A numerical analysis of unsteady behavior of particle aggregation/dispersion was performed in a non-uniform shear flow in a cylinder with a rotating elliptical rotor. In this model, it was considered that an aggregation/dispersion rate depends on Brownian coagulation, shear coagulation and shear breakup. Immersed boundary method was used to calculate a flow with rotating rotor. The diameter of the cylinder was 38 mm. The major and minor axes of the elliptical rotor were 32 and 26.6 mm. The rotor was rotated at 20 RPM. The diameter of particles and the viscosity of disperse media were 2.5 ?m and 195 Pa s. As a result, the cluster was broken by applying shear and the size of cluster decreased with time. The particle dispersion behavior varies with local position. The mean cluster size took a minimum value right after the major axis of the rotor passed. Then the mean cluster size increased again.

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This report shows the fluid property and the velocity profile of carbon nanotube suspension (CNT). The CNT suspension is complex fluid. Complex fluid is defined as any fluid that does not follow the well-known Newton's law. In this study, two-dimensional velocity profiles of CNT suspension were measured between parallel plates and at the upstream region of the contraction. These results show that the velocity profiles in CNT suspension between parallel plates were in good agreement with Newtonian one. However, the results in CNT suspension at the upstream region of the contraction were not agreement with Newtonian one. This is because the elongational viscosities of CNT suspension effect at the upstream of the contraction.

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Magnetic fluids are suspensions of magnetic particles coated with a molecular layer of dispersant. The magnetic particles form chains in a magnetic field, which cause magnetic theological effects. Consequently the behavior of the particles under flow is an important subject for the analysis of the behavior of magnetic fluids. The present study proposed a numerical scheme in which the molecular dynamics simulation for the motion of the magnetic particles and the multi-particle collision dynamics simulation for describing the hydrodynamics interaction among the particles are coupled. This scheme can simulate typical behaviors of magnetic fluids such as the formation of particle clusters and the breaking and reconstruction of the chains under shear.

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To develop the novel microactuator which can change its shape depending on ambient conditions, we have analyzed the dynamics of a liquid crystalline droplet under electric field. The liquid crystalline droplet is placed on a glass plate whose surface is treated with a pair of electrodes and a homeotropic anchoring layer. The movement of the droplet is observed when the voltage is applied between the electrodes. It is found that the moved distance of the droplet strongly depends on the electric field intensity. From the observation of the droplet under a polarized microscope, the reorientation of the liquid crystalline molecules is confirmed and thus the liquid crystalline flow occurs in the droplet. Also, the continuous drive of the droplet is successfully achieved using the patterned electrode.

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The gelation caused by the addition of salt to a cationic surfactant solution was experimentally studied. An aqueous solution of CTAB was used for cationic surfactant solution, and an aqueous solution of NaSal, a salt supplier, was injected into the CTAB solution. The gelation was observed by flow visualization, SALS experiments, and flow birefringence measurement. The experimental results showed that the gelation behavior depends on both the NaSal concentration and the injection rate. The flow birefringence measurement revealed that larger gel structures oriented more strongly in the flow direction appear at a higher NaSal concentration.

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In multi-component soft matter, interface properties often play a key role in determining the properties of the overall system. The identification of the internal dynamic structures in non-equilibrium situations requires the interface rheology to be characterized. We have developed a method to quantify the rheological contribution of soft interfaces and evaluate the dynamic modulus of the interface. This method reveals that the dynamic shear responses of interfaces in bilayer systems comprising polypropylene and three different polyethylenes can be classified as having hardening and softening effects on the overall system: an interface between linear long polymers becomes more elastic than the component polymers, while large polydispersity or long-chain-branching of one component make the interface more viscous. We find that the chain lengths and architectures of the component polymers, rather than equilibrium immiscibility, play an essential role in determining the interface rheological properties.

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In this study, we have been investigating printing characteristics of pastes used for screen printing to form electrode patterns on substrates. Influences of surface treatment of particles in the pastes on the characteristics and their printability are mainly discussed in this report. A stencil with a small hole was utilized as a model of the screen and the pastes were printed to a substrate through the hole. Shapes of the pastes adhered to the substrate was measured. Yield behavior in elongational deformation was observed in the case with low separating velocity. In our experimental conditions, the influences of surface treatment were not so large in the case with high separating velocity.

