Estimation of the contact angle on a moving contact line is one of the important factors for the prediction of the liquid surface geometry contacting with solid. In this study the dynamic contact angle on an accelerating vertical glass rod is investigated both experimentally and numerically to elucidate the effect of the acceleration of the contact line. The experiment was held by using ethylene-glycol and its aqueous solution as test fluid. The measured contact angle in the transient state clearly deviated from that for the steady state, depending on the acceleration of the rod. Numerical simulation shows that the acceleration and the gravity terms in the momentum equation, which are relatively remarkable in macroscopic scale, are not responsible for such deviation in the contact angle. Rather, the dependence of the microscopic contact angle on the acceleration, estimated with the viscous bending model, should be the primary factor on the deviation of the contact angle.
Jet breakup is an important behavior at a core disruptive accident for a sodium-cooled fast reactor. The lattice Boltzmann (LB) method is adopted to simulate the jet breakup behavior. The Multiple-Relaxation Time (MRT) scheme is introduced into the existing three-dimensional 19-velocity (D3Q19) LB model for immiscible two-phase flow to enhance the numerical stability for low kinematic viscosity. The simulation results show that the present LB model using MRT enables to simulate the jet breakup behavior, where the kinematic viscosity is of the order of 10-3. The velocity field and interfacial shape are compared with the experimental result using PIV and Laser-Induced Fluorescence (LIF). The interfacial instability and fragmentation behavior of the jet can be also simulated. Comparison of the LB simulation with experimental data shows that the time series of jet leading edge can be simulated within an error of around 10%.
Dynamic wetting of a solid sphere has been simulated numerically. We have developed the model for the description of the moving contact lines on curved solid surfaces by combining the GNBC-Front-tracking method and the immersed boundary method. The model was applied to the simulation of a quasi-static penetration of a water-repellent sphere into the water. As a result, the shape of the interface for different wettability is in good agreement with the previous experimental results. The residual bubble volume was evaluated under the different sphere size, static contact angle, and penetrating velocity conditions. The present results show a similar trend to the experimental and theoretical ones that the water-repellent sphere has a larger bubble since it incorporates more volume of air into the water, indicating the dynamic wetting of the sphere surface can be reproduced by the model.
Recently it is strongly demanded to manipulate a liquid droplet in lab-on-chip or micro reactors used in chemical engineering devices. This study concerns the control of the movement of liquid droplet on an inclined plate, using the difference of wettability caused by the chemical structure change of a polymer which is resulted from the irradiation of ultraviolet rays. The behavior of droplets was carefully observed experimentally when it enters into the irradiated area having an oblique boundary to the moving direction. Considering the surface tension acting on the perimeter with different contact angles, the movement of droplet was analyzed theoretically. The calculated results approximate well the actual behavior of droplet. Then we proposed a geometric pattern of irradiated area in which the droplet could be moved linearly with a constant angle to the gravitational direction. The experimental observation shows that the droplet behavior was successfully controlled to be moved along on the line of geometric pattern.
Optically induced thermal Marangoni instability in irradiated transparent liquid films on absorbing solid substrates is investigated numerically, where the effects of optical interference between light waves reflected from the gas-liquid and liquid-solid interfaces are taken into account. When a film is exposed to uniform irradiation, the film stability is strongly affected by the energy reflectance that fluctuates with the film thickness, so that the stability varies periodically with the thickness. When a film is exposed to weakly nonuniform irradiation, the behavior of the film is affected not only by the nonuniformity but also by the stability for uniform irradiation with the mean light intensity. While a nonuniformly irradiated film that is stable for uniform irradiation is deformed but remains in the neighborhood of the initial state, a similarly irradiated film that is unstable for uniform irradiation evolves in a manner similar to phase separation behavior observed in uniformly irradiated films.
The disappearance of dryout region observed in liquid film flowing on an inclined wall (50mm width) was investigated experimentally. The film surface profile on the cross section vertical to the flow direction was measured by the needle contact method to investigate the effect of side wall on the critical Weber number WeC at which the dryout region shrinks to disappear. The results show that there exists a minimum film thickness observed at few milli-meters from the side wall, which is much smaller than that of Nusselt' s theory. To avoid such effect of side wall on the dryout phenomenon, the critical Weber number WeC was measured for the film flowing on the vertical plate (200mm width) without side wall and on the outer wall of vertical cylinder. The experimental results show that WeC is much smaller than that for the inclined plate. A simple theoretical consideration was conducted to consider the force balance including the gravitational force at the edge of the dryout region in order to expect the critical Weber number. The results approximate well the tendency of the experimental results dependent on the advancing contact angle and viscosity.
We have conducted large molecular dynamics simulations of the dewetting of ultrathin liquid films on a solid substrate at the nanometer scale. In particular, we observed the visco-inertial regime for the first time; this type of dewetting has never been observed because its specific signal is too small, although it is expected to be observed in a wide range of conditions. Dewetting initially behaves in the inertial regime. After the rim size reaches a crossover length, it transits to visco-inertial dewetting, as predicted by the hydrodynamic theory. The assumption of visco-inertial dewetting with the boundary layer in the liquid film agrees with the simulation results. We evaluated the crossover between the inertial and the visco-inertial regimes at the molecular scale that is larger than the critical time based on the hydrodynamic theory, especially in the range of the appearance of the slip effect.
