In this study, characteristics of phase lag between displacement of forcibly oscillating test body and velocity fluctuation caused by vortex formation behind the test body were investigated. A right isosceles triangular cross-sectioned prism was used as the test body. The base of the isosceles triangle is 100mm and span-wise length of the prism is 300mm. The test body was rotationally oscillated at seven different frequencies from 0.205Hz to 0.235Hz in 0.005Hz interval providing the natural vortex shedding frequency is 0.220Hz. As the results, the behavior of the phase lag is summarized as followings. i) In general sense, it is well known that the phase lag is shifted by π within the lock-in region. Besides that, intermittent slip of phase lag was observed. ii) The velocity fluctuation does not synchronize perfectly with the forced oscillation even when the frequency of the forced oscillation is set to natural vortex shedding frequency of the test body. iii) For the case of forced oscillation frequency is set to the natural vortex shedding frequency, synchronization rate of the velocity fluctuation with the oscillation was found to be 80% at the maximum.
A numerical code based on one of mesh-free particle methods, a Moving-Particle Semi-implicit (MPS) Method has been used for the simulation of free surface flows in a bucket of Pelton turbines so far. In this study, the flow in a stationary bucket is investigated by MPS simulation and experiment to validate the numerical code. The free surface flow dependent on the angular position of the bucket and the corresponding pressure distribution on the bucket computed by the numerical code are compared with that obtained experimentally. The comparison shows that numerical code based on MPS method is useful as a tool to gain an insight into the free surface flows in Pelton turbines.
In this paper, the numerical simulation of incompressible flows around a fish model at low Reynolds number using the seamless virtual boundary method is presented. In order to satisfy the velocity conditions on the virtual boundary points, the forcing term is added not only on the grid points near the boundary but also on the grid points inside the boundary in the seamless virtual boundary method, so that the smooth physical quantities can be obtained. The present approach is verified by the flows around a 2D oscillating circular cylinder and a sphere. The flows around the stationary and swimming fish models are simulated for application to the more complicated flow geometry with moving boundary. These results show that the flows with the stationary and moving bodies can be predicted precisely. Then, it is concluded that the present seamless virtual boundary method is very promising for the numerical simulation of incompressible flows with the complicated geometry and the moving boundary.
The relation between the development of the wall boundary layer in a convergent nozzle and free surface waves on the emanated liquid jet has been investigated experimentally. In the convergent nozzle, which forms a water jet along a flat back wall, the velocity profiles of the wall boundary layer were measured using a laser Doppler velocimeter with jet average velocities of U0 = 5, 10 and 15 m/s. The property of free-surface waves and the intermittency factor of free surface fluctuations were also determined by photography and an optical measurement technique, using laser beam refraction on the jet surface. For the lowest velocity case U0 = 5 m/s, the mean velocity profiles of the boundary layer under the pressure gradient in the convergent nozzle indicated an inverse transition from turbulent to laminar boundary layer, so-called relaminarization. On the other hand, for the higher velocity case U0 ≥ 10 m/s, the relaminarization in the convergent section became incomplete. For these cases, the turbulent intensity near the wall increased significantly and the nozzle-exit boundary layer was restored quickly to a turbulent profile for a short parallel section immediately upstream of the nozzle exit plane. The increase in the velocity fluctuation near the wall promotes wave generation downstream of the nozzle exit plane. Therefore, the intermittence of the wave packet almost disappeared and the jet free surface was covered with continuous capillary waves when the turbulent boundary layer was separated from the nozzle exit.
This paper deals with the development of a numerical approach to predict freestream turbulence-induced (FST-induced) boundary layer bypass transition using an intermittency transport equation. An intermittency based transition model, which is critical for invoking transition onset according to Abu-Ghannam and Shaw correlation, is implemented into the proven Reynolds-Averaged N-S (RANS) solver. The intermittent behavior of the transitional flow is incorporated into the computation by modifying the eddy viscosity µt , obtained from a turbulence model. Wilcox low Reynolds k –ω turbulence model is employed to calculate the eddy viscosity and others turbulent quantities. For validation, the present transition model is applied to the benchmark experiments of flat plate test cases of ERCOFTAC series and to predictions of a modern Low Pressure (LP) turbine flow. It follows from the detail comparisons of the calculated results with the relevant experimental data and other researchers' simulations that the present model is capable to make a reasonable prediction of FST-induced bypass transition.
