In this study, the aim is to clarify the essential vortex interference structure, the flow structure of the wake of the plain weave wire mesh composed of a simple repeating structure is investigated. The aerodynamic characteristics of the wake caused by the wire mesh are assumed to be due to the vortex structure formed in the wake of the wire mesh, which interferes with the jet flow from the open area, however the relationship between the two is not clear. Therefore, the relationship between the vortex structure and the jet flow from the open area was examined. As a result, it was found that the distribution trend of the slow-velocity region behind the wire is different in the longitudinal sections of the open area wake. In this longitudinal section, the slow-velocity area and the high-velocity area were alternately distributed behind the wire crossing point. On the one hand, in the longitudinal section behind the wire, the jet flow from the side of the wire was found. This jet is guessed to be related to vertical vortex of the wake. And, both jets were confirmed to be homogenized in the wake about 10 times the wire diameter.

Experiments were conducted to investigate the relaminarization by visualization and pressure measurement. Two urethan pipes with different diameters were connected with a divergent duct, since the relaminarization is presumed to attain by lowering the Reynolds number. Flow in the pipe was disturbed in the divergent section and the disturbed flow condition continues for a while. After a distance, the flow became more ordered condition under lower Reynolds number in the downstream pipe than in the upstream pipe. The flow in the sufficiently downstream was specified by the local Reynolds number. We obtained the length to establish the ordered condition in the downstream from the viewpoint of entropy change.

Methods of decreasing the CO2 emissions of the internal combustion engine have been suggested. However, these methods generate high combustion temperatures that increase the heat load in general. Therefore, the piston cooling gallery has been proposed as a system for cooling. For supplying the oil to cooling gallery, an oil jet injected from a nozzle placed under the piston flows into the cooling gallery through an entrance hall. It may thus be desirable to control the shape of the oil jet such that it is stable and straight. However, the interface of the ambient air and oil jet may have unstable, and the mechanism is unclear. In this paper, we measured nozzle internal flow using two-dimensional two-component time-resolved particle image velocimetry and visualization of the jet time-synchronously. As the result, we clarified that the characteristic POD (Proper Orthogonal Decomposition) modes of nozzle internal flow makes oil jet wavy.

We conducted three-dimensional flow visualization of concentric/eccentric annular pipe flows in a transitional Reynolds-number regime with a single camera. One of our goals is to provide the first experimental evidence of helically-shaped intermittent turbulent structures in the concentric annular pipe flow, which has been reported only by a numerical simulation [Ishida et al., J. Fluid Mech. 794, R2 (2016)]. Another aim is to reveal the effect of the eccentricity on the intermittent structures that the gap instability caused in an eccentric annular pipe flow. A parametric study was executed for the radius ratios from 0.14 to 0.43 and the eccentricities from 0 to 0.8 (the ratio of the axes distance to a half of the hydraulic diameter). For the concentric cases with high radius ratios of 0.29 and 0.43, we confirmed the helical turbulence and helical puff. With intermediate eccentricities, such helical structures did not appear, instead weak turbulence and staggered vortices survived at much lower Reynolds numbers, i.e., the relaminarization was delayed because of the gap instability. For the highest eccentricity of 0.8, a transitional structure like a puff in a pipe flow without the inner pipe was observed due to the absence of the cross flow through the narrow gap between the cylinders.

In this paper, a temperature recovery factor defined as the ratio of the actual temperature rise at the wall to the maximum possible temperature rise were estimated from experimentally measured wall temperatures and pressures to determine the velocity and temperature of gas through an adiabatic microtube. The experiments were carried out using two pairs of stainless steel microtubes with D=528.4 and 754.2 μm. One of a pair was tested for measurements of local pressures to obtain local Mach numbers and bulk temperatures. And the other of a pair was tested for measurements of local wall temperatures. The stagnation pressure with the atmospheric back pressure was chosen in such a way that the outlet flow reached sufficiently choked. Local total and bulk temperatures were obtained with total and static enthalpies determined with the measured stagnation & local pressures, stagnation temperatures, mass flow rates by real gas manner. In order to estimate the temperature recovery factor, the measured wall temperature and the obtained total temperature were normalized by bulk temperature and represented as Mach numbers. The estimated values of the temperature recovery factor were independent of the Mach number. The present estimated temperature recovery factor was compared with numerical and experimental ones obtained with the assumption of an ideal gas.

