This review describes the researches on flow phenomena at gas-liquid interface subjected to external oscillating force. Especially, surface disintegration, droplet formation, bubble cluster generation, and dynamic behaviour of bubbles in oscillating liquid container are described. And further, surface wave generation, liquid jet generation, jet breakup phenomena, droplet formation of magnetic fluid subjected to vertical vibration or alternating magnetic field are also described. These phenomena are related to interfacial instability of liquids subjected to external oscillating forces.
Treating the moving contact lines (fluid-fluid interfaces on solid surface) in numerical simulations are overviewed. At first, mathematical failure in analytical solution for flow between moving wall and static fluid-fluid interface is introduced. Analytical models for the relation between the dynamic contact angle and the contact line moving velocity are also introduced. Defect of numerical simulations based on implicit interface representations are discussed. Then, a front-tracking method combined with generalized Navier boundary condition is presented. Furthermore, it is shown that an analytical asymptotic relation between macroscopic and microscopic behaviors is very useful for combining the microscopic model into macroscopic simulation method.
In an extrusion process of rubber materials, there are a liquid-liquid phase and a gas-liquid phase. It is difficult to solve viscoelastic flow with high Weissenberg number. In this paper, interface between rubbers is solved by using the commercial software. The difficulty in the numerical simulation is specified. The solution to this problem is proposed. The proposed method is applied to the numerical solution of flow problems for a viscoelastic fluid in arbitrary computation domains. The proposed method is enabled to solve the viscoelastic flow with high Weissenberg number in comparison with the conventional method.
For developing a novel micro-fabrication process of flexible and large-area thin-film display MEMS device, liquid-liquid two-phase slug droplets formation in T-junction microchannel with square cross section is investigated through computational fluid dynamics (CFD) simulation by using a diffuse-interface tracking method based on lattice-Boltzmann model numerical solution scheme plus conservation-modified Allen-Cahn advection equation. The volumetric flow rate ratio is fixed at 1.0 within the range of low capillary, Reynolds and Weber numbers for silicone oil-water system with hydraulic diameter of 100 μm, kinematic viscosity of 1.0 cSt. and interfacial tension of 41.6 mN/m. The major findings are as follows: (1) the continuous and dispersed-phase slugs become shorter at nearly-constant length difference between them as their flow rates are increased; (2) their lengths in the simulation agree well with experimental data; (3) the dispersed-phase volume fraction are well predicted in comparison with experimental and one-dimensional two-fluid model CFD simulation results.
Gas-liquid interfacial flows, such as liquid film flows (also known as wetting flows on plates), are encountered in many industrial processes including absorption, distillation and so on. The present study focuses on detailed descriptions of the flow transition phenomena between the film flow and the rivulet flow, as well as how such phenomena are affected by wall surface texture treatments.This study develops a numerical simulation technique using Computational Fluid Dynamics (CFD) with the Volume of Fluid (VOF) model as well as a lab-scale experimental testing technique. As the liquid flow rate is increased and then decreased, a hysteresis of the transition between the film flow and the rivulet flow is discovered, which implies that the transition phenomenon depends primarily on the history of the change of interfacial surface shape. Further study on the effect of texture geometry shows quantitatively that surface texture treatments added on the plate can help to prevent the liquid channeling and can increase the wetted area. The main reason for increasing the wetted area on the wavy surface is that the liquid film break-up is inhibited due to the liquid holding on the plate and the spreading of the liquid flow in spanwise direction by the wavy surface geometry. Additionally, the simulation results agree well with the experimental results in terms of the gas-liquid interfacial surface shape and the wetted area in the transition region between the film flow and the rivulet flow.
Electrowetting is a technique to modify wettability and meniscus shapes on electrodes by applying voltage to the electrodes. In the present article, we introduce principles of electrowetting deriving the equation connecting voltage and contact angle. Then we reviews related articles on optical devices using electrowetting including our previous works; a liquid prism controlling optic-axis using electrowetting. Numerical simulation solved meniscus shapes between each liquid to predict the relation between applied voltage and optic angle. Results in experiments and simulations are briefly reported.
