A micro scale flow structure induced with AC (alternating current) electric field around a sharp electrode in a particle dispersion fluid was investigated using a micro particle image velocimetry. This flow structure consists of a micro jet from the tip of the sharp electrode and circulation flows around it. This flow structure has great advantage of applications for a DNA or CNT manipulating and this is available for a micro fluidic device and novel nano scale manufacturing process. It is easy that the flow structure is controlled with a shape of the electrode and the AC electric field conditions. In this study, it was clear that the slant severed type electrode is able to generate the stable micro flow structure around a tip of the sharp electrode and the local velocity of this flow structure increases directly with the square of the electric field intensity. These results suggest that this flow induced with the alternating electric field is concerned with the dielectric properties of particles and this flow structure is able to control with the shape of the electrode.
An NMR (Nuclear Magnetic Resonance) imaging technique was employed to visualize the convective motion in a horizontally vibrated granular bed. The size segregation phenomena are observed for a binary mixture of larger and smaller particles. Temporal progress of the size segregation was evaluated for three different combinations of binary mixture. The convective motion was clearly observed by MRI, that is, the smaller particles tended to fall down along the side wall, while the larger particles tended to rise through the central region. The convective motion is observed even in the plane perpendicular to the vibration direction as well as in the plane parallel to the vibration direction for all combinations of binary mixture. It was suggested that the size ratio of larger to smaller particles and of vessel to particle affected the segregation phenomena.
The dynamics of melt migration in partially molten media has long been of interest in physical systems. In order to understand the spatio-temporal patterns of fluid transport in partially molten materials, we explored laboratory experiments on the behavior of viscous flow through deformable porous media. Our experiments were carried out in a transparent rectangular tank (2 × 20 × 18 cm) filled with deformable gel beads (poly acrylamide). A viscous fluid (sugar syrup-water mixture) was injected from a nozzle or a slit placed at the top of the tank. The viscosity and the flow rate of the injected fluid were varied (0.1-11 Pa.s; 0.05-0.33 ml/s). We identified three types of fluid flow in the experiments: (1) homogeneous permeable flow, (2) pulsating flow, and (3) localized continuous plume flow. The transition from the homogeneous permeable flow to the localized fluidized flow depends on injection flow rate, viscosity of the fluid, and deformation behaviors of the porous media. The second type of the flow has a characteristic period equivalent to the pulsation interval. This pulsating phenomenon within the gel mixture is excited by the fluid motion of the injected fluid and is controlled by a self-adjustable hydrodynamic valve composed of deformable gel beads.
Yearly, concerns on environmental problem of the earth are growing on. One of the typical issues is desertification. To inhibit harmful effects of desertification, the prediction methods which clarify mechanism of desertification are required. It is expected that numerical simulations are useful for the purpose. However, the numerical procedure and the physical models for predictions have not been established yet. Hence, the purposes of the present study are to construct the holistic simulation technique which reasonably reproduces a sand transfer, and to apply it to create an effective prevention method of desertification. The computational target is wind tunnel experiments of sand transfer around a cube on the sand surface conducted by Tominaga(1). Numerical results are compared with the experiments. In the computation, it is confirmed that the results are quantitatively similar to the experiments, for example, the eroded sand height around the cube.
A dynamic CT system was developed for visualization of consecutive three-dimensional water behavior in a PEFC stack for neutron radiography. The system is composed of a neutron image intensifier and a C-MOS high speed video camera. An operating stack with three cells based on the Japan Automobile Research Institute standard was visualized using the neutron radiography system at a research reactor JRR-3 in Japan Atomic Energy Agency. The dynamic water behavior in channels in the operating PEFC stack was clearly visualized every 15 seconds by using the system. The water amount in each cell was evaluated by the CT reconstructed images. It was shown that a cell voltage decreased gradually when the water increased and increased rapidly when the water was evacuated. It was estimated that the power generation stopped when the channel of a cell was partly filled with the water because the air supply was blocked to a cell in the stack.
Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. In this study, we improved PB-MTV in its processing speed by utilizing GPGPU(General Purpose Graphic Processing Unit) to increase sample number in measurements. We measured TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number as DNS data provided by Horiuchi, and compared the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. We also measured TKE budget in a bubbly flow in the square duct to examine effects of bubbles on TKE budget. As a result, we found that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) bubbles affect the production and diffusion rates of TKE and do not affect the dissipation rate so much, and (3) the model proposed by Troshko and Hassan can reasonably estimate the production rate of the bubble-induced pseudo turbulence.
