In this paper, we propose a new concept of a magneto-rheological (MR) fluid damper, which is a passive MR fluid damper. The passive MR damper has no electrical devices, such as a sensor, power supply and controller, and hence, it has an advantage in reliability and cost compared with semi-active MR dampers. Moreover, the proposed MR damper can be designed to have a variable damping force in response to its displacement. In this paper, the dynamic performance of the passive MR damper is experimentally demonstrated. The prototype of the proposed damper has been manufactured in order to verify the dynamic performance. The displacement excitation test result of the damper demonstrates that the damping characteristics depend on its displacement amplitude, that is, the damper behaves as a linear viscous damper under small vibrations and develops much higher damping performance under large vibrations.
To predict unstable characteristic of mixed flow pump, two mixed flow pumps were experimentally and numerically investigated. Calculated characteristics agreed well with experimental ones. It was clarified from the numerical results that stall mechanism was different between two impellers. One was the blade stall induced by separation on the suction surface of impeller. The other was wall stall induced by separation on the casing wall. Two stall indices were also proposed from the numerical results. The diffusion factor was the index indicating the inception point of the blade stall. The Taylor's pressure coefficient was the index indicating the inception point of the wall stall. The critical values of those indices for the two pumps investigated in this study were derived from the calculation results and agreed well with those values derived from the experimental results.
A visualization study is made of flow of a constant-density viscous fluid in a closed cylindrical container. Fluid motions are generated by the rotation of one of its endwalls. Vortex breakdown bubble(s) in the meridional plane are visualized by using a high-precision turntable rig. One endwall is a flat disk, and the other endwall is a cone. Cones of inclination angle α=0° (flat disk), 30°, 45°and 60° are employed in the experiment. The visualization photographs, produced by employing fluorescent dye technique, reveal the vortex breakdown characteristics. The breakdown regime diagrams in the aspect ratio-Reynolds number plots are constructed. The changes incurred in the locations and sizes of the breakdown bubble(s) are elaborated. Based on the visualized flow data, plausible physical interpretations are offered.
An experimental investigation is conducted to develop an electrohydrodynamic (EHD) pump based on microelectromechanical systems (MEMS) technology. In EHD conduction pumping, Coulomb force is the main driving force for fluid motion. The non-equilibrium process of dissociation and recombination of dielectric liquid, HFE-7100, produces hetero-charge layers in vicinity of electrodes. The attraction between the hetero-charge layers and electrode surfaces generate the net motion in the dielectric liquid by applying asymmetric electric fields. In order to generate the asymmetric electric fields, two electrode plates are prepared. On one plate, an electrode pattern is designed as a series of planar micro scale comb fingers. The other electrode plate is a non-patterned plane surface. The working fluid is confined between the two electrodes which face each other. The generated pressure and the liquid flow rate are measured for the different asymmetric electric fields. The validity of the conduction pumping mechanism is confirmed by experiments.
An optical servo system is a new control system which can be used in hazardous environments; such as those with electromagnetic influence, radiation and so on. The purpose of our study is to develop such an optical control system. In our previous study, an optical servo valve in which the output differential pressure was proportional to input optical power had been developed. However, the dynamics of the valve depended on the time required to move the flapper membrane of a fluid booster amplifier using the lower flow rate from the photo-fluidic interface. In addition, the lifetime of the valve depends on that of the fluid booster amplifier that has mechanical moving parts. As a next step, we need to improve the dynamics and to get longer lifetime of the optical servo valve and try to develop another type of optical servo valve whose elements have no mechanical moving parts. In this paper, a photo-fluidic control valve which consists of the photo-fluidic interface and fluid amplifier only using fluidics is proposed. As a result, we found that the tested valve generated output differential pressure of + 80 kPa or -80 kPa according to applied optical power. By driving a pneumatic cylinder whose inner diameter is 16 mm with a stroke of 100 mm using the tested valve, we also confirmed that the tested valve has enough output fluid power to drive a small-sized pneumatic cylinder on the market.
An optical servo system is a new control system that can be used in hazardous environments. The purpose of our study is to develop such an optical control system. In a previous study, we had realized an optical control system that executed cart positioning using optical control signals instead of electric signals. We developed an optical servo valve in which the output pressure was proportional to input optical power. As a next step, we need to develop another type of optical valve in order to get higher pressure-gain. In this study, we propose and produce an optical on/off valve that consists of an optical on/off device and a fluid amplifier, and the structure, operating principle and fundamental characteristics of the valve are investigated. As the result, we obtain a higher output pressure of the tested valve compared with the previous one. And we propose the analytical model of the optical on/off device and identify the system parameters. We confirm their validity by comparing them with experimental results. And finally, we improve the dynamics of the device by using a feedback passage plate based on analytical results of the device.
