The final goal of this research is to establish the method for mass and energy transport with inhibiting its diffusion using a coaxial jet. The authors confirmed that the diffusion inhibition effect of the central round jet can be obtained when the velocity ratio of the round jet to the annular jet is set to less than 1. Furthermore, it was clarified that the inhibition effect is related to the change in the vortex structure formation process due to the change in velocity ratio and the advection effect due to the annular jet. This indicates that the vortex structure formed in the jet flow field is deeply related to the diffusion inhibition effect of the coaxial jet. The purpose of this study is to confirm the three-dimensional vortex structure of a coaxial jet at the low velocity ratio experimentally.

The purpose of this study is to control the jet diffusion by using the deforming nozzle. The shape of polypropylene nozzle can change from square shape into cruciform shape smoothly. The velocity measurement of the jet flow was conducted using a PIV method, by changing the nozzle shape variously. The basic characteristics of the jet flow, such as the velocity distribution and the half value width have been obtained. As a result, it has been found that the diffusion of the jet issuing from the deforming nozzle whose exit shape changes to cruciform from square and furthermore returns to square periodically and continuously is promoted in the downstream region, comparing with the case of the square nozzle jet.

Supersonic under-expanded free jet has different flow characteristics than incompressible jet, and there are many studies on practical applications such as jet and rocket engines. However, supersonic under-expanded reattached jet has not been studied enough. In this study, the flow characteristics of a supersonic under-expanded jet from a two-dimensional nozzle that reattaches to the offset side wall are investigated by experimental and numerical analyses. In particular, the effect of offset distance on the flow characteristics of supersonic under-expanded reattached jets are examined. The Schlieren method, Pitot tube, and CFD program were used for flow visualization, measurement of jet velocity distribution, and comparison with experimental results, respectively.

Direct numerical simulations are carried out to investigate the effect of shear layer thickness on jet diffusion and the pressure fluctuations emitted during diffusion by amplifying of helical modes pair in a M=2.0 supersonic jet. In the DNS, a jet is forced by a pair of unstable third helical modes at t=0. The DNS results show that when the start time of jet diffusion is the same, the pressure fluctuation level emitted during jet diffusion is almost the same regardless of the shear layer thickness.

The rainbow schlieren deflectometry is applied for an underexpanded microjet issued from an axisymmetric convergent nozzle with a diameter of 1 mm at the nozzle exit. The nozzle pressure ratio is held constant at 4.0 to produce weak shocks in the jet plume and the Reynolds number based upon the diameter as well as flow properties at the nozzle exit is 5.91×104. The density field of the microjet is reconstructed using the Abel inversion method with the assumption of axisymmetric free jets. The flow feature on cross-section including the central axis of the microjet is demonstrated using various visual representations such as the vertical knife-edge schlieren, the horizontal knife-edge schlieren, the bright-field schlieren and shadowgraphy.

Numerical simulations using the RANS with the SSTk-ω turbulence model are performed to understand the flow feature of an underexpanded sonic jet from a convergent nozzle with a square cross-section of 1 mm × 1 mm at the nozzle exit. The nozzle pressure ratio, which is the ratio of the plenum pressure upstream of the nozzle to back pressure, is held constant at 4.0 and the Reynolds number based upon the flow properties and the height at the nozzle exit is 5.94 × 10⁴. The near-field shock structure in shock-containing square jets is demonstrated with the three-dimensional representations of the magnitudes of the density and pressure gradient vectors.

Jet with Re < 1 is observed downstream of valves in lymphatic vessels of mice and it is an impending research target in developing treatment of noninvasive cancer, especially breast cancer. Traits of the jet are that the pressure and velocity greatly vary at the jet exit because of flows with angle change of ninety degree at the exit periphery. A new theory of Stokesian free jet is presented as a model of the jet from a lymphatic valve to predict the pressure in the jet region. Navier-Stokes equations and continuity equation, expressed by the spherical coordinates, are solved under the Stokes approximation by taking radial pressure and velocity variations at the exit into account. Flow field is divided into two regions by the hemisphere with its origin at the exit center and the exit radius, inner region and outer one. The former is inside the hemisphere and the latter is outside it. The pressures in both regions are given by Legendre polynomials up to the eleventh degree, multiplied by powers of the radial distance, and the velocity components are expanded by two series of unknown functions of the angle, multiplied by the powers of the distance. The outer-region solutions are patched with those in the inner-region ones over the hemisphere, resulting in reasonable solutions for the free jet.

