Iron-nitride magnetic fluid has higher magnetization compared with water-based magnetic fluid W-40 or kerosene-based magnetic fluid HC-50 that are usually used. The effects of magnetic field on oscillation characteristics of a flat plate in iron-nitride magnetic fluid are investigated experimentally and compared with the case of other magnetic fluids especially taking into account the viscous effect. It is clarified that the iron-nitride magnetic fluid has relatively large viscous effect on damping characteristics compared with the commercial magnetic fluids.
The present paper gives a computational method for the flutter analysis of a soft plate placed in two-dimensional subsonic channel flow, remarking the palatal flutter in snoring. The computations were carried out first for the prelimanary case of a hard plate oscillating in windtunnel and then for the case of a soft plate oscillating with standing- or travelling-wave mode, simulating the palatal oscillation. The results obtained for a hard plate are in good agreement with those for the windtunnel wall effects, showing the validity of the present method. The results for a soft plate show that the palatal flutter can be caused by the oscillation of travelling-wave mode, and that the flutter is slightly promoted by the channel walls but slightly suppressed by the hard plate attached fore the soft plate. The effect of mechanical damping is also discussed.
The oscillation of a magnetically levitated liquid droplet in a magnetic fluid is studied theoretically and experimentally. In the theoretical analysis, a small amplitude nonviscous linear theory is adopted. Two models are employed according to the difference in both the magnetic and capillary pressures acting on the interface. Resonance angular frequencies of the levitated liquid droplet are obtained. The responses of the droplet to magnetic field disturbances are discussed. A pair of cylindrical permanent magnetic like poles was set face to face to form a static magnetic field that was used to levitate a diamagnetic liquid or water droplet in a diluted oil-base magnetic fluid, and a pair of electromagnets was used to produce the magnetic disturbances in experiments.
An experimental study to generate T-S (Tollmien-Schlichting) waves and oblique waves in a flat plate boundary layer is carried out. This is considered to be the first step towards controlling boundary-layer transition. In order to generate T-S waves and oblique waves, an array of piezoelectrically driven actuator pieces attached to a flat plate surface was used. The actuator pieces were sufficiently thin to keep the boundary layer in the laminar state. In order to generate various oblique waves, ten actuator pieces were aligned in the spanwise direction, each of which could be controlled separately. It is shown that both two-dimensional T-S wave and oblique waves of various angles could be generated using this device. It is also shown that when a flow pattern similar to the Craik-type three-dimensionalization mode was excited, the onset of turbulence took place early and locally at a certain spanwise location.
Natural convection in a suspension was studied experimentally using both refractive index matching and laser induced fluorescence (LIF) techniques. A rectangular test section was filled with particle suspension and was heated from a horizontal bottom, and two facing vertical walls were kept at a constant temperature. The images of the fluorescent particles were captured by a CCD camera through a sharp cut filter which separates emitted fluorescence from scattered excitation light. The flow velocity fields were analyzed quantitatively using Particle Tracking Velocimetry (PTV). Interface between particle-free fluid and suspension as well as temperature fields was visualized simultaneously with the velocity fields. It was suggested that two-layer convection cells were driven by both heating and sedimentation of particles which drives convection in the overlying layer due to the release of the heated clear fluid at the interface.
A uniform transverse magnetic field is imposed on liquid metal MHD flows. Spatial development of Kármán vortex between two insulator walls is numerically simulated. When a strong magnetic field is imposed, MHD diffusion reduces the vortices including three-dimensional component to quasi-two-dimensional ones in downstream. The condition for development of two-dimensional turbulence is discussed.
This study deals with the development of the lattice BGK model for the Poisson equation. The lattice BGK method, which was derived from lattice gas automata, is a mesoscopic approach for simulating the fluid flow. We can apply this method to several partial differential equations (PDEs) as a numerical solver without losing its advantages such as noise-free calculation, simple algorithm, and high computational efficiency on parallel computers. We develop the new lattice BGK Poisson solver as an example of the elliptic PDE solver and discuss its fundamental properties. By comparing with the finite element method, we confirm the effectiveness of adopted boundary condition rules. We indicate that the extremely favorable speed-up is achieved in parallel computing and that adjustments of the relaxation parameter accelerate the calculation. Our numerical simulations show the potentiality of the lattice BGK method as the solver for the PDEs besides the hydrodynamic equations.
