JP2009301437A - Simulation system and program thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a simulation system accurately obtaining the dynamic property of fluid. <P>SOLUTION: By simulation, a pressure and shear are applied to a molecule group held between two solid atom layers, and the dynamic response of shear stress or the like is obtained, thereby the structural design of the fluid which is molecular fluid is performed. The simulation includes a structural relaxation process (S4) for performing energy relaxation calculation for a molecule group or a molecule included in the molecule group and obtaining a stable structure. Then, the dynamic response is obtained (S11) using the stable structure obtained by the structural relaxation process (S4). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a simulation for designing the structure of a molecular fluid (fluid) by applying pressure and shear to a molecular group sandwiched between two solid atomic layers to obtain a mechanical response such as shear stress.

  The toroidal continuously variable transmission (CVT) obtains a variable gear ratio by changing the inclination of a power roller sandwiched between two disks on the input side and the output side with a strong force. The traction characteristics of the fluid used in the CVT have a great influence on the characteristics of the CVT. Therefore, it is desired to develop a fluid that exhibits a high traction coefficient from the viewpoint of downsizing and increasing the capacity of the transmission, improving the fuel efficiency and reducing the environmental load associated therewith.

  Here, the traction characteristics of the fluid largely depend on the molecular structure of the molecules constituting the fluid. Therefore, various studies have been conducted on the molecular structure of fluid.

JP 2002-150203 A JP-A-8-35831 Japanese Patent Laid-Open No. 10-232860 Hitoshi Awazu, Shuzo Mita, Toshihide Omori, Atsushi Suzuki, "Analysis of Fluid Traction Characteristics by Molecular Dynamics Method (1st Report)-Selection of Appropriate Calculation Conditions", Tribologist 51 (12), 885-891, (2006 ) Hitoshi Awazu, Shuzo Mita, Toshihide Omori, Atsushi Suzuki, “Analysis of Fluid Traction Characteristics by Molecular Dynamics Method (2nd Report) -Analysis of Traction Expression Focusing on Intramolecular and Intermolecular Interactions-”, Tribologist 51 ( 12), 892-899, (2006). H. Washizu, S. Sanda, S. Hyodo and T. Ohmori, N. Nishino and A. Suzuki, "A Molecular Dynamics Analysis of the Traction Fluids", SAE Paper, Transaction, 2007-01-1016, (2007). H. Washizu, S. Sanda, T. Ohmori and S. Hyodo, "Submicron thickness molecular dynamics simulation of lubricating oil film", 1st NAREGI International Nanoscience Conference, pp. 115, Nara, Japan, (2005.6.14-18).

  Here, Non-Patent Documents 1 to 4 do not clearly indicate the entire system, and there is no description about a novel molecule search. In addition, when calculating macro rheological properties of fluid molecular groups by molecular simulation such as molecular dynamics, the number of molecules that can be handled and physical time are limited. In addition, since the regularity caused by the bias of the initial value (for example, the uniformity of the initial structure) occurs for the molecule to be simulated, it was originally a problem that the actual phenomenon having thermal randomness could not be reproduced. .

  Further, in Patent Document 1, a fluid traction coefficient can be calculated to some extent in consideration of shear stress with respect to fluid lubrication characteristics (such as a traction coefficient) under a constant pressure. However, since the pressure is artificially applied by making the distance between the solid wall surfaces constant and the number of lubricant molecules constant, it is not suitable for an application for obtaining a traction coefficient at a constant pressure. Furthermore, since the interfacial slip control between the solid wall and the fluid is not controlled for the viscoelastic behavior inside the fluid layer, the influence of the interfacial slip between the solid wall and the fluid is measured separately when measuring the lubrication characteristics of the fluid itself. Can not do it. In addition, from the viewpoint of limiting the number of molecules, it is difficult to say that the film thickness conforms to the actual phenomenon in elastohydrodynamic lubrication, and the phenomenon cannot be sufficiently reproduced.

  In addition, the initial arrangement of fluid molecular groups has a great influence on the lubrication characteristics. For example, when the molecular group has an artificial crystallinity, the actual phenomenon in elastohydrodynamic lubrication cannot be reproduced. Therefore, there is a problem that the shear characteristics in the simulation may not appropriately reflect the difference due to the molecular structure.

  Patent Document 2 discloses friction between solids, and Patent Document 3 describes interface / adsorption, but does not describe pressure and shear.