To improve the heat transfer performance of the heat exchanger, it is effective to install a vortex generator on the heat transfer surface and break the temperature boundary layer by fluid mixing. The purpose of this research is to investigate the behavioral changes of the flow inside the offset channel with the vortex generator and its effect on the heat transport property. In this report, we show the numerical calculation result of the case where the installation position is offset, considering the interference between the vortex generator fin attached in the channel and the wall boundary layer. The results were compared using the Fanning's friction coefficient and Colburn's j-factor. The reason for the model showing the best heat transfer performance from the distribution of vortex structure and local Nusselt number was investigated. It has been clarified that it is advantageous to improve the heat transfer performance by installing the upstream fin close to the wall surface in the inclined direction. This is because this condition is effective in maintaining the amount of rotation of the vertical vortex extending between the fins and at the same time promoting the development of the horseshoe vortex.

The purpose of the present study is to clarify the torsion effect on mixing in helical pipes. The torsion observed in the flow in helical pipes with circular cross section has been numerically investigated the comparison with the experimental results obtained by our team. This effect causes a complex disturbance in the flow field by the secondary flow generated in the flow path. In this study, we used this secondary flow to achieve a liquid-liquid two-phase flow in the helical pipe with a T-junction using three-dimensional numerical simulation, aiming for mixing in the low Reynolds number region (100 < Re < 1000). The non-dimensional curvature is δ = 0.1, and the non-dimensional torsion effect parameter β₀ (= non-dimensional torsion/(2δ)^1/2) in the range of 1.0-2.0 is applied in several cases. It is found that the fast mixing is obtained in the case of β₀ = 1.6.

Infertility is often cited as one of the causes of a declining birthrate, which has become a serious social problem in recent years. Processes by which motile sperm can be safely and easily sorted are therefore important for infertility treatment. To address this issue, a new sorting method using a microfluidic sperm sorting system has been developed. To improve the separation efficiency of this device, it is necessary to know the behaviors of motile sperm in the microfluidic channels. This study experimentally investigates the behavior of the motile sperm in the straight channel (Poiseuille flow) and the curved channel (Dean flow) using the particle tracking velocimetry method. From the experimental results, if the flow is fast, sperms tend to flow in the same direction as the flow in both the Poiseuille flow and the Dean flow. If the flow is slow, sperms attempt to move in the opposite direction of the flow.

The issues of the interaction between a normal shock wave and a boundary layer are very important in a variety of high-speed aerodynamic applications. Under certain operating conditions the unsteady flows with shock wave oscillations are widely observed in various fluid machineries including supersonic intakes, supersonic diffusers or nozzles, and supersonic ejectors, strongly affecting local heat transfer rates as well as aerodynamics loading. In addition, extreme pressure fluctuation, which is caused by the shock oscillation, induces vibration of the machineries. Understanding and predicting shock oscillation phenomena have stimulated previous many experimental and numerical studies. However, the mechanism of shock oscillations is still elusive. In this study, two-dimensional unsteady behavior produced by the interaction of a shock wave with a turbulent boundary layer in a transonic diffuser is investigated by the detached eddy simulation (DES). The diffuser model used in the present study has heights of 18 mm at the inlet, 6 mm at the throat, and 12 mm the exit. The diffuser flow is demonstrated using time-dependent density contour maps with typical streamlines to investigate the flow features of unsteady shock behavior.

When a compressed air is supplied tangentially to a vortex tube, the total temperature separation is generated between the cold and hot air outlets by the swirling flow field formed inside the cylinder. By using the vortex tube, the industrial local coolers have been applied with the cold air of this device, such as called Ranque-Hilsch tube. Although this system has various advantages in terms of ease of maintenance and environmental protection, its practical application is limited due to the difficulty of temperature control. In this study, for the sake of the improvement of this device, we mainly focus on whistling and intermolecular heat transfer, so we engaged a CFD to investigate this total temperature separation mechanism. The characteristics of the temperature distribution and its change inside the device have been discussed.