Numerical simulations were carried out modeling the bioconvection generated by chemotactic bacteria in a shallow chamber, and the influence of the Rayleigh number on bioconvection was examined. To confirm the present numerical accuracy, the numerical result was compared with a previous analytical solution. The concentration distributions for a stationary flow field were in good agreement with the analytical solution. Next, the variation of flow and concentration fields with the Rayleigh number was examined. At small Rayleigh numbers, the bioconvection does not occur, and the bacterial cells collect near the free surface. The concentration distributions agree well with the analytical solution for the stationary fluid. At a critical Rayleigh number, bioconvection is formed in the center of the chamber. Since bioconvection increases with an increase in the Rayleigh number, a large difference between concentration distributions above the critical Rayleigh numbers and the analytical solution appears. The bacteria are active near the bottom wall in the chamber because adequate oxygen is supplied from the free surface. When bioconvection occurs, convective transport becomes dominant rather than the transport due to oxygen diffusion and bacterial swimming.
The unsteady phenomena in the transonic flow around airfoils are observed in the flow field of fan, compressor blades and butterfly valves, and this causes often serious problems such as the aeroacoustic noise and the vibration. In the transonic or supersonic flow where vapor is contained in the main flow, the rapid expansion of the flow may give rise to a non-equilibrium condensation. In the present study, the effect of non-equilibrium condensation of moist air on the shock induced flow field oscillation around a plate was investigated numerically. The results showed that in the case with non-equilibrium condensation, the flow field aerodynamic unsteadiness is reduced significantly compared with those without the non-equilibrium condensation.
The effect of surfactant solutions on laminar boundary layer over a flat plate has been investigated at Re < 15.3×103 by measuring the velocity profile using a PIV system. It was clarified that the boundary layer thickness of surfactant solutions increases significantly compared to that of tap water. In the upstream region of the leading edge, the boundary layer (or the stagnation area) has already been generated and the thickness of this area is already large. In the downstream region, the velocity profiles of all surfactant concentrations at the near-wall region, are similar, but not identical, to that of tap water. At the far-wall region of the boundary layer, the velocity profile is significantly different. For higher surfactant concentrations, the velocity profile describes like S-shape profile, which is greatly different from that of lower concentration. When the Reynolds number increases, the velocity profile of the surfactant solution gradually develops. The dependence of the boundary layer thickness of the surfactant solution on the distance downstream from the leading edge is similar to that of tap water. Consequently, the surfactant gives large effect for not only development of the boundary layer but also the generation of it.
The aim of this work is to provide an impulse type hydraulic turbine to utilize unexploited water resources as nano hydropower in the mountainous area. The turbine is simplified to make cheap energy and uses inexpensive components for widespread utilization. The turbine model is tested experimentally to reveal the power characteristics. The flow visualization and numerical simulation are conducted to clarify the behavior of free surface flow in the runner with different nozzle positions. The experimental results show that the maximum runner efficiency of the prototype turbine is 0.56. Numerical simulation shows that the output power depends on the nozzle positions with the impingement of the tail of the jet portion to the backside of the blade. This study gives the fundamental information of the turbine performance to acquire a guideline for future practical applications.
Pulsating jets are very common and useful in various industrial aspects. However, they have some different basic characteristics from steady jets. In this research, the authors reveal both the frequency and the amplitude effects of pulsations on a jet, including large-amplitude cases. Experiments are conducted at a Reynolds number of 5,000, Strouhal numbers of 0.011 - 0.27 and velocity-amplitude ratios of 0.1 - 1.0. Using a hot-wire anemometry, the authors show centre velocities, radial profiles, half widths and total flow rates at several positions downward a nozzle exit. From centre velocities and radial profiles, we can see that the streamwise scale of the potential core is less than three times the diameter. From half widths and total flow rates, we can see that the pulsation enhances mixing, and confirm the frequency and the amplitude effects specifically. On the other hand, from centre velocities and radial profiles, we cannot see the both effects clearly. As a result, the authors show the combined frequency-and-amplitude effect upon total flow rate, which consists with small-amplitude results by Crow and Champagne (1971), and find the optimum frequencies.
