Three-dimensional direct numerical simulation (DNS) is applied to a wind-driven air-water two-phase flow, and the turbulence structure in boundary layers on both gas and liquid sides and the scalar transfer mechanism across the air-water interface are investigated. In order to capture the air-water interface behavior, an Arbitrary Lagrangian Eulerian formulation (ALE) method is employed. The results show that compared to the gas side, fluid motion on the liquid side is strongly affected by wind waves and micro-breaking waves (i.e. ripples) formed on the leeward side of the wind waves. That is, the wind waves and micro-breaking waves dramatically enhance the transition to turbulence on the liquid side. The scalar transfer across the wind-driven air-water interface is mainly controlled by the longitudinal vortices related to bursting motions on the liquid side.

View full abstract

An experimental study is performed for dividing turbulent channel flow in 90゜ T-junction. Experiments are conducted for flow rate ratios of 0.2, 0.4, 0.6 and 0.8 (ratio between the flow rates in the branch and main channels) keeping the Reynolds number of the main flow at 4.0×10^4. The width of main channel H_1 is kept to 30mm, and the width of the branch channel H_2 is changed 10mm, 15mm, 30mm and 45mm. The wall static pressure and loss coefficient are measured to quantify the energy loss. The local heat transfer coefficient is measured at the branch channel. To investigate the relationship between fluid flow and heat transfer coefficient, the velocity profiles and turbulent intensities are measured by PIV. It was found that the loss coefficient in the main channel basically remained unaffected by the branch channel width, whereas the loss coefficient in the branch channel increased with decreasing the branch channel width. The dividing flow was separated at the upstream edge of the T-junction and formed a large separation babble. The heat transfer characteristics of the upstream wall at the branch channel were similar to that of the turbulent channel flow over backward-facing step.

View full abstract

A useful computational method was proposed for an incompressible viscous flow simulation around arbitrary geometries on a Cartesian grid system. This method has a remarkable feature that allows us to simulate the flow around geometries which are composed of non-watertight and incomplete polygon elements without any repair. The proposed method can reduce manpower drastically in the process of the mesh generation because the repair of the defective polygon elements can be eliminated. In this method, governing equations are discretized using the extrapolated velocity to satisfy the no-slip condition on the wall surface taking into account the distance between the polygons and the cell center on the Cartesian grid. Moreover, this approach has a higher accuracy of shape approximation compared with the voxel method. In this paper, four different cases were calculated to validate the proposed method. Firstly, the flow around an inclined plate thinner than the mesh size was calculated to show that this method can simulate the flow around the non-watertight geometry. Additionally, in this case, the accuracy of shape approximation was compared between the proposed method and the voxel method. Sencondly and thirdly, flows around a circular cylinder (Re=40, 100) were calculated to confirm the accuracy of solutions in the steady and unsteady flows. Finally, an internal flow in a curved duct was calculated to compare the solutions with other researcher's results including the experiment. Consequently, it was found that the proposed method could simulate the flow around the non-watertight geometry and this method had the reasonably good accuracy compared with the literature.

View full abstract

Our objective is to perform Large Eddy Simulation (LES) of incompressible flows around an automobile with motion using arbitrary Lagrangian-Eulerian (ALE) method without decrease in accuracy. For LES of incompressible flows, the discretization method that conserves not only the mass and momentum but also the kinetic energy is preferable, because such conservation properties enable us to conduct computations without any numerical viscosity. We consider a discretization method as a suitable one to LES using ALE method, which is based on colocated grids, and Crank-Nicolson method is employed for the time advancement. Although it dose not fully conserve the kinetic energy discretely, the error is estimated O (Δt^2), which is expected to be insignificant. On the other hand, we also constructed a fully conservative method of kinetic energy on colocated grids, following the quadratic conservative schemes proposed by Morinishi. Moreover, it is clarified that the conservation properties of the other conventional methods are inferior to the two methods mentioned above. The methods are examined in uniform flow and channel flow calculations with moving and transformed grids. The computational results reveal that both two methods mentioned above give similar results. In addition, the obtained results are better than those with other conventinal methods, where the conservation properties somewhat worsen by using Adams-Bashforth method for the time advancement or using the convection velocity vector that does not fully satisfy the divergence-free condition. The present two methods are expected to make it possible to perform LES using ALE method without any numerical viscosity.