We have performed experiments in a turbulent mixing layer with periodic forcing introduced by a Piezo Film Actuator (PFA). Three different lengths of PFAs have been used, and the effects of various combinations of forcing amplitudes and frequencies are investigated. The forcing at the first and second sub-harmonic frequencies against the natural frequency enhances the development of the thickness of the mixing layer: the mixing layer spreads due to the forcing. On the other hand, the forcing near the natural frequency suppresses the development: the mean velocity gradient becomes steeper than the no control case. The vector pattern of the periodic velocity components indicated the formation of the vortical structure. By forcing at the natural and its first sub-harmonic frequencies, two counter-rotating vortices are clearly observed in one period of forcing. By forcing at second sub-harmonic frequency, the vortical structure is found only in the downstream region. The distribution of the periodic Reynolds shear stress significantly varies with the forcing frequency and it takes a positive value when forcing occurs near the natural frequency. However, the total value of the Reynolds shear stress remains negative due to the contribution of the turbulent components.
An experimental study of counter-current annular two-phase flow of high-viscosity liquid and air in a large diameter pipe was carried out to investigate the inception criterion for entrainment of molten slag droplet in an entrained flow coal gasifier. Liquid film thickness was measured using a high-speed camera for 4 types of liquid with various viscosities and surface tensions. It was clearly shown that the measured average wave amplitude had a good correlation with the gas Reynolds number, and that the predicted critical gas velocity for droplet inception using new formula of wave amplitude had a good agreement with the experimental results under the condition of very low fluid Reynolds number( Ref ≤10). The critical Weber number Wec =1.73 was obtained as inception criterion of droplet entrainment.
Molecular dynamics (MD) study of equilibrium system of a single argon nanodroplet and its surrounding argon vapor is carried out to address a fundamental issue whether the thermodynamic description is applicable to the nanoscale inhomogeneous system. The numerical result is sufficiently reliable so that it can identify the smallest droplet standing stably over 200 ns. The validity of the Laplace equation for nanodroplet is proved by a purely mechanical argument on the basis of directly computed normal and tangential pressures in the transition layer. Furthermore, it is demonstrated that the chemical potentials of liquid and vapor phases are not equal when a droplet is so small that the number of molecules consisting the transition layer may be comparable to that in the droplet. The Kelvin equation does not hold in such a case.
A molecular tagging technique utilizing evanescent wave illumination was developed to investigate the motion of a caged fluorescent dye in the vicinity of the microchannel wall surface in electroosmotic and pressure-driven flows. A line pattern in a buffer solution was written by a pulsed UV laser and the uncaged dye was excited by the evanescent wave with total internal reflection inside the glass wall using an objective lens. The velocities calculated by the measured displacement of the near-wall tagged region were compared with the results of molecular tagging using volume illumination, which represents the bulk flow information. Concerning electroosmotic flow, the micro-PIV technique using a confocal microscope system was applied to the microchannel rinsed by the caged fluorescein beforehand in comparison with a pure glass-PDMS microchannel to examine the effect of dye adsorption to the wall on the electroosmotic mobility. The electroosmotic mobility obtained by evanescent wave molecular tagging (EWMT) showed close to the micro-PIV measurement result near the glass wall for the rinsed case and the uncaged dye at the almost constant velocity remained in the depthwise illumination region. On the other hand, the dye velocity in pressure-driven flow by EWMT increased rapidly with respect to time. The uncaged dye convected to the streamwise direction dispersed toward the wall due to the concentration gradient of the dye, which was confirmed by the numerical simulations.
Impingement process of a nanoscale liquid droplet on solid wall was studied with a molecular dynamics simulation technique in order to investigate one of the elementary processes in inkjet printing technology. We proposed a new wall model consisting of Lennard-Jones (LJ) particles with fixed position, which shows a virtual friction between the wall and the droplet. A droplet consisting of about 14,000 LJ particles was projected with a given speed onto the model wall, and change of the droplet shape was analyzed. After the collision, the droplet spreads on a “hydrophilic” or strongly interacting wall, but bounces on a “hydrophobic” wall. Next we investigated the impinging dynamics on “pre-patterned” walls, which is a promising technique for electronic circuit fabrication with inkjet printing. When the wall is patterned with hydrophilic and hydrophobic regions, the droplet tends to spread only on the hydrophilic area. The droplet is able to trace much complicated patterns when the impinging speed is sufficiently large. In the case that the speed exceeds a certain threshold, however, the droplet breaks up.