In this study, visualization experiments of plane Couette flow were conducted to investigate the spatio-temporal variation of turbulent stripes, the spanwise wavelength, and the process of their generation and breakdown. The experiments were performed with three different film half-widths (δ = 2.90 mm, 3.39 mm, and 4.30 mm). Two cycles of generation and breakdown of turbulent stripe were successfully captured. We defined long-life and short-life cycles based on the order in their appearance, respectively. For each film half-width, the survival time of turbulent stripes, that appeared in the long-life cycle, was t* = 1620, 1960, and 556, respectively, and the number of turbulent stripes was confirmed as 2, 1, and 1. The elapsed time t* since the observation started was non-dimensionalized by the film velocity and half-width. Similarly, the survival time of turbulent stripes, that appeared in short-life cycle, was t* = 970, 1240, and 0, respectively, and the number of bands was 5, 1, and 0. In the case of a film half-width of 2.90 mm, five turbulent stripes were visualized, the largest number in this study. Based on the results, it was found that the number of turbulent stripes decreases and the spanwise wavelength increases as the film half-width increases. This tendency is not considered to be organized on an outer scale, given the standard deviations of the film half-widths. Considering the distribution ratio of the pressure-strain correlation term or friction velocity in turbulent stripes of numerical simulations in previous studies, the tendency for differences in the number of turbulent stripes and spanwise wavelengths at each film half-width to be organized on a viscous scale is considered reasonable.

The airflow in the exhaust pipe of an automobile engine fluctuates periodically in terms of flow velocity and gas temperature, and understanding the complex flow and heat transfer characteristics is extremely important for improving the performance of exhaust systems. In this study, using a horizontal rectangular pipe with a rectangular cross section, we experimentally obtained the time-averaged temperature in the cross section measured by thermocouples and the wall temperature by thermography on the outer wall of the pipe under the pulsation conditions assumed in the actual system, and investigated the effects on the heat transfer characteristics. The influence of flow characteristics on heat transfer characteristics was also investigated by using PIV. The results showed that the heat transfer coefficient peaked at a pulsation frequency of 25 Hz. At higher frequencies, the decrease in velocity amplitude was considered to be responsible for the decrease in heat transfer coefficient. At lower frequencies, the decrease in turbulent energy in the area close to the inner wall was considered to be the cause.

Typical thermal efficiency improvement methods for automotive engines tend to result in higher combustion temperatures. Therefore, it is important to elucidate the characteristics of oil jet behavior in order to develop highly efficient piston cooling technology. The author's research group has reported the effect of unsteadiness of the flow in the nozzle on the jet interface. In this paper, time history PIV results of flow in an acrylic nozzle with a 90° bend are analyzed using Proper Orthogonal Decomposition (POD) and dynamic mode decomposition (DMD), and the characteristic modes obtained by each method are compared and their relationship to the wave behavior of the oil jet is discussed.

In order to better understand how critically the small surface irregularity affects the formation of thermal convection, natural convection in a horizontal slot with surface corrugation was examined experimentally at Rayleigh number based on the slot half-height below 300. The sinusoidal surface corrugations whose amplitude was 2.5% of the slot full-height and wavenumber was 0.5 - 2 was applied on the bottom surface while upper surface was kept smooth. Both walls were heated uniformly at different temperature. The results showed that surface corrugation promoted the occurrence of thermal convection significantly, that is, a pair of convection rolls whose wavelength was the same as that of corrugation was observed at Rayleigh number much lower than the critical one of smooth slot. Beyond critical Rayleigh number, on the other hand, the Rayleigh-Benard convection dominated the flow regardless of corrugation wavelength examined.

Effects of surface roughness on microchannel gas flow are significant since the surface roughness of a microchnnel is relatively large compared to conventional size tubes. Therefore, in this study, the effects of surface roughness on friction factors of nitrogen gas flow through stainless steel microtubes experimentally investigated under atmospheric back pressure and variable inlet pressure. The experiments were carried out with six different diameters stainless steel microtubes whose diameters range from 256 to 867 μm. The average values of Mach number and Fanning & Darcy friction factors between the inlet and outlet considering decrease in gas temperature were obtained from measured stagnation temperatures & pressures and mass flow rates. The effects of surface roughness on the compressibility effect were described with Fanning and Darcy friction factor correlations as a function of Mach number. The arithmetic mean height of the surface (Ra) of the microtube was also measured with a 3D laser scanning confocal microscope for profilometry. Comparing the measured Ra and the equivalent sand grain surface roughness (K_s) determined from Colebrook-White correlation, a correlation between Ra and K_s was obtained.