View full abstract

We evaluate pre-determined controls with temporally- and spatially-periodic spanwise velocity inputs at the wall in a fully developed turbulent channel flow. The spatially-periodic control generally achieves better performance than the temporally-periodic one, which is conventionally called the spanwise wall-oscillation control. Particular attention is paid to the Reynolds stress, which dominates the skin friction. We apply a conditional sampling technique in order to clarify the response of the Reynolds stress around a particular rotation direction of a longitudinal vortex to the spanwise control input. Quadrant analysis of the Reynolds stress shows that the phase-dependencies of Q2 and Q4 events around a longitudinal vortex are totally different. Based on this knowledge, the drag reduction mechanisms in the two controls are discussed.

View full abstract

Flow aspects near a particle are observed using a numerical simulation of periodically rotating spherical particle in a uniform flow or static particle in a periodically oscillating flow, to investigate a modulation of drag force. As a result, a reduction of drag force is found and it depends on period and amplitude of the rotation or the oscillation. A reason of the reduction of drag force is caused by a reduction of friction force on the particle. And the reduction of the friction force is induced by a concentration of the fluctuation energy of the flow by the rotation or the oscillation on the vicinity of the particle in case Stokes layer thickness, which is estimated using a frequency of the rotation or the oscillation, is comparable or smaller than the thickness of boundary layer of the particle that is estimated in a laminar flow.

View full abstract

The two-way interaction between cavitation and turbulence was investigated by the direct numerical simulation of a spatially-developing mixing layer. Namely, the vortical structure and Reynolds stress components were compared between cavitating and non-cavitating conditions. Cavitation mainly occur in the regions of low pressure which are corresponding to vortices. Under cavitating condition, instability of mixing layer is caused more easily due to disturbance by cavitation to the flow field. Therefore, the onset of the instability is shifted into upstream. In more developed region, pitch of roll-cell vortices get longer than non-cavitating condition as a result of shift of pairing process. Longer pitch of roll-cell vortices results in the decreasing of Reynolds stress component which is corresponding to one of the circumferential components of roll-cell vortices. Circumferential component of streamwise vortices, on the other hand, tends to increase in comparison with non-cavitating condition. This is explained by volume fluctuation by cavitation. The modulation of Reynolds stress is consistently described by these changes in vortical structures.

View full abstract

In the present paper the attention is focused on the effect of small cavities inside in-line tube banks on acoustic resonance which occurred in the two-dimensional model of boiler. We measured sound pressure level (SPL), amplitude and phase delay of acoustic pressures and gap velocity. As a result, we found many peak frequencies of sound pressure level with different Strouhal numbers, mainly about S_t=0.15, 0.26 and 0.52. The variation of SPL for S_t=0.26, 0.52 components in tube banks with cavities were the same as the result of no cavities. The existence of cavities inside in-line tube banks caused the resonance of S_t=0.15. And the acoustic resonance of first mode in the transverse direction was generated if the small cavities existed inside tube banks. This resonance was not generated from tube banks of no cavities. The resonance onset velocity in the transverse mode was fairly slower than that of no cavities. It was easy to generate acoustic resonance when there were small cavities inside in-line tube banks. And we have examined the effect of baffle plates for controlling acoustic resonance generated from tube banks with cavities.

