In recent years, it is important to evaluate the debris flow risk with help of run-out analysis, instead of prevention by a control dam as the provision against damage from debris flows. In this study, the result of run-out analysis of debris flows by the practical depth averaged method by using Material Point Method (MPM) is presented in order to help debris flow risk zoning. According to the analysis method suggested in this study, we can simulate the behavior of debris flow by using constitutive law in soil mechanics, namely Drucker-Prager model, with geotechnical parameters, including the aspect of fluid behavior. In addition, this method enables us to do run-out analysis of debris flow in a vast area where an actual debris flow will spread with small calculation cost. In this paper, the outline of MPM and the depth averaged method discretized by MPM (DAMPM) is introduced. Then, the result of simulation of debris flow in the vast area which includes an actual river with a high danger of mud and debris flow is presented. As the result, we could quantitatively calculate the amplitude of velocity, depth and basal force which are brought by the debris flow at a specific point.
The lattice Boltzmann method for solid-fluid two-phase flows with a viscoelastic membrane is improved. In the present model, the elastic force at each lattice node is determined so that the total elastic energy in the whole membrane can be minimized according to the principle of virtual work. The method is applied to the motion of an annular membrane in a shear flow. The calculated deformation of the membrane agrees well with the result by the immersed boundary-lattice Boltzmann method. Next, three-dimensional structural membrane models, spherical and ellipsoidal models, are constructed by means of the mapping system. The motions of these membranes in shear flows are simulated by using the method. In an initially spherical membrane, the steady circulatory flows inside the membrane and the tank-treading motion are observed. In an initially ellipsoidal membrane, the steady shape and orientation of the membrane are almost the same regardless of initial inclination angles.
Two-dimensional interface tracking simulations and photographic experiments were carried out to investigate the effects of film thickness and impingement angle on the splashing event during single-drop impact onto a plane liquid surface. For sufficiently small values of liquid film depth, liquid originally included in the inner region of liquid crown was not transported to the outer region. As a result, the pressure rise at the impact neck became significant, a thin liquid crown was formed, and the crown wall was broken into many secondary drops. Numerical results showed that the pressure rise induced on the bottom wall by drop impact is significantly mitigated if the impacted wall is covered with the liquid film whose thickness is comparable to the primary drop size. With respect to the effect of impingement angle, pressure rise in liquid was equally dependent on the normal and tangential components of impact velocity for slightly and moderately oblique impacts; however, the pressure rise was mitigated for very oblique impacts.
In this study, with the aim of establishment of numerical boiling and condensation model on subcooled boiling, numerical simulation has been carried out in subcooled boiling bubble behavior using the MARS (Multi-interface Advection and Reconstruction Solver) developed by Kunugi. Bubble behavior in subcooled pool boiling is also visualized for the verification of the boiling and condensation model. The subcooled bubble behaviors, especially growing of the nucleated bubble on heating surface, are investigated. It is found that the bubble growth rate obtained by the numerical simulation is very large compared to the experimental one. In order to solve this discrepancy, an effect of unsteady heat conduction on the phase-change phenomena is considered in an enthalpy method. The computational results based on this model show that the bubble growth rate is close to the Rayleigh equation, and it approaches to the visualization results.
In the present paper, the characteristics of adiabatic two-phase flows in a horizontal circular microchannel have been reported. In experiments, in order to determine the effects of fluid properties on the flow characteristics, distilled water and aqueous solutions of ethanol having four different mass concentrations were used as the working liquids, while nitrogen gas as the gas. The nitrogen gas and one of the four liquids were injected through a T-junction type mixer to the test microchannel made of fused silica. To know the effects of flow contraction at the channel inlet, two mixers of different inner diameters of DM = 250 μm and 500 μm were used at a fixed microchannel diameter of D = 250 μm. Bubble velocity data and/or void fraction data were correlated with the well-known drift flux model. The distribution parameter, C0, in the model was found to depend on both the liquid properties and the flow contraction, i.e., C0 increased with increasing of the liquid viscosity and/or decreasing of the surface tension, and was higher for the flows with the contraction. The pressure drop data were correlated with the Lockhart-Martinelli method. The two-phase friction multiplier, φL2, was lower for the flows with the contraction.