A hemispheric-head cyclone separator has higher collection performance of fine particles comparing with a conventional cyclone separator with a cylindrical head. In order to optimize the shape model with hemispheric-head, we have investigated numerically and experimentally the air flow in various shape cyclone models for industrials. The purpose of this study is to develop a cyclone model for fuel cell vehicles based on the results for industrials. Since this model is installed in a fuel cell vehicle, it must be compact due to space constraints. Therefore, as the first stage of development, based on a model of a dome-shaped cyclone separator with a cyclone diameter of 54 mm, we numerically investigated the airflow in the model by changing the dust holder size and the inlet pipe connection height. The motion of fine particles was analyzed considering only drag and gravity, and the effect of internal airflow on collection performance was investigated. As a result, it is shown that the model with the low inlet pipe connection height and the small volume of the dust holder has the highest collection efficiency. It was found that even if the entry particles in the dust holder return to the conical separator part due to small dust holder, the air flow field of the highest collection efficiency model has high centrifugal separation performance by lowering the connection height of inlet pipe. It is also the most suitable in terms of space for installation in fuel cell vehicles.

Penguins have adapted to underwater swimming in the course of evolution. Previous study showed that the riblets mimicking the body feathers of the penguins reduced drag on a flat plate by up to 2.0%. In this study, we measured drag reduction rate of the penguin-mimetic riblets on a 3D body model. Based on a sideview image of a swimming penguin, a 3D spindle model of which cross section was a regular 24-gon shape was created. Riblets were fabricated on flexible polyimide films by laser scanning ablation so that the riblet films were attached to the curved surfaces. Two different cross sections of the riblets were compared: one was the penguin-mimetic trapezoidal cross section (medium-ridge riblet), and the other was a near-rectangular cross section (wide-ridge riblet). Drag of the 3D bodies covered with films with or without riblets was measured in a water channel. As a result, the maximum drag reduction rate of the medium-ridge riblet for the 3D body was 3.5% when s⁺ was 7.29, which was higher than the drag reduction rate for the flat plate with similar s⁺ (0.58% with s⁺ of 7.23) or the maximum value (0.60% with s⁺ of 10.4). Moreover, the medium-ridge riblet showed higher drag reduction ratio than the wide-ridge riblet for full range of flow speed in this study. The results suggest that the penguin-feather-mimetic riblet reduces not only frictional drag but also pressure drag for the 3D body.

We have performed 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. However, the interaction between large-scale turbulence structures and wall-shear stress has not been investigated. In this study, we performed wide-range and high-magnification PIV measurements in order to obtain large-scale turbulence structures and wall-shear stress simultaneously. Decreasing and increasing of wall-shear stress correspond to passing turbulence structures which consist of low and high fluctuation velocities, respectively. This indicates that there would be the correlation between large-scale turbulence structures and instantaneous wall shear stress.

Riblets are micro-grooved structures on the object surface, and it is known that the skin friction drag can be reduced by approximately 10% with the riblets designed to have appropriate size and shape against the near wall turbulence. As for improving the operational durability of riblets, it is necessary to mitigate degradation of the drag-reduction effect. One of the main reasons of decrease in performance is particle clogging on the riblet groove. Although the particle motion near the riblet groove has been studied, the mechanism and characteristics of the particle adhesion have not been fully clarified yet. In the present study, we aimed to clarify how particles accumulate on the riblet grooves. As a preliminary experiment, the riblet plate was exposed in an atmospheric environment. The location and the size of the particles were identified by image analyses. It is found that the most of particles have diameter less than 15 μm, and particles are preferentially found in the corner of the riblet edge. A wind-tunnel experiments are planned to clarity the characteristics of particle adhesion on the riblet surface.

We conducted a detailed analysis on the experimental data of the traveling wave control for zero pressure gradient turbulent boundary layer flow in which the drag reduction rate resulted in 7%. The free stream velocity is 2.0 m/s and Reynolds number based on the momentum thickness of the measurement section is 640 - 900 and the frictional Reynolds number is 300 - 340. It was confirmed that the traveling wave parameters; amplitude, wavelength, and wave speed are the same as that of the previous experiment conducted in the channel flow. We calculate laminar, turbulent Reynold shear stress (RSS), and periodic component in the same period with the control of FIK identity to analyze the contribution of local friction coefficient dividedly. As a result, the contribution of the turbulent component to the friction coefficient was decreased by the control while the periodic component was increased. Although the mean convection term was not directory calculated, the fact that the displacement thickness was thickened by the control suggests that this term was modified by the control to reduce skin friction. However, contributions of the RSS for the friction coefficient might be overestimated because some regions with the strong negative RSS which had been found in the previous DNS could not be measured.

