This paper overviews how the author's group has been exploring on the decision-making process with social dilemma of individual vaccination, namely whether or not to be vaccinated, by means of the model of dynamics for epidemic spreading on a social network applied to evolutionary game theory. On underlying networks, both epidemic and information of agent's strategy are transferred, where the former is modeled by SIR and the latter is emulated as a spatial evolutionary game. Simulation results imply that both the vaccination acceptance fraction and final epidemic size are significantly affected by how strategy updating happens; namely whether he/ she copying from a neighbor or imitating a social trend, and underlying network topologies. The study poses one example of how the social physics helps to understand complex phenomenon taking place in a real world.

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Microorganisms play a vital role in many biological and medical phenomena. Transport phenomena in a suspension of microorganisms are extremely important, because they dominate the growth and distribution of cells. In terms of fluid mechanics, however, mathematical modeling of a suspension of swimming microorganisms is still under way. In this article, I explain recent developments in modeling a suspension of microorganisms. Macroscopic properties of a suspension are strongly affected by the mesoscale flow structures, and the mesoscale flow structures are affected by the interactions between microorganisms. Thus, a bottom-up strategy, i.e. from a cellular level to a continuum suspension level, represents the natural approach to the study of a suspension of swimming microorganisms.

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In a recent article (R. Seto, R. Mari, J. F. Morris, and M. M. Denn, Phys. Rev. Lett., 111:218301, 2013) we found frictional contact forces to be essential for reproducing the shear thickening behavior of non-Brownian suspensions. Although the introduction of frictional contact to a Stokesian Dynamics simulation is speculative and requires the existence of particle-particle contact despite fluid lubrication, the simulation results are in good agreement with experimental data. The model also provides physical insight into the relation between shear thickening and a jamming transition. This article describes the transition from a fluid mechanics perspective to a granular physics perspective.

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To better understand the processes which occur in magma chambers and in liquefied sediments, we conduct laboratory experiments of gravitational instability of a close packed granular medium in a liquid. First we consider a case in which a granular medium overlies a liquid layer. Here the lowermost thin layer of a granular medium dilates and becomes mobile to form a rheological boundary layer (RBL), from which narrow granular plumes grow downwards. We measure the wavelength and the growth rate of the instability and constrain the thickness of the RBL to be only about twice the particle size, and its packing fraction as Φ = 0.42, indicating dilation. When the granular medium is thicker, we find that the granular plumes organize themselves into convection cells. Importantly, the ascent velocity of the boundary between the granular medium and the liquid layer is approximately constant regardless of the thickness ratio of the two layers. A scaling in which the packing fraction of the RBL governs the ascent velocity is found to explain the results well. Next we extend the first experiment and model magma chamber roof melting and subsequent particle settling. An experimental cell consists of a lower layer of a thermally convecting molten wax, and an upper layer comprising of a mixture of particles and solid wax. As the interstitial solid wax of the upper layer melts, the particles become mobile and settle downwards. From experiments with different particle sizes, we find that when the particle is smaller than 0.1 mm, cycles in which melting starts and stops, occur spontaneously. Melting stops due to the formation of a stable stratification in the melt layer which suppresses the vertical heat transfer. Melting resumes by the convective overturning because the cell is heated from below. A dimensionless Buoyancy number is found to explain the critical particle size well. As a third experiment, we consider the instability caused by liquefaction. Here an experimental cell consists of a water-immersed granular medium which is size-graded into 2 layers such that the upper layer has a smaller permeability. When the cell is shaken vertically, liquefaction occurs and the water temporarily accumulates at the 2 layer interface resulting in a gravitational instability. From experiments in which we vary the acceleration and frequency of shaking, we find that there is a critical acceleration for instability to occur, and that this acceleration is minimum at a frequency of about 100 Hz. We show that the frequency dependence can be interpreted from a combined condition of energy and jerk of shaking exceeding their respective critical values.

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Pedestrian dynamics, which has been vigorously studied in traffic engineering, architecture and psychology, also started to attract interest of physicists in the end of the twentieth century. It is almost impossible to predict the movement of an individual pedestrian in detail since she has own will. Physicists, however, have elucidated the macroscopic collective behaviors of pedestrians by dealing them as “self-driven” particles whose destination is clear. In this paper, the author introduces the research on three fundamental pedestrian dynamics, i.e., unidirectional flow, bidirectional flow and egress process, with analysis of cellular automaton models. In congested unidirectional flow, slow rhythm improves pedestrian flow. In bidirectional flow, appropriate anticipation smooth the flow, while excessive anticipation hinders it. Furthermore, in egress process, an obstacle increases the flow if it is set at proper position.

