A statistical method for atomic collision events at hypersonic gas-solid interface

Abstract

The application discloses a kind of hypersonic gas-solid interface atomic collision event statistics method.The method comprises: simulating the high-speed flight of object in gas environment in NAMD software, output the position and speed information of all atoms in system at different time;Using VMD software, all gas atoms that can interact with the front surface in a certain period after system steady state are counted, and the coordinates, speed and potential energy of these gas atoms are output;Define the time when the gas atom turns in the gas-solid atomic repulsive force layer as the collision point, the incident point of the gas atom into the attractive force layer is obtained by pre-searching from the collision point, and the reflection point when the gas atom leaves is obtained by post-searching, which is recorded as a collision event, and the collision event information of different atoms and surface is obtained by circulation.The application proposes a kind of gas-solid interface atomic collision event statistics method under hypersonic strong shock wave condition, which can study the energy transfer mechanism of gas-solid interface from a deeper level and reveal the nature of aerodynamic heating.

Description

Technical Field

[0001] This invention belongs to the field of microscopic gas-solid interface interaction research, and relates to a statistical method for collision events in the process of hypersonic moving surfaces colliding with gases based on molecular dynamics. Background Technology

[0002] Thermal protection design for hypersonic vehicles is a global challenge, and numerous flight tests have failed due to thermal protection system malfunctions. In fact, energy dissipation in the gas-solid interface interaction is crucial in determining aerodynamic heating. Developing a method for studying gas-solid interface interactions in hypersonic flow environments would be of great guiding value for research on aerodynamic heating mechanisms and the design of thermal protection systems.

[0003] Current research on gas-solid interface interactions focuses on collisions between individual gas molecules and surfaces or simple flow problems. The rationale for directly applying these conclusions to complex hypersonic flow fields and strong shock wave problems remains questionable. Furthermore, traditional experimental methods cannot deeply investigate gas-solid interface interactions in complex flow fields. Molecular dynamics methods can accurately resolve atomic collision processes in gas-solid interface interactions, revealing the underlying energy transfer mechanisms at the gas-solid interface. Therefore, a method is needed to accurately study atomic collision events at the gas-solid interface in complex hypersonic flow fields. Summary of the Invention

[0004] To address the challenge of accurately analyzing gas-solid interface interactions in complex hypersonic flow fields and to expand the application of molecular dynamics methods in the study of complex gas-solid interface interactions, this invention provides a molecular dynamics-based method for analyzing atomic collision events at hypersonic gas-solid interfaces. This method can not only accurately capture collision event information between gas atoms and the flight surface under hypersonic flight conditions, but can also be used to study gas-solid and liquid-solid interface interactions in various microscale flows, such as common pipe flows.

[0005] To achieve the above objectives, the technical solution of this invention is as follows: A model of the thermal protection material surface flying in a gaseous environment is established in VMD software. Controlled molecular dynamics (SMD) technology is used to guide the object along a specific direction at hypersonic speeds. The position and velocity information of all atoms in the simulation system are output at a sampling frequency above terahertz. MATLAB software is used to statistically analyze atomic collision events at the gas-solid interface within a specific time range. The specific steps are as follows:

[0006] (1) Establish a thermal protection material surface model in VMD software, including but not limited to common hypersonic vehicle thermal protection materials such as silicon dioxide, silicon, carbon, and silicon carbide. At the same time, put the corresponding number of gas atoms into the simulation system according to the simulated pressure to obtain the initial model of the flying object placed in the gas environment, and output the position file and structure file required for molecular dynamics simulation.

[0007] (2) Import the model file and force field parameter file described in step (1) into the NAMD software for performing molecular dynamics simulation, write the configuration file for running the NAMD software, first minimize the energy of the simulation system under microcanonical ensemble (NVE), then add the Nose-Hoover Langevin piston method to all atoms under isothermal and isobaric ensemble (NPT) to obtain the preset atmospheric pressure, and finally use controlled molecular dynamics (SMD) method under canonical ensemble (NVT) to pull the object to move along a specific direction at a preset speed. After the system reaches steady state, output the position and velocity information of all atoms at a certain frequency.

[0008] (3) Based on the VMD software, the dcd file containing the motion and velocity information of all atoms at different times output in step (2) is processed. According to the cutoff radius of the van der Waals interaction between gas and solid atoms, a tcl script is written to count the number of all gas atoms that appear within the cutoff radius of the front surface of the object in a certain time period, and the coordinates, velocity and potential energy of these atoms in the statistical time period are output.