  The present invention is a simulation system for designing the structure of fluid, which is a molecular fluid, by applying pressure and shear to a molecular group sandwiched between two solid atomic layers and obtaining a mechanical response such as shear stress. In addition, a structural relaxation process is performed to obtain a stable structure by calculating the energy relaxation of the system for a molecular group or molecules contained in this molecular group, and a mechanical response is obtained using the stable structure obtained by this structural relaxation process. It is characterized by that.

  The structure relaxation process preferably includes a structure relaxation process of one fluid molecule.

  The structural relaxation process preferably includes a structural relaxation process that minimizes the distance between the ends of one fluid molecule.

  In addition, it is preferable to include a process of giving different initial velocities to a plurality of fluid molecule groups when the fluid molecule groups are arranged between solid atomic layers.

  In addition, the present invention is a simulation for designing the structure of fluid, which is a molecular fluid, by applying pressure and shear to a molecular group sandwiched between two solid atomic layers and obtaining a mechanical response such as shear stress. A program that includes a structure relaxation process that calculates the energy relaxation of a system for a molecular group or molecules contained in this molecular group to obtain a stable structure, and uses the stable structure obtained by this structural relaxation process. It is characterized by giving a mechanical response.

  According to the present invention, by introducing a structure relaxation process of molecules or molecular groups, atoms and molecules are placed at realistic positions, and the bias of the initial value is eliminated. Can be qualitatively reproduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a flowchart showing the overall configuration of simulation in the embodiment.

  First, the molecular formula of the target fluid molecule is input (S1). Here, the format of the structural formula to be input is in a form suitable for the software used.

  In order to calculate the interatomic interaction in the molecule, a table of intramolecular interaction parameters is created from a database or the like (S2). For example, a force field for molecular dynamics of all atoms such as an Amber force field is used for intramolecular interaction data of fluid molecules (see Chapter 2 of Non-Patent Document 1).

  An initial structure of one fluid molecule is created (S3). The initial structure is a set of atomic coordinates, atomic velocities, and the like. FIG. 2 shows an example of the initial structure for benzene.

  Hereinafter, details of S4 are basically described in Chapter 4 of Non-Patent Document 1. Therefore, the characteristic features of this embodiment will be mainly described.

  A plurality of fluid molecules are arranged in a lattice pattern (S4). At this time, a structure relaxation process of one fluid molecule is executed in advance by a method such as a molecular dynamics method or a Monte Carlo method, thereby obtaining a stable structure.

  3A, 3B, and 3C show examples of a stable structure as a result of performing such structure relaxation calculation (structure relaxation process). 3A is an example of benzene, FIG. 3B is an example of a structure with a large end-to-end distance (n-hexadecane: distance R = 1.34 nm), and FIG. 3C is an example of a structure with a small end-to-end distance (n -Hexadecane: distance R = 0.74 nm).

  Such a stable structure is required by relaxation of the total energy of the system. Here, for a long-chain molecule as shown in FIG. 3B, as shown in FIG. 3C, the inter-terminal atoms (in this case, carbon, in general, the terminal atom excluding hydrogen in the case of the all-atom model, In the case of a coarse-grained model such as the Atom model, a realistic stable structure in which the distance of the terminal residue) is minimized.

  An example of the initial arrangement of molecular lattices is shown in FIG. FIG. 4 shows an example of an initial structure in which fluid molecules are arranged in a lattice form using cyclohexane as the fluid molecules. Then, a structural relaxation process is executed on the lattice-like initial arrangement by molecular dynamics or the like to obtain the structure of the molecular group in the liquid state.

  Here, as a structure relaxation method, for example, annealing is performed by a temperature rising / falling process, and the liquid state is determined from the three-dimensional molecular distribution shown in FIG. At this time, as a method of keeping the temperature of the entire system constant, for example, the heat bath is connected by a Nose-Hover method. As for the interaction between molecules, a Lennard-Jones type interaction as a van der Waals force and a Coulomb force between partial charges on an atom are calculated. Further, the time integration in the movement of atoms is calculated by, for example, an rRESPA (Reversible Reference System Propagator Algorithm) method based on the velocity Verlet method. The boundary condition is a periodic boundary condition in the (x, y, z) direction.

  As described above, in this embodiment, when the fluid molecules are arranged in a lattice shape, the structure relaxation calculation is performed for each fluid molecule, and the structure relaxation process for the molecular group is performed after the fluid molecules are arranged in the lattice shape. To do. Thereby, the initial value dependency can be eliminated.

