Our goal of this study is to develop a soft-landing device using autorotation for a drone. Recent studies are introduced in this paper. At first, we perform motion analysis of autorotating propeller and examine the effect of mass or solidity on terminal velocity. Next, terminal velocity of autorotating propeller is measured by hot-wire anemometer and evaluate the influence of the number of blades on the velocity. Finally, smoke-wire system with light source device using laser diode has been developed to visualize flowfield. To evaluate performance of the developed system, flow visualization around a reduced car model is compared to streamlines around that obtained in CFD.

In this study, we propose a new method to determine the charge distribution of PM2.5 using a Parallel-Plate Particle Separator (PPPS), and discuss the influence of various conditions for the method on its accuracy by using numerical simulation. The PPPS separates particles flowing from the central inlet into three exits based on their electrical mobility. The proposed method reconstructs the charge distribution from two parameters under various sets of various applied voltage: the numbers of particles flowing out to each exit of PPPS; the ratio of outflow particles to inlet particles for each charge number. The charge distribution determined by the proposed method under 2 digits, which are the significant figures in experiments, or more is found to excellently agree with input distribution. The parametric study shows that the accuracy of the determination improves when both the flow field and the set of applied voltages are optimized.

In this study, we analyzed the effect of perforated plates in rectangular tube with sudden enlargement as some local perforated plates in each region. The overall length of rectangular tube is 1.4m and the experimental data for validating numerical simulation is based on the previous work performed by the authors in this study. The flow regimes, 18,700, were selected, and area ratio between plate and holes in the perforated plate is 1:1 at all cases. The numerical analysis was carried out by using the Large-Eddy Simulation to study the instantaneous flow characteristics. The results obtained useful guidelines for securing uniformity and suppressing pressure drop when applying the perforated plate.

We perform direct numerical simulation of turbulent channel flow with traveling wave-like wall deformation at three different friction Reynolds numbers, Reτ = 90, 180, and 360. It is found that the set of control parameters in the maximum drag reduction case is same in wall units: there is little Reynolds number dependency in the mean velocity profiles under this set of control parameters. Based on this scaling, we propose a semi-empirical formula for the mean velocity profile to predict the drag reduction rate at higher Reynolds numbers. Using the proposed formula, it is revealed that about 40% drag reduction effect can be attained even at practically high Reynolds numbers, Re_τ ～ 10⁵ − 10⁶.

We investigated the influence of the wall surface properties on the drag reduction from the viewpoint of the flexibility of the fiber. The flow rate and pressure loss were measured for the water flows in a rectangular channel with an aspect ratio of ten having home-made fiber-flocking surfaces on the upper and lower walls. The test surfaces employed were smooth surface and two fiber-flocking surfaces with different groove heights. We additionally investigated the influence of the fiber height on the velocity profile by using LDV (Laser Doppler Velocimetry) measurement.

The prediction of viscoelastic-fluid turbulent flows within the RANS (Reynolds-Averaged Navier-Stokes) computational framework is required for engineering applications. For such flows the modelling of an additional non-linear term Λ_ij, correlated with the viscoelastic stress dissipation in Reynolds-averaged constitutive equation, is of decisive importance for the flow prediction accuracy. In this paper, a Reynolds-stress model (RSM) was adopted as a background turbulence model within the RANS framework for computing a drag-reducing turbulent flow over a backward-facing step (BFS). Two different model formulations for Λ_ij were investigated by computing three BFS configurations differing in terms of geometry expansion, i.e. expansion ratio (ER). The turbulent intensities were overestimated within the separated shear layer just behind the step edge for the case with ER = 1.25, while they were slightly underestimated for the case with ER = 1.75, irrespective of the Λ_ij modelling. As a result, the reattachment length predicted by RANS became rather short exhibiting an opposite dependency on the ER-parameter compared to the complementary DNS data. In summary, the overall models’ predictive performance in terms of presently investigated Λ_ij-term formulations differs only marginally within the separated shear layer.

