In this study, turbulence structures of the drag-reducing turbulent boundary layer in the surfactant solution at the momentum-thickness Reynolds number Re_θ≃800 were compared between at the large and small drag reduction ratios. Turbulence statistics and structures were obtained by the two-component LDV system and the PIV system for the streamwise and wall-normal (x-y) section and the streamwise and spanwise (x-z) section. The surfactant solution used here was a mixture of cetyltrimethyl ammonium chloride (CTAC) with sodium salicylate as counterion, which was dissolved in tap water. The concentration of CTAC was 65ppm, and the molar ratio of counterion to CTAC was two. The drag reduction ratios DR=78% and 34% were obtained at the solution temperature T=30℃ and 35℃, respectively. The quadrant analysis of streamwise and wall-normal velocity fluctuations revealed that at DR=78%, the positive contribution of the sweep and ejection events to the Reynolds shear stress was comparable with the negative contribution of the inward-interaction and outward-interaction events, so that the Reynolds shear stress was almost zero. At DR=78%, both streamwise and spanwise velocity fluctuations were attenuated largely across the turbulent boundary layer. On the other hand, at DR=34%, turbulence structures were similar to those of water as a whole, although the scale was larger compared to the water.

View full abstract

Due to a translational motion of a dispersed body (e.g., cylinder and particle), the pressure averaged over its wall reveals lower value than in the bulk. Such a depressurization in a uniform flow around a circular cylinder is numerically investigated. In particular, the relevance of the Karman vortex street is discussed. A dimensionless number C_<TDP>, which characterizes the level of the depressurization scaled by the fluid density and the stream velocity, is determined for the Reynolds numbers Re&le;160 in a two-dimensional flow regime. A strong dependence of C_<TDP> upon the hydrodynamic unsteadiness is found: i.e., in the steady state with flow separation (6<Re<47), the slope of C_<TDP> versus Re is reduced with the Reynolds number, while beyond the onset of the unsteady flow at Re&ap;47, the depressurization is enhanced due to the appearance of low pressure regions forming inside the shedding vortices. Both the phase and amplitude in the temporal oscillation of C_<TDP> are well correlated with those of the drag coefficient. The correlation expressions of C_<TDP> are provided as classified according to the flow regimes.

View full abstract

We investigate the fluid-acoustic interactions in fluid-dynamic oscillations of a flow over a two-dimensional cavity by directly solving the compressible Navier-Stokes equations. The upstream boundary layer is turbulent and the depth-to-length ratio of the cavity is 0.5. To clarify the effects of the freestream Mach number on the fluid-dynamic oscillations, we perform computations for the two freestream Mach numbers of 0.15 and 0.3. The oscillations occur with the Mach number of 0.3, while the oscillations do not occur with the Mach number of 0.15. For the Mach number of 0.3, phase-averaged flow fields show that large-scale vortices form in the shear layer. When the large-scale vortex collides with the downstream wall, the low-pressure region spreads along the downstream wall. As a result, the local pressure gradient induces a local downstream velocity, causing the upstream fluid to expand. Finally, an expansion wave propagates to the outside of the cavity. In order to clarify the mechanism for the formation of the large-scale vortices, we also perform the computations of backward-facing step flows with an artificial acoustic source. As a result, it is shown that the large-scale vortices originate from the convective disturbances, which are generated by the acoustic waves. The disturbances grow due to the Kelvin-Helmholtz instability, similar to the growth of the disturbances in a laminar cavity flow. Also, these computations of the backward-facing step flows clarify the reason why oscillations do not occur in the cavity flow for the Mach number of 0.15.

View full abstract

This experimental research concerns the air-bubble-jet flow in water. We try to apply three-dimensional particle tracking velocimetry to the flow. Air bubbles are regarded as tracer particles. Then, we visualise the three-dimensional motions of the bubbles. As we record the stereo images using two high-speed video cameras, we can get timely-consecutive information of the bubbles' motions. As a result, the three-dimensional structures of two simple bubble-jet plumes with different nozzle's tilting angles have been quantitatively revealed near the nozzle exit.

