Masaki Misawa

Simulation of Bubble Motion under Gravity by Lattice Boltzmann Method

We describe the numerical simulation results of bubble motion under gravity by the lattice Boltzmann method(LBM), which assumes that a fluid consists of mesoscopic fluid particles repeating collision and translation and a multiphase interface is reproduced in a self-organizing way by repulsive interaction between different kinds of particles. The purposes in this study are to examine the applicability of LBM to the numerical analysis of bubble motions, and to develop a three-dimensional version of the binary fluid model that introduces a free energy function. We included the buoyancy terms due to the density difference in the lattice Boltzmann equations, and simulated single- and two-bubble motions, setting flow conditions according to the Eötvös and Morton numbers. The two-dimensional results by LBM agree with those by the Volume of Fluid method based on the Navier-Stokes equations. The three-dimensional model possesses the surface tension satisfying the Laplaces law, and reproduces the motion of single bubble and the two- bubble interaction of their approach and coalescence in circular tube. These results prove that the buoyancy terms and the 3D model proposed here are suitable, and that LBM is useful for the numerical analysis of bubble motion under gravity.

Numerical simulation of two- and three-dimensional two-phase fluid motion by lattice Boltzmann method

Abstract This study describes the numerical simulations of two-phase fluid motions under gravity by the lattice Boltzmann method (LBM), in which the fluid motions result from collision and translation of mesoscopic particles and the interface interaction in multiphase fluids can be reproduced in a self-organizing way. Our aims are to examine the applicability of LBM to the numerical analysis of bubble motions in comparison with the two-dimensional results by the Volume Of Fluid (VOF) method based on the Navier–Stokes and the liquid-volume convective equations, and to develop the three-dimensional binary fluids model, consisting of two sets of distribution functions to represent the total fluid density and the density difference, which introduces the repulsive interaction consistent with a free energy function between fluid particles. We included the buoyancy terms due to the density difference between two phases in the lattice Boltzmann equations, and simulated the motions of single bubble and two bubbles rising in a duct, calculating the surface tension from the Laplaces law represented by the non-dimensional numbers, Eotvos and Morton numbers. In the two-dimensional simulations, the results by LBM agree with those by the VOF method. The three-dimensional simulation of two bubble interaction shows that the upper bubble takes a shape of skirt as the lower bubble approaches due to the wake formation, and they coalesce into a single bubble eventually. These results prove the validity of the buoyancy model proposed here and the applicability of LBM to the quantitative numerical analysis of two-phase fluid motions.

A phase-field method for interface-tracking simulation of two-phase flows

For interface-tracking simulation of two-phase flows with a high density ratio, we propose a computational method, NS-PFM, combining Navier-Stokes (NS) equations with phase-field model (PFM) based on the free energy theory. Through the numerical simulations, it was confirmed that (1) the volume flux derived from chemical potential gradient in the Cahn-Hilliard equation of PFM plays an important role in advection and reconstruction of interface, and (2) the NS-PFM gives good predictions for the motions of immiscible, incompressible, isothermal two-phase fluid, such as air-water system, without using conventional interface-tracking techniques.

Lattice Boltzmann simulation on porous structure and soot accumulation

In this study, using the lattice Boltzmann method, we simulate soot accumulation in porous media to examine the particle trap in a diesel particulate filter (DPF). The porous media are numerically formed for different inner structure with given porosity. The porous structure similar to the real sample, Ni-Cr metal, can be obtained, where the pressure fields in both cases are almost the same. In the simulation of soot accumulation, the velocity field is changed when the soot is attached to the porous wall. The pressure drop is largely increased. Interestingly, the friction factor is smaller than experimentally predicted value by Ergun equation.

Lattice Boltzmann simulation on continuously regenerating diesel filter

To reduce particulate matter (PM) including soot in diesel exhaust gas, a diesel particulate filter (DPF) has been developed. Since it is difficult to observe the phenomena in a DPF experimentally, we have conducted a lattice Boltzmann simulation. In this study, we simulated the flow in a metallic filter. An X-ray computed tomography (CT) technique was applied to obtain its inner structure. The processes of soot deposition and oxidation were included for a continuously regenerating diesel filter. By comparing experimental data, a parameter of soot deposition probability in the numerical model was determined.

A Phase-Field Method for Interface-Tracking Simulation of Two-Phase Flows

For interface-tracking simulation of two-phase flows, we propose a new computational method, NS-PFM, combining Navier-Stokes (NS) equations with phase-field model (PFM). Based on the free energy theory, PFM describes an interface as a volumetric zone across which physical properties vary continuously. Surface tension is defined as an excessive free energy per unit area induced by density gradient. Consequently, PFM simplifies the interface-tracking procedure by use of a standard technique. The proposed NS-PFM was applied to several problems of incompressible, isothermal two-phase flow with the same density ratio as that of an air-water system. In this method, the Cahn-Hilliard (CH) equation was used for predicting interface configuration. It was confirmed through numerical simulations that (1) the flux driven by chemical potential gradient in the CH equation plays an important role in interfacial advection and reconstruction, (2) the NS-PFM gives good predictions for pressure increase inside a bubble caused by the surface tension, (3) coalescence of liquid film and single drop falling through a stagnant gas was well simulated, and (4) collapse of liquid column under gravity was predicted in good agreement with other available data. Then, another version of NS-PFM was proposed and applied to a direct simulation of bubble nucleation of a non-ideal fluid in the vicinity of the critical point, which demonstrated the capability of NS-PFM to capture liquid-vapor interface motion in boiling and condensation.Copyright

Effect of platinum nanoparticles on cell death induced by ultrasound in human lymphoma U937 cells.

