Reducing the aerodynamic drag of a competition bicycle-helmet is an important issue because it can affect race results. Since the riding posture of each rider is different, data for simulation analysis of fluid motion is obtained and analyzed for each rider. In addition, the regulations for racing helmets change frequently, so aerodynamic simulation of complex geometries must be performed in a short time span, including the pre-post processing. In this study, we evaluated the accuracy of LBM, which is an orthogonal mesh analysis, when applied to bicycle analysis, and discussed the problems involved. It was found that LBM analysis can achieve the same level of accuracy as NS analysis when the lattice resolution is the same as that of NS analysis. It was also confirmed that there are appropriate values for the object and grid size for object recognition, and that the time required for analysis can be reduced and the size of the computer required for analysis can be reduced by applying LBM.

In high-speed pantograph, it is important that achieving stable current collections and reducing aerodynamic noise. These characteristics are influenced by the pantograph head shape. We developed a multi-objective optimization system using LES and an evolutionary algorithm and conducted the optimization of a three-dimensional panhead shape considering flow between the pan support and the panhead to adjusting the lift force to the target and reducing aerodynamic noise. In the results of the optimization, we obtained the correlation of control points and objective functions.

Snow accretion on electrical wires occurs due to the surface tension of melted snow and ice bonds between the snow crystal and the wire surface. Heavy snow accretion can lead to wire breakage or transmission tower collapse. Additionally, asymmetrical snow accretion causes lift imbalance that can lead to galloping, increasing the risk of wires contacting each other and resulting in short circuits. To quantitatively understand these phenomena, numerical simulations have been conducted to reproduce snow accretion. Shimura et al. employed a particle-based method to predict snow accretion on electrical wires, achieving good agreement between predicted and experimental snow height. However, the previous model did not account for snow body motion, leading to non-physical long-term snow growth. In this study, we modeled the sliding motion of the snow body on the wire surface and conducted simulations to assess long-term snow accretion. We also computed the flow field around the electric wire to investigate the impact of the snow body on the flow field. Our results showed that considering the sliding motion model increased the mass of accreted snow due to the large projected wire area, consistent with the observed result. In addition, the force exerted by airflow on the wires differed significantly with and without the sliding motion model. These findings contribute to a better understanding and estimation of electrical wire galloping.

Owing to investigation of stability of ultra-fine bubbles against external stimulus, we measured number concentration as a function of particle diameter of ultra-fine bubble after temperature change and phase change. Approximately same experimental results until temperature = 50 °C were obtained within the experimental errors. Moreover, number concentration decreased for magnitude of 1/10 after phase change (ice-state). Thus, high stability of ultra-fine bubble was obtained. Furthermore, strongly stability of ultra-fine bubble by a surfactant additive exhibited. For discussing deeply, additional experiment was conducted. Compared with the case of ultrasound, it was found that stability of ultra-fine bubbles against several types of external stimulus.

As a new form of wind energy utilization, we believe that small vertical axis wind turbines that can be installed on the roofs of buildings are promising. When considering installation in residential areas, the ideal type of wind turbine should be a low-speed, high-torque type, that differs from conventional wind turbines in terms of noise and safety issues. In this study, wind tunnel tests were conducted on a small vertical axis wind turbine with NACA 4-digit blades were conducted, and the performance was evaluated based on the power coefficient at each blade shape and installation angle. As a results, it was confirmed that the power coefficient increased at low speed increased as the maximum camber increased. The effects of changing the maximum camber position and increasing or decreasing the maximum blade thickness were also examined.

Three-dimensional convective flow occurs in a single-sided heated rectangular channel depending on the heating temperatures. We investigated the structure and cause of the convective flow based on experiment and numerical analysis at different heating temperatures. In the experiment, a horizontal rectangular channel with controllable heating surface temperature was constructed and flow velocity was measured under multiple heating temperature conditions. Heating was applied only from the bottom surface, and flow velocities in the two-dimensional test section were measured using micrometer-resolution particle tracking velocimetry (micro-PTV). The result revealed the existence of three-dimensional convective flow with complex S-shaped velocity distributions in the spanwise direction of the channel and increased streamwise flow velocity near the heated bottom wall with the heating temperature. We attributed the three-dimensional convective flow to the buoyancy force caused by the density change of the working fluid due to heating from the sidewall surfaces. For verifying the deduction, numerical analysis was performed with the same geometry as in the experiment. In the analysis, the variations in density and viscosity were simulated with respect to temperature dependence. The analysis reproduced a rolled-up flow at sidewall surfaces when the temperature of the heating surface of the channel exterior was higher than that of the channel heating surface. In addition to these, we discuss the origin of the convective flow by considering different types of thermally induced flows including thermophoresis, Rayleigh-Bénard convection, and Marangoni convection.