View full abstract

Recently, optical coherence tomography (OCT) has been being applied to cardiovascular medical diagnosis as a tomographic visualizing technique of unstable atherosclerotic plaque with high resolution of 1 to 10 micrometer. Nevertheless, the vulnerability associated with macrophage invasion into atherosclerotic plaque can never be in vivo detected by any diagnostic approach. Previously, we proposed 2-Color Optical Coherence Dosigraphy (2C-OCD), which can provide microscale tomographic images of drug infiltration. In this study, the vulnerability diagnosing technique of unstable plaque is proposed, by means of 2C-OCD using selective uptake of PDD photosensitizer to lipidrich plaque and macrophage. In order to evaluate the bio-availability, 2C-OCD was ex vivo applied to aorta segments of rabbits with photosensitizer AlPcS administered, comparing between drug infiltration obtained by 2C-OCD and their histological images. Consequently, drug infiltration was only observed to increase significantly at plaque lesion where photosensitizer's fluorescence could be captured. Additionally, AlPcS accumulation appeared to be similar to macrophage invasion visualized by immuno-staining. Therefore, it was confirmed that 2C-OCD could diagnose specific tissue characteristics of unstable plaque as well as the vulnerability attributed to macrophage invasion, based on the obtained drug infiltration.

View full abstract

To simulate the emulsification process in a micro-channel, a computational fluid dynamics (CFD) technique, called the volume of fluid (VOF) method, was used. To improve computational accuracy, the mesh of the simulation model was divided finely enough and a parallel computation for a large-scale CFD simulation was executed to calculate the emulsified droplet generation process in a micro-channel. As a result, it was confirmed that the length of droplet position determined by the VOF method agrees well with the experimentally measured within an error of 5%. In accordance with that agreement between calculation and experiment, a map of the generation of uniform emulsified droplets was constructed by using the VOF method. The map is defined by two dimensionless parameters: Reynolds number (Re) and Capillary number (Ca). It is clear from the map that the generation of droplets peaks at Re of 3.0 to 4.0 and Ca of 10.0×10^<-3>. The map makes it possible to generate emulsified droplets experimentally within a coefficient of variation (C_v) of 5.0%.

View full abstract

For the evaluation of gas entrainment phenomena in the sodium-cooled fast reactor (JSFR), a high-precision simulation method for gas-liquid two-phase flows on unstructured meshes have been developed. In this study, our high-precision volume-of-fluid algorithm on unstructured meshes is introduced into the two-dimensional unstructured adaptive mesh algorithm (described in the 1st report). For that purpose, an appropriate redistribution method for volume fraction values is established. In addition, a calculation procedure is formulated, in which momentum values at both gas and liquid phases are conserved respectively when velocity values at refined or merged cells are calculated. This new unstructured adaptive mesh algorithm for gas-liquid two-phase flows is verified by solving well-known slotted disk rotation problems. As a result, the present adaptive mesh algorithm succeeds in maintaining slotted disk shape after one rotation by relatively small number of cells. In addition, the dam break problem is also simulated. The simulation results showed that the gas-liquid interface is simulated with high resolution and the transient interfacial position is more accurate than the simulation result on the non-adaptive mesh.

View full abstract

To analyze the effect of the flow geometry on a nematic liquid crystalline flow induced by the imposition of an electric field, the numerical simulation of annular backflow between concentric cylinders has been performed using the Leslie-Ericksen continuum theory. For small values of the curvature of the cylinders, the induced backflow profile is anti-symmetric with respect to the center of the flow channel. With increasing the curvature, the backflow shows one-directional profiles. The curvature has little effect on the director orientation profile at a steady state and large effect on the profile at an unsteady state. For small values of the curvature of the cylinders, the shear stresses at the inner and outer cylinder walls are almost same, while for the large curvature, the shear stress at the outer cylinder decreases slightly and the stress at the inner cylinder takes a large value. We have investigated the flow characteristics with various values of the electric field strength, and it is found that the effect of the electric field strength on the backflow profile is similar to the curvature of the channel.

View full abstract

We have investigated aggregate structures and rheological properties of a colloidal dispersion composed of ferromagnetic spherocylinder particles with a magnetic moment along the particle axis direction, by means of Brownian dynamics simulations. In concrete, we have attempted to clarify the influences of the flow field, magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the internal structures of clusters, we have focussed on the radial distribution and orientational distribution functions. The present results are compared with those of the theoretical analysis for non-dilute dispersions by means of the mean-field approximation. The results obtained here are summarized as follows. For the case of a strong magnetic field and strong magnetic interactions between particles, thick and long clusters are formed along the magnetic field direction. As the influence of the flow field increases from such a situation, those clusters dissociate, and the particles are tilted to the flow direction. Although magnetic interactions between particles influence the orientational distribution function in the case of the mean-field approximation, it does not influence significantly the orientational distribution function in the case of the present study because the torque acts on the particles from the other cluster. For a dilute case, as magnetic interactions between particles increase, many step-like clusters are formed so that the viscosity due to magnetic properties and steric layers significantly increases.

