Mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in polymer electrolyte fuel cell (PEFC). It is important to illustrate the water behavior in MPL for improving cell performance in operation. Because MPL is adjacent to catalyst layer and GDL substrate, supposedly influences their liquid water behavior. The paper with MPL was evaluated as specimen GDL. The images of liquid water distribution in the GDL were taken by neutron radiography under a few experimental conditions in order to elucidate the characteristics of water behavior in MPL. The images reveal that neutron radiography can be used to analyze the water behavior in MPL as an effective tool. It is clarified; MPL has two aspects, that is, one aspect is that the water retentivity and nature of network between MPL and paper substrate are formed under partially-saturated condition, another is that MPL acts as hydrophobic under dry condition.
In the first report, we had investigated the characteristics such as the changes of the mean velocity component profiles, half-widths (b_<1/2>) of the mean velocity profile in the downstream direction of the Push-Pull flow. In this report, we have experimentally investigated the characteristics of the turbulence intensity, Reynolds shear stress, and intermittency of turbulence. Turbulence intensity √<<u'>^^^-^2>/U_1 in the axial direction is not affected by the flow rate ratio Q_3/Q_1 (Q_1 is jet flow rate from mozzle and Q_3 is suction flow rate produced by the hood.). At the same time, turbulence intensity √<<v'>^^^-^2>/U_1 in the lateral direction becomes greater as the hood is approached and the flow rate becomes larger. These results are in accord with the values of production term for <u'^2>^^^- and <v'^2>^^^-. Although the variation of √<<u'>^^^-^2>/u^^-_m with respect to dimensionless coordinate y_1/b_<1/2> becomes similar with that for completely developed two-dimensional jet, the value decreases in the downstream direction as the flow rate ratio becomes larger for Push-Pull flow. But the flow rate ratio do not affect √<<v'>^^^-^2/U_1. Reynolds shear stress becomes smaller as the flow rate ratio becomes larger near the hood. Dimensionless distance y_1/b_<1/2>, from the centre axis of the flow to the point where intermittency factory γ becomes constant value (γ=0.3 or 0.5), becomes narrower as the flow rate ratio becomes larger near the hood. At the same time, the distance becomes smaller than that for completely developed two-dimensional jet.
The aim of this study is to investigate on frequency shift of vortex shedding from a circular cylinder by induced jet using dielectric barrier discharge (DBD) plasma. The electrode of a plasma actuator was mounted on the circular cylinder. A wave form with voltage of 2kV and frequency of 10kHz is applied to the electrode. The forward and backward jets for the main flow direction were induced by electrode arrangements. The time-averaged velocity, turbulent intensity, half width and the vortex shedding frequency in the wake behind the circular cylinder were measured using a I-type Hot-wire anemometer and flow field is visualized using smoke wire method and a digital video camera at a Reynolds number of 1.0×10^3. We discussed that the turbulent intensity of backward jet increases near the circular cylinder, and the half width of wakes decreases and increases using the forward and backward jets. Also, the frequency shift of vortex shedding from circular cylinder using plasma actuators changes with respect to no jet.
A Cartesian-mesh approach named Building-Cube Method (BCM) was developed for flow computations. This method has a feature of dividing a computational domain into a number of sub-domains (named as 'Cube') by Quad-tree or Oct-tree, and processing the sub-domains in parallel. In this paper, we propose the modified BCM for changing the subdividing method of the computational domain from Quad-tree to Nona-tree (in two-dimensional case). The purpose of the modification is to subdivide the computational domain more efficient and also to simplify the data exchange method between cubes. The method is evaluated by computing flows around a circular cylinder and an airfoil. It is shown that the modified BCM divides the computational domain more efficiently than the original BCM without degrading the computational accuracy. It is also concluded that the mesh size of about 0.3 Re^<-0.5> near the surface is necessary to reduce the influence of the staircase approximation of surface in the Cartesian-mesh approach.