We propose a novel turbulent eddy viscosity profiler to evaluate the influence of interactions between turbulent eddies and dispersed materials as an effective eddy viscosity in Taylor-Couette geometry for frictional drag reduction studies. This method was conceived from the ultrasonic spinning rheometry which evaluates rheological properties by substituting experimentally obtained velocity profiles into the equation of motion. The eddy viscosity is calculated using a model derived from Reynolds-averaged Navier-Stokes equation. By modeling the Reynolds shear stress using the concept of the eddy viscosity, only the spatial distribution of the eddy viscosity remains to be estimated from the mean velocity distribution. We can extract the contributions of eddies with different time and spatial scales depending on the definition of the averaging operation, since mean velocity profiles include the information of momentum transfer by eddies which have the specific time and spatial scales. In this paper we derive the analytical model equation where velocity profile is averaged in temporal and axial directions and the molecular viscosity term balances with the eddy viscosity term. In this case, we can separate the original flow field into Couette-like mean velocity and fluctuation components corresponding to Taylor vortices and smaller fluctuations. Additionally, we show the experimental apparatus and the result for a glycerin aqueous solution using the equation with the above averaging operation. The distribution of eddy viscosity is obtained as a function of radial position.

Reducing fluid drag and the energy required for fluid transport is an important issue. Riblet is used as a method to reduce frictional drag. Direct numerical simulation (DNS) was used to research the drag reduction effect of the riblet and the torque generated at the base of the riblet. The riblet is a three-dimensional riblet whose height and span vary with respect to the flow direction. The riblet is represented by a superposition of two sine waves, with amplitude and phase determined randomly. The torque and drag reduction effects were investigated for 1108 cases of riblets with different geometries. The friction Reynolds number, defined by the friction velocity uτ [m/s] and the kinematic viscosity ν [m²/s], is 110. The results show that torque is a shape parameter and is most dependent on pressure of the riblet surface. A Paretooptimal solutions that minimizes both the torque and the drag increase ratio was investigated. As a result, four Pareto solutions were found. The drag force generated on each face of the riblet was also analyzed and compared to the Pareto solution in all cases. The results show that the Pareto optimal solution group has a smaller fraction of drag due to pressure than all cases.

In fluid engineering, the reduction of the skin-friction drag and the enhancement of the heat transfer are of importance in turbulent flows. In the present study, we made direct numerical simulations of the concentric annular turbulent pipe flow controlled by the wall oscillation control technique for the skin-friction reduction and heat transfer enhancement. The concentric annular pipe is widely used, and we expected to obtain the control effect by using the different radius of the inner and outer cylinders. Two types of control methods are shown: the wall oscillation technique and the traveling wave-like wall oscillation. In the wall oscillation, the friction drag decreases at all the frequencies, and the similarity between momentum and heat transport is maintained. In the traveling wave, the wave speed significantly affects the friction drag and the heat transfer. In some cases, the dissimilarity between momentum and heat transport is also confirmed.

Experiments were conducted to measure the drag reduction of the complex mixtures in the pipe flow. The friction factor was measured for bio-polymer solutions and aqueous suspensions of fine solid matter. The results indicated that the experimental data points of the test solutions diverged from the maximum drag reduction asymptote at and above Re√f ≅ 200 and aligned parallel to those of Newtonian fluids. Type B drag reduction was observed. The velocity profile in turbulent pipe flow was predicted using a semi-theoretical equation, in which the friction factors were determined using the difference between the experimental results of the tested solutions and Newtonian fluids.

The purpose of this research is to experimentally investigate how increase of the air inlet area of the engine cooling air affects the aerodynamic characteristics depending on the shape of the front-end of the vehicle body. A simplified 1/5 scale vehicle model was used with reproduction of the engine compartment where FF and FR type engines could be installed. In past researches, we used a rectangular front-end model, and in this research we used a round front-end model with a shape closer to the actual vehicle. In the wind-tunnel experiments, the moving-belt ground board was used to capture ground effects, measuring drag force, lift force and body surface pressure. The experiments were conducted at a wind speed and a moving belt speed of 20 m/s. As a result, the reason for the increase in drag due to the increase in air inlet area was the damming effect caused by the interference between the scavenging air flow in the engine compartment and the underfloor flow. The drag force of the round model was lower than that of the rectangular model, but the lift force was not significantly reduced.

Numerical simulations of fully developed turbulent channel flows with riblet surfaces were carried out at the friction Reynolds number of 180. The influence of particles adhering to the rectangular riblets on the drag reduction effect is investigated. The one-way coupling method is employed: the particles are affected by the fluid, but the fluid is not affected by the particles. The simulation consists of three steps. First, we calculate the detailed particles motion and the adhesion position of the particles to riblets in the turbulent channel flow. Second, the riblet shape is modified based on particle adhesion distribution. Here, the condition that the particle adhesion distribution is uniform in the streamwise direction and independent of the number of particles adhered is given. Third, we calculate the flow over the riblet with the adhered particles. As a result, most of the particles adhered to the top wall and the upper half of the side wall surfaces of the riblets. Due to the particle adhesion on the riblets, the drag increases as compared with the case of the initial riblet of the riblet.