A mixed-flow pump with an unshrouded impeller was computed by a one-way coupled fluid-structure simulation to evaluate a prediction accuracy of stress and analyze a flow pattern which caused the largest stress. The stress occurring around a blade root was predicted by a numerical simulation and compared with an experimental one. Five flow rates, Q/Qbep=0,40,70,100 and 120% were simulated and the predicted stresses at all flow rates agreed with the experimental ones within -11∼+6% accuracy. The largest stress occurred around a blade root on a pressure side of blade surface at all flow rates. The stress became largest at 70% flow rate. A flow pattern around the blade was analyzed to investigate how the largest stress occurred at 70% flow rate. It was found in this study that a flow separation occurred around a leading edge on a suction side of blade surface at 70% flow rate and the largest load was acting on an outside region of blade.
This paper reports the result of a primary experimental and analytical study used to explore a reliable technology that is potentially applicable to the inactivation of micro-creatures contained in ship ballast water. A shock wave generated by the micro-explosion of a 10mg silver azide pellet in a 10mm wide parallel test section was used to interact with a bubble cloud consisting of bubbles with average diameter 10µm produced by a swirling flow type micro-bubble generator. Observations were carried out with a high-speed camera, IMACON200, and the corresponding rebound pressures of the collapsing bubbles were measured with a fiber optic probe pressure transducer that provides high spatial and temporal resolutions. We found that micro-bubbles collapse in several hundred nanoseconds after the shock exposure and the resulting peak pressure pulses that repeatedly occurred exceeded well over 200MPa measured at the 20mm distance from the explosion center. These continued for well over 20µs. The experimental pressure responses were explained by solving the one-dimensional bubble Rayleigh-Plesset equation. Such high peak pressures could be used effectively for the inactivation of micro-creatures contained in ship ballast water.
Numerical study on the two-dimensional bioconvection in a deep chamber was carried out under the condition of the oscillating surface oxygen concentration to obtain the basic data for the control of bioconvection by using the surface oxygen concentration. As the numerical results in a large Γ region have an important meaning from a practical viewpoint, the numerical calculations were carried out within the range of 100000 ≤ Γ ≤ 500000, where Γ is a non-dimensional parameter that corresponds to the Rayleigh number in natural convection. The calculation results are summarized as follows. (1) The inactive zone where the bacteria do not swim actively is formed near the bottom of chamber. (2) The oscillating surface oxygen concentration has an effect to enhance the cell sedimentation. (3) The enhancement of the cell sedimentation due to the oscillating surface oxygen concentration becomes remarkable with the decrease in Γ.
A numerical comparison of time-stepping schemes for the solution of the conservation equations of mass and momentum of polymer electrolyte fuel cells (PEFCs) is presented. Darcy drag force is the dominating term for the fluid flow inside the gas diffusion layer (GDL). Permeability of the GDL is usually very small. Convergence of continuity is a major problem for the fluid flow through the GDL, and the situation becomes critical in lower-permeability cases. To overcome this kind of severe situation, an implicit scheme is used to obtain not only faster convergence but also a more accurate continuity condition. Explicit, implicit and semi-implicit treatment of the Darcy drag term is considered, and the schemes are compared for various physical parameters of the GDL, e.g., the Darcy number and the porosity parameter, by one dimensional numerical simulation. Influence of strict convergence on the secondary flow effect and on the pressure loss is described and the superiority of implicit scheme is confirmed for three-dimensional numerical simulation of PEFC channel flow.
A quasi-steady-state approach towards predicting the fluid forces acting on a hand under unsteady conditions yielded errors. The actual motion of a hand in swimming is obviously unsteady, and time-dependent fluid forces must be considered. The purpose of this study is to investigate the relationship between unsteady fluid forces and the vortex behaviors of a three-dimensional airfoil during pitch-oscillating motion. A flow visualization technique was used to examine the flow field near the airfoil edge in a wind tunnel test. The unsteady fluid forces were affected by the shedding behavior of vortices from the airfoil edge during the pitching oscillation, and the flow structure due to the vortex behavior was strongly affected by the reduced frequency.