This study focuses on a power generator that converts pressure pulsations generated in the hydraulic circuit into mechanical vibrations and finally converts them into electric power to absorb them. The authors selected a water hydraulic cylinder because that can easily transmit pulsating force to the outside of the circuit. The pressure pulsation from a swash-plate type piston pump propagates in the water hydraulic circuit. The cylinder locates between the pump and a valve placed just upstream the reservoir. In the experiment, the authors measured the pressure, the flow rate through the pipe and the thrust force on the cylinder rod, and have not done the electric power generation yet. The frequency of the pressure fluctuation and the initial volume of the cylinder is the significant parameter. As a result, the pressure absorption characteristics strongly depend on the frequency and the initial volume.

One of the methods of spraying liquid is the ultrasonic mesh method, in which the liquid is pushed out of a thin metal plate with hundreds to thousands of micropores (also called mesh) by ultrasonic vibration. The frequency of the ultrasonic vibration applied is about 100 to 300 kHz. The distance between the vibrating surface and the mesh is very small compared to the radius of the mesh. Due to the pressure distribution on the mesh surface in contact with the liquid, droplets are formed only from a limited area of the mesh. Determining the active area is important in the development of the mesh atomization equipment. As the first step of the research, we investigated the liquid squeeze flow between the ultrasonic vibrating surface and the static plate without micropores. The compressibility of the liquid becomes remarkable at high-frequency ultrasonic vibration. The pressure oscillation generated by the vibrating surface periodically varied displacement is the product of the fluid density, the sound speed, and the vibration velocity. To accommodate this situation with incompressible flow approximation, it is vital to impose a vibrating pressure boundary condition on the vibrating surface. Modeling and numerical analysis of this flow were conducted by applying a sinusoidal pressure vibration on this surface. The cases including and excluding nonlinear inertia terms and the influence of Reynolds number on this flow were investigated. It has been clarified that the pressure has extrema in the center of the plate and exhibits a parabolic distribution in the radial direction. In the absence of the nonlinear inertia terms, the radial velocity component is positively and negatively symmetric during each period. This symmetry is broken when including the non-linear inertia terms. At a higher Reynolds number, the radial velocity component maintains almost constant across the squeeze gap except for at the vicinity near the walls where boundary layers are observed.

The effects of expansion ratio and Reynolds number on backward-facing step flow are investigated experimentally. The expansion ratio ER is changed to 1.5, 1.75 and 2.0, and the Reynolds number based on the mean velocity and the hydraulic diameter of inlet channel ranges from 1.5 × 10³ to 6.0 × 10³. The Reynolds number range includes laminar, transition and turbulent flows. The forward flow fraction γ and local skin friction factor C_f are measured using a micro flow sensor. In the laminar region, the distributions of γ, C_f and the wall static pressure change greatly between ER = 1.5 and ER = 1.75. In the turbulent region, there is no significant difference in these distributions even if the ER is different.

In this study, an experimental study is performed for multiple-passage channel flows. The multiple-passage is a parallel flow type, and consists of the five branch channels. The channel flows are investigated from the view point of flow uniformity and pressure loss. Experiments are performed for the Reynolds number based on the bulk velocity and hydraulic diameter at the inlet channel from 6.0×10² to 2.0 ×10³. The effect of inlet and outlet manifold volume on the flow distribution is investigated. The wall static pressure is measured, and the pressure loss and flow rate in the branch channel are evaluated. The results show that the uniformity of the flow rate through each branch channel gets worse as the Reynolds number increases. The uniform flow distribution is realized by increasing the volume of the outlet manifold.

Flow measurements and analysis in the low aspect ratio rotating concentric cylinders(Taylor-Couette flow) were performed in terms of the flow structure. Particle image velocimetry was used to obtain the axial and radial velocity fluctuations on a meridional plane of Taylor-Couette flow. In contrast to the high aspect ratio cavity, the waviness of Taylor vortices with lower aspect ratio cavity was suppressed due to the presence of the endwalls, and the critical Reynolds number for the transition from TVF to WVF regime was drastically shifted. Spatial flow structures were extracted by means of proper orthogonal decomposition(POD). Each POD mode characterization enables to understand the governing fluid motion as well as the detailed fluid exchange between two adjacent vortices.

