The flow properties of several types of liquid passing through various sizes of micro-orifices were investigated in this paper. The jet thrust and pressure drops were measured for two polyethylene glycol solutions and four surfactant solutions. Different flow properties were found for the various surfactant solutions depending on the charge of the solute. For an anionic surfactant, the results were similar to those for water, whereas in the case of a cationic surfactant, both the jet thrust and pressure decreased greatly in comparison with the other test liquids. Finally, a nonionic surfactant exhibited a steep rise in the pressure drop at a particular value of the Reynolds number. In explaining this behavior, the liquid-solid interface and alignment of the surfactant molecules are considered, and consequently, it is strongly suggested that the elastic stress on elongational flows is a contributing factor. In addition, the decreases in pressure and thrust for polyethylene glycols are attributed to viscoelastic properties, regardless of the molecular weight of PEG.
Single-relaxation time lattice Boltzmann method lacks the required numerical stability for high Reynolds number flow simulations. The method is also restricted to Cartesian grid, making it difficult to be implemented in flow with curved boundary applications. In this work, we proposed a regularized lattice Boltzmann method with non-equilibrium extrapolation boundary condition as a remedy for both issues. The proposed method was applied to two-dimensional lid-driven cavity flow at high Reynolds number simulation to validate its numerical stability. Flow past two-dimensional circular cylinder at moderately high Reynolds number was simulated to demonstrate the method viability for flow with curved boundary. Results obtained from the proposed method were in excellent agreement with other method in literatures. In conclusion, the proposed method was found to be more effective than the standard lattice Boltzmann method for high Reynolds number simulations and flow with curved boundary.
An experimental investigation on the formation and breakup of a hollow jet issuing from a coaxial nozzle into ambient air has been carried out. The hollow jet consists of an outer jet of water that encloses an inner jet of argon gas. By varying the flow rate ratio of the inner jet to the outer jet, three different breakup patterns of hollow water jets are identified using two types of nozzles: (I) mixed hollow and simple drop formation, (II) single-core hollow drop formation, and (III) multi-core hollow drop formation. These patterns are mapped in a space of Weber number versus flow rate ratio, We-Q. Experimental results in pattern (II) show that increasing the flow rate ratio results in increasing the formation frequency, slightly increasing outer diameters of the hollow drops, and decreasing their wall thickness.
Based on the unified theory by the present authors (Kanagawa et al., J. Fluid Sci. Tech., 5, 2010), the Korteweg-de Vries-Burgers (KdVB) equation and the nonlinear Schrödinger (NLS) equation with an attenuation term for weakly nonlinear waves in bubbly liquids are re-derived from a system of bubble-liquid mixture model equations composed of the conservation equations of mass and momentum, the Keller equation for bubble dynamics, and supplementary equations. We show that the re-derived KdVB equation and NLS equation are essentially the same as those derived from a system of two-fluid model equations except for the coefficients of nonlinear, dissipation, and dispersion terms. The differences in these coefficients are studied in detail, and we find that for the case of KdVB equation, the mixture model is valid only for sufficiently small initial void fractions. On the other hand, for the case of NLS equation, the range of validity of the mixture model depends on not only the initial void fraction but also the wavenumber concerned.
Wave breaking and air bubble generation by a 2-D cylinder moving beneath a free surface were experimentally investigated. Measurements of the free surface profile and visualization of the air bubbles yield the threshold and the regime diagrams of the bubble generation, which depend on the cross-sectional shape of the cylinder, Reynolds number, effective Froude number, normalized depth of the cylinder and angle of attack. As the Reynolds and Froude numbers increase, the surface deformation becomes substantial in the downstream of the cylinder. A breaking wave with air entrainment occurs when the ratio of the wave height to the wave length is greater than ∼0.1. Our results provide useful information for design of facilities such as marine constructions and a hydrofoil air pump for drag reduction of ships.
Cavitation often involves an abrupt phase change phenomenon. This phenomenon causes cavitation erosion on the material surface. In particular, violent bubble collapse near the material surface causes severe erosion. Numerical simulation is a powerful approach by which to clarify the high-speed, complex bubble collapse behavior near the wall boundary. In the present study, numerical analysis of nonspherical bubble collapse behavior and induced impulsive pressure at several initial standoff distance from the wall boundary is performed using a locally homogeneous model of a gas-liquid two-phase medium. The second collapse is confirmed to be more violent than the first collapse, and a circular erosion pattern is predicted owing to a toroidal bubble collapse attached to the wall boundary at a certain standoff distance. The influence of the symmetry breaking of the initial spherical bubble shape on the collapse behavior is investigated, and the asymmetry is found to influence the second bubble collapse, especially the generation of the high impulsive pressure in the central area.
