I introduced two examples of molecular dynamics (MD) simulation of nano-scale bubble with phase change corresponding to cavitation. One is a nucleation-growth process starting from homogeneous bubble nucleation, and a characteristic length reflecting the time change of the size distribution was extracted and compared with a macroscopic theory for Ostwald ripening. The other is a simulation of nano-scale bubble collapsing. In the latter simulation, we evaluated the time change of bubble radius and compared it with some bubble dynamics models derived by fluid mechanics assuming continuity of fluid.
A stirred vessel is a device for mixing fluids. It is widely-used in industry and plays an important role in the chemical process. In the stirred vessels, liquid is stirred by the rotational motion of impeller, and often, there exists large deformation of free surface. The moving particle semi-implicit (MPS) method is a meshfree particle method suitable for such complex physical phenomena because moving boundaries can be handled with relative ease. The devolatilization process is one of the applications of stirred vessel, where dissolved volatile components are removed by inducing the nucleation of bubbles in the tank. This paper explains numerical algorithms for the stirred vessel analysis with the MPS method. Furthermore, numerical simulations of the devolatilization process using the MPS method with incorporated bubble models are presented.
Incompressible flows have much lower flow speeds to compare with the sound speed and called low Mach number flows. So far, we have simulated incompressible flows by using semi-implicit solvers, in which we iteratively compute the linear equation derived from the pressure Poisson equation. In the case of incompressible gas-liquid two-phase flows, the sparse matrix would be very stiff with non-zero elements changing largely from the gas density to the liquid density at the interface. Even if we use a sophisticated Krylov sub-space iteration solver with the multi-grid preconditioner, the convergence becomes poor for large-scale problems. We have developed a weakly compressible gas-liquid two-phase solver based on fully-explicit time integration of compressible Navier-Stokes equation. For several benchmark problems, the results of the weakly compressible solver are in good agreement with those of the semi-implicit incompressible solver. Since the weakly compressible solver uses a local stencil access, it is possible to implement AMR (Adaptive Mesh Refinement) method with less programming cost. Fine meshes are assigned near the gas-liquid interface and we efficiently compute a liquid film generated by a spoon and a growing soap bubble with the AMR two-phase flow solver.
The current status of convection-permitting regional climate models is reported. First, the outline of regional climate models is described. Next, the representation of clouds in regional climate models is mentioned. Subsequently, the performance of numerical simulations using convection-permitting regional climate models is presented. In particular, the effects of high-resolution topography on the performance of simulations are discussed. Finally, an example of future climate projections using convection-permitting regional climate models is introduced.
The severe accident in Fukushima daiichi nuclear power plant revealed the importance of molten core cooling system without the electric power, then cooling system by natural circulation flow has attracted attention. The heat removal performance of this system depends on the natural circulation flow rate, so it is essential to predict the flow rate accurately for the safety design. The purpose of this study is to develop the evaluation method for the natural circulation, and to examine its accuracy dependence on the channel shape. Experiments were carried out at atmospheric pressure, using room temperature air-water flow. We used seven kinds of channels with various shape and diameter, and measured natural circulation flow rate and pressure drop for each channel. Predictive analysis was conducted by our method based on balance between driving force and pressure drop in the loop channel. The major results are as follows: (1) It is possible to predict the natural circulation flow rate with an error of less than 20 percent for various shaped channels. The average and standard deviation for the ratio between calculated and experimental flow rate is 0.98 and 0.047. (2) Two phase prediction method for pressure drop has maximum 15 % error. To improve the prediction accuracy, it is important to improve the prediction of the void fraction and the two-phase multiplier.
In this work, the effects of liquid viscosity on flow patterns, gas slug velocity and gas slug length in a horizontal circular microchannel with an inner diameter of 530 μm have been investigated by using a high speed camera. As the working fluids, nitrogen gas for the gas and aqueous solutions of glycerol with high viscosities the concentrations up to 78.0 wt% for the liquid were used. Kinematic viscosities and surface tensions of liquids were respectively ranging from twice to 35 times and from 0.9 to 1.0 than that of distilled water. The superficial gas velocities, jG, were ranged from 0.01 to 13.0 m/s, and the superficial liquid velocities, jL, were from 0.05 to 0.30 m/s. The flow patterns observed were slug flow and disturbed ring film flow. The velocity of gas slug for the system of nitrogen-distilled water two-phase flow coincided well with that in case of homogeneous flow, on the other hand, velocities in the case of glycerol solution with the higher viscosities showed the higher values than equation of homogenous flow and equation proposed by Hughmark. The correlation of the distribution parameters, C0, of two-phase flow were proposed in the present paper as a function of the ratio of kinematic viscosity of the liquid to that in distilled water. The tendency was observed that the lengths of gas slug became short when the viscosities of the liquid were increased.