To investigate the effects of wavelength of rough surface undulation on turbulent heat transfer over rough surfaces, we measured turbulent heat transfer for turbulent channel flow over three-dimensional sinusoidal rough walls with different wavelength. We used temperature-sensitive paint (TSP) to measure the temperature at rough surfaces. The rough surfaces were heated with constant heat flux, and the roughness Reynolds numbers based on the equivalent sand grain roughness were varied up to 1000. The results showed that the increase in heat transfer rate due to roughness is pronounced for rough surfaces with shorter wavelength. It was also confirmed that the temperature roughness function, which represents the downward shift in the logarithmic mean temperature profiles strongly depends on the wavelength of rough surfaces.

An experimental study was conducted on a transonic jet impinging on a flat plate. When a jet impinges on a flat plate, it generates a high-frequency noise called a screech, which causes oscillations in the flow field. In this study, focusing on the screech generated by a phenomenon called feedback loop, the frequency and sound pressure of a jet exhausted from a circular converging nozzle were measured using a microphone. The jets issued from a circular converging nozzle of diameter D. Experiments were conducted by varying the distance l from the nozzle to the plate and the ratio of pressure in the high-pressure tank to atmospheric pressure. From the obtained low frequencies of sound, the values of the coefficients related to the advection velocity of the disturbance were calculated. Using these coefficients, theoretical values were calculated and compared with the low frequency values obtained in another experiment. The comparison showed that the two values are very close, and this is the fundamental frequency of the screech. The reason for the discrepancy between the theoretical value and the experimental ones shown at certain conditions may be that the location of the plate shock wave moved from the expansion region to the compression region, resulting in a lower frequency.

This study experimentally investigated a radially spreading underexpanded jet impinging on the inner wall of a cylindrical object. When an underexpanded jet impinges on a flat object, the jet oscillates and radiates high-frequency noise. For this reason, studies have been conducted on rectangular or axisymmetric jets impinging on flat plates, which have shown a relation between noise and self-induced oscillation. However, not enough research has been done on radial jets impinging on inner wall of a cylindrical object. Therefore, the purpose of this study is to investigate the relationship between noise and vibration, as well as the characteristics of the pressure oscillation, in the case of radial jets. The jet issued from a convergent radial nozzle composed of a pair of circular tubes with a diameter D. The cylindrical object used for pressure measurement had an inner diameter D_a. The inner wall of object was equipped with eight pressure measurement holes evenly spaced in the circumferential direction, allowing for pressure measurements using transducers. Measurements of the jet's wall pressure and sound pressure levels were taken. The experiments were conducted by varying the nozzle pressure ratio from 2.0 to 4.8 in increments of 0.2. The results revealed that pressure oscillation and sound pressure levels varied at the same fundamental frequency in each condition. Furthermore, in a flow field where three shock cells were formed, the frequency at which pressure oscillations propagated circumferentially around the object was found to be about half of the fundamental frequency.

Supersonic jets are jets with flow velocities exceeding the speed of sound. They are used as assist gases for the laser cutting and cooling jets for heat-strengthened glass production. After impinging on an object, the jet spreads radially over the surface of the object at supersonic velocities. In this study, we experimentally reproduce a radially spreading supersonic free jet. The ends of circular tubes are placed facing each other, and a radial jet issues from the gap between them at a critical pressure ratio or higher. The resulting underexpanded radial jets are visualized and the noise emitted from the jet are measured. Visualization results show that the first cell length of the jet increases with increasing pressure ratio, and becomes shorter downstream. The acoustic measurement results show multiple dominant frequency components. The frequency components with a broadband are considered to be harmonics components of other frequencies. Using the frequency prediction equation, the proportionality constant was calculated from the experimental results. The frequencies and cell lengths that agree well with the proportionality constants for the rectangular jet are α = 0.44 for L₁ and f₂, and α = 0.66 for L₂ and f₄. The sound emitted from the jet is visualized as a density wave, so the frequency was calculated from the density wave, and the radiated sound from L₂ was found to be in good agreement with the frequency component of f₄.

An underexpanded jet radiates noise and oscillates when the jet impinged on the object. These phenomena, the radiated sound and oscillation patterns, depends on the shape of the nozzle and the object that is impinged. In this study, an underexpanded radial jet impinged on an inner wall of a cylinder. Visualization experiment using the shadowgraph method confirmed a strong density change due to the jet impingement. The location where the density wave was generated was analyzed. Therefore, it was estimated that the location was the cylinder end. Acoustic measurement confirmed several dominant frequency components. These frequency components are integer multiples of the fundamental frequency component. The impinging jet and flow pattern are analyzed using the formula of the relationship between the convection velocity of disturbance and the screech. The location of the density wave estimated through visualization is considered to be the sound source. We analyzed the frequency of the feedback loop that forms when disturbances interfere with this sound source. Therefore, oscillations at two and three times the fundamental frequency coexist in the flow field of a radial jet. It is also inferred that these oscillations affect a single frequency component. Thus, these are strongly related to the flow patterns of impinging jet and wall jet.