View full abstract

In microfluidic device such as lab-on-a-chip, flow behavior of liquid is governed by its property, especially by viscosity, due to increasing surface-to-volume ratio. Therefore, an appropriate control on the viscosity can be an effective fluid handling method unique for the microdevice. The aim of the present study is an investigation of the effect of inhomogeneous viscosity on the microflow structure. This study relies on the photothermal technique to induce the local viscosity distribution in the flow field in a microchannel. Optically-induced temperature rise causes corresponding distribution of the viscosity owing to the inherent temperature dependence. We have developed an experimental system to perform the local heating into the microflow and to measure the temperature and velocity fields. Micro-LIF (laser-induced fluorescence) and micro-PIV (particle image velocimetry) are used for the temperature and velocity measurement, respectively. As the results, flow velocity is locally increased at a high temperature area heated by the focused laser beam. Around the hot region, accompanying flow is observed. This change in the flow behavior is attributed to the local reduction of the liquid viscosity. The agreement between the experimental results and the numerical simulation considering the temperature-dependent property elucidated that the primary factor to induce the flow structure variation in microchannel was the local nonuniformity of the viscosity.

View full abstract

The purpose of this study is to predict the motion of fish robot. The undulating caudal fin of the fish produces the lateral and the rotational motions of the body in addition to the propulsive motion. In order to clarify how to influence propulsion by these motions, we researched the interaction between the fish body and the fluid by using the translational and rotational equations of motion of the body combined the two-dimensional numerical analysis based on the ALE finite difference method. In the past researches, the fish model was fixed on the space, or the lateral and the rotational motions of the model were neglected in the analysis. The propulsive efficiency is not estimated exactly in such methods. We computed the complex motion of the fish body by considering the surface force acting on the body. As a result of the present study, it was clarified that the propulsive efficiency of the fish model is increased by 50 percent compared from the non-rotational movement. The reason is that the lateral force (which does not make a contribution to the propulsive force) acting on the body is decreased by the rotational motion of the body.

View full abstract

In this study, we evaluated the lift force generated by the crane-fly's flapping wing with the passive pitching using the dynamically scaled model. Since the wing and the surrounding fluid interact with each other, the dynamic similarity between the model and actual crane-fly flights was measured using not only the Reynolds and Strouhal numbers but also the mass and Cauchy numbers. Although there existed the difference between the mass number due to the constant acceleration of the gravity, but the lift coefficient simulated by our model indicated that the flapping wing with the passive pitching produces the enough lift force to support the crane-fly's weight.

View full abstract

A new subgrid-scale (SGS) model for thermal field is proposed. The model is an extended version of the mixed-time-scale SGS model for velocity field by Inagaki et al. (2005), where the hybrid time scale between the time scales of the velocity and thermal fileds is introduced and the wall-asymptotic behavior is satisfied by incorporating the wall-damping function for LES based on Kolmogorov velocity scale by Inagaki et al. (2006). The model performance is tested in plane channel flows at various Prandtl numbers, and the results show that this model gives Pr_<SGS> similar to that obtained by using dynamic Smagorinsky model with locally calculated model parameters. It is also shown that the proposed model predicts better mean and fluctuating temperature profiles than the Smagorinsky model and the dynamic Smagorinsky model. Since the present model is constructed with fixed model-parameters, it does not suffer from the computational instability with the dynamic model. Thus, it is expected to be a refined SGS model suited for practical LES of the thermal field.

View full abstract

An experimental study of heat transfer on a horizontal rotating cylinder near the flat plate was performed. The cylinder and plate are set in cross-flow. Temperature distribution and coefficients of local heat transfer were measured by Mach-Zehnder Interferometer. Flow visualization was made by smoke. Rotating Reynolds numbers (Re_r) and cross-flow Reynolds numbers (Re_d) were varied from 0 to 2000. The spaces between cylinder and plate were varied from 1×10^<-3>m to 5×10^<-3>m. The rotating direction of cylinder was changed clockwise or counterclockwise. The following results are obtained: When the space between rotating cylinder and flat plate is same as the displacement thickness on the plate, the heat transfer on the cylinder nearby plate has best performance. We have obtained empirical equation of heat transfer from a rotating cylinder near the flat plate in cross-flow.