[0009] (4) Write a script in MATLAB software to process the coordinates, velocity and potential energy information of the atoms obtained in step (2). First, define the moment t when the gas atom i reverses its direction of motion within the repulsive distance between the gas and solid atoms as the collision point, i.e. [ri(t)-ri(t-1)] / [ri(t+1)-ri(t)]<0&ri(t)<d repulsive force. Second, search the preceding sequence of the collision point to obtain the incident point when the gas atom enters the cutoff radius distance, and search the subsequent sequence to obtain the reflection point when the gas atom leaves the cutoff radius distance. Thus, the incident point, collision point and reflection point of a collision event can be obtained, and finally, all collision event information within the statistical time period can be obtained.

[0010] Furthermore, the material, size, geometry, gas atom type and density of the flying object in the simulation system described in step (1) are designable. The simulation system adopts periodic boundary conditions in all three directions, the time step is set to 1fs, the SMD simulation duration is not less than 10ns, and the atomic position and coordinate information in the system is output once every 50fs to ensure high-precision capture of the atomic collision process.

[0011] Furthermore, after obtaining the real-time coordinates of the front surface of the flying object in step (2), the VMD software is used to count all gas atom numbers within the cutoff radius of the front surface at each moment. By taking the union of the sets of gas atom numbers at different moments, all gas atom numbers that may collide with the front surface are obtained.

[0012] Furthermore, the collision points between gas atoms and the surface defined in steps (3) and (4) occur within the distance where the interaction between gas and solid atoms manifests as a repulsive force, and collision points that occur within the distance where gas and solid atoms attract each other are not identified as gas-solid atom collision events; collision events can be classified into incident-immediate reflection type, incident-multiple jump-reflection type, and incident-adsorption-detachment type according to the different number of times gas atoms jump on the surface.

[0013] Furthermore, the molecular dynamics simulation uses the CHARMM36 force field to describe the interatomic interactions, where the van der Waals interaction potentials between gas atoms and surface solid atoms are described by the Lennard-Jones potential function, and the interactions are calculated using a truncation method.

[0014] The method of this invention constructs a molecular dynamics model of an object on the surface of a thermally protective material flying in a gaseous environment using VMD software. In NAMD software, controlled molecular dynamics (SMD) technology is used to guide the object to hypersonic motion along a specific direction, while simultaneously outputting the position and velocity information of all atoms in the simulation system. Combined with VMD software, data analysis is performed to obtain the indices of all gas atoms appearing within the cutoff distance of the object's front surface during a certain time period, and the coordinates, velocities, and potential energies of these atoms are output. In MATLAB software, a collision event statistics script is written to output the incident and reflection information of each collision event.

[0015] The method provided by this invention proposes for the first time a method for studying gas interface atomic collision events under hypersonic flight conditions. By combining VMD, NAMD and MATLAB software, it is possible to study the gas-solid interface interaction within the hypersonic shock layer. This can further facilitate the study of energy transfer mechanisms during atomic collisions and provide guidance for the regulation of aerodynamic heating.

[0016] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0017] (1) This invention studies the gas-solid interface interaction and atomic collision process in the hypersonic shock layer at the atomic scale. Compared with the traditional gas-solid interface research model, it considers the influence of the nonlinear characteristics of the shock wave on the gas-solid interface interaction, and can accurately extract the collision information in the process of gas atoms interacting with the surface.

[0018] (2) The gas-solid interface atomic collision event statistical method provided by the present invention can be used to calculate the energy fitness coefficient and momentum fitness coefficient, providing a basis for determining the boundary conditions of computational fluid dynamics in the field of slip flow. Attached Figure Description

[0019] Figure 1 This is a flowchart of a method for studying hypersonic gas-solid interface atomic collision events in a typical embodiment of the present invention.

[0020] Figure 2 The diagram shows a molecular dynamics simulation of a flying object in a gaseous environment, as well as a diagram of the shock wave structure in front.

[0021] Figure 3 It shows the trajectory of a single gas atom in the direction of the object's flight over a period of time.

[0022] Figure 4 This diagram illustrates a collision event when a single gas atom interacts with the surface of an object. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific preferred embodiments.

