions.

[0085] The inlet condition of the fluid calculation domain adopts the pressure inlet condition, the outlet condition adopts the pressure outlet condition, the axis of the axisymmetric model adopts the axis condition, and the rest of the positions adopt the adiabatic no-slip wall condition, as shown in the following example: Figure 4 FIG. 1 is a schematic diagram of boundary condition settings in an embodiment of the present invention.

[0086] Step 5.3: Set the calculation method.

[0087] A double-precision unsteady density-based solver is used to numerically simulate the dynamic flow process of supersonic gas flow in a flexible extended nozzle of a rocket engine. The convection term in the gas phase governing equation is discretized using a second-order upwind scheme, and the diffusion term is discretized using a central difference scheme. The solver uses a coupled solver to simultaneously solve the coupled equations of continuity, momentum, energy, and species transport equations. The time step is 10 -4s, with 50 iterations per time step. The total calculation time is 0.2 s, of which the nozzle extension section moves continuously to the final working position during the first 0.1 s, and the nozzle extension cone is fixed at the working position during the last 0.1 s.

[0088] Step 6: Perform numerical simulation analysis and post-processing on the supersonic gas flow process of the flexible extended nozzle.

[0089] The numerical results are visualized as follows Figure 5-8 As shown, there are respectively Mach number cloud diagrams of the supersonic gas flow of the flexible extension nozzle of the rocket engine at different times in an embodiment of the present invention, a temperature cloud diagram and density cloud diagram of the local step of the flexible extension nozzle at 0.08s, a distribution curve diagram of the nozzle thrust changing with time, and a distribution curve diagram of the nozzle specific impulse and various specific impulse losses changing with time.

[0090] In summary, the surface expansion simulation method of the flexible extension nozzle of a rocket engine disclosed in this embodiment, by establishing an axisymmetric model of the flexible extension nozzle of a rocket engine, establishing dynamic and static fluid calculation domains and dividing the grid, using dynamic grid technology to simulate the surface expansion process of the flexible extension nozzle, using overlapping grid technology to realize data transfer between dynamic and static calculation domains, and finally realizing a numerical simulation method for predicting the unsteady flow of the flexible extension nozzle of a rocket engine under supersonic gas flow conditions, belongs to the field of flexible extension nozzles of rocket engines. The present invention provides a numerical simulation method for the surface expansion motion process of the flexible extension nozzle of a rocket engine, provides an efficient prediction method for the numerical calculation of the flexible extension nozzle, and the simulation method can maximize the high quality characteristics of the structured grid, while not changing the quality of the boundary layer grid near the wall of the nozzle, significantly improving the accuracy, speed and reliability of the numerical prediction. The present invention can analyze and construct the supersonic flow law of the flexible extension nozzle based on FLUENT dynamic grid and overlapping grid technology, facilitate further optimization of the structure of the flexible extension nozzle, realize the control and adjustment of the surface expansion of the flexible extension nozzle of the rocket engine, and solve the practical engineering application problems of the flexible extension nozzle.