We performed direct numerical simulations (DNS) of turbulent boundary layer flows of viscoelastic fluids with variations of solution concentration and temperature in order to investigate the effects on the turbulent statistics and structures. In this study, we proposed a new constitutive equation model, c-T-FENE-P model, in which the relaxation time varies with temperature. DNS date for 6 cases of increasing and decreasing relaxation times with temperature were investigated. It was found that the dependence of the increasing and decreasing relaxation times on the drag reduction ratio was opposite to each other.

Machine learning based on experimental data was performed to predict the drag reduction effects of various pulsating pipe flows. In the experiment, the voltages input to the pumps were changed over time to generate 2448 different pulsating flows. Pulsating flows with higher pressure gradients than in previous study was investigated with high responsive pumps. Using machine learning model specialized for time series prediction based on experimental data, time series data of flow velocity and differential pressure from the voltages applied to the pumps to calculate the drag reduction rates. It was confirmed that the model could predict the drag reduction effect of complex pulsating flows with the almost same accuracy as in the previous study. Optimization of the voltage waveforms for pulsating control was also performed using the learned model. As a result, the drag reduction rate of optimal voltage waveform within the conditions equivalent to those of the experiment is predicted to be 42%. This is a larger value than the maximum drag reduction rate of 39% which is measured in the experiment.

Ternary fluid flows that single bubbles rise across the interface separating two immiscible liquids (liquid-liquid layer) are investigated through three-dimensional numerical analysis based on the moment-of-fluid (MOF) method. In this study, the effect of the viscosity of both liquids on the bubble rise motion and the behavior of the liquid-liquid interface is discussed. From numerical results, it is clearly shown that the viscosity of the liquids has a significant effect on the motion of a single bubble rising through the liquid-liquid layer. When a bubble rises in the lower layer with the low-viscosity, the bubble rise motion shows dynamic behavior, and the behavior of the liquid-liquid interface is complicated. Also, after passing the liquid-liquid interface, the bubble largely trails the lower layer with the low-viscosity fluid. On the other hand, the dynamic behavior of a bubble cannot be seen when a bubble rises through the liquid-liquid interface between high-viscosity fluids. The MOF method allows us to reproduce the dynamic motion of a single bubble rising across a liquid-liquid layer with a ternary fluid interface.

Researches on free-falling water droplet have been conducted in a long time. However, there are few numerical analyses closing to realistic free-falling phenomenon. In this study, We developed a two-phase flow analyses program based on CCUP method and Level Set method applied by RCIP method. And the numerical analyses of free-falling water droplet in the air were carried out. The droplet diameter is 1mm and 1.6mm. As a result, the falling velocity of two water droplets were close to values based on the experiment. In addition, it is found 1mm water droplet shows spiraling trajectory, which was not shown for 1.6mm droplet water. The spiraling trajectory is related to the vortex existing near droplet surface and moving around the surface.

The water hammer pressure at the high-speed droplet impact has not yet been elucidated, and a reproducible droplet generator is needed to investigate it. We developed a single droplet generation device using airflow with an abrupt contraction pipe. We obtained higher velocity droplets than the smooth convergence tube of the early study. To clarify the reasons, we investigated the details of the droplet acceleration using the fluid analysis software OpenFOAM, especially focusing on the pressure gradient force in the abrupt contraction pipe. Comparing the experimental and simulation results, we found that the pressure gradient force has a small contribution to the droplet acceleration and the drag force is dominant for the motion. After examining various shapes, such as a combination of a smooth convergence with a straight pipe, we found that applying as large a drag force as possible at the beginning of acceleration leads to efficient acceleration.

Liquid droplets adhered to a rotating disk will deform by the centrifugal force and then detach from the surface when the centrifugal force exceeds the critical force. Previous studies about the onset motions of droplets were conducted at the constant rotational speed of the disk. In this study, the deformation process of the droplets until sliding is investigated at various wettability and angular acceleration α = 50, 150 rpm/s by using a high-speed camera. The droplets of aqueous ethanol solutions are placed on substrates with various wettability. The droplets volume are V = 5, 10 μL. It is shown that the deformation and sliding processes of the droplets could be roughly classified into two types, which the droplet before sliding has a trail or not. However, there are droplets that cannot be classified into the two types. Those droplets are sometimes, but not always, trailing even under the same conditions. For classified droplets, the critical sliding contact angles depend on the initial contact angle, and the approximate equations for critical contact angles are shown as function of initial contact angle at each type. The Furmidge equation for critical sliding force on an inclined plate is found to be valid for droplets on a rotating disk. Therefore, if the deformation process before sliding can be classified, the sliding critical force can be predicted from the static parameters, which are the initial contact angle and the initial droplet length (Diameter of droplet).