View full abstract

When a train runs through a tunnel at high speed, large pressure changes are generated in the tunnel. It is necessary, therefore, to predict these pressure changes accurately in planning speed-up of Shinkansen or Maglev trains. For this purpose, we have developed a one-dimensional flow simulation program to calculate the pressure changes and wind velocity in a high-speed railway tunnel. This program can make the prediction for tunnels with shafts and/or stations/terminal up to about 500km/h of the train-speed. The results of the simulation are found to be in good agreement with those of model experiments performed for verification.

View full abstract

Atomization characteristics of surfactant aqueous solutions injected from a fan spray nozzle have been clarified. Surfactant solutions generally have unusual properties, that is, surface tension depending on time (dynamic surface tension) and strong stability of a thin liquid sheet such as a soap film. These special effects result from the adsorption of surfactant molecules to newly formed liquid surface. In this study, the influence of the dynamic surface tension and surface stabilizing effect by the surfactant molecules on the liquid sheet atomization was discussed. The dynamic surface tension and surface-stabilizing effect were measured by oscillating jet method and Ross-Miles method, respectively; Ross-Miles method is generally used to estimate the abilities to form and maintain foams. The break-up length of the liquid sheet and wave amplitude inducing break-up were also measured by photography. Test liquids with several surfactant concentrations indicated that increase of surfactant concentration reduced the relaxation time of the dynamic surface tension, expanded the break-up length and amplify the wave amplitude. This means that adsorption of surfactant molecules increases the break-up length regardless of large wave amplitude. To investigate this inconsistency, two test liquids, which were basically ethanol and surfactant solutions, were specially produced so that they have different surface-stabilizing effect with same density, viscosity and surface tension. The break-up length of the surfactant solution became longer than that of the ethanol solution. This indicates that the surface-stabilizing effect by the surfactant molecules suppresses the liquid sheet atomization.

View full abstract

An experimental study has been carried out to investigate the self-excited shock wave oscillation on a 2D symmetrical circular-arc airfoil in internal transonic flow, and a new determination model of the shock wave oscillation frequency is proposed. Shock wave positions are measured by using schlieren optical system and two line scan cameras. The result shows that the phase difference between shock wave position fluctuations of each side are almost 180 degrees, and oscillation frequencies can be successfully estimated by the proposed determination model. In this model, it is almost equal to the experiment value to set the characteristic point as X_b=1.5.

View full abstract

PIV measurements are carried out for turbulent flows in a channel with a porous bottom wall. In the highest permeable wall case among the measured, it is observed that the flow is not laminar and tends to be turbulent even at the bulk Reynolds number of Re_b=1300. It is also found that the flows become more turbulent in the cases of higher permeability. To confirm the effects of the wall roughness and the wall permeability, detailed discussions are made assuming the effective wall roughness of the porous media. The results clearly indicate that the turbulence is induced by not only the wall roughness but the wall permeability. The experimental correlation of the mean velocity with the permeability Reynolds number is then proposed. The measurements have also revealed that as Re_b and or the wall permeability increase, the wall normal fluctuating velocity is less damped near the porous wall due to the effects of the wall permeability. This leads to the increase of the turbulent shear stress resulting in the higher friction factor of turbulent porous wall flows.

View full abstract

The promotion of jet-stream diffusion may enhance the efficiency of mechanical performance in various industrial devices such as fluid machinery and combustion equipments. Both active and passive means were applied for the control of jet streams. Our previous studies disclosed that diamond-shaped cylinder bundles produce a self-excited oscillating jet-stream flow field having multiple uniform flow-rate groups. The present visualization work deals with flip-flop flow oscillation from a diamond-shaped cylinder bundle with wall concavity. It is disclosed that the wall concavity has induced a substantial change in the flow patterns in the exit jet-stream field with strong turbulence and jet-stream dispersion.

View full abstract

This paper describes a detailed study of a synthetic jet governing flow field. The synthetic jet, known as a zero net mass-flux jet, is generated by a vibrating diaphragm to oscillate flow through a small orifice. The synthetic actuator consists of the orifice and cavity installed the PZT disk of 31.2mm diameter. The test is conducted in variable input voltages and frequencies to PZT disk. The time-mean velocity and phase-locked velocity is measured by Laser Doppler Velocimeter. In the test condition of the input voltage of AC 30.0V at the actuation frequency of 990Hz, the time-mean data show the entrainment from vortexes increases the mass flow with the high blowing velocity away the orifice exit. Phase-locked velocity vector and vorticity fields reveal the periodic vortexes behavior, ejecting from the orifice, extending itself with loosing the coherent structures, and diminishing in the ambient air.