The net rotor torque generated by a straight-bladed vertical axis wind turbine has temporal variation for an azimuth angle of the blade. The torque variation should be investigated to understand the performances of the wind turbine. The blade camber and thickness are important to determine the characteristics of the wind turbine. We have studied effects of the blade camber and thickness on the mean characteristics and temporal torque variation at any azimuth angle of the blade. The mean torque and power increase with the smaller camber and the larger blade thickness over relatively lower tip speed ratio. The maximum mean torque and power coefficient take the largest value at certain blade thickness. The maximum value of the torque variation emerges at an azimuth angle of the blade located in upstream, and it has significant contribution to the mean torque. In particular, over relatively lower tip speed ratio, the maximum value of the torque variation remarkably increases with the smaller camber and the larger blade thickness.
The laminar-turbulent transition of a mixing layer induced by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. A mixing layer was formed between the jet from the nozzle and the surrounding quiescent fluid. The plates oscillated vertically in relation to the mean flow. The oscillation frequency was two orders of magnitude smaller than the fundamental frequency of the velocity fluctuation. Mean and fluctuating velocity components in the streamwise and normal directions were measured by hot-wire anemometers. The oscillation was found to be effective in enhancing the mixing, though the amplitude was the same order as the momentum thickness of the boundary layer at the nozzle exit. The disturbance traveled downstream as the convective instability, though it was damped only far downstream. The downstream development rate of fluctuating velocity in the normal component was larger than that in the streamwise one. Thus, the need for linear instability analysis of non-parallel flow was suggested. Streamwise variations were examined in the fluctuating velocity and perturbation energy production and convection rates, which contribute to the velocity. The streamwise variation in the streamwise component did not correspond to that of the normal component.
The laminar-turbulent transition of a boundary layer induced by jet flow ejection in the inlet region of a circular pipe was experimentally investigated. The jet flow was periodically inserted radially from a small hole in the inlet region into the pipe flow. Axial velocity was monitored by a hot-wire anemometer. The difference of properties in laminar-turbulent transition from developed Hagen-Poiseuille flow was examined. Isolated turbulence patches were generated by the jets, and then they propagated downstream. The leading edge of the turbulent patch was definite, whereas its trailing edge was not. This characteristic was similar to that of a turbulent spot in a flat-plate boundary layer. The threshold value of the jet flow rate to generate the turbulent patch was then obtained. The threshold value decreased and saturated finally with the increase in jet flow duration. The normalized jet flow duration when the threshold value was saturated increased with the increase in Reynolds number, contrary to the developed region. The normalized threshold flow rate tended to vary with the Reynolds number among three regions. All tendencies were different from those of the developed region. With the increase in jet flow rate beyond the threshold value, the duration of the turbulent patch increased, though the fluctuating velocity within the patch did not. The propagation velocities of the leading and trailing edges, and the duration and fluctuating velocity within the turbulent patch were almost constant irrespective of the jet flow ejection frequency.
In the present study, the authors consider a control of the flow past a semi-infinite plate with a blunt front. The flow is one of the simplest separated-and-reattaching flows, and can be a model such as heat exchangers and flow straighteners commonly seen inside power plants, chemical plants and household appliances. As a control device, a rotating object, which is a small flat plate, is placed in the upstream of the semi-infinite plate. In a wind tunnel, the authors conduct flow-velocity-fluctuation measurements using a hot-wire anemometer for various control-object positions and rotation speeds, showing (1) root-mean-squared values near the leading edge of the semi-infinite plate and (2) dominant frequencies in the downstream. The present test range of the control-object rotation speed is low; specifically, the object's tip velocity is less than the uniform mainstream velocity. Referring to such results, we can classify the controlled flow into four regions. Moreover, in order to clarify the characteristics for each region, the authors carry out (1) flow visualisations with particle-image-velocimetry analyses, and (2) velocity-profile measurements concerning time-mean and turbulence-intensity values.