  In this way, when the structure relaxation process is executed and the initial arrangement is obtained, two sets of solid atomic layers are created (S5). Here, the solid atomic layer is, for example, a single crystal of α-iron, 10 × 10 × 3 atoms in the (x, y, z) direction, and the lattice spacing is 2.87 cm.

  Between the solid atomic layers, the fluid molecular group in the liquid state is arranged in one or several groups (S6). The fluid simulation cell to be simulated is a rectangular parallelepiped, and a periodic condition is imposed as a boundary condition in the (x, y) direction in the orthogonal coordinate system.

  Hereinafter, x is the sliding direction, and z is the direction of the oil film thickness. FIG. 6 shows an example of the simulation cell. This example is an example of the initial arrangement of the compression process.

  Here, in the case where a plurality of fluid molecule groups are arranged, by providing different initial velocities for each atom, each molecule, or each molecule group, it is possible to reproduce an actual phenomenon that originally has thermal randomness. Thereby, the initial value dependency can be eliminated.

  In the case of the sliding surface where the solid atomic layer is in contact with the fluid, for example, α-iron, the (100) plane is used. In addition, the vibration of iron atoms in each layer may be frozen for efficient simulation.

  The number of molecules to be arranged is an amount that reproduces the actual phenomenon in elastohydrodynamic lubrication with respect to the oil film thickness. FIG. 7 shows the film thickness / shear rate dependence of the traction coefficient in n-hexane. Thus, as the shear rate decreases, the traction coefficient μ decreases accordingly. Further, the traction coefficient μ decreases as the film thickness increases. This system indicates that the film thickness is required to be at least 5.0 nm or more (see Chapter 4.1.2-4.1.4 of Non-Patent Document 1). Therefore, when the oil film thickness is set to 5 nm or more in accordance with the actual phenomenon, the viscoelastic-elasto-plastic shear behavior of the fluid is observed, not the solid behavior like the stick-slip motion in the ultrathin film. Is preferable.

  Regarding the interaction between the solid atomic layer and the fluid, the interaction between the solid atomic layer and the fluid is considered to be the main factor in the expression of traction under EHL (elastohydrodynamic lubrication) conditions. A large interaction that does not occur is given (S7). For example, when the solid atom is α-iron, to provide a large interaction force, the Lennard-Jones parameter is set to approximately σ = 12 kcal / mol. On the other hand, when σ is small (σ <0.1σM), an interface slip occurs as shown in FIG. 8 (see Chapter 4.1.5 of Non-Patent Document 1). Thus, by setting the solid-fluid interaction that does not cause interfacial slip for the viscoelastic behavior inside the fluid layer, the lubrication characteristics of the fluid itself can be measured without being affected by interfacial slip. .

Here, in FIG. 8, σ _ij = 12.14 kcal / mol≡σ _M for the Lennard-Jones parameter of the solid atom. FIG. 8 shows a velocity profile when σ _ij = σ _M , 0: 5σ _M , 0: 1σ _M , 0: 05σ _M , 0: 01σ _M in a system with a film thickness z ₀ = 5.0 nm. Yes. When σ _ij = 1.0, 0: 5σ _M , no interface slip between the solid atomic layer and the fluid occurs. When σ _ij = 0: 1σ _M , some interface slip is observed, and when σ _ij = 0.1σ _M or less, the slip increases as σ _ij decreases.

  Next, a periodic boundary condition is given only in the horizontal direction (x, y) and a constant pressure is given in the vertical direction to compress the fluid and obtain a constant pressure state (S8). For example, an external pressure of 1 GPa is applied to the solid atomic layer in the z direction to compress the fluid. The pressure is given as a pressure applied to the upper solid atomic layer in the z-axis direction. The constant pressure state is set when the internal pressure and the external pressure become constant and the film thickness becomes substantially constant. Here, if necessary, the temperature of the system is set to a condition that assumes an actual sliding condition by temperature control. From here on, the temperature is constant. FIG. 9 shows an example in which the constant pressure state of the fluid molecular group is obtained.

  In FIG. 10, the pressure dependence of the traction coefficient of n-hexane is shown (refer nonpatent literature 4). It can be seen that it is necessary to apply an external pressure of 1 GPa or more in order to perform a simulation under the limit shear stress, which is a characteristic of elastohydrodynamic lubrication.

  A shear field is applied by horizontally moving the upper and lower solid atomic layers in the x direction at a constant speed with a relative speed difference v0 (S9).