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The motion of a solid sphere sedimenting at its terminal velocity through worm-like micelle solutions is experimentally investigated. A rotational motion of a sphere is detected by tracking marker points on the surface of the sphere. Abrupt acceleration in sedimenting velocity of a sphere is found to be accompanied by its sudden rotation followed by sudden deceleration in sedimenting velocity and cessation of rotational motion. We suggest that the onset of this motion is caused by the partial detachment and adhesion of flow induced structured fluid on the interface of the sphere and unbalanced tensile forces lead to the rotational motions.

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The motion of single bubbles rising in hydrophobically modified alkali-soluble associative (HASE) polymer solutions are experimentally examined. The non-Newtonian liquid is made up of 1.1 and 1.4 wt% HASE polymer solution, which are adjusted to pH ? 7.0 by adding a sodium hydroxide solution. Rising bubbles with threadlike shapes are observed, regardless of small Deborah number conditions. It is shown that unique shapes of the bubble largely depend on the HASE solution and the bubble size. Consideration for unique shapes is made based on enlarged pictures taken with a high-speed video camera.

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Behavior of bubbles is important in process for production of materials. In process for production of materials, bubbles are mixed in the materials spontaneously. To remove bubbles from high viscous fluid, clarify of behavior of bubbles is needed. In the previous study, behavior of bubbles is observed. However relation between bubbles and surrounding fluids, and pressure are not studied. In this study, to understand behavior of bubbles, we observe shape and velocity of injected bubbles. As a result, the rising velocity of bubble upward flow depends on bubble diameter. Shape of bubble is spherical in high viscous fluids. When two bubbles coalesce, the lower bubble catches up with the upper one. After catching up, bottom of upper bubbles is deforms. After deforming, the lower bubble is absorbed. It can be reproduced to some extent the actual phenomena make the numerical calculations for rise of bubbles was confirmed. It is also confirmed that the numerical simulation can reasonably reproduce bubble rising velocity in high viscous fluid.

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When strong pressure-oscillation was applied to the two vertically aligned air bubbles in viscoelastic fluids from the bottom side, the lower-bubble rose faster than the upper-bubble. It is thought that a hydrodynamic interaction may be occurred in the fluid between air bubbles. To clarify the stress contribution on the interaction, we tried measuring the retardation profile near and between the vertically aligned small air bubbles under pressure-oscillating fields, by using the flow birefringence technique with CTAB / NaSal solution. In the case of near the bubbles (2μL for each), it was found that the strong retardations by the uniaxial strain between small air bubbles was observed at the time of the air bubbles shrinkage. The biaxial tensile flow was seen in vicinity of small air bubbles when the bubbles expanded.

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Structural change of a polymer solution in a planer channel flow after an abrupt expansion have been investigated. Flow of 0.5wt% xanthan gum aqueous solution through 1:4 expansion channels with height of 1 mm and 0.2 mm has been tested. Distribution of birefringence and orientation angle at a center line of the channel were measured. Flow-induced birefringence and orientation angle after the expansion were depending on the channel height. Influence of elongational effect generated at the center region in height was disappeared in the case of small gap.

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The polarized thin film formation by application of chromonic liquid crystal was investigated. The thickness and extinction of the formed dry films was measured and compared with the conditions of the meniscus region at the application process. The traditional shape of the application which has about 50 degrees escape angle of the back wall causes a wall climbing effect and the thickness in the meniscus region becomes large. However, the dry file thickness and extinction are almost the same in the case of the edge type applicator that does not cause the wall climbing effect.

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The present study is the experimental investigation of flow structures in the modified surfactant channel flow by simultaneous PIV and PLIF. In order to modify the near-wall region in the surfactant drag-reduced flow, water is dosed into the surfactant drag-reduced channel flow from a whole surface of the channel. By water dosing, drag reduction rate is enhanced compared to that without water dosing. The results show that the reason for this enhancement is the large decrement in the viscoelastic stress in the near-wall region while the slight increment Reynolds shear stress. In terms of velocity and concentration field, the feature in the near-wall region is similar to that in a usual Newtonian turbulence, however, the feature in the region remote from the wall is similar to that in a usual viscoelastic channel flow.