View full abstract

In this paper, natural convection in an enclosure which two parallel heated plates are included is investigated experimentally. We consider the experimental model stated as follows: two heated plates arranged in the vertical direction: the surface of heated plates have constant heat flux: the temperature of ceiling is kept for cooling: the other sides have thermal insulation boundary condition. The relevance of the flow in the enclosure and the heat transfer characteristics from each heated surface was presented. In the area of between cooled ceiling and heated surface, vortex motion arises when the ascending flow from the upper plates and the accompanying flow to the rising flow from lower area are interfered. Heat transfer around heated surface is enhanced by the vortex motion. In the other areas, the flow is circulated along the heated surface, and average Nusselt number is proportional to modified Rayleigh number at the power in general.

View full abstract

The experimental approach to the heat transfer mechanism of nucleate boiling has been studied with a MEMS thermal sensor and a numerical analysis. The MEMS sensor includes gap electrode and eight micro thermocouple on a silicon or a glass substrate. The electrolysis of water by the gap electrode was used to trigger an isolated boiling bubble growth. The electrolysis trigger features low thermal influence and high repeatability in the bubble initiation, thus the temperature variation under a similar condition to the ordinary nucleate boiling can be measured. Moreover, the evaluation method of the heat transfer beneath the bubble has been developed through the transient heat conduction simulation of the heating wall. With the measured temperature data as an interface boundary condition, local heat flux, heat flow and transferred heat were derived for an isolated boiling bubble generation. It was demonstrated that the detailed information of the heat transfer beneath the boiling bubble can be evaluated quantitatively through the measurement with the MEMS sensor and the numerical analysis with the measured temperature data.

View full abstract

The thermal boundary resistance between a single-walled carbon nanotube (SWNT) and surrounding argon has been investigated by using molecular dynamics simulations. With a non-stationary approach, the thermal boundary resistance was quantified for a wide range of temperatures and argon densities, which covers various argon phases i.e., gas, liquid, solid and supercritical phases. The results show that, when the surrounding argon is in fluid phase, thermal boundary resistance is determined by the local density of the argon layer adjacent to the SWNT independently of the phase. On the other hand, when the surrounding material is solid, the modal thermal energy transfer manifests, which contributes to the density effect on the thermal boundary resistance.

View full abstract

Polymer electrolyte fuel cell (PEFC) is expected to applications for various usages such as a power source for small robots and personal computers because PEFC has high energy density and can generate electric power under low temperature environment. As the application, swimming fish robots with PEFC are useful for various usages such as ecological investigation in water etc. In the case that rechargeable batteries are used for supplying electricity to robots, they are not able to continue swimming for a long time because of low energy density of the batteries. Therefore, a small and ultra-light passive-type polymer electrolyte fuel cell called "Power Tube" has been developed. On the basis of this fuel cell technology, the authors have created low energy consumption small fish robots powered by Power Tubes on a float or a buoy. The fish robot with a float swims for approximately 50 minutes by only Power Tubes with a voltage booster and the other fish robot with a submersible system can also swim for about 50 minutes by a hybrid system of a lithium polymer battery and Power Tubes.

View full abstract

In this research, the objective of our research was to convert heavy fuels or solid fuels into lighter liquid fuels with better fuel properties. We have proposed the fuel reformulation technique through an ultrasonic Sonochemistry application. However, we could get little volume of the improved fuel. Therefore, we need a larger apparatus to put the fuel reforming by ultrasonic waves into practical use. As the first step toward the introduction of the new fuel reforming method by Sonochemistry into industry, we set up a long cylindrical apparatus in direction of the ultrasonic waves which improves the standing waves. And then, we examined the influence on the reforming effect of the fuel containing one single component by liquid level change. We obserbed that the reforming efficiency was dependent on the liquid level, the efficiency becomed higher under the resonant state of the ultrasonic waves. We concluded that the collapse intensity of the bubbles was intensified due to formation of the standing wave in the resonance condition.