Pressure drop and heat transfer characteristics of graphene oxide nanoparticle suspensions in circular pipe flow were investigated. The concentrations of test suspensions are 0.1wt%, 0.3wt% and 0.5wt%. Friction coefficient of test suspensions in turbulent flow was reduced as compared with water. The drag reduction effect increased with increasing the concentrations of test suspensions. The maximum drag reduction was 31% at 0.5wt%. In addition, heat transfer coefficient of graphene oxide nanoparticle suspension was increased compared to water. The effect of heat-transfer enhancement increased with increasing the concentrations of test suspensions. It was found that the graphene oxide nanoparticle suspensions not only reduce the pressure drop but also increase the heat transfer in a circular pipe flow.

We performed direct numerical simulation of pulsating controlled flow for drag reduction and predicted time evolution of vortical structure distribution using deep learning. The datasets for the deep learning is the distribution of vortical structure defined by second invariant of velocity gradient tensor in circumferential cross sections. There are the correlation between the time variation of the friction drag and the ratio of the vortical structures occupying in the sections. We predicted the time variation of the ratio by predicting the time variation of the vortical structures using convolutional neural networks with convolutional long short-term memory (ConvLSTM). It was revealed that the model can predict the periodic variation of the ratio.

An experimental study was performed to investigate the relationship between the extensional rheological properties of polymer solutions and vortex deformation in turbulent flow. In this study, we used 2D flow that is advantageous as it examines the effect of the extensional rheological properties of flows. Polyethyleneoxide was added to two-dimensional (2D) turbulent flow: Vortex shedding and vortex deformation in the 2D flow were visualized and analysed. The vortex shedding and deformation were categorized in three types, which was affected by relaxation time of each solution and neighbour vortices. Particle Image Velocimetry was also used to analyse variation of extensional rate in the velocity fields.

Localized and sustained heavy rainfall caused by the training over Hiroshima, Japan in August 2014 is simulated by the Multi-Scale Simulator for the Geoenvironment (MSSG) model. Subsequently, we seek for a control method toward the weakening of the heavy rainfall. In the high-resolution simulation case, the rainy area formed a line around Hiroshima, and the maximum precipitation per 1 hour reached 99 mm. The vertical cross-section of the hydrometeor has confirmed that the line-shaped surface precipitations are caused by several training cumulonimbus. These results agree with the data obtained with Japan Meteorological Agency radar. We then conduct sensitivity analysis of the rainy area and the precipitation intensity with differently-controled initial conditions.

In order to suppess the vortex shedding from a pantograph head of bullet trains, which is known as one of the main sources of aerodynamic noise, we extend the suboptimal control technique so that it can deal with a body of arbitrary shape by numerically computing the impulse response of the adjoint equation. The methodology is first assessed by considering a flow around circular cylinder, for which the analytical solution of the impulse response can be obtained. The resultant blowing/suction profile based on the numerically computed impulse response is found to reasonably agree with the analytical solution. Subsequently, we apply this technique to a flow around a pantograph head to obtain the suboptimal blowing/suction profile to reduce the pressure drag. It is observed that the vortex shedding disappears and the fluctuation of lift coefficient nearly vanishs when the maximum blowing velocity is set at 20% of the free-stream velocity.

In recent years, a dielectric barrier discharge plasma actuator (DBD-PA) which is a fluid control device, has been investigated for achieving both aerodynamic functionality and design shape in transportation equipment. In this study, we installed a DBD-PA to a bluff body that reproduced the characteristic flow structures of a sedan-type automobile in order to reduce the aerodynamic drag. In particular, we controlled the flow generated from the curved shape in the rear end of the body side, which has high sensitivity of design and aerodynamic forces. As a result of the wind tunnel test, we showed that the aerodynamic drag can be reduced by 3% with controlling the vortex that causes increment of aerodynamic drag while maintaining the curved shape.

In this study, we focused on the wing with sinusoidal leading edge (SLE) to control flow separation at low Reynolds Number. In previous study, it was found that the effectiveness of SLE on wing performance changes depending on the wing cross section and angle of attack. So, in this study, we predicted the effective region of SLE in advance. From the results of wind tunnel experiments and numerical simulations, we found that the boundary profile between separation and unseparation region. By using the boundary profile, we could predict the effective region of SLE in advance even if the maximum wing camber changes.