A novel two-phase flow measuring technique based on local electrical conductivity measurement was developed for clarifications of three-dimensional flow structure in gas-liquid two-phase flow in a narrow channel. The measuring method applies the principle of conventional wire-mesh tomography, which can measure the instantaneous void fraction distributions in a cross-section of a flow channel. In this technique, the electrodes are fixed on the inside of the walls facing each other, and the local void fractions were obtained by the electrical conductivity measurement between electrodes arranged on each wall. Therefore, the flow structure and the bubble behavior can be investigated by three-dimensional void fraction distributions in the channel with narrow gap. In this paper, a micro Wire-Mesh Sensor (μWMS) which has the gap of 3 mm was developed, and the instantaneous void fraction distributions were measured. From the measured distributions, three-dimensional bubble distributions were reconstructed, and bubble volumes and bubble velocities were estimated.
For the high-performance, large-capacity and downsizing of electronic equipments, the thermal management becomes more important by the increasing heat generation density from semiconductors densely integrated. Flow boiling heat transfer is one of effective methods because of its high heat removal ability. Experiments on the increase of CHF for flow boiling in narrow channels by improved liquid supply were conducted for the development of high-performance cold plates for space applications. A large surface of 150mm in heated length and 30mm in width with fine grooves was employed. A structure of narrow heated channel between parallel plates with an auxiliary unheated channel was devised and tested by using water as a test fluid for different combinations of gap sizes and volumetric flow rates, where five different liquid supply modes were compared. Values of CHF larger than 2×106W/m2 were obtained only for a gap size of 2mm, indicating that higher vapor velocity in the main heated channel was more important than the enlargement of gap size to increase CHF in the experimental range. Under several conditions for gap size of 2mm, the extensions of dry-patches are observed at the upstream location of the main heated channel resulting burnout not at the downstream but at the upstream. It was clarified that there is an optimum flow rate distribution for both channels to obtain the highest values of CHF. For a gap size of 2mm, high heat transfer coefficient as much as 7.4×104W/m2K are obtained at heat flux 1.5×106W/m2 under the mass velocity 720kg/m2s based on the total volumetric flow rate and on the cross section area of the main heated channel. It is a useful method to supply the cooling liquid from the auxiliary unheated channel in the transverse direction perpendicular to the flow in the main heated channel in order to also obtain high heat transfer coefficient.
Partial length rods (PLR) are used in fuel bundles of BWR to reduce pressure drops in two-phase regions and to optimize the power distribution. Since little is known about effects of PLR on two-phase flows, air-water two-phase flow around PLRs in a four by four rod bundle is visualized by using a high-speed video camera. The experimental apparatus consists of acrylic channel box and transparent rods. Air and water at atmospheric pressure and room temperature are used for the gas and liquid phases, respectively. The ranges of the gas and liquid volume fluxes, JG and JL, are 0.4 < JL < 2.0 m/s and 0.06 < JG < 8.85 m/s, which cover typical two-phase flow patterns appearing in the fuel bundle. As a result, the following conclusions are obtained. (1) In churn flows at high JL, the flow pattern in the downstream of PLR transits to slug flow, and the flow patterns in the surrounding subchannels transit to bubbly flow due to the redistribution of gas flow. (2) In annular flow, the liquid film on the PLR forms a liquid column above the end cap of PLR. Drops are generated by column breakup and deposit on liquid films on the neighboring rods. (3) The liquid film thickness on the surface of neighbor rods facing the PLR increases and it reduces that on their opposite surface in the downstream of PLR.
Void drift between triangle tight lattice subchannels has been studied experimentally and analytically. In the experiment, a channel having two subchannels simplifying the triangle tight lattice subchannels was used. Experimental data were obtained for both air-water and air-water with a surfactant flows to know the effects of the reduced surface tension on flow and void redistributions due to the void drift. In the analysis, the experimental data were compared with the calculation by a subchannel analysis code based on a one-dimensional two-fluid model in which various constitutive equations of wall and interfacial friction forces were tested. The results of such experiment and test of analysis will be reported.