View full abstract

Effect of motions of atoms or molecules on dissociation probability was analyzed by Molecular Dynamics (MD) method. Platinum (111) surface and hydrogen were chosen to be the metal surface and the gas molecule, respectively. Embedded Atom Method (EAM) was used as the interaction between the surface and the atoms in order to express the dependence of electron density. The parameters were determined so that the results obtained by EAM method were consistent with that obtained by Density Functional Theory (DFT). In this 1st report, the EAM potential was improved to express the characteristics of each site, that is, electron density or dissociation barrier. These characteristics obtained by DFT calculation were reproduced by the EAM potential and it was verified that the dissociation phenomena of a hydrogen molecule on a Pt (111) surface can be simulated accurately by this potential.

View full abstract

Flame propagation in a small diameter tube has been experimentally investigated. Methane/air and propane/air mixtures were injected into the small diameter tube from a swirl generator installed at one end of the tube, ignited at the other open end, and the propagation limits have been determined as function of air flow rate and the mixture equivalence ratio. Results show that, when the tube inner diameter is greater than 3.6mm, flame through the vortex bursting mechanism become possible. An increase in the air flow rate leads to an increase of the propagation range in equivalence ratio, whereas a decrease in the air flow rate leads to a decrease of the propagation range, and eventually a flame cannot propagate upstream for any equivalence ratio of the mixture. Further experiments have showed that, as the swirl intensity is increased, flame propagation range becomes widened in the mapping of the equivalence ratio and the air flow rate. The range of equivalence ratio Φ for which the flame propagation due to vortex bursting becomes possible is on relatively fuel lean side between 0.75 and 1.1 for methane/air mixtures, whereas the range is quite limited on very fuel rich side between 1.1 and 2.2 for propane/air mixture. These ranges are much narrower than those for the vortex induced flame propagation in an open air or in a very large diameter tube, suggesting strong influence of the Lewis number of a deficient species for the occurrence of vortex induced flame propagation in a small diameter tube.

View full abstract

In this study, the authors aim to develop a small gas engine system for biomass gas by modifying the control system of a conventional spark ignition engine. Before developing a control algorithm, combustion experiments with various components fuels assuming component of real biomass gases like as fermentation gas and pyrolysis gas were carried out. It was clarified that the relationship between dimensionless combustion duration and equivalence ratio is expressed in first order liner function regardless of fuel components. Indicated thermal efficiency can be also expressed by combustion duration and volumetric efficiency under the condition that COV of IMEP was lower than 5% and pumping loss decreased against volumetric efficiency linearly, and the relationship between combustion duration and MBT can also be expressed in first order function. By using these relationships, a gas engine control algorithm, which can define target values of equivalence ratio of premixture and ignition timing realizing high thermal efficiency for fuel compositions automatically analyzing in-cylinder gas pressure data in real time, is developed in order to use gaseous fuels produced from biomass resources effectively. Thus, biomass fueled gas engine system is developed by applying the algorithm to the automobile gasoline engine, hardware modification of which were only fuel supply system and flywheel. The engine system was connected to a gasification plant using wood chip and operation test was carried out. As a result, the engine system could set optimum premixture condition and ignition timing, which realized stable and high thermal efficiency operation automatically.

View full abstract

Homogeneous Charge Compression Ignition (HCCI) engine has the potential to achieve lower NO_x and PM emissions with high thermal efficiency. Although it has these merits, operation region is still limited by knocking. Combustion phasing retard is one of the methods to avoid knocking. However, excessive retard of CA50 will lead to unstable combustion. In this study, we developed a combustion control system for HCCI engine fuelled with DME. The cyclic dispersion mechanism on the excessive retard of CA50 was analyzed. And the controlling road map for reducing combustion variations was obtained. Based on these results, reduction of cyclic dispersion is realized by combustion control.

View full abstract

To obtain a strategy for combustion control to achieve low NO_x emission with high thermal efficiency in DI-PCCI operation of diesel engines, numerical study was conducted using a combustion model including a stochastic-PDF mixing model with a reduced chemical kinetic scheme. Effects of injection conditions on heat release and NO_x formation processes were investigated. The results indicate that the combustion model successfully describe the trends of NO_x, maximum pressure rise rate and thermal efficiency. The mechanisms which control pressure rise rate and NO_x formation were discussed paying attention to the effects of fuel-air mixing.

View full abstract