Air lubrication systems have gained considerable popularity as a promising drag reduction technology in recent years. However, the numerical simulations of intermediate-sized bubbles is quite challenging because of the numerical diffusion of the conventional method and the high deformability of the bubbles. This hinders the study of the physical mechanisms involved in a variety of phenomena in such types of bubbles, such as the bubble–liquid interaction effect, high bubble deformation, and flow in the liquid film generated above the bubble. In this study, direct numerical simulation is conducted for intermediate-sized bubble in turbulent channel to investigate the drag reduction around the bubble. To capture sharp-interface between gas and liquid, a solver viz. interIsoFoam of OpenFOAM, which is directly captured by the improved Volume of Fluid method, was applied. The numerical results for skin friction trends around the bubble are shown as the Weber number. The drag reduction is achieved from rear part of liquid film and frontside of secondary flow.

We performed stereoscopic PIV measurements for the drag-reducing turbulent boundary layer flows by injecting surfactant aqueous solution. For the case of the high drag reduction ratio, large-scale turbulence structures were often observed in the fluctuating velocity field. In this study, we discussed two kinds of machine learning methods for the quantitative evaluation of such large-scale flow structures. One is the identification by using the second invariant of the velocity gradient tensor whose differential values are calculated by machine learning. The other is the cluster analysis method by using experimented data on streamwise fluctuation velocity.

We performed PIV measurements in the flow in the cylindrical container of drag-reducing surfactant solutions due to the agitator. We investigated the vortex inhibition (VI) by measuring the vortex depth. The surfactant aqueous solution with the drag reduction (DR) effect showed the VI effect. The effects of the number of rotation and solution temperature on the flow were clarified. Then, we discussed the relation between the VI effect and velocity profiles in the cylindrical container.

Numerical simulations of turbulent channel flows with sinusoidal superhydrophobic surface are performed. Influence of particles adhering on the superhydrophobic surface to the drag reduction effect is investigated. As results, the adhering particles decrease the drag reduction effect of the superhydrophobic surface. Initially, number of adhering particles on the sinusoidal type of the superhydrophobic surface is smaller than that on the straight type. However, as time advance, the sinusoidal type exceeds the straight type. This is because the particles adhere on the slip area uniformly in the straight type, whereas they tend to adhere upstream side of the slip area in the varicose type.

Direct numerical simulations of turbulent flows along a long-thin cylinder and polygonal prisms are performed. We aim to design skin-friction drag reduction method for flow along the long-thin cylinders and prisms and the present study shows uncontrolled flows to clarify the flow characteriscs. The cylinder and polygonal prisms are represented by an immersed boundary method. The cross-sectional area of the computational domain along the cylinder and polygonal prisms are set to be equal. The mean velocity, Reynolds shear stress, and vortical structures are found to depend on types of the cross-sectional shape. In the quadrangular prism, the root-mean-square value of the streamwise velocity and Reynolds shear stress are large around the corners. Above the corners of the quadrangular prism, the root-mean-square value of the streamwise velocity and the Reynolds shear stress are larger than those of the cylinder. On the other thand, above the surface of the prism, these statistics are smaller. The two point correlation of the streamwise velocity minimized at about 75 degrees. It corresponds that there are two or three pairs of velocity streaky structures and it is similar trend with the flow along the cylinder. However, in the case of the poligonal prism, the mean velocity, the Reynolds shear stress, and vortical structures are affected by corners. Accordingly, we concluded that it is possible to reduce the drag of the polygonal prisms by the control of the flow around the corners.

Direct numerical simulations of incompressible turbulent flows over a backward-facing step are performed. To control the flow separation, a body force distribution in form of a traveling wave is given on top of the step. In this study, we investigate the effects of control parameters of the traveling wave. In the controlled flow, a time-averaged reattachment distance is affected by the control, e.g., when the wave travels in the downstream direction, the time-averaged reattachment point moves in the upstream direction and the pressure recovery occurs on the upstream side.