Friction drag reduction effect of a passive blowing on an airfoil is investigated experimentally. The passive blowing is driven by the pressure difference on the surface between the leading edge and the rear part of the upper surface. The Reynolds number based on the chord length is 6.45 × 10⁵ and 1.55 × 10⁶. The angle of attack is set to 0° and 6°. The streamwise mean velocity profile in the blowing region is shifted away from the wing surface. This behavior qualitatively suggests reduction of friction drag by the passive blowing. As a result of a quantitative assessment, which evaluates the effects of pressure gradient and the roughness of the wall caused by the perforated metal plate used for blowing, it is confirmed that the passive blowing reduces the local friction drag by 4% - 23%.

The suppression of wing-tip vortex is desirable to reduce the induced drag of airplane. In the present study, the decay of the wing-tip vortex is enhanced by introducing wing-tip vibration. A vibrating wing-tip was attached to a NACA0012 half wing model. The tangential velocity of the vortex was measured at x/c = 4 by 2D2C PIV. The vibration frequency and the angle of attack of the wing are selected as parameters. The vortex strength is evaluated by the circulation calculated by line-integral of the tangential velocity of the vortex. When the tip vibration is introduced, the circulation significantly drops at a certain frequency. This critical frequency increases with the angle of attack. It is found that the maximum tip velocity at the critical frequency corresponds to approximately 30 % of the maximum tangential velocity of the vortex for all the angle of attacks tested. The vortex completely decays when the ratio exceeds around 65 %.

Effects of uniform blowing on an airfoil are investigated experimentally aiming at turbulent friction drag reduction. In order to examine the control effects, velocity measurements on the airfoil by a hot-wire anemometry are conducted. The velocity gradients on the airfoil surface are found to be reduced by uniform blowing, which suggests that local skin friction are suppressed qualitatively. We have also attempted a quantitative assessment of the control effect using the mean velocity profiles in the boundary layer; the modified Clauser-chart method and the method using the law of the wall with pressure gradients. Though these assessments, it is found that these methods are consistent with each other and 21%−66% of the local friction drag are reduced by this control. From these investigations, it is also confirmed that the uniform blowing is effective for friction drag reduction on the airfoil quantitatively.

Suboptimal control proposed by Lee et al. (J. Fluid Mech., vol. 358, pp. 245–258, 1998) is one of the effective feedback control schemes for turbulent skin friction reduction. In 2017, Nakashima et al. (J. Fluid Mech., vol. 828, pp. 496–526, 2017) elucidated the suboptimal control effect by using resolvent analysis. Building on the implication obtained from their result, we try to improve the performance of the control law developed by Lee et al.. However, this attempt does not result in drag reduction. Examination for spectral budget equation attributes this lack of efficacy to nonlinear energy transport between different scales or wall-normal locations. Then, we propose a new feedback control law via resolvent analysis and assess its effect by direct numerical simulation at the friction Reynolds number of Re_τ = 180. As a result, about 10% drag reduction is obtained by the new law. Conditional averaging method reveals the control successfully suppresses near-wall coherent structures. Physical meaning of the control law is finally derived from the theoretical viewpoint.

The research on the effect of upstream tetrahedron on horseshoe vortex around circular cylinder was conducted. The difference between no tetrahedron and tetrahedron introduced case was investigated by PIV measurements. Velocity measurements conducted at spanwise center plane and spanwise shifted plane. It was confirmed that upstream tetrahedron reduce horseshoe vortex in both plane and lower height tetrahedron works better to reduce vortex.

When droplets impact on a heated wall at sufficient high temperature, droplets can levitate owing to vapor stream generated by droplet evaporation in the vicinity of the heated wall. This levitating phenomenon of impacting droplet is known as the Leidenfrost effect, and it is reported that the occurrence of the Leidenfrost effect is affected by wettability of the solid surface. We investigated the levitating mechanism of the impacting droplet on heated wall by using molecular dynamics (MD) simulation. In this MD study, we changed wettability of the solid wall by varying the liquid-solid intermolecular force to investigate influence of wettability on the occurrence of the Leidenfrost effect. We found that the strong intermolecular force led large amount of evaporation of the impacting droplet. As the intermolecular force became stronger, the droplet could levitate easier because of the intensive evaporation in the vicinity of the three-phase contact line on the heated wall.