Impact force around a cavitating jet can be utilized for surface modification to improve fatigue strength of metallic materials. A peening method by cavitation impacts is called cavitation shotless peening (CSP), as shots are not required. For making practical use of CSP, enhancement of capability of the cavitating jet is required. In the present paper, an optimum nozzle outlet geometry that the mass loss shows the maximum was found by changing the nozzle outlet geometry through erosion test. In addition, the discharge frequency of cavitation clouds, Strouhal number and cavitation impacts by changing with nozzle outlet geometry were measured. It was revealed that nozzle outlet geometry affected the capability of the cavitating jet, the frequency of impacts, Strouhal number and the maximum value of cavitation impact force.
Popular microbubble generators need great mechanical power to produce microbubbles. It is a disadvantage when the microbubble distributors are applied in industries. A novel microbubble generation system using direct-contact condensation of the mixed vapor bubbles was invented. The system has simpler construction, easier maintenance and less mechanical energy consumption than the other ones. To understand the microbubble generation, the phenomena of the direct-contact condensation of mixed vapor bubbles were observed using a high speed video camera attached to a microscopic lens. The microbubble size depends on the superficial vapor velocity, vapor composition and inner nozzle diameter. The microbubble generation mechanism consists of the mixed vapor bubble dispersion from jet plume, the breakage of the bubbles and the bubble shrinkage by condensation of steam component. By the correlation of the experimental results, the empirical equation which can estimate the median mean diameter of microbubbles distributed in water within an accuracy of ±5% was proposed in this study.
Microbubbles were examined in vitro to find whether antibacterial activities would be exhibited against Saccharomyces cerevisiae, Escherichia coli, Salumonella typhimulium, Staphylococcus aureus, and Bacillus subtilis. Among them, Saccharomyces cerevisiae cells tended to grow well under both aerobic and anaerobic conditions by the action of microbubbles. To clarify the underlying mechanism(s), interaction of microbubbles with a model protein, bovine serum albumin (BSA), was studied using fluorescence spectroscopy. Collisional quenching of fluorescence was observed in the microbubble-BSA system. In addition, a taste sensor consisting of several kinds of lipid/polymer membranes for transforming the information on taste substances into electric signal was applied to Shochu made from microbubble water. The sensor output showed an electric pattern similar to an umami substance, monosodium glutamate.
Effects of electrolytes on bubble coalescence are investigated experimentally. Collisions of bubbles with free surface of three kinds of liquids, namely, super purified water, NaCl solution and KCl solution, are observed by using a high speed camera. It is found that the numbers of bounces on a free surface in electrolyte solution are larger than those in super purified water with the same equivalent bubble radius. This result qualitatively indicates that electrolyte works to prevent bubbles from coalescing. It is also revealed that a bubble can coalesce if the Weber number, which is based on radius of curvature of fore side of the bubble and approach velocity just before collision with free surface, is less than a critical value. The decrease of the critical Weber number, which corresponds to the inhibition of the coalescence, is observed with the increase in the concentration of the electrolyte solutions.
Characteristics of the surrounding liquid motion induced by a single bubble and a swarm of bubbles were experimentally investigated. Two types of bubbles with different diameters were precisely generated by using a bubble generator and hypodermic needles. PIV measurement was carried out to capture the liquid motion disturbed by the bubbles. The liquid motion was analyzed based on turbulence intensity defined by the standard deviation of liquid-phase velocity fluctuations and the bubble velocity. As a result, the bubble-shape oscillation significantly affects the area of disturbance region. Large shape oscillations on a bubble induce a wide range of fluid disturbances. In addition, it is also clarified that the turbulence intensity measured via 2-dimensional PIV is significantly affected by the bubble motion. Turbulence intensity decreases due to transition from zigzag motion to helical motion of bubbles.
Bubble rise characteristics, including the shape of, rise velocity of, and pressure around a single bubble, are examined using a two-dimensional column. Of particular interest are the effects of column thickness on these rise characteristics. The column thickness tested for simulating—based on the volume-of-fluid (VOF) method—the dynamic variations in the flow around/the boundary of deformable bubbles with area-equivalent bubble diameter of 8-10 mm covers a range from 12 to 3 to 1 mm. Besides reasonable agreements in the said characteristics with those obtained experimentally, it is found that the wake vortices, fate of which have a strong influence on the pressure field, remain almost intact over 2-3 cycles of vortex shedding in the 12-mm thick column, just 1 cycle for 3 mm, and in effect with no formation (as well as shedding) of organized vortical structure in the 1-mm thick column.
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