[0024] One aspect of this invention provides a method for statistical analysis of hypersonic gas-solid interface atomic collision events, the flowchart of which is shown below. Figure 1 As shown, it includes:

[0025] Step 1: Establish a surface model of thermal protection materials in VMD software, including but not limited to common hypersonic vehicle thermal protection materials such as silica, silicon, carbon, and silicon carbide. At the same time, place the corresponding number of gas atoms in the simulation system according to the simulated pressure to obtain the initial model of the flying object placed in the gas environment, and output the position file and structure file required for molecular dynamics simulation.

[0026] Step 2: Import the model file and force field parameter file into the NAMD software for molecular dynamics simulation. Write the configuration file for running the NAMD software. First, minimize the energy of the simulation system under the microcanonical ensemble (NVE). Then, add the Nose-Hoover Langevin piston method to all atoms under the isothermal and isobaric ensemble (NPT) to obtain the preset atmospheric pressure. Finally, use the controlled molecular dynamics (SMD) method under the canonical ensemble (NVT) to guide the object to move along a specific direction at a preset speed. After the system reaches steady state, output the position and velocity information of all atoms at a certain frequency.

[0027] Step 3: Use VMD software to process the output dcd file containing information on the motion and velocity of all atoms at different times. Based on the cutoff radius of the van der Waals interaction between gas and solid atoms, write a TCL script to count the number of all gas atoms that appear within the cutoff radius of the front surface of the object within a certain time period, and output the coordinates, velocity and potential energy of these atoms within the statistical time period.

[0028] Step 4: Write a script in MATLAB software to process the coordinates, velocity and potential energy information of the atoms obtained in step (2). First, define the moment t when the gas atom i reverses its direction of motion within the repulsive distance between the gas and solid atoms as the collision point, i.e., [ri(t)-ri(t-1)] / [ri(t+1)-ri(t)]<0&ri(t)<d repulsive force. Second, search the preceding sequence of the collision point to obtain the incident point when the gas atom enters the cutoff radius distance, and search the subsequent sequence to obtain the reflection point when the gas atom leaves the interaction. Thus, the incident point, collision point and reflection point of a collision event can be obtained, and finally, all collision event information within the statistical time period can be obtained.

[0029] In step one, the material, size, geometry, gas atom type and density of the flying object in the simulation system are designable. The simulation system adopts periodic boundary conditions in three directions, the time step is set to 1 fs, the SMD simulation duration is not less than 10 ns, and the atomic position and coordinate information in the system is output every 50 fs to ensure high-precision capture of the atomic collision process.

[0030] In some instances, the solid blocks are made of common thermal protection materials such as silicon oxide, silicon carbide, silicon, carbon, and aluminum oxide. The size of the objects ranges from 1 nm to 100 nm, and the surface morphology includes planar, convex spherical, concave, and rough surfaces. The gas pressure ranges from 0.1 atm to 20 atm, and the types of gas molecules include, but are not limited to, nitrogen, oxygen, carbon dioxide, argon, and helium.

[0031] In step two, after obtaining the real-time coordinates of the front surface of the flying object over a period of time, the VMD software is used to count all gas atom numbers within the cutoff radius of the front surface at each moment. By taking the union of the sets of gas atom numbers at different moments, all gas atom numbers that may collide with the front surface are obtained.

[0032] In some instances, the process of an object moving from the left end to the right end of the simulation system is recorded as a statistical time period, and collision event information is collected within multiple time periods.

[0033] In step three, the collision points between gas atoms and the surface are defined as occurring within the distance where the interaction between gas and solid atoms manifests as a repulsive force, while collision points occurring within the distance where gas and solid atoms attract each other are not identified as gas-solid atom collision events.

[0034] In step four, collision events can be classified into incident-immediate reflection type, incident-multiple hops-reflection type, and incident-adsorption-detachment type, depending on the number of times gas atoms jump on the surface.