Reduction of air resistance is a very important design element in aircraft engineering, and at subsonic speeds, skin friction resistance accounts for half of the total resistance. Surface shearing stress can be measured by Oil Film Interferometry. In the OFI it is important to know the direction in which the shearing stress acts, and corrections must be made if the direction is different from the mainstream direction. In this study, the shearing stress direction was identified using a path of a center of the ring-shaped fringes, the OFI is applied the region upstream of the center of fringes, and the shear stress was estimated. The flow field along the flat plate was the object of measurement. The flow field was formed using a wind tunnel, and a tripping wire was placed in the nozzle rip of the wind tunnel to form turbulence. A drop of oil placed 320 mm downstream from the front edge of the flat plate gradually spread and was transformed into an oil film of about 15 mm in diameter over time. The center of the ring fringe moved linearly through the oil film for about 300 seconds after the wind tunnel was started. The film gradually became thinner upstream of the oil film and thicker downstream. The skin friction resistance was calculated and was underestimated by 5 % compared to the theoretical value.

Cellulose nanocrystals (CNC) are rod-like nanomaterials extracted by acid hydrolysis from cellulose, with lengths in the range of hundreds of nanometers. CNC exhibits unique properties such as high specific stiffness, light weight, and birefringence, making it a promising material for multifunctional applications, especially when dispersed in fluids. CNC aqueous suspensions show shear thinning behavior, and depending on concentration, they can form cholesteric liquid crystalline phases. The internal structure of the CNC suspensions was investigated using polarized light microscopy, and rheo-optical measurements were performed to study the structural changes under different conditions. Rheological measurements were conducted with a stress-controlled rheometer, and the flow history was carefully controlled to avoid measurement discrepancies. The suspension was exposed to a controlled shearing process and temperature variations during the experiments. Creep tests and hysteresis loop experiments were carried out at different temperatures to study the viscosity response under applied shear stress. Small angle light scattering (SALS) was also used to capture the scattering patterns during the rheological measurements, providing insights into the internal structural changes. The results indicated that at lower temperatures, the cholesteric domains were more prominent and influenced the flow behavior, whereas at higher temperatures, the viscosity decreased due to structural rearrangements. These findings highlight the importance of understanding both the transient rheological behavior and the internal structure for industrial applications of CNC suspensions.

When a surfactant is added to a colloidal suspension, the shape of the particle precipitate after drying changes depending on the type of surfactant. In this study, a one-way drying model that simplifies the drying phenomenon was introduced for suspensions with CTAB. the volume fraction measurement method using the Lambert-Bael rule was applied to colloidal suspensions with CTAB to evaluate the transition of the solid-liquid layer boundary due to one-way drying. In unidirectional drying, CTAB did not affect the rate of transition of the solid-liquid layer.

Fragmentation of magma is a fundamental process in volcanic eruption. Since Maxwellian viscoelasticity plays an essential role in magma fragmentation, we performed analog experiments on the fracture of viscoelastic worm-like micelle (WLM) solutions. In the experiment, the fracture caused by the movement of a cylinder inserted perpendicular to the free surface of the solution was observed by high-speed photography. Two fractures were observed, one in the fluid region and the other in the solid region. In the former, the fracture is caused by entrapped bubble in the wake of the cylinder. The fracture occurred when the aspect ratio of the bubbles reached a critical value regardless of the solution relaxation time and the diameter of the cylinder. This suggests that the free surface shape is an important parameter on the fracture.

Solid-like fragmentation of viscoelastic magma is an important factor in triggering explosive eruptions. This fragmentation occurs near the free surface of flowing magma. Therefore, we focused our attention on the fracturing phenomena that occur in the flow around a moving cylinder of a Maxwellian fluid, a wormlike micellar solution. The time variation of the free surface was measured by the fast checkerboard demodulation (FCD) method, a type of background-oriented Schlieren imaging. The results show that the curvature of the free surface just before fracture is similar for different diameters and moving speeds of the cylinder.