Void wave, which is a spatio-temporal fluctuation of the void fraction of air bubbles in turbulent flow of water, appears beneath the bottom of a ship with an air-lubrication system when the frictional drag is reduced. Controlling the void wave has a potential to improve the drag reduction rate by air-lubrication. However, it is hard to generate the void wave in a normal channel facility generally used for laboratory-scale experiments on air lubrication. As the reason for this, it is expected that there is the boundary layer without spatially development in such channel flow. We, hence, designed an expanded channel facility where the boundary layer thickness developed and realized the generation of the void wave. To estimate the spatial development of the generated void wave in our channel, we observed the void wave at several positions in the streamwise direction by a high-speed camera and then, analyzed the frequency spectrum of the void fraction by Walsh transformation. Two dominant frequencies of the void wave were revealed at the midstreamposition regardless of the superficial velocity of gas phase, and the frequencies decreased in the downstream-position.

The two-fluid jet method, which impacted droplets accelerated by a high-speed air stream, is widely used in cleaning the semiconductor manufacturing process. A wafer must always be covered with a liquid film during the cleaning process because local drying may cause some defects. However, the liquid film thickness formed by the high-speed airflow and droplets and whether a liquid film exists on the impingement surface have not been investigated. In this study, we tried to measure the liquid film thickness using an optical fiber probe and a laser displacement meter. We discuss the return light signal from the optical fiber probe installed on the surface irradiating the two-fluid jet.

The ice accretion on an aircraft threatens navigation safety because it causes deformation of the wing shapes and reduces the aerodynamics performance. In the glaze ice conditions, where the ambient temperature is about -10 to -3 ℃, impinged droplets do not freeze instantly, and they runback along the wing surface as a liquid film. In addition, splashing and rebounding occur, which generate secondary droplets, if supercooled large droplets (SLD) whose diameters are more than 40 μm impinge on an aircraft. Furthermore, liquid film on an aircraft wing at glaze ice conditions significantly affects these phenomena. The purpose of this study is to model the droplet impact behavior and the formation of secondary droplets on a wall with a thin water film in terms of SLD icing under glaze ice conditions by using the three-dimensional E-MPS method. In a vertical impact case, the crown is formed by the impingement, and no secondary droplet is generated by the finger jets. The crown height decreases with decreasing water temperature, which is attributed to the decrease in Weber number. In an oblique impact case, the total mass of secondary droplets increases as the impact angle decreases. The secondary droplet mass generation attains the maximum at about 39 degrees impact angle. Moreover, the secondary droplets do not appear as the impact angle decreases around 30 degrees, resulting from Kelvin-Helmholtz instability. The velocities of secondary droplets decrease as the secondary droplets are generated later.

Two cavitation bubbles generated by laser irradiation in water generate microjets due to their mutual interference. The characteristics of microjets vary depending on the size and distance of the bubbles, as well as on the time difference between bubble generation and microjet generation. Therefore, the purpose of this study is to clarify the effect of the bubble generation time difference, T^*_D in dimensionless form, on the bubble motion and microjet by numerical calculations and experiments. Numerical calculations are performed using the boundary element method and the VOF method, both of which have conflicting characteristics, and two numerical methods are used for efficient analysis. In this study, cavitation bubbles are generated at the top and bottom, and the lower bubble that is generated first is bubble A, and the upper bubble that is generated after a delay is bubble B. When the distance between the bubbles and the bubble size ratio are kept constant and the bubble generation time difference is varied, it is found that there are three types of microjets generated in bubble A due to the mutual interference between the two cavitation bubbles. Upward microjets are generated in bubble A and penetrate upward when the bubble generation time difference is small. This occurs when the expansion of bubble A is slightly disturbed, causing the underside of bubble A to become concave. A downward microjet is generated in bubble A and penetrates downward when the bubble generation time difference is large. This is caused by the upper side of bubble A being pushed in due to the strong influence of the expansion of bubble B. When the bubble generation time difference is in the range of 0.16 ≤ T^*_D ≤ 0.20, bubble A is separated into two parts, resulting in the creation of upward and downward microjets. This is caused by the predominant horizontal nipping of bubble A.

Urolithiasis is a disease in which stones form in the kidneys, ureters, bladder, and other organs. Transurethral lithotripsy (TUL or URS) is one of the treatments for urinary lithiasis. Stone crushing is caused by laser-induced bubble collapse and laser-induced heat generation, which may cause damage to the surrounding tissues and complications. In this study, we observed the collapse behavior of laser-induced bubbles that formed near the wall with different hardness. The results showed that the bubbles collapsed while moving toward the rigid wall during the collapse.

Simultaneous high-speed observations in two-directions and pressure measurements using a Fiber Optic Probe Hydrophone (FOPH) are conducted to investigate the collapse of a laser-induced bubble near a rigid wall and the corresponding impulsive pressure distribution. It is shown that the measured pressure distributions next to the wall and the observations of top and side views reveal a strong correlation with the pitting damage due to the bubble collapse by Philipp and Lauterborn (1998). The results also show that asymmetric bubble collapse is observed at the second collapse.