[0035] This implementation example uses a statistical method for atomic collision events at the hypersonic gas-solid interface, as follows:

[0036] (1) The solid material used is silicon crystal with dimensions of 8.8 × 8.8 × 2.2 nm³, and the simulation box has dimensions of 30 × 30 × 120 nm³. All three directions are periodic boundary conditions. The box is filled with argon atoms at a pressure of 10 atm, and both the gas and solid temperatures are set to 300 K. The results are as follows: Figure 2 The model shown in (a) is analyzed; the average velocity of the gas region is obtained as follows: Figure 2 (b) shows the shock wave structure;

[0037] (2) The CHARMM36 force field is used to describe the interatomic interactions. The van der Waals interaction potentials between gas atoms and surface solid atoms are described by the Lennard-Jones potential function. The interaction is calculated using the truncation method, with a truncation radius of 8. :

[0038] (3) Molecular dynamics simulation was run in NAMD software. First, the energy of the simulation system was minimized, and then the system was brought to the preset temperature and pressure under the NPT ensemble. In the NVT ensemble, the SMD method was used to tug the block along the z direction at a speed of 2000 m / s. The time step of the simulation was 1 fs, and the atomic position and velocity information were output every 50 fs. The simulation duration was 10 ns.

[0039] (4) Using VMD software, obtain the atomic numbers of all possible gas atoms that may collide with the front surface of the object, and output the position and velocity files of these gas atoms. Figure 3 The image shows the trajectory of a gas atom in its flight direction within 0-2 ns. During the periodic motion of the object, there are a total of 4 complex interactions with the surface.

[0040] (5) In MATLAB, statistically analyze the information of each collision event according to the above definition, and obtain the following results: Figure 4 The diagram shows five collision events between a gas atom and the surface of an object within 0.04 ns, including the collision point, incident point, reflection point, and parameters such as interaction time and depth.

[0041] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all fall within the protection scope of the present invention.

Claims

1. A statistical method for hypersonic gas-solid interface atomic collision events, characterized in that, Includes the following steps: (1) Establish a molecular dynamics model of micro-nano scale objects moving in a gaseous environment. In NAMD software, the controlled molecular dynamics (SMD) technology is used to guide the object to fly at a preset speed. After the system reaches steady state, the position and velocity information of all atoms in the system are output at a certain frequency. (2) Import the file containing all atomic positions and velocities output in step (1) into VMD software. Based on the cutoff radius of the van der Waals force between gas and solid atoms, write a script to count all gas atom numbers that appear within the cutoff radius of the front surface of the object in a certain time period, and output the coordinates, velocities and potential energy of these atoms in the statistical time period. (3) Based on MATLAB software, a script is written to process the coordinates, velocity, and potential energy information of the atoms obtained in step (2). The moment t when the direction of motion of gas atom i reverses within the repulsive distance between gas and solid atoms is defined as the collision point, i.e., [r i (t)-r i (t-1)] / [r i (t+1)-r i [(t)]<0&r i (t)<d 排斥力 ; (4) Based on the collision point in step (3), the incident point of the gas atom entering the cutoff radius distance is obtained by the preceding search, and the reflection point of the gas atom leaving the cutoff radius distance is obtained by the subsequent search. Thus, the incident point, collision point and reflection point of a collision event can be obtained. By traversing all the gas atom motion trajectories that interact with the surface in the above manner, all collision event information within the statistical time period can be obtained.

2. The statistical method for hypersonic gas-solid interface atomic collision events as described in claim 1, characterized in that, Step (1) The material, size, geometry, gas atom type and density of the flying object in the simulation system are designable. The simulation system adopts periodic boundary conditions in three directions. The atomic position and coordinate information in the system is output once every 50 femtoseconds to ensure high-precision capture of the atomic collision process.

3. The statistical method for hypersonic gas-solid interface atomic collision events as described in claim 1, characterized in that, After obtaining the real-time coordinates of the front surface of the flying object in step (2), the VMD software is used to count all gas atom numbers within the cutoff radius of the front surface at each moment. By taking the union of the sets of gas atom numbers at different moments, all gas atom numbers that may collide with the front surface are obtained.

4. The statistical method for hypersonic gas-solid interface atomic collision events as described in claim 1, characterized in that, The collision points between gas atoms and the surface defined in step (3) occur within the distance where the interaction between gas and solid atoms manifests as a repulsive force. Collision points that occur within the distance where gas and solid atoms attract each other are not identified as gas-solid atom collision events.

5. The statistical method for hypersonic gas-solid interface atomic collision events as described in claim 1, characterized in that, The collision events described in step (4) can be classified into incident-immediate reflection type, incident-multiple jump-reflection type, and incident-adsorption-detachment type, depending on the number of times the gas atoms jump on the surface.

6. The statistical method for hypersonic gas-solid interface atomic collision events as described in claim 1, characterized in that, The van der Waals interaction between gas atoms and surface atoms is described by the Lennard-Jones potential function, and the interaction potential is calculated using the truncation method.

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