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The deformation of single Newtonian drops under simple shear flow of shear-thinning fluids is examined by numerical simulations with a volume-of-fluid (VOF) method. In our study, the Carreau model is used for modeling shear-thinning fluids. From computational results, it is clearly shown that the decrease in the viscosity makes a large impact on drop deformation: the deformation behavior of a drop is constrained when shear-thinning property emerges depending on the shear-rate. Effective (representative) Reynolds and Capillary numbers and a modified viscosity ratio defined using effective shear rate and viscosity are proposed in order to organize and understand drop deformation in shear-thinning fluids. The usefulness of effective dimensionless numbers is demonstrated.

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The aim of this study is to clarify the large deformation characteristics of a droplet in oscillating shear flow. Oscillating shear flow is generated between parallel plates which move opposite directions with sinusoidal waveform with motor sliders. A droplet is set between two plates and it receives shear stress from the plates. A deformed droplet is illuminated by green-laser sheet light and its shape is recorded by a high-speed video camera. The experimental results show that time variations of deformation parameter D have viscoelastic response to strain. Phase advance of maximum deformation parameter to strain θ_<ave> was described as a function of maximum capillary number Ca_<max>.

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Experimental and numerical flow visualizations on density destratification process provided by a bubble plume in horizontally long environment are presented. The experiment has shown that the initial density stratification can be broken broadly by creation of intermediate density layer inside turbulent bubble plume since the intermediate layer naturally induce density current in the horizontal direction. The process has been simulated in a 2-D rectangular tank, which further revealed that oscillation of bubble plume promoted the current. 3-D simulation applied for a dam has shown activation of destratification by the intermediate density layer regardless of geometry of the dam.

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Characteristics of flow patterns and frictional pressure drop and the flow phenomena of gas-liquid two-phase flow in microchannels were investigated experimentally by using three kinds of circular microchannels with the inner diameter of 0.10 mm, 0.15mm and 0.25mm. The effect of tube diameter on the flow patterns was made clear by using high speed video images. In the flow patterns maps the boundaries of the transition from slug flow to ring film flow or churn flow in each channel shifted to the side of smaller values of the superficial gas velocity when the tube diameter decrease. It was found that there were not so remarkable difference between the frictional pressure drops in each microchannel due to the sizes of tube diameters.

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The discharge flow into the atmosphere is used for industrial use such as a sprinkler and a thaw system of track. For high efficiency and high precision of these systems, it is necessary to spray the same flow rate for dispersion domain. However, characteristics of discharge from side holes of a pipe have not been clarified. The purpose of the present study is to perform numerical analysis of pipe-inside flow including gas-liquid two-phase discharge flow from the side holes of the pipe and is to clarify a key factor of pipe flow with side holes.

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We report a series of experimental study on the flow behavior of a buoyant jet in a non-Newtonian fluid with gel beads. A viscous fluid is supplied from a nozzle at a constant volume flux in the gel beads layer. The hydrogel beads are deformable and the volume fraction of the interstitial fluids is about 15%. In our experiments, we identify at least three types of fluid flow: homogeneous permeable flow, pulsating flow, and localized continuous flow. The flow behavior depends on the injection flow rate, the rheological properties of the mixture, and the volume fraction of the interstitial fluid. Our experimental model explains the origin of the intermittent nature of multiphase flow observed in food industries and magma transport in nature.

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Measurements of local void-fraction around tube in liquid-gas two-phase flow were carried out using a void probe technique. Horizontal tube bundle was set in a vertical rectangular channel with 90 X 90 mm^2, and the tube layout was in-line. Outer diameter of the tube was 15 mm, and the pitch to the diameter ratio was 1.5. Void fraction distributions were compared between 1^<st>, 4^<th> and 7^<th> rows at the center column. Furthermore, the differences of the void fractions were compared around between the center and wall side. Void fraction distributions at inlet of the tube bundle strongly affected on the void fraction distributions in the bundle. Void fraction deviation increased with gas-flow rate. The flow regime changed from bubbly to churn-turbulent flow with increasing of the gas-flow rate, and it increased bubbles' horizontal movement in the bundle. Therefore, the void fraction at 0° and 180° increased with gas-flow rate. As a result, the void fraction distributions around a tube became flat in churn-turbulent flow regime.

