Takashi Tokumasu

Dynamic molecular collision (DMC) model for rarefied gas flow simulations by the DSMC method

The Dynamic Molecular Collision (DMC) model is constructed for accurate and realistic simulations of rarefied gas flows of nonpolar diatomic molecules by the Direct Simulation Monte Carlo (DSMC) method. This model is applicable for moderate temperatures (up to a few hundred K for nitrogen), where most molecules are in the vibrational ground state and the vibrational degree of freedom can be neglected. In this range, moreover, the rotational energy can be considered as a continuous one. The collisions of diatomic molecules are simulated many times by the Molecular Dynamics (MD) method at various initial conditions. The site to site potential is used as an intermolecular one. The collision cross section is developed from the database obtained by MD simulation and kinetic theory of viscosity coefficient of diatomic molecules. The probability density function of energy after collision is also developed using the database. In order to verify the DMC model, two flow fields are simulated. First, the DMC model is...

Thermodynamic Effect on a Cavitating Inducer in Liquid Nitrogen

For experimental investigations of the thermodynamic effect on a cavitating inducer, it is nesessary to observe the cavitation. However, visualizations of the cavitation are not so easy in cryogenic flow. For this reason, we estimated the cavity region in liquid nitrogen based on measurements of the pressure fluctuation near the blade tip. We focused on the length of the tip cavitation as a cavitation indicator. Comparison of the tip cavity length in liquid nitrogen (80 K) with that in cold water (296 K) allowed us to estimate the strength of the thermodynamic effect

Effect of molecular elongation on the thermal conductivity of diatomic liquids

The effect of molecular elongation on the thermal conductivity of diatomic liquids has been analyzed using a nonequilibrium molecular dynamics (NEMD) method. The two-center Lennard-Jones model was used to express the intermolecular potential acting on liquid molecules. The simulations were performed using the nondimensional form of the potential so that the molecular elongation, d/σ, was the only parameter varied in the simulation. The simulations were performed for five values of this parameter. First, the equation of state of each liquid was obtained using equilibrium molecular dynamics simulation, and the critical temperature, density, and pressure of each liquid were determined. Then, NEMD simulations of heat conduction in the five liquids were performed using values for temperature and density which were identical among the five liquids when they were reduced by their respective critical temperature and density (T=0.7 Tcr and ρ=2.24 ρcr). Obtained thermal conductivities were reduced by the critical t...

Effect of bound state of water on hydronium ion mobility in hydrated Nafion using molecular dynamics simulations

We have performed a detailed analysis of the structural properties of the sulfonate groups in terms of isolated and overlapped solvation shells in the nanostructure of hydrated Nafion membrane using classical molecular dynamics simulations. Our simulations have demonstrated the correlation between the two different areas in bound water region, i.e., the first solvation shell, and the vehicular transport of hydronium ions at different water contents. We have employed a model of the Nafion membrane using the improved force field, which is newly modified and validated by comparing the density and water diffusivity with those obtained experimentally. The first solvation shells were classified into the two types, the isolated area and the overlapped area. The mean residence times of solvent molecules explicitly showed the different behaviors in each of those areas in terms of the vehicular transport of protons: the diffusivity of classical hydronium ions in the overlapped area dominates their total diffusion at lower water contents while that in the isolated area dominates for their diffusion at higher water contents. The results provided insights into the importance role of those areas in the solvation shells for the diffusivity of vehicular transport of hydronium ions in hydrated Nafion membrane.

Energy dissipation in non-isothermal molecular dynamics simulations of confined liquids under shear.

Energy is commonly dissipated in molecular dynamics simulations by using a thermostat. In non-isothermal shear simulations of confined liquids, the choice of the thermostat is very delicate. We show in this paper that under certain conditions, the use of classical thermostats can lead to an erroneous description of the dynamics in the confined system. This occurs when a critical shear rate is surpassed as the thermo-viscous effects become prominent. In this high-shear-high-dissipation regime, advanced dissipation methods including a novel one are introduced and compared. The MD results show that the physical modeling of both the accommodation of the surface temperature to liquid heating and the heat conduction through the confining solids is essential. The novel method offers several advantages on existing ones including computational efficiency and easiness of application for complex systems.