Turbulence characteristics in the spatially developing fractal-generated turbulence are investigated by means of the direct numerical simulation (DNS). The square type fractal grids are numerically constructed using the immersed boundary method. The effects of solidity σ and thickness ratio t_r, which are ones of the most important grid parameters for fractal grids, are investigated in detail. The results show that the increase of σ leads to the improvement of the uniformity and isotropy of flow fields. The integral length scale in the decaying region of the fractal-generated turbulence remains monotone increase at high σ and t_r, whereas it is approximately constant at low σ and t_r. The derivative skewness and flatness in the streamwise direction are different from those in the transverse direction for all σ and t_r investigated. These results will be useful for designing fractal grids for practical applications.
For the achievement of low-cost computations of the fluid flow through micro-nano sized porous media such as catalyst layer of fuel cell, lattice Boltzmann method (LBM) has been improved for high Knudsen number flows by considering the diffuse scattering boundary condition, effective relaxation time related to Knudsen number and the regularization procedure. These treatments have been validated in simple geometries such as two-dimensional Couette flow and channel flow. In this paper, above micro-flow LBM is extended to three dimensional flow. A nano-mesh structure is presumed and its applicability is discussed in terms of discrete velocity (Q19 and Q39 models) by comparing to the molecular dynamics simulation. It is found that the resulted velocity profile by LBM agrees with that by molecular dynamics including the velocity slip at the porous wall. The Q19 model has enough accuracy compared to Q39 model, which suggests that the advantage of the simple model for the application. The less difference between Q19 and Q39 models implies that the third order term in Hermite polynomials becomes less effective for the complicated flow regime.
The unsteady behavior of a vortex generated on the diffuser leading-edge, which is called the leading-edge vortex (LEV), is discussed through experiments and numerical simulation. The LEV is different from the separating vortex of the diffuser leading-edge and passage vortex of the diffuser, develops rapidly with a decrease in the compressor mass flow rate, and forms a flow blockage in diffuser passages. Therefore, the evolution of the LEV may become a cause of diffuser stall. Additionally, in one attempt to control the LEV, two types of tapered diffuser vanes, which are shroud- or hub-side tapered diffuser vanes, were adopted. Though the shroud-side tapered diffuser vane can effectively reduce the compressor noise level, the compressor performance deteriorates remarkably. On the other hand, the hub-side tapered diffuser vane not only reduces the compressor noise level but also improves the compressor performance. According to the visualization results of the oil-film methods and numerical simulations, the hub-side tapered diffuser vane can suppress the evolution of the LEV in the compressor low-flow-rate operation.
A theoretical approach to the rheological properties of a particle model which is widely applied as a numerical method to analyze the behaviors of a continuous media has been constructed. The present study treats a particle system which describes the mechanical behaviors of the continuous media by means of the computational mechanical simulation. The behaviors of the particles in the system have been investigated analytically in order to conduct the numerical analysis effectively. A basic equation which describes the motion of the particle is derived from the particle system. Since the basic equation is the nonlinear equation which contains the thermal effect, it is analyzed theoretically by using some mathematical techniques. As a result, the time dependent behaviors of the stress and strain which are derived from the particle system are shown analytically. Especially, it is found that the viscoelastic behaviors emerge from the interactive behaviors of the particles under the constant temperature.
Numerical analyses are performed for attenuation of weak shock waves generated by explosion of a unit mass of TNT. Viscous and heat conduction loss and molecular vibrational relaxation effects of oxygen and nitrogen molecules are considered. The effect of each diffusion term on the wave attenuation and the wave form change is investigated in detail. In addition, the characteristics of each attenuation effect are investigated by the technique of frequency analysis.
A three-dimensional direct numerical simulation (DNS) with Level-Set method is applied to the impingement process of a single droplet with diameter of 1.2〜2.2mm on the air-water free surface, and the characteristics of vortices induced below the free surface are investigated by comparing with the experiments. The results show that the present DNS can capture the generation and transportation process of the vortices. The predicted vortex intensity corresponds with the experimental bestfit curve proposed for a larger droplet with diameter of 2.2〜5.6mm. In addition, the comparisons of velocity vectors and scalar concentration fields between the vertical and oblique impingements show that the momentum and mass transfer mechanism are strongly affected by the angle of droplet's impingement.