View full abstract

Single-blade centrifugal pump is widely used as a sewage pump. However, a single-blade is acted on by a large thrust during the pump operation because of the geometrical asymmetry of the impeller. Therefore when the reliability of the pump is secured, it is necessary to grasp a radial thrust quantitatively and elucidate the behavior and the dominant factors. This study investigated the radial thrust acting on two kinds of single-blade centrifugal impeller of different impeller outlet angle by an experiment and CFD analysis. Furthermore, as for the radial thrust to act on two kinds of impellers, the radial thrust was modeled by a combination of three components, unsteady, momentum and pressure components. As a result, the effect of the impeller outlet angle on the radial thrust and the each divided components was clarified.

View full abstract

A sirocco fan using contra-rotating rotors in which an inner rotor was settled inside the sirocco fan rotor and each rotor rotated in an opposite direction was proposed for the purpose of getting the higher pressure and making the structure of a sirocco fan compact more. If the high discharge pressure is obtained with the adoption of the contra-rotating rotors, it could be used for various purposes. Pressure coefficient of the sirocco fan with contra-rotating rotors was 2.4 times as high as the conventional sirocco fan and the maximum efficiency point of contra-rotating rotors shifted larger flow rate than the conventional sirocco fan. On the other hand, it was clarified from the flow measurement results that the larger back flow region than conventional one occurred at the shroud of the casing in the case of contra-rotating type and this would induce decrease of the efficiecy. In the present paper, the performance of the conventional sirocco fan and the sirocco fan with contra-rotating rotors are shown and the internal flow field at the outlet of the outer rotor of both cases is clarified Then, the effect of casing width on the performance and the internal flow field is investigated and the casing width which would be suitable for the contra-rotating type would be discussed.

View full abstract

In this paper, effect of the baroclinic torque and buoyancy on vorticity generation in a boundary region between the ambient air and burnt gas of a two-dimensional premixed laminar flame is investigated through shadowgraph, laser tomography and PIV (Particle Image Velocimetry) techniques. It is found that vorticity is generated only on a boundary region in the dual-density non-reaction flow field. Then, the jet flow is contracted and accelerated by the induced vortices. In addition, a numerical study is carried out by using the discrete vortex method in the condition that is the same as this experiment. In this analysis, a boundary region in a dual-density is replaced by a vortices, and the baroclinic torque and buoyancy is generated in the boundary region. The simulation results of vorticity development process is compared with the experimental results. As a result, it is found that flow configurations of the burnt gas can be simulated successfully by the present method.

View full abstract

Chemical materials such as mineral oil, which may impact the environment and ecosystem, are included in cooling water for cutting asphalt roads. In our paper, cooling water used for asphalt road improvement was extracted and analyzed chemically. A treatment system of cooling water for cutting asphalt roads was developed. It is clarified that the cooling water treated by the system can clear away the environment quality standard for drainage.

View full abstract

The origin of Sonochemistry is an acoustic cavitation process in the liquid phase; those are the nucleation, growth and violent collapse. Here, instantaneous hot-spot with high temperature and pressure is generated in the local region by the bubble collapse. The authors have proposed to apply the sonochemistry into a heavy fuel oil mixed with a lower boiling point fuel for reformulation of the heavy fuel oil and reported the advantage of the mixed fuel. Thus, the several experimental results relating to the fuel reformulation in the previous studies. Then, it is necessary to clarify a theoretical analysis for reformulation process based on cavitation dynamics. Therefore, the authors develop an energy modeling on the cavitation bubble collapse have process for reformulation of two-component fuel by use of Sonochemistry. And it is confirmed that reformulation efficiency of mixed fuel is higher than that of the pure fuel case.

View full abstract

Soot particles formation process in pyrolysis of hydrocarbon fuels, highly diluted with argon, behind reflected shock waves was studied by using optical methods. The laser light extinction measurement was conducted to obtain total soot volume. Time history of soot particles temperature was estimated based on spectral dependence of monochromatic emissive powers from thermal radiation of the soot particles. The final soot particles temperature, corresponding to the final ambient temperature, varies from temperature behind the reflected shock wave, which is due to endothermic reactions of fuel decomposition and exothermic processes of soot particle formation. The experimental results show that the soot particles temperature governs the final soot yield. The homogeneous nucleation theory can explain the temperature dependence of soot particles formation qualitatively.