A DFT Study of Bond Dissociation Trends of Perfluorosulfonic Acid Membrane

The bond dissociation energy of a perfluorosulfonic acid (PFSA) molecule was investigated by density functional theory (DFT) to provide some dissociation trends of the PFSA molecule and the basic information for the design of more durable PFSA membranes. As the preliminary analysis, the chemical bond strengths of the PFSA molecule were analyzed exhaustively, and the vulnerable bond was identilied by comparison of the results. The same calculations were performed for an ionized PFSA molecule to determine the influence of the ionization state on the bond strength. The C-S bond was the weakest among the side chain backbone in the neutral molecule; however, the C-S bond became stronger when ionized while it weakened the C-0 bond connecting the side chain with the main chain. Analysis of the C-F bonds in the side chain showed that the dissociation energy decreases in the order of primary, secondary, and tertiary bonds, as also reported in the literature. Analysis of the main chain showed that the secondary C-F bonds neighboring the tertiary C-F bond connecting the side and main chains were the weakest. Ionization of the PFSA molecule weakens the average dissociation energy of C-F bonds. As the realistic analysis, the same calculations were performed considering the solvation effects to analyze the effects on the dissociation trend. Moreover, the possibility for improving the durability of PFSA membranes was investigated by partial substitution of H atoms or CH 3 groups for F atoms.

Parallel computing of diatomic molecular rarefied gas flows

A parallel algorithm for direct simulation Monte Carlo calculation of diatomic molecular rarefied gas flows is presented. For reliable simulation of such flow, an efficient molecular collision model is required. Using the molecular dynamics method, the collision of N2 molecules is simulated. For this molecular dynamics simulation, the parameter decomposition method is applied for parallel computing. By using these results, the statistical collision model of diatomic molecule is constructed. For validation this model is applied to the direct simulation Monte Carlo method to simulate the energy distribution at equilibrium condition and the structure of normal shock wave. For this DSMC calculation, the domain decomposition is applied. It is shown that the collision process of diatomic molecules can be calculated precisely and the parallel algorithm can be efficiently implemented on the parallel computer.

The dynamic effects on dissociation probability of H2–Pt(111) system by embedded atom method

The effects of the motion of atoms or molecules on the dissociation probability of the H2–Pt(111) system were analyzed by molecular dynamics. The embedded atom method (EAM) was used to model the interaction between a Pt(111) surface and an H2 molecule to consider the dependence of electron density. Initially, the EAM potential was constructed to express the characteristics of the system, such as the electron density or dissociation barrier at certain sites and orientations, as obtained by density functional theory (DFT). Using this potential, simulations of an H2 molecule impinging on a Pt(111) surface were performed, and the characteristics of the collision were observed. These simulations were performed many times, changing the orientation of the H2 molecule, and a dynamic dissociation probability at each site against impinging energy was obtained. On the other hand, a static dissociation probability was defined from the dissociation barrier of a hydrogen molecule obtained by the EAM potential. These re...

Molecular dynamics study of the effects of translational energy and incident angle on dissociation probability of hydrogen/deuterium molecules on Pt(111)

The dissociation probabilities of H2 and D2 molecules on a Pt(111) surface with thermal motion were analyzed using the molecular dynamics (MD) method. The potential constructed using the embedded atom method was used as the interaction potential between a gas molecule and the surface. The effects of changing the translational energy and incident polar angle of D2 molecules impinging on a Pt(111) surface were analyzed using MD simulations. The effect of initial orientation, incident azimuthal angle, rotational energy of gas molecules, and the impinging points on the surface were averaged by setting the initial values in a random manner. When the molecules approach normal to the surface, the dissociation probability increases with the initial translational energy. At larger incident angles, the probability becomes smaller. The impinging processes were categorized in terms of reaching the chemisorption layer by analyzing the repulsion forces from the surface. The effective translational energies for impingem...

Reaction analysis for deprotonation of the sulfonic group of perfluorosulfonic acid molecules at low hydration levels.

A reaction analysis for deprotonation of the sulfonic group in a model molecule of perfluorosulfonic acid (PFSA) at low hydration levels was performed. PFSA is usually adopted as a polymer electrolyte membrane in polymer electrolyte fuel cells. In hydration level three, the deprotonation reaction certainly occurs. The deprotonated state produced is more stable than the predeprotonated state by 3.72 kcal/mol. In addition, its activation energy is very low. Although quantitative discussion of this activation energy is difficult considering the computational error, it can be said qualitatively that H(+) is abstracted smoothly from the sulfonic group because of a low activation energy. From the results of bond-order analysis, the produced H3O(+) is strongly bound by the SO3(-) group. Thus, diffusivity of H3O(+) would be low. In hydration level four or more, we found a possibility that the diffusivity of H3O(+) increases because the hydrogen-bond strength between H3O(+) and SO3(-) is lower or SO3(-) cannot bind H3O(+) directly by forming an Eigen cation.