In this paper, the Cartesian grid approach with virtual flux method based on the usual incompressible Navier-Stokes equations is proposed for simulating the incompressible multiple flows with heat transfer. In this approach, the fluxes on the closest grid points to virtual boundary are estimated to satisfy the velocity, pressure and temperature conditions on the virtual boundary. First, the present method is validated in flows around a circular cylinder and a pair of circular cylinders in transverse arrangement. As a result, this approach gives the good flow fields quantitatively in comparison with the reference results. Then, the method is applied to the consolidated simulations of multiple flows with heat transfer between heated and cooling flows. Therefore, it is concluded that the present approach is very fruitful for the consolidated simulation of incompressible complicated multiple flows with heat transfer.
Computational fluid dynamics (CFD) simulations were carried out for an oscillatory flow in a 3D realistic model of the human central airways to reveal the effect of airway geometry on the oscillatory flow structure. This 3D realistic airway model is a computational model of multi-branching airway constructed based on X-ray CT images of actual human airways in which the airway diameter ranged from approx. 2 to 14 mm, and the flow in this airway model was simulated using CFD software (Fluent). The resultant inspiratory flow patterns in this 3D realistic airway model were relatively similar to patterns observed in a simplified planar multi-branching airway model, whereas the expiratory flow patterns strongly depended on the realistic airway geometry and showed more complicated secondary flow patterns. Secondary flow velocities were higher in the realistic airway model than in the simplified airway model in both the inspiratory and expiratory flows. Langrangian fluid particle tracking clarified the convective dispersion due to asymmetric inspiratory and expiratory velocity profiles.
We studied the drag reduction effect and wettability of surfaces with microscale cylindrical pillars. As the ideal surface that satisfies these two antinomy conditions, improvement of drag reduction effect and maintenance of air layer, we propose the microstructured surface with dual length pillars. Pillars are partly made shorter (a half of original ones) so that the pillars are arranged densely while the liquid solid contact at the tip of pillars is reduced. Test surfaces with dual-length pillars demonstrated greater drag reduction (over 10%) compared to a surface with uniform length pillars. We also examined the temporal variation of area of air layer retained on the tested surface that was exposed to water with or without flow. The dual-length pillar surface demonstrated better durability against wetting compared to that without short pillars.
A confined jet sometimes causes a self-exited oscillation due to the existence of a downstream target. In this work, the authors study this phenomenon. More specifically, the authors deal with a simple fluidic oscillator; namely, a two-dimensional confined jet into an abruptly-expanding channel with a downstream target of a square cylinder. The authors conduct the velocity measurements by an UVP (ultrasonic velocity profiler). Besides, the flow patterns in the fluidic oscillator are observed by a PIV (particle image velocimetry). As a result, the authors reveal the geometrical effects upon the range of stable jet's oscillation, such as the aspect-ratio effect, the channel-breadth effect, the cylinder-size effect and the cylinder-distance effect, together with the Re effect.
In this study, we demonstrate the transportation of small liquid droplet on vertical parallel electrodes using electrowetting phenomena and the interfacial oscillation. The transportation of liquid droplets, which can be applied to droplet-based biochemical devices, is performed by applying AC voltage that ranges from 0 to 100 Vpp and from 20 to 100 Hz. The flow characteristics are observed by a high speed camera, and the effects of interfacial oscillation and the dynamic contact angle on the droplet motion are investigated in detail. The experimental results show that the vertical velocity of the droplet can be controlled by changing the amplitude and the frequency of the applied AC voltage. Furthermore, the dynamics of the droplet are estimated theoretically considering the interfacial tension due to the dynamic contact angle and the gravitational force. The predicted force and the displacement of the droplet reasonably agree with the experimental values obtained from the high speed observation of the droplet motion.
Sewage pumps are designed with a wide flow channel by, for example, sacrificing some efficiency and reducing the number of blades, in order to prevent plugging with foreign bodies. Authors have been proposing a new type of sewage pump impeller designed to further improve pump efficiency and performance in passing foreign bodies. This sewage pump impeller has a structure in which the suction flow channel of closed type non-clog pump is wound in a helical spiral. The focus of this research was to investigate internal flow in this single blade sewage pump impeller. The results clearly indicated the following facts. As a result of having compared experimental results by LDV with calculated results by CFD, there is the difference about the radial component of absolute velocity, but agrees about pump performance and the tangential component comparatively well. A flow to circulate through in the impeller again is accepted by an experiment, and the sign is accepted by CFD in a specific impeller position.