The PIV(Particle Image Velocimetry) probe was developed in order to investigate behavior of liquid droplet in a steam turbine. The feature of the PIV probe is built in the light sheet optics system and the borescope, it can measure the liquid phase flow velocity. This probe can estimate particle size from the relational equation of the gaseous phase velocity, the liquid phase velocity, and the liquid droplet size. The relational equation was derived by the wind-tunnel test. Furthermore, it was shown that the liquid phase velocity is measured with appling of the PIV probe to actual size steam turbine, the gaseous phase velocity is measured using a pitot tube, and liquid droplet diameter can be derived.
A high-speed water jet ejected into water forms a cavitating water jet accompanied with cavitation clouds in a periodic manner. A powerful impulsive force can be caused at collapse of an unsteady cavitation cloud when a cavitating water jet impinges against solid wall. In the present experiment, cavitation clouds are observed to investigate the details such as impinging and collapsing behaviors using a constrained-type test section. The present purpose is to investigate about the behavior of cavitating water jet in the near impinging wall region and the relation of cavitation cloud collapse with pressure wave formation as well as the propagation of pressure wave. In order to estimate the high speed phenomena such as rapid and consecutive collapses of cavitation clouds and pressure wave formation, the frame difference method for cavitating flow is used in the present image analysis for cavitation cloud. The usefulness of the method is experimentally verified for the behavior analysis of high speed liquid flow accompanied with growth and collapse of bubbly clouds. As a result it is experimentally found that 1) the present image analysis method based on the frame difference method makes possible to grasp the motion of pressure wave propagation in cavitation cloud, 2) cloud collapse causes a pressure wave which propagates toward the surrounding area and as a result causes secondary local collapses in a chain-reaction manner.
As for cavitating jets injected from an orifice nozzle of inner diameter 0.15 mm at pressure 10-200 MPa into still water at atmospheric pressure, we conducted experiments of flow visualization of free jets with high-speed CCD camera at 50×104 fps and erosion tests of aluminum specimens. From these experiments, the following results were obtained. In erosion tests of cavitating jets, it is known that there are two peaks in mass loss curves, and besides, the peak of the diameter of eroded area was found to exist between two mass loss peaks. The distance may be suited to cleaning material and peening on metal for large damaged area and controlling erosion effect by adjusting injection pressure. The mass loss of downstream peak owing to collapses of cavitation clouds was highest at certain pressure. A high pressure pulse wave was generated 1-5 μs before the volume of cavitation clouds becomes smallest, the incidence frequency was of the order of 10 kHz, and the propagation speed of pressure wave was 1000-1100 m/s. Cavitation noise and erosion could be caused by these behaviors of cavitation clouds.
The evolution of a plume formed by ascending micro-bubbles is investigated by using quantitative visualization to understand the elementary processes that contribute to the inverse energy cascade as small scale flow grows to a large convective flow process. To make the visualization manageable, the plume is confined between two close parallel plates so that the plume-driven convection is restricted to two-dimensions. The results indicate that the bubble plume self-organizes, with a negative diffusion coefficient of buoyancy distribution. A mechanism for the enlargement of the plume is given in terms of two-dimensional flow interaction with the liquid phase. It is confirmed that the kinetic energy is concentrated in the low wavenumber flow from the energy spectra of the liquid phase.
In this paper, nanobubble shrunk from microbubble generated by electrolysis was observed by optical microscope in order to distinguish a bubble from impurities or contamination in the water. To keep microbubbles in focus, it is necessary to generate microbubble whose diameter is less than 5μm in focal position directly since its rising velocity is negligibly small and shrink rapidly. Therefore, diameter distribution of bubble generated by electrolysis was firstly investigated under the different configurations of electrode diameter, concentration of solution and voltage. In the result, when diameter of electrode was less than 1μm, distribution of bubble diameter was decreased to generate smaller bubble. Observation of Brownian motion of nanobubble was succeeded. Root mean square of displacement and diameter of nanobubble were simultaneously measured and compared with a behavior of the ZnO solid particle. As a consequence, root mean square of displacement of bubble is smaller than that of ZnO particle due to different structure of interface.