[0091] The above specific description further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A flexible extension nozzle profile development prediction method based on dynamic and overlapping grids, characterized by: The following steps are included: Step 1: Predict the surface expansion motion process of the rocket engine flexible extension nozzle under supersonic gas flow conditions. First, it is necessary to simulate the relative motion between the basic nozzle and the extension section. The surface expansion of the flexible extension nozzle is achieved by the continuous extension of the nozzle extension section and the gradual stretching of the gas expansion skirt. The memory alloy on the inside of the gas expansion skirt maintains the integrity of the nozzle inner surface and enhances the surface stiffness. In view of the above characteristics of the surface expansion motion process of the rocket engine flexible extension nozzle, the rocket engine flexible extension nozzle is modeled as an axisymmetric model. The relative motion between the basic nozzle and the extension section is simplified to the relative parallel movement of two stacked rigid bodies. The geometric parameters of the axisymmetric model are determined. Step 2: Establish dynamic and static fluid calculation domains for the geometric model of the flexible extension nozzle of the rocket engine established in step 1, and mesh the fluid calculation domain in blocks; according to the characteristics of the surface expansion movement process of the flexible extension nozzle of the rocket engine, set the nozzle extension section and the external flow field area as the dynamic calculation domain, and the remaining areas as the static calculation domain, wherein: the dynamic calculation domain is used to improve the prediction accuracy of the unsteady flow characteristics of the flexible extension nozzle of the rocket engine under supersonic gas flow, and the static calculation domain is used to improve the prediction efficiency under the premise of ensuring the prediction accuracy of the unsteady flow characteristics of the flexible extension nozzle of the rocket engine under supersonic gas flow; mesh the dynamic and static fluid calculation domains to obtain the calculation grid of the flexible extension nozzle of the rocket engine; in the calculation grid, the boundary layer grid of the nozzle extension section area of ​​the dynamic calculation domain is encrypted to make the boundary layer grid meet y + The grid is refined in the nozzle inner wall, throat and nozzle exit areas in the static calculation domain to improve the calculation accuracy and better capture the nozzle near-wall parameters; Step 3: Based on the computational grid for the flexible extended nozzle of the rocket engine divided in Step 2, the overlapping grid technology is used to couple the dynamic and static computational domains. Data transfer between the dynamic and static computational domains is achieved based on the overlapping grid coupling setting, thereby improving the prediction efficiency of the unsteady dynamic flow of the flexible extended nozzle while meeting the prediction accuracy. Step 4: Dynamically simulate the deployment motion of the flexible extended nozzle profile of the rocket engine flexible extended nozzle calculation model using dynamic mesh technology; the dynamic simulation of the deployment motion of the nozzle profile is realized based on the dynamic mesh technology of FLUENT; the dynamic mesh technology is used in the dynamic calculation domain to realize the movement of the grid nodes, and the movement of the grid is realized by the movement and update of the grid nodes, thereby realizing the dynamic movement of the extended section of the flexible extended nozzle; the movement of the grid nodes is carried out in the entire dynamic calculation domain, while the update of the grid nodes is only carried out on both sides of the dynamic calculation domain, so that the boundary layer mesh quality near the nozzle extension section can be kept unchanged, ensuring the accurate prediction of the unsteady flow characteristics of the flexible extended nozzle of the rocket engine under supersonic gas flow, thereby realizing the performance estimation of the flexible extended nozzle under supersonic gas flow; Step 5: Based on steps 1 to 4, carry out unsteady numerical calculations of the flexible extended nozzle under supersonic gas flow conditions, that is, realize efficient and high-precision prediction of the flexible extended nozzle profile based on dynamic grids and overlapping grids.

2. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 1, wherein: The method also includes step six, which includes performing numerical simulation analysis and post-processing on the supersonic gas flow process of the flexible extended nozzle of the rocket engine based on the unsteady flow characteristics of the flexible extended nozzle of the rocket engine under the supersonic gas flow obtained by simulation in step five, obtaining a Mach number cloud map, a temperature cloud map, a density cloud map of the supersonic gas flow of the flexible extended nozzle of the rocket engine, a distribution curve map of the nozzle thrust changing with time, and a distribution curve map of the nozzle specific impulse and various specific impulse losses changing with time, analyzing and constructing the supersonic flow law of the flexible extended nozzle, facilitating further optimization of the structure of the flexible extended nozzle, realizing the on-the-fly control and adjustment of the profile deployment of the flexible extended nozzle of the rocket engine, and being able to solve the actual engineering application problems of the flexible extended nozzle.

3. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 1 or 2, wherein: y described in step 2 + is an indicator to measure the grid accuracy, where y is the thickness of the first layer of grid, μ is the molecular viscosity, ρ is the fluid density, and τ w is the wall shear stress; the reasonable range for the nozzle supersonic gas flow should be 0.1≤y + ≤2; The near-wall parameters described in step 2 include wall pressure and friction; The data transfer between the dynamic and static computational domains described in step three includes the transfer of mass, momentum, and energy.

4. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 3, wherein: The flexible extension nozzle of the rocket engine is modeled as an axisymmetric model using CAD software.

5. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 3, wherein: In order to improve the accuracy and efficiency of numerical calculations, structured grids are used in both dynamic and static areas.

6. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 3, wherein: In order to ensure the rationality of the calculation model and improve the calculation accuracy and efficiency, the Matching method is used to couple the grids at the interface between the fluid domain and the fluid domain, and the Coupled method is used to couple the grids at the interface between the fluid domain and the solid domain.

7. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 3, wherein: FLUENT is used to perform unsteady numerical prediction of flexible extended nozzle under supersonic gas flow conditions.

8. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 3, wherein: In order to improve the accuracy of mesh node motion, UDF is used to compile the mesh motion conditions and link them to the FLUENT solver.

9. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 3, wherein: In order to ensure the rationality of the calculation model and improve the prediction accuracy and efficiency, the SSTk-ω model is used as the turbulence model, the pressure inlet and pressure outlet conditions are used as the boundary conditions, and the second-order upwind double-precision solver is used as the solution method.

10. The method for predicting the profile development of a flexible extending nozzle based on a dynamic grid and an overlapping grid according to claim 2, wherein: Since the calculation model of the flexible extendable nozzle is different from that of the traditional fixed nozzle, in order to accurately obtain the thrust, specific impulse and various specific impulse losses of the flexible extendable nozzle, the calculation formulas of the nozzle thrust, nozzle specific impulse and various losses need to be modified; the nozzle thrust, nozzle specific impulse and various losses described in step 6 are calculated using the following formula: The formula for calculating the rocket nozzle thrust is: in, is the mass flow rate, v e is the exhaust velocity at the nozzle outlet, P e is the nozzle outlet pressure, P a is the external pressure, A e is the nozzle outlet cross-sectional area; The calculation of thrust using (1.2) requires obtaining the aerodynamic parameters of the nozzle outlet cross section. The position of the extension section of the flexible extension nozzle is different at each moment, and the nozzle outlet cross section changes in real time, so it is not convenient to use this formula for calculation. For the flexible extension nozzle model, the thrust calculation formula should be Among them, P n is the cross-sectional pressure of the nozzle fixed expansion section, A n is the cross-sectional area of ​​the fixed expansion section, P i is the inner wall pressure of the nozzle expansion section, A is the cross-sectional area of ​​the expansion section; In (1.3), The term characterizes the thrust of the fixed section nozzle, The term represents the thrust provided by the moving expansion section. This term is directly obtained by the numerical software, so it is easier to obtain the thrust of the flexible extension nozzle at each moment. The formula for calculating the specific impulse of a rocket engine is: (1.4) where F is the nozzle thrust, is the mass flow rate; The expansion loss calculation formula is: Fx=2π∫r(P+ρU x 2 )dr (1.5) (1.5) where P is pressure, ρ is gas density, and U x is the axial velocity, r is the radial distance from the axis; (1.6) U y = radial velocity; Expansion loss is the difference between the total thrust and the axial thrust and is expressed in terms of specific impulse as When the propellant gas passes through the nozzle, friction is generated between the airflow and the nozzle wall. The resistance is equal to the impulse loss of the gas. This part of the lost impulse cannot generate nozzle thrust and is defined as the surface friction loss. The shear stress at the wall is defined as (1.8) where μ EFF is the effective eddy viscosity; The axial resistance caused by surface friction is F drag =∫0LT ω cosθd(A s ) (1.9) (1.9) where θ is the angle of the wall, A s is the wall surface area, and the viscous loss is expressed in terms of specific impulse as In summary, the loss caused by the extension section of the flexible extension nozzle is I(sys)=I0-Isp-I(div)-I(vis) (1.11) Where I0 is the ideal specific impulse calculated from the one-dimensional isentropic flow, Isp is the calculated specific impulse of the nozzle, I(div) is the nozzle expansion loss, and I(vis) is the viscous loss in the fixed section of the nozzle.

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