This paper theoretically examines an weakly nonlinear propagation of plane progressive waves in an initially quiescent compressible liquid uniformly containing many spherical microbubbles. We focus on a wave propagating with a large phase velocity exceeding the speed of sound in a pure water (i.e., 1,500 m/s), which is accomplished by the consideration of compressibility of the liquid phase. For simplicity, the attenuation of wave owing to the viscosity in the gas phase and heat conduction in the gas and liquid phases are ignored, and wave dissipation is thereby owing to the liquid viscosity and compressibility. The set of governing equations for bubbly flows is composed of the conservation laws of mass and momentum for gas and liquid phases, equation of motion for radial oscillations of representative bubble, and the equation of state for both phases. By using the method of multiple scales and an appropriate determination of size of three nondimensional parameters, i.e., the bubble radius versus wavelength, wave frequency versus eigenfrequency of single bubble oscillations, and wave propagation speed versus speed of sound in a pure liquid in terms of small but finite wave amplitude (i.e., perturbation), lead to derive a nonlinear wave equation describing the wave behavior at a far field.

Boiling in quenching process is an important and significant phenomenon, due to the variations of cooling rate and finally the deformation of the product, which has been paid much attention in the past few decades. However, it is still difficult to conduct accurate measurements, because of the small spatial scales and rapid time constants. Recently, a new solver “interThermalPhaseChangeFoam” in OpenFOAM has been proposed for liquid-vapor thermal phase change flows by using improved VOF method. The computational domain is a cube with the heating wall at the bottom for both the changed and fixed wall temperature cases. The three stages that is the vapor film stage, boiling stage and convection stage have been reproduced from the results of time evolution of heat flux.

The viscoelasticity of soft tissue surrounding bubbles play an important role in bubble dynamics in the context of ultrasound applications in medicine. Resonant bubbles oscillating with large amplitude often lose their sphericity in shape; the Rayleigh–Plesset formulation designed for analysis of spherical bubble dynamics will fail in this case. In this study, we experimentally observe the non-spherical oscillation of a gas bubble in gelatin gels, as a tissue phantom, under 28-kHz ultrasound irradiation; we treat the gelatin concentration and the equilibrium bubble radius, and the pressure amplitude of the ultrasound as experimental parameters. A spherical bubble is generated by focusing a laser pulse into a gel that is supersaturated with the dissolved air; the nucleated bubble shows gradual growth due to influx of the dissolved gas, thereby varying its equilibrium radius (from 50 μm to 200 μm). The ultrasound with varying its pressure amplitude is irradiated toward the bubble and the bubble oscillation is recorded with a high-speed camera. The experiments show that an increase of the gelatin gel concentration leads to an increase of the natural frequencies of non-spherical mode oscillations and suppression of their oscillation amplitude.

Droplet impact against a solid surface is a very important phenomenon in the fields of semiconductor cleaning and spray cooling. However, the detailed mechanism has not yet been clarified; hence, clarification and control of the dynamics of droplet impact are required. In this study, we consider the influence of the air film formed between the solid surface and the droplet on the droplet collapse process at droplet impact. We observed the air film at droplet impact by using color interferometry. We measured the duration when the air film existed and the diameter of dimple formed under the droplet.

A thermo-capillary flow in a rectangular liquid film which has two free surfaces was studied by means of a numerical simulation. In order to consider the surface deformation, we simulated the gas-liquid two-phase flow, allowing to determine the change of flow pattern with a varying volume ratio. Through the present two-phase flow simulations, we confirmed two patterns of the basic flow in the thin liquid film: one was double-layered, and the other was single-layered flow. The selection of the basic flow depends on the initial thickness of the liquid film. In the case of a thin liquid film accompanied by a single-layered flow, when the volume ratio of the liquid film was less than unity, a similar flow occurred in the liquid film. However, when the volume ratio was more than unity, the flow direction of the single-layered flow reversed.

The use of bubbles as a way of reducing skin frictional drag has long been a focus for engineers. The existence of gas bubbles into a boundary layer of a liquid phase has multiple different impacts on the original flow structure. In the present study, drag force was measured acting on the elliptic cylinder when it was accelerated under three conditions. The cylinder was immersed in tap water and in tap water with microbubbles, and two types of microbubble generator (slit type and pressurized dissolution type) were used in this experiment. The cylinder was accelerated from 0 to 4.2 or 6.3 cm/s. It was shown that the presence of microbubbles reduced drag on the cylinder in tap water. Microbubbles generated by the slit type that had stronger zeta potential was more effective than that by the pressurized type.