This experimental research concerns a simple air-bubble jet from a bottom nozzle in water. We try to conduct the measurements of the flow by applying a three-dimensional particle-tracking-velocimetry (3D-PTV) technique, in order to specify both bubble (air) and liquid (water) velocities. As tracer particles, we regard bubbles themselves in bubble-velocity measurements, and polyethylene particles suspended in water in liquid-velocity measurements. Then, we visualise the three-dimensional motions of the bubbles and the liquid. As we record stereo images using a pair of high-speed video cameras, we can get temporally-consecutive spatial information both bubble's and the liquid's. As a result, we quantitatively reveal the three-dimensional and instantaneous behaviour of the unsteady bubble-jet flow. And, using the obtained three-dimensional information, we show the fundamental flow structure of the bubble jet from a statistical view point.
The paper describes the further improvement of interferometric laser imaging for droplet sizing technique that is the novel Lagrangian approach for the measurement of droplet size and velocity vector by means of a high-speed and high-resolution camera in conjunction with the double-pulsed high-frequency Nd:YLF laser. Measurement uncertainties due to the geometric optical approximation as well as the refractive index modulation were numerically analyzed. The technique was applied to the measurement of the evaporating ethanol spray. The size transition of the single droplet within a few milliseconds was experimentally investigated. The results led us to evaluate the mass transfer rate at the gas-liquid interface of the individual droplet.
Three-dimensional reconstruction of two-phase interfacial structure in a narrow channel was conducted by using the liquid film thickness and void fraction distributions measured by liquid film sensors. This sensor based on the electrical conductance measurement between the electrodes. A pair of liquid film sensors is installed on the opposing walls of the rectangular flow channel, and the electrical conductance between the electrodes on a sensor and between the sensors was measured. Then, the liquid film thickness on the walls and the void fraction in the narrow gap are obtained in two-dimensional area. The liquid film sensor with a spatial resolution of 2×2 mm2 was applied to measure the air-water two-phase flow in the narrow rectangular channel with a gap of 1.5mm and a width of 32mm. The measured distributions of the liquid film thickness and the void fraction were used to estimate the thickness of the gaseous phase, and the gas-liquid interfacial structure was visualized in bubbly, slug and churn flows.
The lattice Boltzmann method is briefly described from its basic theory to some applications. The basic equations, the discrete BGK equations, are introduced, and the concept of this method is explained Two types of models, that are non-thermal and thermal models exist, and some thermal flows are simulated by two-particle models. Examples of the aero-acoustics and the multi-phase flows are shown. Some new topics are also briefly described, and the references are presented.
A passive pitch-angle control system for a small horizontal axis wind turbine (HAWT) has been proposed in our previous study. The main component of the system is the rubber pipe reinforced by metal fibers arranged with oblique angle, and the system is installed at the root of the blade. When the rubber pipe is subjected to centrifugal force due to the rotation of the rotor, the oblique fibers cause the torsional deformation of the pipe and the change in the pitch-angle of the blade. In this study, FEM analyses for the fiber-reinforced rubber pipe are carried out, and the influences of the structural specifications on the torsional deformation are examined. Additionally the electric power generation of the turbine with the system is estimated in order to confirm the effectiveness of the proposed system for the prevention of the overspeed of the turbine. Through the study, it is found that the fiber-angle of the fiber-reinforced rubber pipe affects the torsional deformation remarkably, and the influences of the dimensions of the rubber pipe on the torsional behavior can be simply shown as a relation between the average stress and the average shear strain. It is also confirmed that the proposed pitch-angle control system prevents the overspeed of the turbine in the strong wing condition and the stall control is more effective than the feather control.
This paper propose two new flow visualization methods based on the Background-Oriented Schlieren (BOS) technique for compressible flow. One is “Simplified background-oriented schlieren (S-BOS) ”. This technique does not require a cross-correlation algorithm, which is typically used in BOS. The data processing is much simpler than that of the original BOS, since image displacement associated with density gradient is algebraically calculated from intensities of the images with the periodic background pattern. Moreover, it easily allows us to automate the data processing, because it is neither necessary to remove incorrect vectors nor to optimize parameters such as the interrogation window and search window. The other is referred to “Wavelet-based background-oriented schlieren (W-BOS)”. This technique provides a schlieren image using continuous wavelet transformation for the periodic background pattern. By transforming the periodic intensity pattern into the phase, a schlieren image can be obtained easily. Since the optical setup in both techniques is simpler than that of a conventional schlieren imaging, they could be used in various situations including field tests. A wind tunnel test was conducted in a 1 m × 1 m supersonic wind tunnel. Their usefulness was demonstrated by comparing with the conventional schlieren images.