In this study, we report on the potential use of platinum nanoparticles (Pt-NPs), a superoxide dismutase (SOD)/catalase mimetic antioxidant, in combination with 1MHz ultrasound (US) at an intensity of 0.4 W/cm(2), 10% duty factor, 100 Hz PRF, for 2 min. Apoptosis induction was assessed by DNA fragmentation assay, cell cycle analysis and Annexin V-FITC/PI staining. Cell killing was confirmed by cell counting and microscopic examination. The mitochondrial and Ca(2+)-dependent pathways were investigated. Caspase-8 expression and autophagy-related proteins were detected by spectrophotometry and western blot analysis, respectively. Intracellular reactive oxygen species (ROS) elevation was detected by flow cytometry, while extracellular free radical formation was assessed by electron paramagnetic resonance spin trapping spectrometry. The results showed that Pt-NPs exerted differential effects depending on their internalization. Pt-NPs functioned as potent free radical scavengers when added immediately before sonication while pre-treatment with Pt-NPs suppressed the induction of apoptosis as well as autophagy (AP), and resulted in enhanced cell killing. Dead cells displayed the features of pyknosis. The exact mode of cell death is still unclear. In conclusion, the results indicate that US-induced AP may contribute to cell survival post sonication. To our knowledge this is the first study to discuss autophagy as a pro-survival pathway in the context of US. The combination of Pt-NPs and US might be effective in cancer eradication.

Oligonucleotide Microarray Analysis of Age-Related Gene Expression Profiles in Miniature Pigs

Miniature pigs are useful model animals for humans because they have similar anatomy and digestive physiology to humans and are easy to breed and handle. In this study, whole blood microarray analyses were conducted to evaluate variations of correlation among individuals and ages using specific pathogen-free (SPF) Clawn miniature pigs. Whole blood RNA is easy to handle compared to isolated white blood cell RNA and can be used for health and disease monitoring and animal control. In addition, whole blood is a heterogeneous mixture of subpopulation cells. Once a great change occurs in composition and expressing condition of subpopulations, their associated change will be reflected on whole blood RNA. From 12 to 30 weeks of age, fractions of lymphocytes, monocytes, neutrophils, eosinophils, and basophils in white blood cells showed insignificant differences with age as a result of ANOVA analysis. This study attempted to identify characteristics of age-related gene expression by taking into account the change in the number of expressed genes by age and similarities of gene expression intensity between individuals. As a result, the number of expressed genes was less in fetal stage and infancy period but increased with age, reaching a steady state of gene expression after 20 weeks of age. Variation in gene expression intensity within the same age was great in fetal stage and infancy period, but converged with age. The variation between 20 and 30 weeks of age was comparable to that among 30 weeks individuals. These results indicate that uniformity of laboratory animals is expected for miniature pigs after 20 weeks of age. Furthermore, a possibility was shown that whole blood RNA analysis is applicable to evaluation of physiological state.

LATTICE BOLTZMANN SIMULATION ON FLOW WITH SOOT ACCUMULATION IN DIESEL PARTICULATE FILTER

Since diesel exhaust gas has more ambient air pollutants such as NOx and particulate matters (PM) including soot, the special treatment for exhaust emission standards is needed. Recently, a diesel particulate filter (DPF) has been developed to reduce PM in the after-treatment of exhaust gas. However, since the structure of the filter is small and complex, it is impossible to examine the phenomena inside the filter experimentally. In this study, we conduct fluid simulation in the diesel filter. We use the lattice Boltzmann method. The soot accumulation is considered to simulate the PM trap in the filter. For the wall-boundary in the simulation, the inner structure of the filter is obtained by a 3D-CT technique. Results show complex flow pattern in the diesel filter. Due to the soot accumulation, the velocity is changed and the pressure is increased. The pressure drop becomes larger as soot concentration at the inlet is higher.

Elasticity Evaluation of Regenerating Cartilage Sample Based on Laser Doppler Measurement of Ultrasonic Particle Velocity

It is important for regenerative medicine to evaluate the maturity of regenerating tissue. In the maturity evaluation of regenerating cartilage, it is useful to measure the temporal change of elasticity because the maturity of regenerating tissue is closely related to its elasticity. In this study, an elasticity evaluation method for the extracted regenerating cartilage sample, which is based on the laser Doppler measurement of ultrasonic particle velocity, was experimentally investigated using agar-based phantoms with different elastic moduli and the regenerating cartilage samples extracted from beagles in animal experiments. In addition, the experimentally-obtained elasticity was compared with the result of a static compression test. These results verified the feasibility of the proposed method in the elasticity evaluation of regenerating cartilage samples.