Jets are used in various fluidic devices, and it is necessary to understand jet control to enhance its performance for different purposes. Initial velocity optimization is one of the methods for jet control. Considering the feasibility, it is essential to clarify the impact and importance of optimizing the ejection angle, because it is relatively easy to change the magnitude of the velocity, but precise angle modification is very challenging. Therefore, this study aims to clarify the impact of angle component of initial velocity optimization by using deep reinforcement learning together with computational fluid dynamics simulation. Two-dimensional incompressible fluid simulations were performed. The streamwise (x) and vertical (y) lengths are 15d and 20d, respectively. Here, d is the width of the jet exit and d=20mm. The fluid is the air and initial flow rate q₀ was fixed to 0.004 m²/s. In this case, the Reynolds number based on the mean initial velocity and jet width is 2700. As the jet temperature, T, was set to 303.15K while the initial ambient temperature T₀ was set to 298.15K. Figure 1 shows that the distributions are close to uniform in the cases of temperature maximization and velocity minimization. In contrast, in the other three cases, the distributions are significantly different from uniform. It is worth noting that the first two cases correspond to the optimization to suppress entrainment, and the rest correspond to the optimization to promote entrainment(1). With respect to the spatial distribution of the velocity, in the cases for entrainment promotion, although the number of peaks and positions are not the same among the three cases, the large velocity generally directs inward (toward the centerline) in the case with angle optimization. On the other hand, when it is aimed at suppressing the entrainment, the velocity at the edges is large and tends to direct outward.

Round jets roll up the shear layer near the nozzle exit and form vortex ring street. The formed vortex ring street then merge to form large vortex rings. The large-scale vortex rings eventually break down and become disordered due to the azimuthal instability of the vortex rings. Since the formation of vortex rings is a periodic phenomenon based on the instability of the shear layer, acoustic excitation, which is one of the active control methods using periodic sound waves, is highly effective for controlling vortex rings. Acoustic excitation can cause the regular merging of vortex rings, which occurs irregularly in the natural transition of jets. We have previously shown through experiments by flow visualization that four consecutive vortex rings can be merged by acoustic excitation with a single sinusoidal wave of a single frequency, and have shown the condition of Re and St of jets for the merging of four vortex rings. The streamwise distance between the vortex rings when the four vortex rings merge has been presented. Here, PIV measurements were conducted to investigate the advective velocity of the vortex rings in the above merging process. As a result, we found that there was a significant difference in both the distance between the merged vortex rings and the advection velocity when the four vortex rings merged compared to when they did not.

The spray injection of lycopodium powder, which simulates apple pollen, was numerically simulated with a Lagrangian discrete phase model (DPM) to design a rotary-wing type UAV for the artificial pollination of apples. The lycopodium powder stored in a powder tank was suctioned by airflow through an inner air nozzle and injected through the injection nozzle. The computed results showed a good agreement with the injection experiments in terms of spray foam. However, further investigation was required on the flow rate of the powder and unsteady spray foam variation.

Dynamic behavior of vortex ring is the generation of fluid force by the motion and elastic deformation of objects, that why the grasping the behavior of vortex ring is very important. That behavior is measured by the visualization of the flow field around the moving object mainly, and from those results in previous research, it is clarified that the growth of vortex rings is effected by the acceleration and elastic deformation of object. Those results are measured directly, then, there is no established method yet to theoretically grasping the growth of vortex ring using physical property that represent the motion and elastic deformation of object. If it becomes possible to theoretically grasp the growth of vortex ring based on the motion and elastic deformation of objects, it will be possible to indirectly grasp the growth process of a vortex ring from the motion and elastic deformation of object without measuring the flow field of the fluid. In this study, we attempted to theoretically capture the growth of vortex rings generated by downstroke and upstroke of butterfly wing, which occur the different elastic deformation of wing, in terms of the fluid force of added mass flow.

Numerical analysis of flow around vegetations with different heights was carried out to reduce windthrow damage. The velocity, pressure distribution and drag depending on the wind direction and the wind velocity were investigated by steady and unsteady flow simulations. The tree models were arranged in a configuration based on previous experimental setups, and the analysis was carried out using the RANS method for steady flow simulations and the LES method for unsteady flow simulations. The results of the steady flow simulations showed that the drag coefficient of the trees varied depending on their position relative to the wind direction. The drag acting on the tree in the wake behind another tree with small crown tends become more larger. The results of the unsteady flow simulation showed that vortex shedding and vortex interaction with trees affected acting force of the transverse direction. The pressure drop associated with vortex formation was found to significantly influence the force acting on the trees.

In this study, in order to understand the vortex shedding characteristics from an asymmetrical bluff cylinder, the flow features of vortex shedding from a square cylinder oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/fK=1~6, amplitude ratio 2a/d=0.67 and 1.00, and main flow velocity U=0.0654m/s. Experimental apparatus consists of the closed circuit water channel apparatus, an oscillating apparatus, a visualization apparatus, and a video recording apparatus. Use those equipment, the variations of mean vortex shedding frequency were investigated and the existence range of the lock-in was shown. As a result of the experiments, Even if it used the cylinder of asymmetrical cross-sectional shape for the flow axis, producing the lock-in phenomenon was observed. It was shown that the lock-in range is in the tendency which will narrow if oscillating half amplitude becomes large. It was found that distribution of the flow pattern of the lock-in region and changes process change with degrees of angle of attack.