Effects of dynamic surface tension on the droplet formation of surfactant solutions were studied. Test fluids used were aqueous solutions of CTAB at several surfactant mol concentrations (CD) and CTAB/NaSal aqueous solutions at CD=1.0 mM and at three mol concentrations of NaSal. A droplet formed when a surfactant solution was injected into air from a capillary tube was investigated and the relation between the droplet diameter and the injection velocity V was measured. The size of droplet was evaluated by an equivalent droplet diameter Dexp, which is the diameter of sphere whose volume is the same as that of a droplet injected. For the CTAB systems, Dexp increased with increasing V at relatively low velocities because the dynamic surface tension also increased. However, the diameter decreased with increasing the velocity at relatively high velocities. This phenomenon can be qualitatively predicted from an equation of the force balance at a capillary exit when the effect of surface tension is evaluated using the dynamic surface tension. For the CTAB/NaSal systems, Dexp increased with increasing V at relatively low velocities and reached a constant value. This phenomenon was also predicted qualitatively with the force balance equation.
In this study, we construct an experimental apparatus for a prototype artificial heart and lung (AHL) by installing hollow fibers into the cylindrical tube of the vibrating flow pump (VFP). The oxygenation characteristics are investigated both by experiments using bovine blood and by numerical analyses based on the computational fluid dynamics. The analyses are carried out at the Reynolds numbers Re ranged from O(1) to O(103), which are determined based on the experimental conditions. The blood flow and the diffusion of oxygen gas are analyzed based on the Newtonian/non-Newtonian, unsteady, incompressible and axisymmetric Navier-Stokes equations, and the advection-diffusion equation. The results show that the oxygenation rate increases in proportion to Re1/3, where the phenomenon corresponds to the decreasing thickness of the concentration boundary layer with Re. Although the effects of the vibrating flow and the rheology of the blood are clearly appeared on the velocity field, their effects on the gas exchange are relatively small at the ranges of prescribed Reynolds numbers. Furthermore, the numerical results in terms of the oxygenation rate are compared with the experimental ones. The basic design data of VFP were accumulated for the development of AHL in the clinical applications.
We developed a numerical simulation code and a method of estimation for predicting the cavitation erosion in pumps. Cavitation erosion is closely related to cavitation intensity based on bubble dynamics. A “bubble flow model” simulates detailed bubble behavior in a cavitating flow. Cavitation intensity was estimated by analyzing the bubble pressure and bubble nuclei distribution in a centrifugal pump. We simulated the impulsive bubble pressure that varied in microseconds. The impulsive pressure was considered to be related to actual bubble collapse, which caused cavitation erosion. The erosion area was experimentally detected using a paint method. The predicted high cavitation intensity area agreed well with the experimental erosion area, since the predicted and experimental areas were both located between the shroud and mid-point of the blade near the leading edge. Our code was thus effective for estimating the cavitation intensity and predicting the erosion area around the impeller of a centrifugal pump.
The present study aims to control the leakage flow and total pressure loss in high pressure turbines. A new tip shape is proposed. It is based on the triple squealer shape by adopting a new middle squealer, along the camber line, whose first and last thirds were removed. An unstructured, finite volume, multiblock, 3-D, compressible Reynolds-Averaged Navier-Stokes equations solver was used to compute the flow through a high pressure turbine cascade. The turbulent viscosity was calculated by the Delayed Detached Eddy Simulation (DDES) model which is based on the Spalart-Allmaras one equation turbulence model. The performance of the new shape is compared to that of flat tip, double and triple squealers. The results successfully demonstrate that the new shape has the least total pressure loss among them.
In this study, process of vorticity decay caused by viscous interaction in a separated shear layer was investigated based on velocity measurements and flow visualization in the immediate vicinity of the separation point of a test body. The experiment was carried out at a fixed Reynolds number of 19,000. The research results innovate the important aspects of clarification on vorticity decay including i) part of the vorticity in the separated shear layer behind a body are caused by viscous interaction associated with transition of velocity distribution from boundary layer like shape to free shear layer like shape in the immediate vicinity of the separation point; ii) Reynolds shear stress associated with turbulent diffusion is not concerned in the vorticity decay process in the immediate vicinity of the separation point; iii) since the Reynolds shear stress around an intermediate size vortex produced by pairing of smaller vortices is considerably large, the flow field becomes turbulent and three dimensional.
An experimental study was conducted in order to clarify the response of the fully developed pipe flow to d- and k-type wall roughness of various streamwise lengths. The measurements were set to emphasize on the response processes, which are deformation and relaxation of the mean velocity profile related to the strength and type of roughness. Under the same effective pressure drop, comparison of the mean velocity profiles and three common characteristics of boundary layer thicknesses (displacement, momentum, and energy) revealed that the initial stage of the response to the flow depends on the type of roughness. The total recovery length until the fully developed state depends only on the effective pressure drop caused by the rough wall.