Low-frequency underwater ultrasound is conventionally used to degrease metallic parts. It is important to reveal its cleaning mechanism in order to achieve more efficient cleaning for practical purposes. In this study, we set up a simple ultrasonic cleaning test and optically visualize acoustic cavitation bubbles. Glass plates on which an oil drop is deposited are used as a cleaning sample. The sample is soaked in a water container and subjected to 28-kHz underwater ultrasound. The ultrasound-induced cavitation bubbles near the glass surface are recorded by a high-speed camera. It is observed that cavitation bubbles nucleate preferentially in the water (not in the more viscous oil) and subsequently penetrate into the oil, perhaps, due to Bjerknes force. The visualization suggests that the dynamics of bubble clouds in the oil play an important role in degreasing.

When a droplet impacts on a solid surface, splash can be often observed. Wide varieties of physical parameters must be considered for the splash generation. In this study, we investigate the effects of the impact velocity and surrounding gas pressure on the generation of splash. We experimentally observed high-speed droplet impact on a cover glass in a reduced-pressure chamber using the high-speed video camera with the frame rate up to one million f.p.s. We discovered a different type of splash from those previously reported in the high-speed droplet impact under a low-pressure. The ejection of this splash can be found when the impact velocity is greater than a threshold velocity regardless of the surrounding gas pressure and surrounding gas species.

Adsorption of surfactants at an interface of a single drop affects the boundary condition and changes the drag force acting on the drop. However, our knowledge on the relationship between the drag force and the local interfacial forces in a contaminated system is still unsatisfactory. In this study, we measured velocity distributions in the vicinity of a single spherical drop in containing surfactants using spatiotemporal filter velocimetry (SFV) with boundary-fitted measurement region. Interfacial viscous stress, Marangoni stress and pressure distributions on an interface were evaluated from the measured velocity distributions. The drag force acting on the drop was also evaluated by integrating the stress and pressure over the interface. The results indicated that he present method is useful for understanding the relationship between the drag force and the local interfacial forces acting on a drop.

Technology using micro bubbles and technology using ozone that are environmentally friendly are attracting attention.We suggest water treatment system, decolorization system of wastewater and oil removal system that use microbubble ozone.

In water treatment, the BOD increased more than the conventional system. Thus, microbubble ozone is effective as water treatment. In decolorizaiton, the chromaticity of waste water showed a lower value than the conventional system when using microbubble ozone. In removal, residual rate has increased when using microbubble ozone. It suggests that microbubble ozone is not effective to removal. It is thought that the chemical property changed due to the oxidation by ozone. As a result, bubbles did not remove oil.

In separation technology, liquid film flow is used in applications such as absorption tower, for example. In general, packings are used to increase the wetted surface area because increase of wetted surface area is one requirement to improve the separation performance. Therefore, It is important to estimate critical Weber number at which the triple-phase contact line starts to advance and wet the wall. A simple theoretical model which estimates critical Weber number was proposed and the calculated results approximate well the experimental results measured in this study.

Spherical bubbles rising along an inclined flat wall has been investigated experimentally to analyze the bubble-bubble interaction and the clustering behavior. In this study, free-slip and no-slip conditions on the bubble surface are controlled using MgSO₄ solution and TritonX-100 solution, respectively. The bubble-bubble interaction at a bubble Reynolds number of 100 and the effect of boundary condition are discussed through the quantitative evaluation of the bubble velocities and the distance between two bubbles. It is observed that the bubbles tend to be arranged in horizontal line as they rise. First, the trailing bubble is accelerated under the influence of the wake of the leading bubble. Then, the trailing bubble moves around the leading bubble, and two bubbles take side-by-side arrangement. This interaction does not depend on the boundary condition on the bubble surface qualitatively. The quantitative differences due to the boundary condition especially appear in the faster rising velocity of the trailing bubble and longer distance between side-by-side arranged bubbles in case of no-slip bubbles. The velocity of the bubble cluster is also analyzed.