In order to apply the lattice Boltzmann method to a flow problem of magnetic suspensions, we have investigated the feasibility of the viscosity-modifying method that is expected to be a technique for sophisticating the activating method of the particle Brownian motion based on fluctuation hydrodynamics. We have addressed a magnetic suspension in thermodynamic equilibrium to clarify the influences of various factors such as the roughness of a lattice system and the volumetric fraction of magnetic particles on the scaling coefficient of viscosity. From the snapshots and pair correlation functions of magnetic particles, it is seen that the viscosity-modifying method can show good agreement with the results of Monte Carlo method in both quantitative and qualitative points. This good agreement is almost independent of the roughness of a lattice system if a relatively fine lattice system is used. The scaling coefficient of viscosity is almost constant and independent of the strengths of magnetic particle-field and particle-particle interactions, and also is almost constant for the variation of the volumetric fraction and the number of particles for a given lattice system unless a coarse lattice system is used. We may conclude from these results that the lattice Boltzmann method with the viscosity-scaling procedure is quite a possible technique for simulating a flow problem of magnetic particles under a non-uniform applied magnetic field.
The generation of compression wave produced when a high-speed train enters a tunnel is studied by solving the two dimensional incompressible irrotational flow over the train nose. The Schwarz-Christoffel conformal mapping is applied to model the train and tunnel portal configuration. The complex velocity potential of the flow field is defined by a distribution of sources on the nose. The contribution from the exit-flow shear layer vortex is estimated by modeling a pair of point vortices placed at short distances from the corner of the tunnel portal and imposing the Kutta condition that the velocity should be finite at the corner. The unsteady potential equation is solved to obtain the pressure field at each step of train translation. The results are compared with numerical calculations of the pressure rise obtained by the finite difference lattice Boltzmann method (FDLBM), which recovers the compressible Navier-Stokes equation of fluids. It is shown that the contribution from the exit-flow vortex increases up to approximately 7 % during the time in which the train nose enters the tunnel. The prediction with point vortices shows close agreement with the FDLBM.
Flow fields with coherent turbulent structures behind a backward-facing step have been investigated with Laser Doppler Velocimetry to clarify a mechanism of flow control with synthetic jets. Synthetic jets with a frequency which is observed in the velocity fluctuation of the backward-facing step flow without the jet have been fed perpendicularly to the main flow from the vicinity of the step edge. Periodic component of velocity fluctuation with the same frequency of the jet is extracted with procedure of phase locked averaging. Vortices shed from the step edge have an organized structure which is synchronized with the jet and these vortices are advected immediately above the separated stream line at a speed of around 45% of the free stream velocity. Amplitude of non-periodic velocity fluctuation are larger than those of periodic fluctuations due to the vortices. The kinetic energy of the non-periodic velocity fluctuations comes from the main flow and the periodic velocity fluctuations are just the beginning of such taking in. The reattachment position closely correlates with total kinetic energy of the periodic and non-periodic velocity fluctuations. This implies the flow reattachment is promoted by the mixing of the fluids in the main stream and the reversed flow region.
A phenomenon has been observed in which intracellular Ca2+ concentration in endothelial cells increases upon application of shear stress (Ca2+ response). It is therefore assumed that Ca2+ is the second messenger in the transfer of shear stress stimulation into cells. The Ca2+ response is also known to spread to surrounding cells (Ca2+ wave). We investigated the effects on Ca2+ wave among cultured bovine aorta endothelial cells (BAECs) upon inhibiting the main intercellular signaling pathways, such as gap junction and paracrine pathways by inducing Ca2+ wave using D-myo-inositol 1,4,5-trisphosphate, P4(5)-(1-(2-nitrophenyl)ethyl) ester trisodium salt (Caged IP3) due to an intracellular IP3 elevation. In addition, we investigated the Ca2+ wave among BAECs under shear stress loading. Using Caged IP3, local release of ATP from BAEC induced Ca2+ wave. The Ca2+ wave was inhibited by the inhibitors of paracrine pathways. Furthermore, the Ca2+ response spread in the direction of the downstream under shear flow. These results suggest that paracrine pathway is dominant in both of flow and no flow conditions.