The present paper describes the development of the magnetostrictive flow-induced vibrational power generator using a circular cylinder. Effects of a fin plate behind a cylinder and ribs on the side surface of a cylinder on the vibrational characteristics and the power generation of its generator were investigated by the wind tunnel test. The fin height L_f was changed from 0.25L, 0.5L, 0.75L, to 1L, and the rib height W_r was also changed from 1, 2, 4, 6, to 10 mm. The vibration for circular cylinders with a fin plate of L_f/L = 0.75 and 1 begins from Vr = 7. The vibration ceases at Vr > 14. Although the reduced velocity region for vibration increases more than without a fin plate, the maximum amplitude decreases less than without a fin plate. The onset velocity for a circular cylinder with ribs is Vr = 3.6. Although the reduced velocity region for vibration does not increase more than without ribs, the power generation for a circular cylinder with ribs of W_r ≥ 6 mm increases more than without ribs.

In recent years, there is increasing anticipation for the development of unmanned micro air vehicles (MAV) capable of exploring narrow spaces within collapsed buildings. Attention has focused on insect-inspired flapping wing MAV, which exhibits excellent aerodynamic performance and stability at small scales. In particular, configurations with four flapping wings as dragonflies are referred to as tandem flapping wing configurations. There are various flight mechanisms within the tandem flapping wing flight, and these have not yet been fully clarified. The objective of this study is to conduct computational fluid dynamics (CFD) analyses around the tandem flapping wings and to investigate the influence of the positional relationship between the fore and rear wings, determined by the distance and the phase difference between the fore and rear wings. In this study, we conducted analyses by varying the distance, which is normalized by the chord length from 0.2 to 1.0 in 0.1 increments and the phase difference between the fore and rear wings from 0 to 360 deg in 10 deg increments. CFD analyses were conducted based on the three-dimensional incompressible Navier-Stokes equations and continuity equation. This study focused on a hovering condition without uniform flow. The obtained results indicated that the thrust force of the rear wing was sensitively influenced by positional relationship between the fore and rear wings. Result showed that the phase difference changed the effects obtained from negative pressure regions formed in the lower side of the fore wing. The smaller the phase difference, the larger the effects from the negative pressure regions when the rear wing follows the fore wing, and the highest thrust force was obtained at the condition of φ=30 deg. In addition, higher thrust force was generated with smaller distance between the fore and rear wings, and the highest thrust force was obtained at the condition of d= 0.2.

Accurate non-intrusive force measurement is often challenging, particularly in applications involving animals and vehicles. This paper presents a non-intrusive technique leveraging the vortex force map (VFM) method, which calculates forces from snapshot velocity and vorticity fields obtained through particle image velocimetry (PIV). The VFM method is applied to two different kinematic families: surging flat plates and pitching NACA 0018 airfoils at Reynolds numbers of O(104 ). These flowfields are characterised by the shedding of coherent leading-edge and trailing-edge vortices. In both scenarios, the VFM method demonstrates remarkable agreement with direct force measurements. Additionally, we offer physical interpretations of the relationship between forces and vortical structures, drawing on visualised force contributions from each vortex. Notably, the VFM method exhibits high robustness to noise, a crucial feature in experimental fluid mechanics.

An aeroelastic analysis system for a vertical-axis butterfly wind turbine (rotor diameter: 14 m) is under development using MATLAB Simulink/Simscape. A method is proposed to extract the forces and moments acting at any part of a multibody model for structural analysis using a weld joint block. The behaviors of the rotor rotational speed and the movable arm tilt angle in a constant or turbulent wind, as well as the axial force and moment acting on the joint part between the hub and a slant blade, were analyzed for a model incorporating a suspended movable arm type over-speed control system. In a constant wind of 7 m/s, the three movable arms tilted almost simultaneously, however in a turbulent wind, only one movable arm tilted. The average values of the span-wise force and bending moment at the joint part between the upper slant blade and the hub were almost equal in constant and turbulent flows, however the fluctuation amplitude depends on the rotor rotational speed.