After termination of natural circulation during hypothetical accidents in light water reactors, vapor generated by decay heat forms two-phase flow in stagnant coolant liquid and rises in a reactor core. In these conditions, void fractions in the reactor core affect the two-phase mixture level and cooling of fuel rods. In this study, numerical simulations were carried out for a 4×4 rod bundle which was a mock-up of the PWR core by using the computational fluid dynamics (CFD) software, FLUENT 6.3.26. Simulated void fractions showed a similar trend to air-water data but were about 0.10 higher than measured void fractions at large air volumetric fluxes. Over-prediction of void fractions might be due to severe flooding at the upper end of the bundle.
Air-water tests on counter-current flow limitation (CCFL) were carried out using vertical pipes with the diameter of D=20 mm and a sharp lower edge which simulated the lower part of U-tubes in a steam generator. Flow patterns were observed to identify the location of CCFL occurrence, and effects of the pipe length L and the water level in the upper tank h on CCFL were measured under the conditions of L=100-700 mm and h=100-600 mm. As a result, CCFL appeared at the upper end under the conditions of L/D=5 and h/D=5, but at the lower end for L/D≧15 and h/D=5～30. In the case of the CCFL occurrence at the lower end, L and h did not affect CCFL characteristics. Using the Wallis parameters, JG* and JL*, a CCFL correlation for the sharp lower edge was derived. The CCFL constant, which is the value of JG*1/2 at JL*1/2=0, was C=0.73±0.02.
Experiments on a bubbly channel flow utilizing silicone oil as the test fluid were performed to elucidate mechanisms of frictional drag reduction by bubbles: Using silicone oil overcomes problems of contaminations on bubble interfaces and allows large deformations of bubbles even at small Reynolds numbers (Rem) because of its small surface tension. The local friction drag coefficient measured by a shear stress sensor indicated that the bubble injection introduces drag reduction even at low Reynolds number turbulent flows (Rem ~ 1900) at which the reduction has never been observed in water channel. We also introduced evaluations of the drag reduction using a gain factor that includes energy spent for bubble injections. The gain factor for an actual ship was estimated on the experimental results to investigate applicability of the drag reduction technique for vessels.
Skin frictional drag of the gas-liquid two-phase flow is investigated using silicone oil as the test fluid in a horizontal channel. In this 2nd report we focus on time-series characteristics of wall shear stress in addition to conventional average-based evaluation of skin friction characteristics. Influence of the bubble injection in both transition and turbulent flow regimes are examined. Wall shear stress fluctuations indicate that the bubble injection produces quasi-random fluctuations that obey Gaussian distribution and eliminates positive spikes on the fluctuations representing sweep events in turbulent flows. This elimination takes primary role on the drag reduction in the present case that the bubble deformations are allowed. A simultaneous measurement of the both wall shear stress and projection void fraction indicates two important things: (1) the void fraction has significant time fluctuations even by continuous bubble injections; (2) there is a time delay between local maxima of the void fraction and local minima of the wall shear stress.
This paper presents a study of the gas wiping process, which is used in coating processes to control the final coating weight applied on a substrate. Multi-slot type gas wiping nozzle was proposed for improving coating weight controllability, and investigated using numerical simulations recent years. We made two types of 3-slot wiping nozzle and measured 2-dimentional free-jet characteristics with a hot-wire anemometer. Then, gas wiping model experiments with paraffin wax coating were carried out. We found that a gap between main and auxiliary slot has significant effect on the jet stream characteristics. Although each slot gas pressure conditions were the same, velocity distribution of the jet becomes sharp if the gap is narrow. The coating thickness after 3-slot gas wiping decreased with a sharp pressure gradient of the jet.