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The bubble departure mechanism, which in general is dominated by the buoyancy force, has been shifted to the surface tension force dominance by employing a two-dimensional saw-tooth surface. The experiment was conducted in a carbonated water pool, and spontaneously generated and detached bubbles on test walls were visualized. Consequently, three different bubble departure modes (spontaneous, double contact, and top contact) were observed, and bubble growth and departure mechanisms were analyzed in terms of instantaneous bubble diameter and surface geometry. The results showed that bubbles departed from the surface in double contact mode shaped almost perfectly spherical, which implies the departure mechanism was surface tension force dominant.

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We experimentally studied a cavitating flow in a two-dimensional convergent-divergent nozzle with a diverging angle of 8.4° using HFE (hydro-fluoro-ether), by which we can expect to observe thermal effect at room temperature. But the amount of dissolved air in HFE is very large, so we have to take account of the effect of air inclusion inside cavitation bubbles. Then we redefined the modified cavitation number to remove the effect of dissolved gas assuming that pressure at nozzle throat equals to cavity pressure. Except the lowest throat velocity case, the more cavity length was gradually increased, the faster velocity at throat was, compared to same modified cavitation number, revealing the thermodynamic effect of cavitation. In the lowest velocity case, the cavity length seemed to tend to be overestimated due to many tiny air bubbles downstream of sheet cavities formed from separations of dissolved air.

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Taylor vortex flow with a small aspect ratio has characteristics such as the different vortex structure, because of the Ekman effect in the top and bottom boundary layers and the acceleration of the inner cylinder. This feature is expected to develop a new typed mixing bioreactor system, but this vortex flow mechanism like a chaotic flow has not been investigated much. Furthermore, the multiphase flow such as the solid-liquid flow is important to be analyzed with a measurement system. A ultrasound measurement method is the one of useful device for observation of such kind flows. In this report we mainly analyzed the vortex structure in higher Reynolds number by using the ultrasound measurement system.

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We constructed an experimental setup of focused ultrasound Field for water with uniformly distributed lipid microbubbles, and measured nonlinear propagation properties caused by oscillations of microbubbles. An ultrasound transducer with a concave PZT ceramic element with a focal length of 40 mm was fixed in a water tank. The therapeutic sensitizer Sonazoid with a diameter of 2 to 3 μm was used. A 30 mm thick polyacrylamide gel containing microbubbles was fixed to include the focus to suppress bubble mobility during the measurement. Elements with four center frequencies l, 2.23, 3.35, and 4.32 MHz, and a moderately weak ultrasound intensity (focal pressure 0.15 MPa) that suppresses the destruction of microbubbles, were used. Then, we conducted one-dimensional measurement and the results showed that the waveform attenuated with the increasing incident frequency and void fraction, which indicates the nonlinearity of the oscillations of microbubbles. Higher harmonics were generated at low frequencies. While focal-pressure increased with the applied voltage to the transducer at high frequencies (near resonance), no increase in focal-pressure was observed in the case of low frequencies.

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Abstract Two-dimensional simultaneous measurement of the interface of the liquid film with a thickness of millimeter order is required for verification of a theoretical model on the development of the fingers. We measured a water film flowing down an inclined plane using color coding method, developed for the measurement of interface wave, to observe a behavior of the finger. The contact angle is largest at the tip position of the finger advancing fastest and is variable at the straight and trough position depending on a level of growth of the finger.