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The present article overviews some granular/multiphase flows observed in civil engineering fields and discusses their grain-scale mechanics. In particular, solid/fluid phase transition behaviors are highlighted to characterize the mechanics of granular materials. In liquefaction phenomena, uni-directional shear causes solidification of the system, while cyclic shear leads to liquefaction. This behavior can be understood as an induced anisotropy of granular packing structure. In rapid flows, kinetic stress increases with increasing shear rate, which may result in drastic reduction of shear resistance of the flow. Emphasis is also put on the significance of the analysis of long-term geological formation whose micro-mechanism is governed by the grain/fluid interaction during erosion, transformation and sedimentation process.

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The hydrodynamic structure of the high-speed water jet utilized in the jet grouting was experimentally investigated. The nozzle diameter of water jet ranged from 2 to 6mm, and the outlet pressure is up to 20 MPa, so the jet velocity reaches to several hundred meter per sec. The double-pulse Nd:YAG Laser and CCD camera with image intensifier were used to capture clear images of the water jet. From the captured images, velocity of water jet was measured using LIF technique combined with PTV at various locations from outlet of nozzle. The result showed the velocity of water jet remains its initial velocity at 400D (D: nozzle diameters) from nozzle outlet. Radial velocity profile inside the jet showed flat velocity profiles for pressures and locations from outlet in the present experiment.

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We investigated the freshness-keeping effect of water containing fine bubble (FBW, < 100 μm) on cut flowers such as a gentian, a lisianthus, and a small chrysanthemum. Pure water was also used for comparison. The effect of the glycogen on freshness keeping with FBW was also investigated. Although there were statistical dispersions in experimental results, FBW was effective in keeping the freshness in the experiments. Additionally, we also confirmed that FBW water improves the coloring rate of the lisianthus. These results suggest that fine bubble improves the absorption of water by the plants.

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Numerical simulations were done for counter-current flow limitation (CCFL) at the lower end of a vertical pipe simulating lower part of steam generator U-tubes by using the volume of fluid method (VOF) implemented in the CFD software FLUENT6.3.26. The simulated CCFL characteristics agreed well with air-water experimental data but flooding in simulations appeared at the upper end of the vertical pipe. To avoid flooding at the upper end, water was supplied through the pipe wall simulating condensation on the inner surface and flooding at the lower end was successfully simulated. However, computations by the standard k-ε turbulence model became unstable for pressures lower than 1.0 MPa and significantly underestimated falling water flow rates. On the other hand, computations by the laminar flow model were stable even for low pressures and significantly overestimated falling water flow rates. Computations by the k-ω SST turbulence model were unstable for pressures lower than 1.0 MPa but gave good agreement of a CCFL value with a steam-water experimental value at 1.0 MPa.

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Particle discharge rate R_out is simulated with Euler-Lagrange approach under accelerated air ventilation circumstances. Two factors of Stokes number St and acceleration are taken into account to investigate their influences on R_out. The particle mass flow rate q_m as an important parameter is used to evaluate the essential of the R_out. St is rescaled by changing particle diameter d_p. Consequently, the tendency of R_out is clarified under accelerated air ventilation circumstances. In the simulation, the unsteadiness of the fluid velocity field produces the unsteady forces, which include pressure gradient force f_PG and virtual mass force f_VM. Unsteady forces provide a time delay to particle movement and reduce R_out. Moreover, smaller St (St=0.001 and 0.1) increases the fine R_out, and larger St (St=1.2 and 5.0) is adverse. In the case of St=0.1, the R_out are fundamentally identical regardless of the accelerations. Nevertheless, in the case of St=1.2, the increase of the acceleration of the air ventilation circumstances reduce the R_out .

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Flow characteristics of a gas-solid spouted bed with a modified draft tube have been investigated experimentally. In this work, four different types of draft tubes were employed and spherically shaped silica gel particles having low particle density were used as the bed materials. The flow characteristics were compared with those obtained using spherically shaped glass beads. The results showed that the conical-cylindrical porous draft tube was the best option for gas-solid contact and solids circulation, and the gas flow rate through the annulus reached the level of a conventional spouted bed without a draft tube. Also, the minimum spouting gas velocity, bed pressure drop and gas flow rate through annulus for the silica gel particles were smaller than those for the glass beads. Although the silica gel particles descended in annulus faster than the glass beads because of high sphericity, the particle density of the silica gel was so small that the solids circulation rate was smaller than that for the glass beads.

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