Bubble drag reduction (BDR) is expected as a solution to ship energy saving. However, it has a problem that the combination of bubbles at aft disturbs the drag reduction. This study aims to development of bubble splitter to make bubbles fragmented without external force for superior performance by BDR. We designed three types of protrusions as bubble splitters and examined their performance in a gas–liquid two-phase channel flow through observation of a downstream region of the bubble splitters. The bulk speed in channel flow was about 1 m/s. We also conducted an experiment with surface active agent to consider effects of the size of the splitters. Unfortunately, bubbles did not fragment enough with nor without surface active agent and it led to the conclusion that the bubble splitters do not work in the flow. However, the large air film right downstream from the device was a finding which is expected to enhance performance of gas layer drag reduction or gas cavity drag reduction.

This study experimentally investigated the cooling performance of fine bubbles using a recirculating pipeline system. Purified water was passed through a fine-bubble generator; the test fluids used in this study contained 2 × 10⁸ bubbles with an average bubble size of ~100 nm in 1 mL. Experiments were conducted under turbulent flow conditions, and the temperature change of the circular pipe was measured by flowing the test fluid at 20 ℃ through the circular pipe heated to 80 ℃. It was found that the addition of fine bubbles reduced the cooling rate by up to 66%. Furthermore, the improvement rate of the cooling performance was found to depend on the flow rate. Moreover, the improvement rate increased as the flow rate decreased.

Cooling processes are ubiquitous in food process engineering. Effected by surface shape and cooling medium characteristics, turbulent boundary structures are generated in the flow around a heated body. Turbulent boundary structures have a close relationship with local heat transfer coefficient because turbulent mixing in turbulent boundary layer promotes heat exchanges and local heat transfer. In chocolate cooling process, the chocolate would solidify unevenly due to differences in local heat transfer coefficient, and quality of chocolate would be poor. To elucidate local heat transfer of the chocolate cooling process, hence, it is necessary to measure three-dimensional (3D) and complicated turbulent structures in boundary layers. In this paper, we investigated boundary layer structures using 3D three-component color PIV with three color layers by the equation of continuity. We measured three types of target: a mold for bar chocolate, a mold for grain chocolate and a flat plate. From color PIV results, we evaluated mean velocity and distribution of turbulent kinetic energy in the boundary layer. Standard deviation of two chocolate molds are higher than the flat plate. It means that turbulent boundary structures of two chocolate molds were dispersed and heat transfer distributions would be uneven.

Aiming at applying vortex generators (VGs) to a piston surface of a spark ignition engine for cooling the air-fuel mixture, direct numerical simulation of a laminar boundary layer flow was performed to investigate heat transfer enhancement effect by two VGs. Triangular pyramid shaped VGs are installed on the lower wall. The height of VG is the same as inlet boundary layer thickness (δ₀) and arranged at 10.0δ₀ interval in the longitudinal direction. Laminar boundary layer flow is given as the inlet boundary condition. In this study, angle of the inlet flow is varied from 0°, 30°, 37°, 60°, 120°, 150° to 180°. Here, 37° means one of the angles at which the downstream (second) VG lines up behind the upstream (first) VG in the streamwise direction except 0° and 180°. The Nusselt number (Nu) increases by installing VGs. High Nu can be obtained from 0° to 120° due to vortices generated by VGs. When flow impinges VGs with large angle, Nu increases in broader area. In addition, when the wake of the first VG impinges the second VG, heat transfer enhancement effect increases highly. It is because more vortices generated in the downstream of the second VG.

Skin-friction drag reduction in wall-turbulence is of importance to reduce energy loss and many flow control techniques to obtain the drag reduction have been studied. It is known that the streamwise large-scale vortical structure decreases the skin-friction drag in the wall turbulence. In this study, we performed direct numerical simulations of the turbulent pipe flow controlled by the large-scale vortical structure to investigate its drag reduction effect. We impose the temperature distribution on the pipe wall to generate buoyancy force and the large-scale vortical structure. As results, a maximum drag reduction rate is 11.4% when two pairs of the large-scale vortices are generated. In this case, the Reynolds shear stress increases where the upward flow from the wall toward the pipe center occurs. However, the negative Reynolds shear stress is observed where the downward flow from the center of the pipe to the wall is induced. The visualization of the vortical structures corresponds the distribution of the Reynolds shear stress: they increase at the upward flow region and vanish at the downward flow region.