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We newly developed a measurement technique for a liquid film flow via a single-tip optical fiber probe (S-TOP). The measurement method for the liquid-film thickness is as follows; the S-TOP with a tapered tip was installed parallel to the main stream, and detected a wavy surface; the liquid phase fractions were calculated in every installed position of the S-TOP. Through a computational method of the S-TOP, we obtained the relationship among the liquid phase fractions, the installed positions and the wave heights. Moreover, the wave velocities were accurately measured with our original micro-fabricated S-TOP that has two sensors. The experimental and numerical analyses were executed in order to understand the complex signals of the S-TOP. Finally, the simultaneous measurement technique of the thickness and wave velocity was obviously demonstrated. When the liquid phase fraction was 0.52, the installed-position equals the average film thickness. A difference in the results of the wave velocity between the S-TOP and the visualization was -10.3 %.

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A fixed bed reactor that operates in gas-liquid co-current down flow is called Trickle Bed Reactor (TBR). In this study, authors employed the lab-scale TBR, made of 68, 100mm inner diameter acrylic column, packed with particles of two sizes (3, 5 mm) that are used in the actual reactor. Liquid and gas were injected from the top of the column and cross-sectional liquid distribution was captured at the bottom of the column by electrical resistance tomography (ERT). ERT is a tomographic technique that provides the cross-sectional conductivity distribution at the rate of about 50 frames per second by injecting current and measuring voltages between the 16 electrodes that are attached around the column. By analyzing the spatial and temporal characteristics of the liquid distribution obtained by ERT, the relationship between operation condition and flow regime transition and property of pulsing flow was revealed.

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In the frictional drag reduction by using bubbles, fluctuations of void fraction take an important role to enhance the effect. We have investigated the mechanisms on formation of bubble clusters that provide the large fluctuation. The objective of this study is establishing a new measurement system of passing interval of bubbles with ultrasonic wave in order to find out the property of void fluctuation. We conducted synchronous measurement of ultrasonic pulse emission and video camera recording of bubble images in a horizontal bubbly channel flow. The present technique provided reasonable results on the passing interval in comparison with the estimates from the images. Probability distributions of the passing bubbles obtained at three different points in the downstream represent influence of bubble coalescence.

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In order to reduce the energy necessary for supplying air bubbles for ship drag reduction, we propose two types of devices for bubble generators using hydrofoils. One is the winged air induction pipe (WAIP), which has an angled hydrofoil with an air introducer, utilizes the low-pressure region produced above the hydrofoil as the ship moves forward, which drives atmospheric air into water. Another type of bubble generator is the device that consists of hydrofoils with air holes provided on their surface. Small bubbles are generated from the air holes when the hydrofoil moves to create low pressure region around the air hole. In this presentation we describe the principles of bubble generation processes based on a simple fluid dynamic theory and on towing tank experiments. We also report the optimization of the hydrofoil bubble generators using flaps.

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Oil mist in the pipeline is usually removed by using clean gas (purge). However, the oil mist behavior in the pipe during this purge is not revealed enough. Therefore, we built the test pipeline with 80.7 mm diameter and 145 m long and injected the oil mist at upstream which purged with various flowrate and the change of mist concentration were measured. The results indicate that the concentration profiles of oil mist highly depend on Reynolds Number. It is also found that diffusion process of mist-gas two phase flow and gas-gas flow are qualitatively similar because mist was small enough.

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Skin-friction drag in turbulent pipe flows significantly decreases by a pulsating due to a relaminarization phenomenon. In a straight pipe flow,63% drag reduction rate has been obtained experimentally. The purpose of the present study is to investigate the drag reduction effect by the pulsating in a turbulent pipe flow with a branch. The cycleaveraged friction Reynolds number is set to be 200 before the branch. The flow rates at a main and a sub branches are set to be equal. The range of control parameters, i.e., a period of the pulsating and an acceleration parameter, that produces the drag reduction effect, is scaled with a local skin-friction velocity and a mean pressure gradient averaged for an acceleration phase. This scaling is found to be suited not only the flow before the branch, but also that after the branch. The drag reduction effect in a deceleration period mainly contributes to the drag reduction effect of pulsating.

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Direct numerial simulations for periodically diverging-converging turbulent pipe flows are perfromed to examine a drag reduction effect. The friction Reynolds number is set to be 110 and five different pipe shapes are examined. The maximum drag reduction rate of 14% is obtained when the ratio between the pipe length and the maximum deformation thickness is 797. The turbulent puff is observed in the pipe flow by the visualization of vortical structures.