Sewage pumps are designed with a wide flow channel by, for example, sacrificing some efficiency and reducing the number of blades, in order to prevent plugging with foreign bodies. Authors have been proposing a new type of sewage pump impeller designed to further improve pump efficiency and performance in passing foreign bodies. This sewage pump impeller has a structure in which the suction flow channel of closed type non-clog pump is wound in a helical spiral. The focus of this research was to investigate internal flow in this single blade sewage pump impeller by LDV measurement and CFD. As a result of having examined influence of impeller blade loading and volute casing, it clearly indicated the following facts. As well as influence of impeller blade middle loading and influence of the secondary flow, by increase of slip by influence of volute casing, static pressure and total pressure suddenly decrease together towards the end from impeller blade middle neighborhood.
Pulsating jets are very common and sometimes useful in industrial fields, due to some differences in basic characteristics from steady jets. In this research, the authors focus upon the mechanism of the frequency effect on a finite-amplitude pulsating jet. Experiments are conducted at a Reynolds number of 5,000, Strouhal numbers of 0.13, 0.20 and 0.27, and a velocity-amplitude ratio of 0.5. Using olive-oil smoke, the authors visualise the flow from a nozzle exit with a circular cross section, and get quantitative information by a PIV technique. As a result, the authors have succeeded in defining the locations of main and subsidiary ring vortices. And, the authors have revealed the vortices' convection manners at three frequencies. Besides, the instantaneous measurements on entraining flow with a conditional-sampling technique have shown the complexity of the frequency effect, which is affected by two factors in a trade-off relation.
In this study, we propose a novel technique to measure electric fields and electrokinetic phenomena in microchannels using two kinds of tracer particles with different electrophoretic mobilities. In the present technique, electric fields, whose precise measurement is quite difficult, is obtained from the velocity differences between two kinds of tracer particles. The principle of electric field measurement is based on the flow visualization and the mathematical analysis of the flow fields of different particles. Each tracer particle with different mobilities due to surface charge densities is mixed in a buffer solution to analyze the flow by particle image velocimetry (PIV). Experimental demonstrations are successfully conducted both in straight and T-shaped microchannels with rectangular cross sections. The experimental results of the electric field are compared with the roughly estimated values based on the numerical analysis of the Laplace equation. The comparison indicates the applicability of the proposed technique because they are in reasonable agreement. Moreover, the detailed electrokinetic phenomena, e.g., the electrophoresis and electro osmosis flow, are successfully investigated using the electric field obtained here. These results lead us to conclude that the proposed method can be used as a practical tool to access the details of electrokinetic phenomena in microchannels.
Creep tests, flow birefringence measurements, and flow visualization experiments in shear flows between parallel plates were carried out for aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) with excess of NaSal. The mol concentration of CTAB was fixed to 0.030 mol/l and the concentration ratio of NaSal to CTAB was varied. The creep tests indicate that it requires long time to develop micellar network structures for low to moderate concentrations at low shear stresses. Under conditions of shear rate at which the shear-thickening property in viscosity is observed in a flow curve, periodical changes in flow birefringence and shear stress occur and these frequencies are almost the same as that of the emergency of white turbidity observed in the flow visualization. These results indicate that relatively large structures of micellar network are induced by the shear flow and repeatedly appear and disappear.
A high-precision simulation algorithm for gas-liquid two-phase flows on unstructured meshes has been developed to simulate gas entrainment phenomenon in a sodium-cooled fast reactor. In this study, it became clear that unphysical behaviors near gas-liquid interfaces were caused by conventional algorithms. Then, physics-basis considerations were conducted for mechanical balances at gas-liquid interfaces to derive appropriate formulations. By defining momentum and velocity independently and developing the momentum transport equations for both gas and liquid phases, the physically appropriate formulation of momentum transport was derived, which eliminated the unphysical pressure distribution caused by the conventional formulation. In addition, the physically appropriate formulation was derived for the pressure gradient to satisfy the mechanical balances between pressure and surface tension at gas-liquid interfaces. As the validation test, the rising gas bubble in liquid was simulated by the developed simulation algorithm with the physically appropriate formulations, and the simulated terminal bubble shapes on the structured and highly-distorted unstructured meshes coincided with the experimental data under each simulation condition determined by the Morton and Eötvös numbers.
The adoption of the contra-rotating rotor has been proposed in response to a demand for the axial flow pump with higher specific speed. The internal flow field and blade rows interactions between front and rear rotors should be considered in the design for higher performance and more stable operation, but has not been clarified yet. The flow field in contra-rotating axial flow pump was measured at various flow rates with the LDV and wall pressure measurements. In the present paper, the experimental and numerical results in the flow field between front and rear rotors are shown at the design flow rate and the flow behaviors are discussed, related on blade loading, blade tip leakage flow, wake and blade rows interaction.