The asymmetric toroidal bubble collapse causes the asymmetric pitting damage distribution.

Fiber-optic probes have advantages such as high time resolution on the order of GHz, high spatial resolution on the order of hundreds of microns, corrosion resistance, heat resistance, and pressure resistance. Due to these advantages, application to fluid pressure measurement has been studied. This is useful in the medical field for the development of ultrasonic therapy equipment. However, the signal change of the fiber-optic probe is small concerning the pressure change of the liquid, and it is difficult to measure with high sensitivity. Here, we focused on the tip shape of the fiber-optic probe and investigated the effect on the sensitivity. This study prepared the types of cleave, convex sphere, cone, and truncated cone. We experimented on each shape by immersing it in 6 different concentrations of glycerin-water solutions. We found that the cleave and the convex spherical surface polished with a roughness of 0.02 μm have good sensitivity. Moreover, the cone showed a sensitivity exceeding the theoretical line. From there, the surface roughness and shape of the probe should be selected correctly in pressure measurement.

High Intensity Focused Ultrasound (HIFU) is utilized for a minimal invasive method to ablate tumors by the temperature rise at the focal point of the focused ultrasound. In HIFU therapy, the treatment time tends to be long due to the narrow region of the ablation. Hence, the ways to enhance the efficiency of HIFU therapy have been studied, and the usage of a dense cavitation bubble cloud is expected to enhance energy dissipation from acoustic energy to heat. However, the cavitation cloud may cause the damage of surrounding tissue due to shockwaves when it collapses. Therefore, it is necessary to control cavitation clouds for effective usage. Ogasawara et al. (2018) investigated the cavitation cloud formation in water by using a laser-induced bubble as a reflector of HIFU. In the present study, we investigate the influence of medium elasticity on the cavitation cloud formation by the backscattering of HIFU from microbubbles created by a focused laser beam in the elastic medium. As a tissue mimicking phantom, agarose gel is chosen because of its high optical clarity for photography and its wide usage in tissue engineering. Medium elasticity varies from approximately 2.7 kPa to 200 kPa. By using residual microbubbles in the gel as reflectors of HIFU, the cavitation cloud generation point can be controlled. It is shown that as gel stiffness increases, the maximum value of the top interface position of cavitation clouds decreases and the collapse time of cavitation clouds becomes shorter.

In recent years, Additive Manufacturing (AM) technology such as 3D printing has attracted much attention. In order to produce high-quality metal powders as printing materials, it is necessary to elucidate the phenomena of liquid metal splitting accompanied by solidification, which is manifested in the powder manufacturing process. So far, various experiments have been conducted on the atomization method, but the mechanism for determining powder properties has not been clarified. In particular, there is insufficient knowledge about the phenomenon of molten metal splitting along with solidification. Therefore, in this study, we investigate the breakup process of a single ligament of 42Sn-58Bi, which is a kind of fusible alloy with a melting point of 139 ℃. A high-speed camera is used to visualize the pinch-off process of the ligament. The variation of the neck diameter of the ligament is traced during the fragmentation process. When the initial temperature is high, we found that the neck diameter thins down following a power law of 2/3, corresponding to the trend derived from capillary time scale. On the other hand, as the initial temperature gets lower, the neck diameter becomes thinner than that predicted by the capillary time scale just before breaking up, which is attributed to the influence of solidification.

Non-uniform distribution of interfacial mass flux across an evaporating droplet will subsequently induce a temperature gradient across the liquid-air interface, and result in thermal Marangoni stress that reforms the flow field inside the droplet. Recent study (Shiri, et al. Phys. Rev. Lett. 2021, Yang, et al. Langmuir 2022) confirmed the role of thermal Marangoni effect on the shape of evaporating single component droplets on completely wetting substrates. Nevertheless, a comprehensive evaluation of the interacting mechanisms is still lacking due to the intrinsic limitation of experimental techniques to decompose the different influence factors. To this end, we formulate a mathematical model to simulate the evaporation of volatile droplets on completely wetting substrates based on the lubrication theory. With this model, we elucidate the interacting physical mechanisms that governs the droplet kinetics during phase change, which includes the capillary effect, the thermal Marangoni effect, the interface motion due to evaporation, as well as the removal of energy barriers by precursor film. A phase diagram is summarized in terms of the Knudsen number and liquid-to-solid Biot number, which quantifies the prevalence of the interacting effects.