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In order to reduce the pressure loss and to improve the flow uniformity simultaneously under the condition of a limited diffuser length of the automobile catalytic converter or the automobile diesel filter, the authors have proposed a flow deflector placed inside the diffuser part (Hirata et al., 2006 & 2008). And, the authors have succeeded in both of the pressure-loss reduction and the flow-uniformity improvement. However, such past studies do not show whether the flow deflectors are effective in actual exhaust systems. So, in the present study, the autors try to reveal more practical performance of such a flow deflector, concerning the actual exhaust-gas flow from two types of gasoline engines. As a result, a multi-cylinder engine shows better performance than a single-cylinder engine.

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In recent years, low fuel consumption attracts much attention in the automotive industry from the viewpoint of environmental problems. The aerodynamic drag reduction of a car is one of the most important issues for low fuel consumption. However, the flow mechanism in a wheel house has not been clarified in detail. On the other hand, recently it is shown open?source CFD toolbox OpenFOAM is usefulness. Therefore in the present study the flows around a car are simulated by RANS using OpenFOAM, in order to investigate the flow in wheel houses. Two cases are computed; tire with wheel and tire without wheel.

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Flow around a square cylinder controlled by plasma actuators is numerically investigated by means of two-dimensional direct numerical simulation. The Reynolds number based on the cylinder diameter and the freestream velocity is set to be 100. Three different arrangements of plasma actuators on the cylinder surface and two different actuation amplitudes are investigated. The drag and the root-mean-square of lift fluctuations are found to reduce in some cases, accompanying a suppression of vortex shedding. Such an effect is most pronounced in the case where the plasma actuators are installed on the rear surface to induce inward flows.

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Turbulent drag reduction over riblets with trapezoidal cross-section was examined by wind channel experiment and direct numerical simulation. The maximum drag reduction of 6% compared to the smooth surface was confirmed for the straight groove with spanwise interval of 18 in wall units. The good agreement between experiment and DNS was demonstrated. Drag reduction over wavy riblets was also examined.

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We have carried out experiments to measure velocities of turbulent water flow over two wavy plates on the bottom of a shallow open channel at a high Reynolds number. These plates mimic the folded skin of fast-swimming dolphin. The hydrophilicity of these plates are different. Also, we have measured total drag acting on the test plates by the flow. Wall-shear stress is evaluated from the gradient of mean velocity on the plate surface. The pressure drag is calculated from the difference between the total drag and the friction drag. The friction drag of the wavy plate made of the hydrophilized polycarbonate sheet is 12.1% lower than that of silicone rubber sheet.

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We carried out direct numerical simulation on turbulent flow over wavy walls, which mimic the skin folds of fast-swimming dolphins. We changed the amplitude and the angle of ridgelines of the wavy wall. The computational results show a decrease of 6.0 % in the total shear stress at the wall in the case of A^+ = 6 and γ = 0 degrees. The regions of high - and - low wall - shear stress becomes smaller in the case of the angled wavy wall than that in the case of the simple wavy wall. This is due to the secondary flow along the valley.

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Detached eddy simulation of fully developed turbulent channel flows at high Reynolds numbers with a uniform blowing and suction was carried out aiming at skin friction drag reduction. The friction Reynolds number was set to be 180, 640, 2000, 8000 and 20000. The mean velocity profiles are shifted from the blowing side to the suction side. As a result, the skin friction drag was reduced on the blowing side, while increased on the suction side. The drag reduction rate on the blowing side is found to increase with the Reynolds number and the blowing amplitude.

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A flow around an airfoil controlled by using blowing and suction is studied by means of direct numerical simulation. A uniform suction is applied near the leading edge in order to delay the transition, while a uniform blowing is applied near the trailing edge to reduce the friction drag of a fully developed wall-turbulence. As a result, both the friction drag and the pressure drag are reduced. The reduction of the pressure drag reduction is attributed to an increase of the pressure on the rear part of the airfoil, while the reduction of the friction drag is due to the modification of the velocity profile on the controlled surface.