Three-dimensional direct numerical simulation (DNS) is applied to a gas-liquid two-phase flow with a non-mobile wavy gas-liquid interface, and the turbulence structure in boundary layers on both gas and liquid sides and the scalar transfer mechanism across the gas-liquid interface are investigated. The results show that the scalar transfer across the non-mobile wavy gas-liquid interface is mainly controlled by the longitudinal vortices related to bursting motions on the liquid side, whose mechanism is similar to those across the flat and realistic wind-driven gas-liquid interfaces. However, since the scalar transfer is strongly affected by the fixed interface configuration, the low-scalar-flux streaks observed in the cases of the flat and realistic wind-driven gas-liquid interfaces do not clearly appear. This means that in order to precisely predict the scalar transfer rate across the realistic wind-driven gas-liquid interface, the non-mobile wavy gas-liquid interface is not suitable to be employed.
A variable displacement pump is a high-efficient pump for hydraulic power steering systems. In order to develop a quiet variable displacement vane pump which leads to lower vehicle interior noise, we need to reduce pressure pulsation and vibration of the pump. So, we have developed a new simulation method for predicting pressure pulsation and vibration caused by a variable displacement pump. This simulation method combines hydraulic simulation and vibration simulation to predict vibration in the design phase. To improve accuracy of the hydraulic simulation, the deformation of the pump shaft was modeled. Moreover, method of characteristics using block diagram was used to calculate pressure pulsation of rubber-made pump outlet hose. This hydraulic simulation results were used as excitation force acting on the pump shaft in vibration simulation. As a result, hydraulic simulation predicted pressure pulsation of 11th order which comes from the number of vanes and its harmonics of the pump revolution accurately. The differences of predicted and measured results were within 0.1dB at 1500min^<-1>, 9MPa. And vibration simulation using the results of hydraulic simulation predicted vibration of 11th and 22nd order of the pump revolution that agreed well with experimental results.
The present study is concerned with numerical simulation of the movement of gas progressing with the constant velocity in a horizontal channel filled up with water. The dependence of the gas-liquid interface shape and the liquid flow in the neighborhood of the gas phase tip on the channel height and the gravity force are analyzed by the numerical simulation program FIDAP. The three cases that the height of the channel is greater than, almost the same as and less than the bubble limiting thickness are studied. The relation between the residual liquid film thickness on the wall after passage of the gas phase tip and the tip velocity is also discussed.
To estimate an influence of velocity on thermodynamic effect, we conducted experiments, in which inducer rotational speed was changed in liquid nitrogen. The experiments in liquid nitrogen and in cold water allowed us to estimate the amplitude of thermodynamic effect. In the experiment with lower rotational speed, the suction performance was improved in liquid nitrogen. The cavity length at lower rotational speed was shorter than that at higher one at the same cavitation number. Thus, we have confirmed that the degree of thermodynamic effect depends on the rotational speed as a suppression of cavity length. The temperature depression of liquid nitrogen was estimated from the comparison of cavity length between in liquid nitrogen and in cold water. We found that the amplitude of temperature depression became smaller when the rotation speed was lower. In addition, from arrange data using modified non-dimensional thermodynamic parameter Σ^*_<mod>, the effect of nonlinearity of vapor pressure/temperature curve should be considered to evaluate the thermo-dynamic effect especially in the case of higher temperature and rotational speed.
The self-consistent equations of the piston effect are used to calculate the behaviors of the unsteady airflows caused by the moving trains in the subway system. The system is composed of the tunnel for trains, pathways or stairways for passengers, and ducts for the ventilation or the air-conditioning, and strongly has the one-dimensional aspects. In order to make the one-dimensional pipe line net-work, every component of the system is transposed to the equivalent pipe, and is connected up each other to form the three station model. By making use of the theory of the piston effect and the interfering model of airflows with division or combining, the dynamic behaviors of the airflows are calculated as the computer simulation, and are compared with the field data measured in the real subway system. The result of simulation has a good agreement with the field measurements.