  The change with time of the traction coefficient is observed as shown in FIG. 11, and a determination is made that the state in which the values converge is regarded as the steady state of the system (S10). FIG. 12 shows an example of molecular distribution in a constant pressure / constant shear state. Note that the processes of S9 and S10 may proceed simultaneously.

  The time average of the traction coefficient in the steady state is obtained and output (S11). If the target physical quantity is other than the traction coefficient (friction coefficient) μ, the coefficient is obtained and output.

  It is determined whether or not the molecule expresses the target physical property (S12). If appropriate, the process ends. If inappropriate, the process returns to S1, and the calculation is performed again with another molecular structure.

  As described above, in the simulation according to the present embodiment, an external field having a constant pressure is applied to the fluid molecule group sandwiched between the solid atomic layers in a direction perpendicular to the solid atomic layers, and the shear field is generated by horizontal movement. By applying it, it is possible to simulate the behavior of a fluid molecular group in a constant pressure / constant shear state and to determine its physical quantity (for example, traction coefficient).

  The number of molecules and physical time that can be handled are limited to the calculation of macro rheological properties of fluid molecular populations by molecular simulation such as molecular dynamics. In such a method, an unnatural regularity due to the deviation of the initial value (for example, the uniformity of the initial structure) occurs, and the actual phenomenon cannot be reproduced. In this embodiment, by introducing a structure relaxation process of molecules or molecular groups, atoms and molecules are placed at realistic positions, and the bias of the initial value is eliminated, so that the shear behavior of the oil film in the actual phenomenon is reduced. It became possible to reproduce qualitatively.

  Specifically, as a process of structural relaxation, calculation is performed to minimize the total energy from an arbitrary initial spatial arrangement of atoms on one fluid molecule, and at this time, the end-to-end distance of one fluid molecule is minimized. Moreover, by arranging different initial velocities to a plurality of fluid molecular groups when the fluid molecular groups are arranged between solid atomic layers, the structure of the fluid molecular group in an actual phenomenon is reproduced to solve this initial value problem.

  FIG. 13 shows an example of comparison of the calculated values with the experimental values for eight types of hydrocarbons (A to H) having different molecular structures. This embodiment shows that it is possible to predict the order of traction coefficients for each molecular structure.

It is a flowchart which shows the procedure of the simulation which concerns on embodiment. It is a figure which shows the example of an initial structure. It is a figure which shows the example of the stable structure of a fluid molecule | numerator. It is a figure which shows the example of the initial structure which has arrange | positioned the fluid molecule | numerator in the grid | lattice form. It is a figure which shows the example of the fluid molecule | numerator group of a liquid structure. It is a figure which shows the example of the initial stage arrangement | sequence of a compression process. It is a figure which shows the film thickness and shear rate dependence of a traction coefficient. It is a figure which shows the influence of a solid atomic layer-fluid interaction. It is a figure which shows the example of a constant pressure state. It is a figure which shows the pressure dependence of a traction coefficient. It is a figure which shows the example of the calculation result of a traction coefficient. It is a figure which shows the example of a constant pressure and a constant shear state. It is a figure which shows the comparison with the experimental value of the traction coefficient in the fluid from which molecular structure differs.

Claims (5)

A simulation system for structural design of fluid, which is a molecular fluid, by applying pressure and shear to a molecular group sandwiched between two solid atomic layers and obtaining a mechanical response such as shear stress,
It includes a structural relaxation process that calculates the energy relaxation of a system for a molecular group or molecules contained in this molecular group to obtain a stable structure, and obtains a mechanical response using the stable structure obtained by this structural relaxation process A simulation system. In the simulation system according to claim 1,
The structure relaxation process includes a structure relaxation process of one fluid molecule. In the simulation system according to claim 1,
The structural relaxation process includes a structural relaxation process that minimizes a distance between terminals of one fluid molecule. In the simulation system according to claim 1,
A simulation system comprising a process of giving different initial velocities to a plurality of fluid molecular groups when the fluid molecular groups are arranged between solid atomic layers. A simulation program for designing the structure of fluid as a molecular fluid by applying pressure and shear to a molecular group sandwiched between two solid atomic layers and obtaining a mechanical response such as shear stress,
On the computer,
It includes a structure relaxation process that calculates the energy relaxation of a system for a molecular group or molecules contained in this molecular group to obtain a stable structure, and obtains a mechanical response using the stable structure obtained by this structural relaxation process. A characteristic simulation program.