Gas flow in high frequency oscillatory ventilation (HFOV) was numerically investigated. This artificial respiration technique has features of low tidal volume and high frequency. Although molecular diffusion appears to be influential in gas transports at lower airways, the effect of flow convection never vanishes. The present paper is aimed to discuss the contribution of flow convection to gas transports in such lower airways and obtain physical understandings of old air evacuation in the lower airways. For calculating flow in the lower region airway, boundary conditions based on compliance and resistance of airways were introduced. The obtained air flows appeared to be laminar and reversible. However, particle trackings revealed that the pathlines deviated slightly from one respiratory cycle to another, envisaging irreversible trajectories. The Lagrangian analysis also manifested the longitudinal gas redistribution characterized by the incoming central flow and the outgoing near-wall flow. Those effects would be accumulated through repetitive respiration, and result in the effective gas mixing in HFOV
The present work numerically characterizes a microfluidic device with pumping and mixing functions for lab-on-a- chip applications. The planar fluidic network of the device consists of two inlet channels, a pump chamber, a neck channel, and an outlet channel, all of which connect at a junction. The pump chamber is enclosed by a soft polymeric diaphragm. Pump performance is based on a new valveless principle. Mixing enhancement relies on the unsteady flow obtained from the pumping function. Numerical simulations are performed on two different floor structures in the neck channel: with and without an oblique ridge. The pumping result achieved shows that the flow rate increases notably with increasing deflection of the diaphragm and that the ridge has a negligible effect on the pump performance of the device. The results also demonstrate that the fluidic network of the device rectified flow better than its conventional nozzle/diffuser counterpart. For the measurement of the mixing, the intersection map, mixing variance coefficient, and mixing efficiency are analyzed. Analysis shows that mixing is substantially improved with the ridge and the device can suitably perform mixing for a wide range of flow rates. Without the ridge, mixing relies only on pure diffusion processes; thus, it is very poor. With the integrated pumping and mixing functions, the device serves as a promising step toward the realization of lab-on-a-chip or microfluidic systems.
For the application to optimal design and effective operation of air-conditioning system in an aircraft cabin, we propose an indicator to represent the air quality. It was derived from the result of the large-eddy simulation (LES) of turbulent flow with the immersed boundary (IB) method, which took into account the respiration of passengers. The cabin air quality is defined as an oxygen concentration of inhalation air (OCIA) per each breath. Degree of air renewal in reference volume is a dominant factor of air quality, OCIA. Intensity of time-averaged velocities by cross-sectional components closely correlates with the degree of the air replacement, and becomes the most suitable indicator of air quality. In case of decrease of the intensity, each of two different flow conditions, namely stagnation and circular flow just around a mouth, was found out at the reference volume as inactivation elements of the air renewal.
Influence of the flow induced by dielectric barrier discharge (DBD) plasma actuators on the wing-tip vortex is investigated numerically and experimentally. The plasma actuators are installed on the suction side of the NACA0012 airfoil and operated in blowing and suction modes. For the numerical simulation, direct numerical simulation (DNS) based on the finite-difference immersed-boundary method is used. The DNS shows that the circulation parameter, which measures the strength of wing-tip vortex, is reduced by blowing as well as suction. At the same time, however, the lift-to-drag ratio is found to decrease by the actuation. In the experiment, a wing model with plasma actuators is set inside the wind tunnel and the velocity fields are measured using a PIV system. Although suppression of wing-tip vortex is not confirmed due to insufficient strength of actuation and insufficient length of measurement area, the change of streamwise mean velocity profile is found to be similar to that of DNS.
Numerical study on the mass transfer in a two-dimensional channel with the transversely oscillating wall was carried out as the basic study to understand the mass transfer of the infused drug into the blood flow. It is clear that the actual oscillation of the blood vessel in vivo includes the many frequency components with a wide range. So, the numerical calculations were carried out varying the wall oscillation frequency widely and we considered the role of the Strouhal number mainly. The calculation results are summarized as follows. (1) In a low Strouhal number region, the wall oscillation has an effect to enhance the mass diffusion. However, further increase in the Strouhal number brings the restraint effect of the mass diffusion and brings the increase in the mass concentration at the wall. (2) The increase in the Schmidt number has an effect to clarify the restraint effect of the mass diffusion in a high Strouhal number region.