We washed a solid surface (SUS304) attatched with starch paste as the soiled model because of investigating washing effect of fine bubble mixtures via high swirling method and pressurized dissolution method, respectively. As compared with water one, relative high washing effect of microbubble mixtures was observed. Furthermore, good agreement between the resultant wasing rates of high swirling method and those of pressurized dissolution method was obtaind. For understanding the experimental results, particle diameter change and static contact angle of interface between test fluids and solid surfaces was measured and discussed.

The demand of LNG is extending in recent years and requirement to store LNG with different density in the same tank is increasing. In that situation double-diffusive convection and phenomenon called as “Rollover” will occur in LNG tank by the heat input and LNG tank will be damaged by large amount of BOG (Boil off Gas). We must consider the rollover phenomenon when we design LNG tank and plant system. The purpose of this study is to establish analytical technology which can predict rollover. We calculated double-diffusive convection in the two and three dimensional numerical analysis and evaluated the numerical analytical results concerning the density and the temperature field by comparison to subscale tank experiment. We achieved that numerical analysis qualitatively realized experiment of double-diffusive convection.

This study performs the derivation of a nonlinear wave equation for plane progressive quasi-monochromatic waves in a compressible liquid containing many spherical microbubbles. Main assumptions are as follows: (i) the wave frequency is larger than an eigenfrequency of single bubble oscillations. (ii) The compressibility of the liquid phase is incorporated. (iii) The effect of viscosity in the gas phase, heat conduction in the gas and liquid phases, phase change across the bubble wall, and thermal conductivities of the gas and liquid, are neglected. The basic equations for bubbly flows are composed of a set of conservation equations of mass and momentum in a two-fluid model, the equation of bubble dynamics, and so on. From the method of multiple scales with an appropriate choice of scaling relations of some physical parameters, i.e., wavelength, wave frequency, propagation speed, and amplitude of waves concerned, we can derive the nonlinear Schrödinger (NLS) equation with an attenuation term and some correction terms, which describes the long-range wave propagation, where the phase velocity is larger than the speed of sound in a pure liquid. We clarified a decrease of the group velocity in a far field.

The present study aims to experimentally and numerically investigate the effect of vapor condensation on the splash generation mechanism after droplet impact. First, we experimentally observed ethanol droplet impact in a vacuum chamber in air or ethanol vapor. At the same ambient pressure, the splash was suppressed in ethanol vapor. We considered the cause of the difference in the two results to be vapor condensation. Next, we numerically simulated the droplet impact with or without vapor condensation. The results show that when the droplet impacts, the ambient pressure with condensation near the impact point becomes lower than that without condensation. Finally, we investigated the influence of condensation coefficient on the ambient pressure of vapor near the impact point. From the results, as the value of condensation coefficient increases, the ambient gas pressure after the impact decreases. We concluded that the vapor condensation suppresses the generation of splash after droplet impact.

Many studies on the dynamic behaviors of bubble in the jet in cross flow have been already reported in both experimental and numerical approaches. However, the break-up mechanism has not been understood. The authors are trying to understand the break-up mechanism from the bubble cloud to small bubbles. In this study, the authors tried to clarify the break-up process from the bubble cloud in the jet in cross flow to small bubbles using by VOF method using ANSYS CFX 17.2. The waves on the surface of the bubble cloud synchronized with the karman vortex street. Moreover, the break-up of the bubble from the bubble cloud also synchronized with the dynamic behavior of the karman vortex. Based on these results, it is predicted that the dynamic behavior of the karman vortex rolled up from the cylinder contributes to the break-up of the bubble from the bubble cloud.

Deformation and sound emission of bubble in water are simple phenomena, but contain important physics. In this study, at first, deformation behavior and emitted sound from a bubble were measured at detaching from a nozzle installed in water with varying the diameter of nozzles. After that, CFD simulation using VOF method was used to reproduce these data. As a result, by setting the simulation conditions appropriately, not only deformation behavior but also sound pressure level were agreed well with experimental data and knowledge on emitted sound and flow characteristics around bubble was obtained. Especially, simulation models and conditions were discussed in detail.

It is sometimes required to control bubble generation in still liquid in order to investigate the flow contains bubbles. One of the methods is to use acoustic pressure wave and a slit elastic tube. In this study, we investigated the effect of aqueous surfactant for this bubble generation method. Especially we focused on the effects of the adsorption of surfactant to gas-liquid interface, i.e. dynamic surface tension. As a result, there were differences of generated bubble size by changing the kind and concentration of surfactants. This indicate that the surface tension change in milliseconds time scale is affected to the bubble generation. In addition, the shape oscillation of generated bubble was suppressed, and the bubble was adhered to the elastic tube. This result supports a hypothesis of our proposed bubble generation mechanism.