In this study, two Savonius wind turbines were placed in close proximity, and their relative positions were varied to investigate the effect on wake profile. Each turbine consists of three semi-circular blades, and the analysis was conducted by changing the combination of the rotational direction of the turbines and the distance between them as parameters. First, the temporal variation of the instantaneous velocity profiles was obtained for each arrangement across different measurement regions. Next, a Proper Orthogonal Decomposition (POD) analysis was performed on the data. To extend the POD analysis to the entire measurement region, two-point velocity correlations measured in each local region were combined. The results showed that the velocity deficit and its fluctuations were minimized when the turbines were spaced 1.5D, where D is turbine diameter, apart in a counter-rotating configuration, where the gap flow moved upstream. Furthermore, a comparison between the POD modes of each local region and the entire region revealed that the mode profiles were consistent with one another. These findings suggest that information extracted independently from local regions can effectively represent the temporal and spatial variations across the entire measurement region.

The cylinder blade wind turbine is driven by longitudinal vortex generated near the intersection of the cylindrical blades and the wake plate. In this study, the effect of blade pitch on steady lift force was examined through fluid analysis. The lift force calculated through fluid analysis indicated that reducing the blade pitch decreases the lift force per blade. While increasing the number of blades enhances the overall performance of the wind turbine, once the blade pitch exceeds a certain threshold, the driving force decreases. Visualization of the flow field revealed that blade pitch affects the stable formation of longitudinal vortex, causing fluctuations in lift force.

The lift force of the cylinder blade wind turbine driven by longitudinal vortex improves with an increase in blade diameter d and ring plate width W, but existing wind turbine systems don't account for the differences that arise when transitioning from a cylinder parallel movement system to a cylinder rotation system. Therefore, this study aims to clarify the effect of ring curvature and propose performance prediction indicators based on representative geometric parameters. The lift force is evaluated through wind tunnel experiments by varying the ring plate diameter D in arbitrary combinations of d and W. As D decreases or W increases, the difference in rotational speed between the inside and outside of the blades within the ring should increase, causing the rotational speed of the wind turbine to relatively rise. However, it was found that when D becomes smaller than a certain value, the rotational speed decreases to balance with the form drag of the blades. It was found that when D increases and the longitudinal vortex generation region approaches the blade tip, the slope of the torque curve decreases at low rotational speeds. By mapping these results, it became possible to estimate performance based on geometric parameters.

Round jets have symmetric and asymmetric modes, and symmetric mode typically appears near the nozzle. However, by controlling the jet, asymmetric mode, i.e., helical mode, can be formed near the nozzle. Here, a round nozzle has three holes for the synthetic jets. The three holes are positioned at equal intervals around the circumference of the nozzle. By sequentially driving the three synthetic jets, a helical structure is formed near the nozzle exit of the round jet. The frequency of the synthetic jets is set to the frequency of vortex-ring formation in the natural transition. The results of numerical simulation using OpenFOAM and experimental results, such as, flow visualizations and PIV measurements, are presented for this flow field.

A device that reproduces breathing was created. This device was composed of an electric actuator and an air cylinder. The change and distribution of flow velocity at the nozzle outlet under the flow rate and period conditions of human breathing at rest were investigated. As a result, it was found that the flow direction changes between discharging and suctioning, and that the flow velocity during suctioning is slower than the flow velocity during discharging. The shape of the velocity distribution was almost the same as that of a circular jet. The velocity waveform during discharging was almost the same as that was observed the row of puff in an intermittent jet. It was suggested, therefore, that a leading vortex and a trailing jet are formed when the flow was discharging.

A ball mill is a type of grinding machine that grinds materials by the behavior of hard balls in a rotating cylindrical container. It can be divided into wet condition, in which liquid is added, and dry condition, in which liquid is not added. When comparing the two, it is said that the wet condition is superior to grinding speed and fine grinding, but the factor of the improvement of grinding rate by liquid has not been clarified from the behavior of the liquid and balls during the grinding process. In this study, the behavior of the liquid and balls was visualized by PIV and visual PTV measurement, and it was confirmed that the vortex flow generated at ω = 150 rpm coincided with the position of the dense balls. In addition, under wet conditions, it is considered that the difference in behavior of the hard balls between the outer and center parts is larger than that of the dry condition because the behavior of the balls on the outer part is promoted by the vortex flow. Therefore, it was expected that the number of contacts between the hard balls increases, and the grinding rate improves. Therefore, it is considered that the vortex flow by the liquid contributes to the improvement of grinding rate.

Blade-free planetary mixing is a method that mixes materials in a container by precessional rotational motion, which is a combination of the rotation and revolution of the container. This study investigates a method for optimizing the mixing conditions to improve the mixing performance of blade-free planetary mixer. A numerical analysis combining the volume of fluid method and the particle tracking method was used to evaluate the mixing performance of the blade-free planetary mixer for liquids with a free surface. The precession rate, which is the ratio of the rotation speed to the revolution speed, and the container angle were optimized using a genetic algorithm. As a result, the genetic algorithm searched for mixing conditions with high mixing performance with a relatively small number of cases For liquids with a free surface, mixing performance was improved when the rotation speed and revolution speed were the same level and the container angle was small. In addition, under these conditions, the liquid surface was significantly tilted and irregular swirling flows of particles in the fluid were observed.