The aim of this experimental study is to investigate the diffusion of jets, with particular attention focused on the relationship between the static pressure and the stream wise mean velocity on the development of a two-dimensional air jet. A jet with three injection Reynolds numbers (Re=1.0×104, 2.0×104, 3.0×104) was generated in a two-dimensional wind tunnel. The velocity distributions were measured by an X-type hot-wire anemometer. The static pressure distributions were measured by a static pressure probe developed in our laboratory, which incorporates a hot-wire anemometer. There are two important characteristics for the hot-wire static pressure probe. The one is that mean static pressure and the static pressure fluctuation can be measured at the same time. The other is that the probe is not affected by the flow and the changes of physical properties (thermal conductivity) of gases, because the sensing hot-wire is always in the air bias flow. The sensitivity of the fstatic pressure probe is 92.3 mV/Pa. The frequency response is flat from 16 Hz to 2.5 kHz. As a result of the experiment, it was found that negative static pressure exists in the turbulent shear layer. It is considered that the entrainment processes from the negative static pressure by the vortex structure motion of the turbulent shear layer.
The accuracy of a scalar probability density function method, which is able to deal with reaction exactly, has been investigated in terms of a second order closure model for flow field evaluation. Numerical calculations have been conducted in the configuration of a hydrogen/nitrogen jet nonpremixed flame. The second order closure model is based on a Reynolds stress model. The present solution is a combined PDF/moment method, in which PDFs are evolved to match both the means and variances of mixture fraction from both the PDF and moment methods. Then the two moment equations of mixture fraction are based on the second order closure. The present evaluation is based on a comparative study with the first order closure model based on a k-ε 2 equation model. The present scalar PDF model with the second order closure model evaluates adequately anisotropy and then the anisotropy affects strongly the mixing process in the near field. The spread of the jet flame is able to be evaluated adequately by the second order closure model than by the first order one. The early flow filed is extremely important and the present initial low turbulences affect the downstream flame structure.
The thermal boundary layer near the combustion chamber wall of internal combustion engine has the important key to decrease in the cooling loss. And it is thought that the pollutant emission such as unburnt hydrocarbons exists near the wall because combustion flame is quenched when it approaches the wall. Therefore, the thickness and temperature distribution in the boundary layer are important. Then, resistance wire thermometer with tungsten wire of 5μm diameter is developed for the purpose of measuring boundary layer temperature directly. The measurement of temperature distribution of boundary layer is carried out, by using a rapid compression-expansion machine (RCEM). The nitrogen gas is used in order to oxidation prevention of tungsten wire. The measurement of the various positions in the combustion chamber is possible to make the measurement position movable in the vertical direction to wall. As a result of experiment, thermal boundary layer thickness in compression stroke was 0.5mm in maximum. Next, we try to measure a quenching distance of the combustion flame in homogeneous charged compression ignition engine of a mixture of di-methyle ether and air. The quenching distance is shorter than the thermal boundary layer thickness of non-combustion conditions.
To clarify the impact of surface treatment and gas channel pitch of separator on in-plane temperature distribution and power generation performance of a single-cell polymer electrolyte fuel cell (PEFC), we have measured the in-plane temperature distribution on backside of separator at cathode by thermograph under the power generation with the operation of relative humidity of supply gas of 100 %. We have also investigated the influence of gas flow rate at inlet of the single-cell PEFC on heat transfer phenomena and power generation performance. As a result, the best power generation performance is obtained for the gas channel pitch of 1.0 mm irrespective of surface treatment of separator. The power generation performance of hydrophobic separator is better than that of hydrophilic separator since the performance of discharging the water produced in gas channel is better and the pressure loss of gas channel caused by gas flow is smaller. The temperature drop at the last turned edge of gas channel in observation area with increasing gas flow rate for hydrophobic separator is larger compared with hydrophilic separator by 1-2°C. The higher improvement of power generation performance due to increasing gas flow rate is obtained for gas channel pitch of 1.0 mm compared with gas channel pitch of 0.5 mm irrespective of surface treatment of separator.
For analysis of in-cylinder flow from intake to compression process, a optical engine with a quartz sleeve for high engine speed was developed. This engine was able to be driven up to 10000rpm. We measured and analyzed in-cylinder flow using PIV. According to the result, 2 pairs of vortex were observed under intake valves on intake process. This vortex is a feature flow of engine with high volumetric efficient intake port. We also evaluated in-cylinder flow of 2 valves and 1 valve deactivation. A turbulent intensity of 1 valve deactivation was about 3 times one of 2 valves and it was confirmed to obtain the combustion promotion by turbulence at high engine speed on both of optical and metal engine.