Skimmed milk powder and water are mixed to make viscoelastic droplets, and a solid surface impact experiment is performed. Observe the droplet impact behavior using a high-speed camera and investigate the influence of the difference of liquid moisture content and the difference of droplet temperature. As a result of the experiment, it became clear that the difference of the moisture content of the droplets influences the presence or absence of attachment. It was confirmed that the effect of droplet moisture content on the contact time was small, but it became clear that increasing the droplet temperature affected the contact time.

Liquid manipulation by photoisomerization attracts much attention as a new active droplet control method for microchemical analysis. Such a manipulation can be realized by a use of solution liquid of surfactant that exhibits the cis-trans isomerization due to light irradiation with a specific wavelength such as ultraviolet light. This isomerization provides also changes in the contact angle and surface tension of the liquid. The manipulation of liquid can be performed by utilizing this property: the specific-light irradiation on a one-side surface of the liquid column in a tube would generate differences in the contact angle and surface tension between the both sides of the liquid column. Although the control have been demonstrated experimentally, its flow dynamics and developments of each isomer distribution are not fully understood. In order to reveal the phenomenon, we performed direct numerical simulation of air-liquid flows and its scalar transport of the isomer, using a software of OpenFOAM.

The purpose of this research is to control the behavior of microbubble by using electric or magnetic field. The electric charge is given to the microbubble by the surfactant. The magnetic charge is given to the microbubble by the magnetic particle. The water tank and the fluid passage are used in these experiments. The water which contains the microbubble is in the tank or flows in the passage. The sides of tank and the walls of the passage are made of the iron plates. The iron plates are used as the electrode or the magnetic pole. The results show that four dates. In the tank, the microbubble moves to the electrode. In the passage, the microbubble follows the flow and moves to the electrode. In the tank, the microbubble moves to the magnetic pole. In the passage, the microbubble follows the flow but does not move to the magnetic pole.

The present study deals with the dynamics of a laser-induced cavitation bubble generated in the middle of two parallel horizontal rigid walls which are closed at one end with a vertical rigid wall by using a boundary element method with an incompressible formulation. A complex jetting behavior depends on the relative size of the bubble with respect to the distance between the parallel walls and the initial location of the bubble from the vertical rigid wall. A bubble generated far from the vertical rigid wall develops a radial jet during its collapse phase, which leading to bubble splitting. When the bubble is generated close to the vertical rigid wall, a planar jet is formed during bubble collapse which is towards the vertical rigid wall. A larger size of the parallel walls results in an increase of the bubble oscillation period. Nevertheless, the bubble shape and jet formation are not affected by the size of the parallel walls.

Ultrasonic cleaning plays a key role in manufacturing process. Recent studies suggest that its cleaning mechanism is supported dominantly by the dynamics of acoustic cavitation bubbles. In this context, we favor to promote surface cavitation by having more gas bubble nuclei at cleaning surfaces. In this study, we experimentally study the effect of gases dissolved in solid substrates on the stability of gas bubble nuclei. In our experiments, polypropylene plates are used to create surface bubbles. Since their surface is hydrophobic, the surface bubbles are hemi-spherical in shape. Here, we consider the cases with the plates saturated or subsaturated with dissolved gases (air). It is confirmed from the experiments that the dissolution rate of the surface bubble is accelerated by having the gas-subsaturated plates. This is because of the gas transport from the bubble into the (subsaturated) solid phase. In the context of ultrasonic cavitation cleaning, it may be favorable to put cleaning samples under pressurazation before cleaning, which allows for having a larger number of stabilized gas bubble nuclei at their surfaces.

Gas-liquid two-phase flow are frequently encountered phenomena in various engineering field, and monitoring of flow condition in pipeline is required for accident prevention and flow control in the application field. The objectives of this research are the estimation of void-fraction and comparison with the performance by the two methods. One is estimation by image reconstruction using the electrical impedance tomography method. The other is estimation of void-fraction by comparing pre-training data using a neural network. Training and image reconstruction of the artificial neural network were performed based on current data obtained through multiple electrodes phantom for void-fraction and the results were compared.