The flow of a liquid film sheared by gas stream in a horizontal rectangular bent duct was investigated using a high-speed camera. Compared to the straight horizontal duct condition, the local gas-phase velocity distribution in the bend and the effects of interactions such as reflections of waves impinging on the wall remain unknown. In this study, we investigate the unique scattering and splitting behavior that occurs at the bend by varying the gas-phase flow velocity and the bend angle of the rectangular duct. Specifically, Bag breakup was quantitatively evaluated using We number and compared with Bag breakup generated from a single droplet using characteristic time t^*_bk. The results showed that Bag breakups were unevenly distributed due to bend effects, and the We number varied depending on the location of the Bag breakup. It was also found that the possible range of the characteristic time of bag breakup was smaller than that of a single droplet.

Microbubbles are tiny bubbles with a 1–100 μm diameter. They have unique chemical and physical characteristics compared to ordinary bubbles that have diameters on the order of several tens of micrometers．However, the effect of generation method on microbubble characteristics has not been fully understood and these findings can have important implications in a wide range microbubble applications. The aim of this study is to investigate the characteristics of microbubbles generated by different generation methods and evaluate the application effect. Two types of microbubble generator (shear type and pressurized dissolution type) were used in this experiment. The shear type generator used in this study was a slit shear type generator developed in our laboratory. It was confirmed that microbubble size distributions and the microbubble adsorption to substances in water were affected by the generation method.

Visualization experiments are performed to examine nucleation, growth, and detachment of bubbles at hydrophobic surfaces of a beer can. Solutions of our concern are carbonated water and beer. The solution under cooling is poured into a (hydrophilic) glass container; the container is then sealed. A piece of a beer can whose inner coating is hydrophobic is inserted inside the container. To trigger gradual growth of bubbles nucleated preferentially at the piece of the beer can, dissolved gas supersaturation is created by heating the solution (with the container being sealed). From the visualization experiments, we report on the evolution of bubble population and size (from the front view) and the shape of attached bubbles (from the side view) at surface, exploring the role of surface tension and proteins in detachment of the bubbles.

Liquid film flow widely appears in the industrial field, affecting the machinery's reliability and lifetime. The notorious erosion in the steam turbines is one example, which derives from coarse droplets atomized by the liquid film on the turbine blade. There are fewer methods for measuring the liquid film in such a condition directly. We have proposed an Optical-fiber-based reflective probe (ORP) for this issue. We demonstrated a thickness measurement for the liquid-film flow driven by airflow and found an uncertainty caused by the local curvature of the interface. In this study, we investigated the signals delivered by the ORP during the measurement of the wavy film flow. The ORP measured the wavy interface simulated by a stainless-steel specimen machined into a sinusoidal waveform. We also conducted high-speed visualization and LIF imaging to confirm their positional relationship; then, we compared the time-series variation of the film thickness and the ORP signals. As a result, we found the ORP has a significant sensitivity to the interface's angle even, which is hard to visualize. Further, the waveform of the ORP signal depends on the intensity distribution of the light emitted from its tip. These are beneficial information for optimization of the ORP for future studies.

In the aerospace field, liquid hydrogen and oxygen which are “cryogenic fluid” are used as the propellant for rockets and hypersonic engines. These fluids become easily a gas-liquid two-phase flow in pipes and fluid machines. We have developed small capacitive void fraction sensors which can measure in real time, which is used to control the flow rate of cryogenic fuel and to understand typical two-phase flow phenomena such as the cavitation. The conventional capacitive sensors have a structure in which two electrodes sandwich a pipe made of dielectric material. However, the dielectric material has problems such as insufficient strength under high pressure condition and decreasing measurement accuracy due to changes in dielectric constant with temperature change. We developed a new capacitive sensor which consists of only metal parts to solve these problems. On the other hand, it may cause a decrease in measurement accuracy due to the structural limitation. This paper presents the results of the electric field analysis and the static fluid experiment using silicon oil and air to verify the accuracy of the sensor. The mean errors obtained by the analysis and the experiment are 3.47 % and 5.32 %, respectively, which almost satisfy the required specification. However, it was found that the accuracy is poor in some void fraction ranges, and the sensor needs to be improved.

In the cleaning and coating processes of product manufacturing, some cases require filling liquid into closed-end holes. We have developed a liquid infiltration method, i.e., gas removal, into closed-end holes by irradiating acoustic waves. The method uses gas oscillation inside the holes; however, the final removal process of break-up gas columns is still unclear. This study observed the multiple gas column oscillation inside a closed-end hole during acoustic wave irradiation and its removal process. We also visualized the liquid flow between columns using the PIV. As a result, we found that the gas removal depends on the size and number of gas columns. The removal case of air columns approached and coalesced and then ejected. The oscillation near the gas column near the hole exit induces the flow between columns, and the other gas column also oscillates. Therefore, the smaller flow velocity cases for small and long-distance gas columns could not achieve gas removal due to liquid inertia.