Haruka Yasuoka

Confinement effects on liquid-flow characteristics in carbon nanotubes.

Liquid flow dynamics through the armchair (6,6)-(160,160) carbon nanotubes (CNTs) is elucidated by molecular dynamics simulations. The liquid is modeled by nonpolar argon atoms to understand the fundamental flow physics. The velocity profiles and slip lengths are discussed considering the radial distributions of the fluid density by the presently proposed finite difference-based velocity fitting method. It is found that as the CNT diameter D increases, the slip length and the flow rate enhancement show three-step transitional profiles in the region of D≤2.3 nm. The slip length and the flow rate stepwise increase at the first transition while they drop at the second and third transitions. The first transition corresponds to the structural change from the single-file chain to single-ring structures of the molecule cluster. The second and third transitions take place when the ring structure starts to develop another inner layer.

Thermal lattice Boltzmann method for complex microflows.

A methodology to simulate thermal fields in complex microflow geometries is proposed. For the flow fields, the regularized multiple-relaxation-time lattice Boltzmann method (LBM) is applied coupled with the diffusive-bounce-back boundary condition for wall boundaries. For the thermal fields, the regularized lattice Bhatnagar-Gross-Krook model is applied. For the thermal wall boundary condition, a newly developed boundary condition, which is a mixture of the diffuse scattering and constant temperature conditions, is applied. The proposed set of schemes is validated by reference data in the Fourier flows and square cylinder flows confined in a microchannel. The obtained results confirm that it is essential to apply the regularization to the thermal LBM for avoiding kinked temperature profiles in complex thermal flows. The proposed wall boundary condition is successful to obtain thermal jumps at the walls with good accuracy.

Molecular Dynamics Simulation of Slip-Transitional Flows in Nano-Channels

The detailed near wall flow behavior in nanochannels depending on the surface structure is investigated by performing equilibrium molecular dynamics simulations. The nanoCouette flows and force driven nano-Poiseuille flows are considered. The simulated flow conditions are in the slip-transitional flow regime since the estimated Knudsen number is Kn=0.2. Due to the effects of the surface structure, it is confirmed that the velocity profile very near a wall varies depending on the surface structure. It is also found that the Knudsen layer profile in nanoflows deviates from that by the gas kinetic theory in the vicinity of the wall surface. By the interaction between a fluid molecule and the wall molecules, the fluid molecule staying in the wall vicinity is less strongly stuck to the wall than those at a little away from the wall surface. This effect produces a hook like velocity distribution near the wall which is hardly described by the Boltzmann equation for rarefied gas flows.