In this research, the ultrasonic velocity profile (UVP) measurement method is applied to the narrow gap flow inside the radiator. A fundamental study of UVP flow measurements in simulated radiator samples was carried out with the aim of understanding flow and phase variations in the radiator. An understanding of the flow conditions in a radiator will lead to an improvement in the performance of heat exchange. However, it is difficult to measure the flow rate inside the narrow gap of a stainless-steel radiator. Laser Doppler Velocimetry (LDV) and Image Processing Velocimetry (PIV, PTV) are widely used fluid velocity measurement methods, but they are difficult to apply to measurements in vessels with small capacities because of the need for an optical measurement window. This research focuses on real-time flow measurement methods that employ ultrasound, which does not necessitate the use of an optical measurement window and can be applied to opaque liquids. A narrow gap flow sample composed of PVC resin and stainless-steel plate was manufactured to simulate a narrow gap in stainless-steel radiators. It could be used to verify the feasibility and accuracy of using UVP to measure stainless steel radiators. At the same time, PIV measurements are also used to measure the flow in the sample. The accuracy of UVP flow measurements in narrow gaps has been verified through a comparison between UVP measurement results and PIV measurement results.

A dielectric elastomer actuator was fabricated to realize a traveling wave-like wall that reduces turbulent friction drag. The actuator is rectangular, with one actuating part (electrode part) at one end of the rectangle, and the other parts are composed of elastomers only. The actuator was operated at high voltage, and the displacement was measured with a laser displacement meter. The results showed that the actuating part with electrodes on both sides of the elastomer showed a displacement of 100 μ m at the input frequency. This displacement was sufficiently more significant than the change in the thickness of the elastomer layer estimated from the Coulomb force acting between the electrodes and is thought to be the result of the elastomer sheet itself being deformed by the force of area expansion due to the constant volume of the elastomer. When the electrodes were operated at a higher frequency (100 Hz), the displacement was about 10 μ m. The vibration generated at the electrode part propagated downstream, and the 100 Hz vibration was dominant even at a position more than 100 mm away. The change in phase difference between the electrode part and an arbitrary position in the streamwise direction demonstrated the generation of a 100 Hz traveling wave.

Streamwise vortices increase friction drag in a turbulent boundary layer by vigorous momentum transport near the wall. This study aims to clarify the change of vortical structure with decreasing turbulence in turbulent boundary layer flows where drag is reduced by traveling wavy control. A three-dimensional velocity field was constructed for a flow with a developed turbulent boundary layer using stereo PIV of the channel cross-section multiple times and the Taylor hypothesis. Although traveling wave control did not eliminate the streamwise vortices throughout the entire region, the vorticity in the streamwise direction tended to decrease. Furthermore, the instantaneous vorticity fields at each cross-section revealed that the streamwise vortices were restricted below y+ < 50 and that their shape and position in the wall-normal direction changed according to the wall deformation.

When riblets are used in an atmospheric environment, the adhesion of particles to the grooves reduces the friction reduction performance. In this study, we aimed to clarify the change in particle adhesion characteristics due to the superhydrophobic surface, to verify the effect of the superhydrophobic surface on the cleaning effect of accumulated particles, and to clarify the change in particle adhesion characteristics due to the presence of a superhydrophobic surface. The cleaning effect was evaluated by spraying water mist onto the riblets, counting the number of particles attached before and after cleaning using image analysis, and performing statistical processing. The riblets with the superhydrophobic surface had a removal rate of 90.5%, while the normal riblets had a removal rate of 88.1%, showing no significant difference. It is thought that the shape of the riblets affected the behavior and surface structure of the water droplets.

Direct numerical simulations of a plane-impinging jet were carried out to investigate the suppression of heat transfer and mixing promotion by pre-combustion in a gasoline engine combustion chamber. Combined structures of ribs perpendicular to the mean flow and riblets parallel to it were installed on the lower wall of the flow system. When only ribs were installed, the jet inflow between the ribs and the flow along the bottom wall was prevented, and the turbulent energy near the wall decreased. As a result, it was revealed that the total heat transfer was reduced. When only riblets were installed, the riblets reduced the turbulent energy near the wall. However, as jets flowed between the riblets, the contact area with the high-temperature fluid increased, increasing total heat transfer. The structure combining ribs and riblets did not achieve a heat transfer suppression effect compared to the rib-only case because the increase in the heat transfer surface due to the installation of the riblets exceeded the ribs' heat transfer reduction effect. The heat transfer suppression effect might be further increased by installing riblets designed to more effectively prevent the inflow of jets between the riblets in the turbulent flow behind the ribs.