It is particularly important to know the deformation behavior of the erythrocyte passing through a micro channel (MC) like the blood capillary. In this paper, the deformation behavior of an erythrocyte passing through the pseudo MC was calculated and observed by a two-dimensional numerical simulation of blood flow. The pseudo MC wall was created by assuming that the wall was one of the fluids which had no velocity at all times. The erythrocyte's movement and deformation were calculated by using an immersed boundary method. The following results were obtained by the simulation. An erythrocyte smoothly deformed and passed through the pseudo MC. In the pseudo MC, its shape changed to bullet-like shape and its moving velocity was almost constant. In addition, a narrow layer of only blood plasma, what we call the plasma layer, was formed between pseudo MC's wall surface and the erythrocyte membrane. It was possible that the plasma layer puts the erythrocyte into fluid lubrication. On the other hand, in our previous researches the deformation behavior of the erythrocyte passing through the MC was observed by experiments. The simulation results had good agreement particularly on an erythrocyte's deformed shape and process in MC with the experimental results.
We developed a computational model that expresses a deformation behavior of a red blood cell (RBC) in a high shear flow. The RBC was modeled as a capsule of spring-networks that has a mechanical nature of lipid bilayer and spectrin. Besides bending and stretching of the membrane, area and volume constraints were imposed in order to assure their incompressibility. Fluid forces exerted by external plasma and internal hemoglobin were estimated based on the momentum conservation and Newton's law of viscosity. Given a flow field, the behavior of RBC was determined toward the minimum energy state. In the Couette flow, the RBC was stretched with a tank-tread rotation of the membrane. A good agreement with experimental data was obtained by using a nonlinear spring for the stretch where a spring constant changes as a function of stretch ratio. In the cyclically reversing unsteady shear flow at a frequency of 1, 2, 3 and 5Hz, the RBC deformed in concert with a change in the flow field. The time lag on the RBC deformation as well as the deformation index well agreed with experimental results both qualitatively and quantitatively. Those results demonstrate capability of the proposed RBC model.
The carotid bifurcation is found to be a major site of Atherosclerotic plaque formation and intima-media thickening. In the present study, oxygen mass transfer in the human carotid bifurcation has been numerically investigated, focusing on effects of the bifurcation angle and the volumetric flow ratio between the internal carotid artery (ICA) and the external carotid artery (ECA) on the magnitudes and distributions of the oxygen wall flux. Three-dimensional models of bifurcations with two different bifurcation angles mimicking the real bifurcation anatomy were constructed to perform simulations of steady blood flow under the wall boundary condition of a constant oxygen concentration. Results reveal that the axial flow separation at the outer common-internal carotid wall can significantly alter the distribution and the magnitude of the oxygen wall flux, depending strongly on the bifurcation angle. The magnitude of the lowest Sherwood (Sh) number (non-dimensional oxygen wall flux) at the ICA sinus, where Atherosclerotic plaque is likely to develop, is sensitive to the change in the value of the volumetric flow ratio rather than the change in the bifurcation anatomy.
A new method to actuate a small droplet on a plate was proposed. In order to reduce the resistance to the droplet movement, which is typically represented by the resultant of surface tension acting on the three-phase contact line, i.e., σ (cos θ_R-cos θ_A) (σ: liquid surface tension, θ_R, θ_A: receding and advancing contact angles), a supersonic oscillation was imposed on the plate to suppress the contact angle hysteresis (θ_R-θ_A). The experimental results using SAMs (Self-Assembled Monolayers) plates showed that the resistance was reduced by 80% due to the oscillation having the frequency of 28kHz and the amplitude of few micrometers. Then the Laser beam was directed to one end of the droplet. The contact angle was remarkably decreased due to the local heating of SAMs plate and the droplet moved to the end of the Laser beam, since the tangential component of surface tension at the heated end overcomes that on the other. The droplet can be actuated by about 0.6mm/s by the method combining the oscillation and the local heating by the Laser beam.