Injection of bubbles into Taylor–Couette flow creates new flow modes that do not appear in single-phase flow conditions. A spiral vortical array is one such mode departing from the original toroidal Taylor vortices. The aim of the present study was to explore the mode transition of the two-phase flow using particle tracking velocimetry. Vertical-axis concentric cylinders with a rotating inner cylinder were used to measure the motion of both phases in the range 300 < Re < 5000. From time–space mapping of the experimentally measured stream functions, we found that bubble injection amplified the axial displacement of the wavy Taylor vortices in the toroidal array, and there was a smooth switch to a spiral array when the azimuthal travelling velocities coincided. During this vortical reconnection, we found that the two counter-rotating vortices were attenuated asymmetrically.

In order to investigate interaction between microbubbles and coherent structure in a turbulent boundary layer, we visualized microbubbles injected into the boundary layer of a horizontal channel flow. A laser sheet illuminated a cross section slanted to the streamwise direction (x). This illumination gives us space–time 3D velocity distribution (t, y, z, u ,w) of microbubbles by PTV method. We introduced Taylor’s frozen flow hypothesis to correct the time t’. Statistics from the velocity distribution suggest that impact by turbulent diffusion could not be ignored about microbubble distribution and that microbubbles existed in low-speed and high-speed streaks rather than vortical structures.

Scrub cleaning with PVA brushes is widely used in the post CMP cleaning process for semiconductor device manufacturing. PVA brushes are hydrophilic porous sponges, and the PVA brush scrubbing is a complex system in which a gas-liquid-solid interface exists. To elucidate the lubrication condition between a PVA brush and contacting surface, we observed the real contact area of the brush using a total reflection optical device. We investigated the real contact area of the brush by applying a normal load and sliding with various speeds. As a result, the real contact area increased with the increase of brush compression, however the increase was non-linear. It was also confirmed that contact points concentrated in the front part for moving direction and detached in the rear part on surfaces. Additionally, stick-slip motion was observed under low rotating speed by observing the contact surface of PVA roller brushes.

We have succeeded in grasping changes in shear stress by using a Laser Doppler Shear Stress Sensor to directly measure the time fluctuation of shear stress in turbulent flow. A decrease in stress is confirmed in the two-phase flow. However, it has not been investigated what kind of bubbles will affect the stress. In this research, we investigated the influence of bubbles on wall shear stress by measuring shear stress using direct measurement method of bubble flow and visualizing bubbles covering sensor surface. At the void fraction α=1.0, it was found that the shear stress did not decrease but rather increased at 5 m/sec, 7 m/sec. It was found that to reduce the shear stress, it is necessary to cover the area of more than 5% with bubbles in the local area.

In order to measure void distribution in opaque two-phase flows such as liquid-metal, ultrasonic tomography was developed. In the tomography, the measurement accuracy depends on the number of sensors. However, with increases the number of sensors, the temporal resolution decreases for obtaining a cross-sectional information. Therefore, lower number of sensors is suitable for two-phase flow measurement. In order to reduce the number of sensors, a reflection method was employed for the tomography. An ultrasonic sensors which can emit ultrasonic wave at wide-angle was developed. It was shown that the sensor could emit ultrasonic wave in the angle ranging of ±55°. Furthermore, the sound pressure thickness was almost constant at 4~6 mm. As a result, it was confirmed that echo method could be applied for the tomography algorithm, and it enabled to reduce the number of ultrasonic sensors for the tomography.

Amount of drag reduction by bubbles depends on the void fraction provided in boundary layers. It means that certain power must be consumed to generate bubbles in water and it worsens the total power-saving performance. To obtain sufficient drag reduction with lower amount of the bubbles, RBI (Repetitive Bubble Injection) is suggested. In order to estimate effect of RBI, the wall shear stress and velocity vector fields around bubble cluster generated by RBI in a horizontal channel flow are measured. This multiple measurement leads to deeper understanding of bubble-originated drag reduction phenomena, in particular for unsteady process of boundary layer alternation. In the present experiments, the results have demonstrated that the bubble cluster promotes the drag reduction more than conventional type injection, named CBI (Continuous Bubble Injection), for the same void fraction supplied.