It was well known that the boundary layer has a preferred wavenumber of perturbation. Therefore, it may be possible to delay the transition to turbulence and reduce the turbulent frictional drag by artificially deforming the hairpin vortex to narrow the spacing in the flow direction and increase the wavenumber. The purpose of this study is to investigate the differences in the downstream development of the hairpin vortex when it is uniformly deformed in the width direction by obstacles placed on the wall surface. The results obtained show that the boundary layer was also unstable for obstacles with a height of about the displacement thickness of the boundary layer, and that they were effective in promoting transition to turbulent flow of the hairpin vortex, regardless of the cross-sectional shape of the obstacle. However, only for a triangular obstacle with a height 10 times the viscous length, the development of the hairpin vortex was slightly weakened. From these results, it was found that the height of the obstacle should be limited to less than 10 times the viscous length to deform only the hairpin vortex, and the gradient of the leading edge of the obstacle should be gentle to weaken the development.

It was well known that in the process of breakup of a vortex structure, the transition to turbulent spots rapidly progresses triggered by the generation of vortices in the spanwise direction. However, since multiple hairpin vortices exist even just before the breakup, it is not possible to confirm whether the vortices generated in the spanwise direction are the direct cause of the breakup. In addition, it is interesting to observe whether reconnection or marge occur of vortices when multiple hairpin vortices are in the vicinity. Therefore, to simplify this phenomenon, we artificially generated two hairpin vortices close to each other in the laminar boundary layer and investigated behaviors that occur between these vortices pair. In this study, we investigated two cases in which the distance between the hairpin vortices was about the width of the vortex and about twice the width of the hairpin vortex. If the distance between the two hairpin vortices was Δz+=40, which is about the same as the width of the vortex, the heads of the two hairpin vortices approach each other and join together. On the other hand, if the distance between the two hairpin vortices was Δz+=59, which is about twice the width of the vortex, the two hairpin vortices did not approach each other and traveled parallel to each other. Therefore, it was clarified that the two hairpin vortices joined when the distance between the two vortices was within twice the width of the vortices. Furthermore, the existence of a high-speed region in the center of the joined vortices implies that the two hairpin vortices did not merge into a single hairpin vortex, but rather that the legs of each vortex reconnected and formed a duplex vortex structure consisting of a large hairpin vortex with a counter-rotating U-shaped vortex inside it.

Some polymers are highly susceptible to degradation caused by mechanical shear forces. This study focuses on the impact of stirring power during solution preparation on the turbulent drag reduction effect. PIV measurements of turbulent boundary layer flow were conducted to clarify the influence of stirring rotation numbers on drag reduction by injecting polymer solutions with different stirring rotation numbers. Extensional viscosity measurements were also performed to investigate the effect of mechanical degrada tion on the rheological properties of the polymers. Based on these insights, we investigated the relation between the mechanical degradation of polymers and their drag reduction effect.

We examined the air lubrication by bubble injection with a new experimental system simulating a zero-pressure-gradient turbulent boundary layer beneath a moving belt at the water surface in a lab-scale facility. By varying the air injection rate and wall speed, we estimated the impacts of the injected bubble on turbulence statistics and wall frictional drag. Velocity field measurements showed an increase of mean velocity indicating the reduction of the wall frictional drag, which was qualitatively validated by measurements of the torque on the motor driving the belt. This means that we can estimate the local drag reduction from the mean velocity increment. The Reynolds shear stress was suppressed for the case of the highest wall speed for which a detailed investigation on instantaneous velocity fluctuation is ongoing.

An experimental study of pulsation control for reducing frictional drag in turbulent pipe flow was conducted. Some numerical studies have demonstrated that relaminarization was observed during the entire period of acceleration and deceleration. Although turbulence suppression is observed, it is challenging to achieve relaminarization in experiments. Our aim is to consider pulsation control to settle the flow into a completely laminar flow. PIV measurements were performed to reveal the difference between the enhancement and suppression of turbulence corresponding to acceleration and deceleration. In this study, we measured turbulent flow fields subjected to a pulsation that exhibited a sufficiently high drag reduction effect. The results showed that the Reynolds shear stress decreased in the pulsating flow compared to the uncontrolled flow except near the wall in the time-averaged statistics, which indicates that the turbulence was suppressed. In addition, from the coefficient of frictional drag in the uncontrolled and controlled conditions, it is possible to show that the turbulence decreased during acceleration and during deceleration, and there was a tendency for re-laminar flow. However, the turbulence did not completely disappear and increased again in the late deceleration phase. These results suggest that turbulence decreases during the acceleration phase and tends to relaminarize in the early deceleration phase, leading to a drag reduction.