A slot coating application is an attractive coating method for creating a uniform coating thickness over a substrate material. However, a variety of disturbances in operational conditions (e.g., pumping fluctuations, motor system cogging) may lead to unacceptable variations in the final coating thickness. The dynamic response characteristics of the final coating thickness resulting from such disturbances can be computed using a numerical analysis technique that employs a visco-capillary model. In addition, there are two different methods for controlling the thickness of the final coating. One passive control method changes die lip geometry, and the other active control method uses a flow feedback system coupled with a final coating thickness sensor. The passive technique can suppress final thickness variations by 20-30% in a limited number of cases, whereas the active technique substantially reduces thickness variations in every case where the disturbance frequency is less than 10 Hz.
This paper presents results of an experimental study on incompressible turbulent flow over a rectangular cavity. The experiment was carried out to clarify the strength and three-dimensional nature of cavity oscillations for fully turbulent inflow. Particle image velocimetry (PIV) and fluctuating pressure measurements were made in a closed, re-circulating water tunnel for cavity length-depth ratio from L/D=1 to 4 at a momentum thickness based Reynolds number, Reθ=8,300. Measured power spectra of pressure fluctuations show dominant frequencies with broad peak width indicating the absence of resonant oscillations. Pressure fluctuations at two spanwise separated locations are well correlated at the frequency range in which dominant peaks are found. Instantaneous flow fields reveal strong three-dimensional distortions. Although formation and passage of turbulent eddies are observed in the instantaneous x (streamwise)-y (vertical) plane flow field, instantaneous x-z (spanwise) plane flow fields show strong spanwise variations.
The effect of an intermediate standing baffle on the structure of flow in a rectangular open channel has been successfully captured by an Ultrasonic Velocity Profiler (UVP). Various spatial distributions such as on-axis time-averaged velocity profiles and turbulent intensities at different streamwise positions indicate the flow structure of the uprising flow at the upstream of baffle, vortex shedding and flow separation, change of the effective cross-section at immediate downstream of the baffle and recirculation flow at the downstream of the baffle. These phenomena are also reflected in the peak values of the relative turbulent intensity profiles. Space-Time on-axis velocity color-map of upstream and downstream sections confirms the existence of periodic change of flow direction near the edge of the baffle at downstream sections. The captured phenomena were also categorized by observing four types of Phenomenological Zero Crossing Points (PCP). Comparison of space-dependent power spectra of upstream and downstream sections of the baffle indicates the existence of some peak structures concentrated near the edge of the baffle for downstream sections whereas such peak structures have not been observed for the downstream sections. Also for downstream sections mainly the existence of peak values in the space distribution of two frequency modes could be confirmed which can be attributed to the vortex shedding due to the existence of the baffle.
The three dimensional structure of the wake vortex was investigated in relating to the aeolian tone from the inclined blade. It was pointed out that the aeolian tone is largest not at zero, but at ten degree of the inclined angle. The correlation length of the wake vortex in span becomes large at small inclined angles. The large scale structures of the wake vortex are generated in zero and ten degree of inclined angle. The shape of the structure is influenced with the span-wise velocity in the dead air region. There are small scale spots of the pressure fluctuation at the trailing edge. The spots are interacted each other and make a large cluster of the pressure fluctuation. It is proposed that the scale of it is decided by the timing of the wake vortex formation. And the shape of the wake vortex structure is closely related with the scale of it. The correlation length of the wake vortex is calculated from the shape of the structure. The estimation of aeolian tone by using the length with Fukano's model is well coincided to experiments.
The sewage pumps are demanded a high pump efficiency and a performance in passing foreign bodies. Therefore, the impeller used by these usages requires the large passed particle size (minimum particle size in the pump). However, because conventional design method of pump impeller results in small impeller exit width, it is difficult to be applied to the design of single-blade centrifugal pump impeller which is used as a sewage pump. This paper proposes a design method for single-blade centrifugal pump impeller. As a result, the head curve of the impeller designed by the proposed design method satisfied design specifications, and pump efficiency was over 62% more than conventional single-blade centrifugal pump impeller. By comparing design values with CFD analysis values, the suction velocity ratio of the design parameter agreed well with each other, but the relative velocity ratio did not agree due to the influence of the backflow of the impeller entrance.
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