In this study, the bubble growth process with lateral bubbles merging in a high viscous liquid during nucleate boiling is computationally examined. The Moment-of-Fluid (MOF) method is used to track the evolution of bubble interface. The numerical simulations are carried out by solving the conservation law equations for mass, momentum, and energy in the liquid and vapor phases. Nucleate boiling in liquids with 10, 102 and 103 times higher than viscosity of water are considered. From numerical results, it is shown that the diameter of bubble released from the heated wall and the bubble departure time become larger in nucleate boiling in liquids with 103 times higher than viscosity of water. Also, it is shown that the growth rate of merged bubble in liquid with 103 became larger than that for water. This is because the high viscosity of the liquid suppressed the oscillatory motion of the merged bubble and a high temperature region is formed around the merged bubble.

Weakly nonlinear propagation of pressure waves in an initially quiescent compressible liquids uniformly containing many spherical microbubbles was theoretically studied by deriving the KdVB (Korteweg–de Vries–Burgers) equation. In particular, the energy equation at the bubble-liquid interface (Prosperetti, J. Fluid Mech., 222, 587, 1991) and the effective polytropic exponent were newly introduced into our model (Kanagawa et al., J. Fluid Sci. Technol., 6, 838, 2011) to clarify thermal effect inside the bubbles mainly on wave dissipation. Thermal conduction was investigated in detail by using some temperature-gradient models. The main results are summarized as follows: (i) Two types of dissipation term appeared: one was a well-known second-order derivative comprising the effect of viscosity and liquid compressibility (acoustic radiation), and the other was a newly discovered term without differentiation comprising the effect of thermal conduction. (ii) The thermal effect contributed to not only the dissipation effect but the nonlinear effect, and nonlinearity increased compared with that in Kanagawa et al. (2011). (iii) There were no significant differences among four temperature-gradient models for milliscale bubbles. However, thermal dissipation increased in four models for microscale bubbles. (iv) The thermal dissipation effect in this study was comparable with that in a KdVB equation derived by Prosperetti (1991) although the forms of dissipation terms describing the effect of thermal conduction differed.

The Korteweg-de Vries-Burgers (KdVB) equation describes an weakly nonlinear propagation of pressure waves in liquids containing many spherical gas bubbles. As one of disadvantages, the coefficients of the KdVB equation derived from the mixture model did not have dependency on the initial void fraction and the initial bubble radius. On the contrary, the coefficients of the KdVB equation derived from the two-fluid model depended on the initial void fraction and the initial bubble radius. We numerically calculated the KdVB equation derived from the two-fluid model via a split-step Fourier method and investigated evolved waveform for the initially Gaussian waveform and the dependency of an evolved waveform on an initial void fraction and initial bubble radius. In the range that the initial void fraction was from 0.001 to 0.05 and the initial bubble radius was from 0.1 to 0.5 mm, the evolved waveform became the oscillatory shock waveform or the soliton pulse waveform. The initial void fraction and the initial bubble radius strongly affect the propagation speed of the pressure wave and the evolved waveform type, respectively. The numerical waveforms were good agreement with the experimental waveforms in the case that the initial pressure perturbation was 0.29 bar or less and this implied the limitation of the assumption of the weak nonlinearity.