View full abstract

In this study, combustion field with intermittent fuel jet has been investigated. A combustor consists of inner central fuel nozzle and surrounding air nozzle. To reveal an effect of fuel injection, ratio of fuel injection time to total period (R) is changed. When fuel supply rate is kept constant, fuel injection velocity is increased at larger R. In experiments, NO_x concentration was measured. The fuel is propane. To discuss flame structure and flame characteristics in detail, numerical simulation was conducted. The fuel injection time and velocity were changed to in the range of R=0.14, 0.20, 0.33, and 1.0. A flame index was obtained to examine premixed and non-premixed combustion regions. The so-called jet diffusion flame is formed at R=1.0. Experimental results show that, the combustion area is very fluctuating by intermittent fuel injection. As R is decreased, EINO_x is reduced. Based on numerical simulation, by decreasing R, the flame region is detached at the burner port. In this case, some of the fuel is re-ignited in burned gas area of high temperature. At this condition, it is found that fuel and air are mixed to form premixed combustion zone, with lower flame temperature. Hence, the low level of NO_x emission by intermittent fuel injection is explained by this partial premixing effect of fuel and air.

View full abstract

Miniaturization of combustor is one of the important development subjects for high-performance and high-power combustion equipment. We had examined the center-air burner consisted of three concentric tubes, in which air flows in both inner (central) and outer tube and fuel flows in the annulus between these air tubes. In this burner, fuel and oxidizer are mixed more efficiently than normal burner consisted of two concentric tube. We expect that the combustor using center-air burner achieves more highly loaded combustion and overcomes unavoidable technical problems of miniaturization of combustor: combustion efficiency reduction and incomplete combustion degradation. In this study, we carried out the numerical calculation of center-air burner by using detailed chemical kinetics, and examined the effects of preheating temperature and ejection velocity of center-air on combustion efficiency, combustion intensity and emission index of CO. Then, it is found that the use of the center-air burner is very effective way to miniaturize the combustor.

View full abstract

Effect of fuel properties of kerosene on deposit accumulation was studied experimentally. Five types of kerosene, which distillation and composition properties were different, were used as the test fuel. Deposit accumulation processes were investigated by using wick combustion burner. Deposit accumulation rate was estimated from the mass of deposit which accumulated on wick and the fuel consumption. As the results, it was confirmed that the deposit accumulation rate increased with the content of aromatic hydrocarbons and naphtheno benzenes. Main component of the deposit was heavy hydrocarbons formed by thermal decomposition and polycondensation of the fuel within the wick. To estimate the effect of peroxide on deposit accumulation, test fuels which contain peroxide generated by sun exposure were used. The peroxide-containing fuels significantly increased the deposit mass than normal fuels.

View full abstract

This study has examined the influence of the addition of hydrogen to propane mixtures on its local burning velocity. Hydrogen added propane mixtures having nearly the same laminar burning velocity with different rates of addition of hydrogen δ_H and equivalence ratios Φ(0.8〜1.4) were prepared. A two-dimensional sequential laser tomography technique was used to obtain the temporal statistical relationship between the flame shape and the flame displacement. The local flame displacement velocity S_F and curvature of turbulent flames were quantitatively measured as the key parameters on turbulent combustion. The Markstein number Ma was also obtained from outwardly propagating spherical laminar flames, in order to examine the effect of positive stretch on burning velocity. It became clear that the trends of the mean values of measured S_F with respect to δ_H and Φ gave good agreement with the turbulent burning velocity. The trends of Ma with respect to δ_H, Φ and fuel types could also explain qualitatively the local burning velocity and turbulent burning velocity.

View full abstract

Spray combustion model with a flame boundary and two NO formation regions was developed for NO exhaust estimation of a large-sized marine Diesel engine. The basic prediction characteristics were studied by using an actual engine specification and the parameters such as a droplet initial diameter, a fuel injection period, and a flame angle. The simulation could show the change of chemical species concentration in the inside and the outside of the spray. As a result, it is found that this model predicts that the NO formation is frozen after the termination of combustion and the flame angle influences the NO exhaust rate and the thermal efficiency more sensitively among the parameters.

View full abstract

In order to mitigate the global warming, a system dissolving huge amount of CO_2 gas captured from fossil fuel fired power plants into the ocean with high acceptance of the ocean environment is indispensable. We propose a sequestration system of CO_2 in the deep ocean water using gas-lift effects (so-called GLAD System). The system is an inverse-J pipeline set in the ocean at a depth of 200-3000m. In the system, a pumping effect by buoyancy of the dissolving CO_2 bubbles is used to transport CO_2-rich seawater to great depths. In the present paper, we outline the GLAD system and summarize its related fundamental studies for the purpose of designing a highly-efficient and most-reliable system.

View full abstract