arXiv subjects

H. Otomo

Publications and source records attributed to H. Otomo.

4 recordsLinked to original sources

arXiv2025

Enhanced numerical models for two-component fluid flow in multiscale porous structures

Multi-component fluid flow simulations in multi-scale porous structures often involve regions that are under-resolved at practical computational resolutions. Accurately capturing the contributions from these unresolved regions is critical. Previous studies proposed a model to account for viscous and capillary forces in under-resolved regions, showing permeability, capillary pressure, and relative permeability comparable to fully resolved high-resolution cases. In this study, we extend the model to handle diverse structures and capture detailed fluid behavior. We introduce controllable surface tension in a pseudo-potential lattice Boltzmann model while keeping interface thickness and spurious currents constant, improving interface dynamics resolution. A method is developed to capture residual fluid components smaller than cell size using local constitutive relations, including absolute and relative permeability and capillary pressure curves. Additionally, a tensorial resistivity model is implemented for heterogeneous structures such as fiber bundles, aligning resistivity along principal axes determined from the Hessian and gradient of local porosity fields. Benchmark tests, including practical rock geometries, validate these enhancements, demonstrating improved transient interface dynamics, accurate capture of irreducible fluid components, and correct directional effects in under-resolved structures.

physics.flu-dyn↗

arXiv2025

Lattice Boltzmann models for the hydrodynamic equations in multiphase flow with high density ratio

Multiphase flows with high density ratios, such as water and air flows, have recently been simulated using the lattice Boltzmann (LB) method. This approach corresponds to solving the phase field equations, such as the Cahn-Hilliard and Allen-Cahn equations, and the hydrodynamic equations, typically the Navier-Stokes and pressure equations for pseudo-incompressible fluids. Due to the high density ratio, the higher-order numerical truncation errors associated with spatial density gradients can become significant. These errors can lead to problems such as inaccuracies in shear stress, violations of Galilean invariance, and undesirable dependencies on absolute pressure for the pseudo-incompressible solutions. To overcome such problems, the moments of the distribution function and the equilibrium state must be carefully designed while ensuring robustness. In this work, we propose a new scheme based on the lattice kinetic scheme (LKS), which directly solves the velocity and pressure fields in the similar discrete space as the LB method. When mapping the LKS-based models to the LB models, the original LKS models are simplified for computational efficiency and the filter collision operator is implemented. Benchmark test cases confirm that the proposed scheme effectively addresses these issues, achieving high accuracy and robustness while eliminating the iterative steps typically required in the LKS. One of the most significant improvements is the accuracy of the airflow field induced by water motion, likely due to improved momentum transfer across the interface.

physics.flu-dyn↗

arXiv2025

Capillary flow simulation with the phase-field-based lattice Boltzmann solver

The phase-field-based lattice Boltzmann (LB) model has been developed to perform high fidelity multiphase flow simulations. Its ability to accurately handle high density ratio and surface tension effects is expected to be beneficial for capillary flow simulation, leading to accurate reproduction of flow patterns such as slug flow, droplet flow, and film flow. This is critical in many engineering cases because the flow patterns significantly affect the velocity and pressure fields. In this study, on top of the LB models based on the conservative Allen-Cahn equation and the volumetric boundary conditions for the complex geometries, an optimized wettability and friction model are implemented. With these models, we conducted a set of benchmark test cases, including static and dynamic multiphase flow scenarios such as the droplet on the curved surfaces, water-filling channel for the Lucas-Washburn law, and the critical pressure in the three-dimensional channel, an air-driven multiphase flow in the experiments. In all of these cases, the solver produces results that are consistent with both theory and experiment, even with respect to the pressure field accuracy, which has often been overlooked in many previous studies.

physics.flu-dyn↗

arXiv2008

Shear viscosity of hadronic gas mixtures

We investigate the effects of baryon chemical potential μon the shear viscosity coefficient ηand the viscosity to entropy density ratio η/s of a pion-nucleon gas mixture. We find that ηis an increasing function of T and μ, while the ratio η/s turns to a decreasing function in a wide region of T-μplane. In the kinematical region we studied, the smallest value of η/s is about 0.3.

hep-ph↗