Clustering of spherical bubbles rising along an inclined flat wall has been investigated experimentally to analyze the bubble-bubble interaction. To avoid bubble coalescences, the electrolyte (MgSO₄) solution and the surfactant (Triton X-100) solution are utilized as a liquid phase. In MgSO₄ solution, free-slip boundary condition of bubble surface is maintained, on the other hand, the boundary condition of the surface of rising bubbles in Triton X-100 solution becomes no-slip due to well-know Marangoni effect. The development of bubble clusters and the bubble-bubble interaction of bubbles are discussed through the statistical analysis of the bubble velocities and the distance between neighboring bubbles. The bubbles tend to be arranged horizontally as they rise, which induces bubble cluster formation. The horizontally aligned bubbles tend to rise more stably in case of the no-slip condition. The drag coefficient increases with increasing number of horizontally aligned bubbles. When the approach distance L is short, the effect of wake-induced lift is confirmed only in the case of free-slip condition. In case of no-slip condition. Even in the longer separation distance of side-by-side bubbles, the separation velocity was still induced on no-slip bubbles.

The cavitation bubble cloud formation due to the backscattering of high-intensity focused ultrasound (HIFU) from a laser-induced bubble and the bubble collapse near a gelatin-water interface have been investigated experimentally. High-speed video camera and fiber optic prove hydrophone (FOPH) were used for the observation of the bubble and for the pressure measurement, respectively. 1.1 MHz HIFU was utilized and a laser-induced bubble was generated at the focus of HIFU as a reflector bubble of HIFU. Cavitation inception occured due to the stronger negative pressure generated near a bubble by the backscattering of HIFU and the bubble cloud was formed along the opposite direction of HIFU propagation. The cavitation inception pressure has been evaluated as the pressure at the tip of the bubble cloud, which decreases with increase in water temperature. As a preliminary step to experiments using gelatin, the collapse of a laser-induced bubble near a gelatin-water interface has been observed. There was a large difference in the growth and shrink behaviors of bubble in gelatin and in water, and large bubble deformation was observed near the gelatin-water interface at maximum expansion. Also, the bubble translation became remarkable when the bubble collapses near the interface.

Weakly nonlinear (i.e., finite but small amplitude) propagation of plane progressive pressure waves in a compressible water flow uniformly containing many spherical gas bubbles is analytically investigated. Gas and liquid phases are flowing with each velocity. Drag force and virtual mass force are incorporated in an interfacial transport term in momentum conservation laws based on a two-fluid model. As bubble dynamics, translation and spherically symmetric oscillation are considered. The bubbles do not coalesce, break up, extinct, and appear. For simplicity, the gas viscosity, the thermal conductivities of the gas and liquid, and the phase change and mass transport across the bubble-liquid interface, are ignored. From theoretical and numerical analyses, the following results are obtained: (i) By the use of the method of multiple scales, two types of Korteweg–de Vries–Burgers (KdVB) equation with a new term without a differentiation due to the drag force are derived from the basic equations for bubbly flows in the two-fluid model. (ii) The translation of bubbles affects the nonlinear effect of waves. (iii) The drag force acting bubbles affects the nonlinear and dissipation effects of waves. (iv) The dissipation effect due to the drag force is strongly dependent on both the initial void fraction and the initial bubble radius.

This study theoretically investigates one-dimensional weakly nonlinear interaction between short and long waves in a compressible liquid uniformly containing many spherical gas bubbles. Important assumptions are summarized as follows: (i) Two different wave modes coexist, one is the short wave and the other is the long wave; (ii) Bubble does not coalesce, break up, extinct, and appear; (iii) The effect of viscosity in the gas phase, heat conduction in the gas and liquid phases, and phase change across the bubble wall, are neglected for simplicity. The basic equations for bubbly flows are composed of a set of conservation equations of mass and momentum for the gas and liquid phases in a two-fluid model, the Keller equation, and so on. Appropriate choices of scaling relations of some physical parameters, i.e., wavelength, wave frequency, propagation speed, yields the result that amplitudes of the short and long waves are of the same order of magnitude. By the use of a singular perturbation analysis based on the method of multiple scales, we can derive the coupled equations from the set of governing equations for bubbly flows, one is the nonlinear Schrödinger (NLS) type equation for the short wave with a nonlinear interaction term and the other is the Korteweg–de Vries (KdV) type equation for the long wave with a nonlinear term of the short wave.