The large-eddy simulation (LES) of a volcanic plume based on a multi-fluid approximation was performed. After the impact of subgrid scale (SGS) turbulence models on the structure of a volcanic plume was assessed, the transport process of volcanic ash inside a plume was investigated. The simulation results showed that the selection of SGS turbulence models could lead to impact on the plume structure such as the plume height and ascending velocity. Concerning the transport process of volcanic ash, the spatial distributions of particles with different sizes was not identical mainly because of the difference in mean vertical velocity. This supports the validity that the sedimentation rate is used as a parameter in existing source term models for long-range transport simulation of volcanic ash. However, this also suggests that existing source term models based on the one-fluid approximation can cause considerable approximation error for large particles.
The aim of this study is to confirm the existence of a spontaneous evaporation molecular mass flux which takes a constant value independent of the degree of net evaporation/condensation. We carried out the numerical simulation based on the mean field kinetic theory during net evaporation/condensation and estimated the evaporation and reflection molecular mass fluxes by using a pair of boundaries. The simulation results showed that the reflection molecular mass flux varies with the increase of the degree of nonequilibrium, and the evaporation molecular mass flux takes a constant value during net evaporation and condensation.
The acoustic levitation is one of the levitation techniques to control a sample in sound pressure. The acoustic levitation induces the internal and external flow of the levitated droplet. It is considered that these flows affect heat and mass transfer of the levitated droplet. The purpose of this study is to investigate the relationship between convective heat transfer and flow behavior of the levitated droplet. The convective heat transfer between ambient air and the droplet is calculated by an evolution of the surface area and temperature. The convective heat transfer increases as the temperature of water droplet increases. The external flow structure is measured by PIV. The non-heated water droplet has a toroidal vortex below a levitated droplet. As the temperature of water droplet increases, the external flow structure changes from that of non-heated droplet. The larger the diameter and temperature of droplet become, the larger external flow velocity near the surface of the levitated droplet becomes. It is suggested that the increase of external flow velocity enhances the convective heat transfer and the convective heat transfer is more enhanced by the increase of temperature.
A bubble generation method that uses a slit elastic tube and an acoustic pressure wave in the gas phase can produce single bubbles of various sizes. In the method, the bubble size greatly depends on the tube length and the boundary condition of the tube end. In this study, we investigated the effects of acoustic pressure waveforms on the bubble generation. We changed the tube length and the boundary condition of the tube end and measured the waveform using the electret condenser microphone (ECM) and the generated bubble diameter. The time length that the positive pressure was maintained on the waveform depends on the tube length and the boundary condition. The diameter of the generated bubble also changed with the time length. We demonstrated that this phenomenon was caused by the change of the phase of reflected wave at the tube end by using FDTD simulation.
We experimentally investigated the motion of a pair of bubbles initially positioned in-line configuration in ultrapure water or an aqueous surfactant solution. The bubble motion was observed by two high speed video cameras. The Reynolds number of bubbles were ranged from 50 to 200, and the bubbles hold a spherical shape in this range. In ultrapure water or small concentration of surfactant solution, initially the trailing bubble deviated from the vertical line on the leading bubble owing to the wake of the leading bubble. And then, the slight difference of the bubble radius changed the relative motion. When the trailing bubble slightly larger than the leading bubble, the trailing bubble approached to the leading bubble due to it' s buoyancy difference. The bubbles attracted and collided only when the bubbles rising approximately side by side configuration. On the other hand, the trailing bubble was drafted to the wake region of leading bubble when the bubbles were rising in high concentration of surfactant solution. In this case, the bubbles always collided in line configuration. We consider that these phenomena are caused by the changes of lift force direction in the wake region owing to the surface contamination. In addition, bubble coalescence was inhibited in surfactant solution, even the relative motion was similar to the clean bubble case.
Countercurrent flow limitation (CCFL) may occur in a hot leg (consisting of an inclined pipe, a vertical 50-deg elbow, and a horizontal pipe) and a pressurizer surge line (consisting of a vertical pipe, a vertical 90-deg elbow, and a slightly inclined pipe with elbows) under postulated accident conditions in a pressurized water reactor (PWR). In our previous study, we developed a one-dimensional (1D) computation method to predict CCFL characteristics in an inclined pipe with elbows, to generalize the prediction method for CCFL in piping systems. In this study, we did 1D computations for CCFL experiments in horizontal pipes (diameters of D = 0.03-0.65 m and ratios of the length to the diameter of L/D = 4.5-63) to evaluate prediction accuracy of the 1D computation method. The differences of the CCFL constant C (value of JG*1/2 at JL*1/2 = 0 where JG* and JL* are the Wallis parameters) between measured and computed values were within dC = ±0.055 for C = 0.4-0.7.
We carried out two-dimensional numerical simulation for downward Poiseulle water flow with a carbon-dioxide bubble or multiple carbon-dioxide bubbles in a vertical channel. A phase-field method was used for capturing the interface. The concentration equation including the phase-field function and the time evolution equation of the phase-field function including the velocity field were solved simultaneously by using Graphics Processing Units. The results showed that a bubble in the offset position moved away from the left-hand side wall, while it was rising, due to the lift force. This motion causes the development of asymmetrical, complicated structure of wake flow. Then, the bubble moved towards the wall due to the change in the flow velocity field. These time changes in the wake flow enhance the diffusion and absorption of carbon dioxide in flow. In the case of four bubbles, the decreasing rate of bubble mass is higher than that for a single bubble. This is partly because of the difference in the interface length, and partly because of the renewals both of the concentration boundary layer and the wake-flow structure caused by the impact of bubble coalescence.