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Drag Reduction in the energy consumption for liquid transportation has attracted much interest in recent years. Previous study shows that the pressure drop in pipe decreases under the stratified condition. The objective of this study is to clarify the drag reduction phenomena of thermal stratification in pipe flow. It has been shown that the stratification of pipe flow decreases the pressure drop in 3000 < Re < 5000. Furthermore, several types of junctions are tested to promote the drag reduction by stratification.

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In the frictional drag reduction using bubbles, we proposed a periodic bubble injection technique to improve the performance and the new technique improved the performance 66% in Re 2200 at a horizontal channel flow. In this paper, the void fraction fluctuation in higher Reynolds number condition is investigated by visualization. The uncontrolled fluctuation of the void fraction fluctuation doesn't have any specific frequency, however the artificial fluctuation of void fraction maintain its frequency in streamwise. We indicate that it is possible to control density and shape of a swarm of bubbles by controlling the injection frequency and gas volume.

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Micro bubble has unique characteristics. One of them is the drag reduction. This study investigates the effect of the pipe diameter on the micro bubble drag reduction. The pressurized dissolution method is applied. The frictional pressure drop is measured in vertical pipes with different diameters. As the flow rate increases, drag reduction decreases. Drag reduction is higher in larger pipe than in smaller pipe for the same Re number. On the other hand, drag reduction is lower in larger pipe than in smaller pipe for the same JL. The lower the void fraction becomes, the higher drag reduction becomes. Pipe diameter has some effects on the drag reduction.

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We studied the effect of hydrophobic microstructural shape on laminar drag reduction experimentally. Deep truncated cone holes are machined into a test wall surface, and the surface is coated with triazine thiol to induce hydrophobicity. The aperture ratios of the test walls are 10% and 20%. Drag reduction was estimated by measuring the pressure loss in microchannels. The Reynolds number range for drag reduction in the case of the test walls was found to be wider than that in the case of conventional walls. Additionally, the percentage of drag reduction for test walls is comparable to that for conventional walls with smaller aperture ratios.

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Drag reduction have been reported in a lot of experiments in microchannels with hydrophobic cavities. In order to understand the mechanism, analytical simulations is performed. The analytical model is a rectangular channel of height h = 5 μm and width w = 20 μm with hydrophobic micro cavities on bottom and top of the channel. In this channel, water does not penetrate into micro cavities, and air remains in micro cavities. As a result, air and water form gas-liquid interface in micro cavities. It was found that the gas-liquid interface plays an important role in the drag reduction effect, and the drag reduction rate depends on the shape of the gas-liquid interface. Moreover, it was obtained that the penetrating depth of water is important for drag reduction.

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Thread-like micelles of cationic surfactant aggregate to SIS (Shear-induced structure), and the SISs are seems to attribute to drag reduction effect. In this study, we experimentally investigated drag-reducing flow using cationic surfactant solutions. The SISs were visualized in turbulent flow to elucidate the induced mechanism and role of a micelle structure in drag reduction. The streamwise changes in the SISs and the drag reduction were measured and their relationship was discussed. In our experiments, the SISs were generated from a wall and had gel-like characteristics. The number and thickness of SISs increased downstream, and this increase was correlated with the streamwise increase in the drag reduction rate.

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Drag reduction of turbulent flow by surfactant solutions is well known. However, most of studies about it have been carried out using homogeneously premixed solutions. In this study, drag reduction by injection of dense surfactant solution into a turbulent pipe flow has been studied. As a result, it is found that higher drag reduction is obtained by injection of dense surfactant solutions than that of homogeneously premixed solutions. It may be important for drag reduction by injection that surfactant solutions mix considerably with main flow.

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Pressure drops of water and surfactant solutions and their drag reduction effects were investigated in slit flows. For water, the resultant pressure drops agreed with the numerical predictions. For aqueous solutions of surfactants, the experimental results differed according to their polarity. Cationic surfactant caused the biggest decrease from the predicted values, followed by non-ionic surfactant. Anionic surfactant gave the same results as water. These effects were attributed to a liquid-solid interface phenomena.

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