For investigating stability of ultra-fine bubble (UFB) under practical conditions (relatively high and low temperature), we measured particle diameter of UFB in liquid-state and ice-state conditions. In the liquid-state, the number density did not change up to 60°C. The peak diameter did not changed, neither. Although there was a difference in the critical temperature of 50°C, a similar tendancy was obtained in the ice-state. It was found that UFB was highly stable existence up to 50°C when it was boiled in hot water. On the other hand, number density was reduced by approximate an order of magnitude after ice-state. For understanding the experimental results, we discussed comparison with ultrasonic irradiation and dissolved oxygen.

The drainage and rupture of the thin liquid film formed between a bubble and a flat mica plate have been investigated. It is known that the bubble coalescence can be prevented in electrolyte aqueous solution, and its effects strongly depends on the type of electrolyte. In this study, by using two types of electrolyte aqueous solutions (0.05 M MgSO₄ aqueous solution and 0.05 M CH₃COONa aqueous solution) and surfactant aqueous solution (10 ppm Triton X-100 aqueous solution), the effects of impurities in water on the behavior of the liquid thin film and the effect of the approaching velocity of bubble toward the mica plated have been evaluated. By using an optical interferometer and a high-speed video camera, interference fringe patterns can be observed and the shape of the liquid thin film formed between a bubble and a flat mica plate can be obtained. There was almost no difference in the rim width and depth between the case with two electrolyte aqueous solutions, while the increase in the rim width was suppressed and the depth became smaller in case with Triton X-100 aqueous solution due to the Marangoni effect. The rim width and depth tend to increase as the bubble approaching velocity increase in every solutions.

In the recent development of the micro- and nanofluidics, the prediction of the flow with the slip velocity on the walls plays a significant role. However, the computational methods that deal with the arbitrarily shaped slip boundaries have not been established. The immersed boundary method is one of the most popular method to simulate the flows with the arbitrarily shaped surfaces where the no-slip boundary conditions are imposed. In the immersed boundary method, the boundary forces defined on the immersed surfaces are added to the Navier-Stokes equations to impose the boundary conditions. Since the immersed surfaces are not fitted on the fixed grid where the flow fields are solved, the boundary conditions are described with the interpolation operator and the boundary forces are smeared on the fixed grid with the regularization operator. These operators compose of the discrete delta functions. In the case of the Navier slip boundary condition containing the velocity gradient, however, the conventional combination of these operators cannot appropriately evaluate the velocity gradient. In order to impose the Navier slip boundary condition on the immersed boundaries, we introduce a new regularization operator based on the immersed boundary projection method, in which the boundary force are determined implicitly to satisfy the boundary conditions. The flow inside two concentric cylinders are calculated to validate the present method. The resulting velocity profile shows a good agreement with the analytical solution.

Drug delivery system (DDS) is a technology to control the effective area and the amount of drug, in order to suppress the harmful side effects. Microbubble-containing liposome (MCL) is one of the possible DDS carriers. MCL is manipulated by acoustic radiation force, which is applied to internal microbubbles, to be trapped around the target area and to be broken to release the internal drug. Previous studies succeeded in disrupting MCL using ultrasound, however, the results did not show enough reproducibility. Also, the method has not been established to manipulate perfectly without contact including positioning. In this study, we aim to realize the contactless positioning of MCL and the selection of MCL with a certain condition to unify the behavior toward the acoustic field. As its first step, we observed and evaluated the behavior of MCL under the standing wave.

The present study deals with the growth of bubble nuclei in a pressure field formed by high-intensity focused ultrasound using the equation of motion for a spherical bubble. In the analysis, two kinds of waveforms are used: one is a pressure waveform measured in an experiment for cavitation inception pressure by the backscattering of focused ultrasound from the interface of a laser-induced bubble, and the other is a sinusoidal wave. The analysis with the former waveform shows that once bubble nuclei of a few nanometers in radius grow explosively lager than about 40 microns by a negative pressure of the pressure wave, they grow gradually without showing oscillatory motions as the wave is continuously applied to the nuclei. On the other hand, when a sinusoidal wave is applied to bubble nuclei, they show periodic oscillations taking the period-doubling bifurcation with increase in the initial nuclei radius. These differences in bubble growth between them are due to the differences of pressure waveforms: the gradual increase in radius for the former waveform is caused by its nonlinear waveform in which the negative pressure amplitude is larger than the positive one.