In this study, a compact synthetic jet generator using electromagnetic micro flap actuator as a driving mechanism is developed and tested. Firstly, the effects of flap's width and length on static and dynamic responses of the driving mechanism are numerically investigated. The results show significant effects of the flap length rather than width. In experiments, the electromagnetic micro flap actuators are fabricated, and the static and dynamic responses for 3 mm wide flaps with 6 and 7 mm long are examined using a high resolution photographing technique and capacitance probe. The significant differences in their static and dynamic responses are observed as expected. After assembling the electromagnetic flap actuator to the synthetic jet generator, its generated jet velocity is examined using CTA anemometer. The maximum jet velocity of 0.6-0.8 m/s at 2 mm downstream from its orifice is obtained at about 600 Hz driving frequency, and no significant effect of driving signal shape, namely sinusoidal and square signals, on the jet velocity is observed. In addition, the jet velocity is comparable to a synthetic jet generator driven by a diaphragm actuator.
In this paper, the flow behavior of viscoelastic fluid in asymmetric planar contraction geometry is studied by flow visualization and PIV measurement. Experiments are carried out for three channels of different contraction widths. The experimental results indicate that the flow field in the contraction channel is characterized by the Weissenberg number to Newtonian-like flow, transitional flow and vortex formation flow. It is found that the vortex formation flow occurs over the critical Weissenberg number, which is independent of the Reynolds number. However, the critical Weissenberg number decreases with increasing the contraction width due to the influence of velocity acceleration in the entry channel in the upstream of the contraction.
We performed numerical simulations of a non-steady separated shear layer with and without cavitation to examine the effect on flow dynamics. Particular focus was given to the interaction between vortices and cavitation. In the simulations, streamwise and spanwise vortices generated by the instability of a shear layer flow, typical features in such a flow field, were formed further upstream in cavitating than in non-cavitating flows. In cavitating flows, the vortex shedding frequency and the Reynolds stresses increase, and we observed vortex weakening caused by cavitation. We found that a Burgers-type vortex can become the origin of a cavity. Contrarily, the growth of the cavity attenuates the corresponding vortex. We described this influence of a cavity on a streamwise vortex with a simple inviscid model.
Omnidirectional reductions in drag and fluctuating forces can be achieved for a circular cylinder subjected to cross-flow by attaching cylindrical rings along its span at an interval of several diameters. In this work, the effects of ring diameter (D), spanwise width (W), and spanwise pitch (P) on vortex shedding suppression were investigated. It was found that the periodicity of the pressure fluctuation on the sides of the cylinder disappeared at higher Reynolds numbers (Red ≥ 20,000) for the ring configuration with D/d = 1.3, W/d = 1, and P/d ≈ 3, where d is the base cylinder diameter. In this configuration, the fluctuating lift force was about 1/30 that of a 2D cylinder due to the suppression of periodic shedding and the reduction of spanwise correlation. The mechanism behind this was explored through flow visualization and particle image velocimetry and is considered to be as follows: A spanwise pressure gradient originating from a stepwise change in diameter induces spanwise flow, which brings the corner vortex to the side of the ring. This promotes laminar-turbulent transition in the shear layer separated from the ring at Red ≥ 20,000. As a result, the wake behind the ring shrinks significantly, which induces a pair of large transverse vortices just behind the ring edges. Consequently, the formation of two-dimensional spanwise vortices is obstructed, and the periodicity of the vortex shedding is suppressed.
An immersed boundary method based on a finite difference lattice Boltzmann method (IB-FDLBM) is presented. The FDLBM solves the discrete Boltzmann equation including an additional collision term by using finite difference schemes. The additional term works as a negative viscosity in the macroscopic level and allows us to alter the fluid viscosity while keeping the other relevant parameters of the simulation fixed. The immersed boundary method employs a direct-forcing method, which utilizes external forces at Lagrangian points embedded on immersed boundaries to impose the no-slip boundary condition. Several benchmark simulations are carried out to validate the developed method, i.e., flows past a circular cylinder, a falling particle, and interaction between two falling particles. Couette flows between a stationary and a rotating cylinder are also simulated at various values of the relaxation time for collision. The main conclusions obtained are as follows: (1) steady flows past a stationary circular cylinder are well predicted, (2) the motions of particles falling through liquids predicted using IB-FDLBM quantitatively agree well with those obtained using immersed boundary methods based on the lattice Boltzmann equation (IB-LBM), (3) the developed method well predicts the interaction between two particles falling through a liquid, e.g., the drafting-kissing-tumbling motion, and (4) distortion of velocity fields in circular Couette flows at high relaxation times is removed by the additional collision term.
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