In this paper, we describe the development of a lattice Boltzmann scheme for incompressible thermohydrodynamics. Being based on kinetic theory, the scheme simulates macroscopic fluid flows and heat transfers with the use of distribution functions. A systematic derivation of the lattice Boltzmann scheme from the continuous Boltzmann equation is discussed in details. We find that a 5-velocity model can be employed to simulate heat transfer in such a case where the viscous and compressive heating effects are negligible. As a benchmark, numerical simulations of natural convection in a square cavity are carried out. Through the results, the scheme is found to have a second-order convergence rate. In addition, the scheme is verified to be as accurate as conventional methods over a wide range of Rayleigh numbers.
A low-Reynolds-number second moment closure is improved with the aid of DNS data and applied to turbulent channel flows with a spanwise rotation. Shima’s pressure strain model is refined and a multiple source model is introduced for the exact ε equation. Computational results for the turbulent plane channel flows rotating in orthogonal mode are compared with the DNS, LES and experimental data. The present second moment closure achieves a level of agreement with DNS data similar to those for the non-rotating flow cases. In particular, it yields the improved performance in predicting the distributions of ε in the near wall sublayer.
Compressible Navier-Stokes equations are numerically solved to study spatially developing plane Poiseuille flows undergoing transition to turbulence. Navier-Stokes Characteristic Boundary Conditions and Compact Finite Difference Scheme are used in the streamwise (x) and the vertical (y) directions, however a classical Fourier method is employed in the periodic (z) direction. Several subsonic simulations, with isothermal walls, for different Mach numbers and forced with different random-disturbances are carried out to study the effects of compressibility and disturbances on the transition mechanism. Some of the simulations in their early transitional state, (i.e. DNS of Re=2 500 and M=0.5 plane Poiseuille flow perturbed at the inlet with 5% of the streamwise velocity amplitude) show the appearance of ωy vorticity component in the vicinity of the walls. This near wall vertical vorticity along with the generated streaks, located between the counter rotating vortices, evolve downstream with a longitudinal elongation.
Critical heat flux (CHF) of Subcooled Flow Boiling with water in a tube was investigated from the viewpoint of mechanistic models. The Weisman-Pei bubble crowding model was selected to predict CHF in a short tube and in a tube with an internal twisted tape under nonuniform heating conditions, Based on the results of bubble behavior observation and preliminary analysis. The original Weisman-Pei model was modified in order to explain the physical phenomena of CHF. The modified model predicted well CHF in a smooth tube including the very short tube and the tube with an internal twisted tape under uniform and nonuniform heating conditions.
Turbulent transition mechanisms and heat transfer characteristics of the natural convection over heated, horizontal plate were investigated both experimentally and analytically. An unsteady, 3-D numerical analysis has been performed on the water flow over 150 mm-wide plate heated with constant heat flux. The analytical results showed that longitudinal vortices play a crucial role on the turbulent transition over the plate. The vortices appear first in the laminar boundary layer at certain distance from the leading edge. Then, they detach from the plate and become distorted toward downstream. Meanwhile, the flow and temperature fields over the plate were visualized experimentally. The results confirmed that the above transition actually occurs over the plate. Moreover, the local heat transfer coefficients predicted by the analysis coincide well with those measured by the experiments.
The present study is conducted to investigate the local heat/mass transfer characteristics and local film cooling effectiveness around a conical-shaped film cooling hole with compound angle orientations. The cylindrical hole and two types of shaped hole with conically-enlarged hole exits are used to investigate the effect of film cooling hole geometry. One shaped hole (shaped hole #1) expands 4° in all directions from the middle of hole to the exit. The other shaped hole (shaped hole #2) has the tilted center-line by 4° between the conical and metering holes and is enlarged by 8° to downstream side. The hole area ratios of the exit to the inlet are 2.55 and 2.48, respectively. The film cooling jet is ejected through the single hole, which is inclined at 30° to the surface based on the metering hole. The lateral injection angle is changed from 0° to 90°, and the blowing rates from 0.5 to 2.0. The naphthalene sublimation technique is used to obtain local heat/mass transfer coefficients and adiabatic/impermeable wall film cooling effectiveness around the film cooling hole. For the shaped holes, the penetration of jet is reduced, and higher and more uniform cooling performance is obtained even at relatively high blowing rates because the conically expanded hole exit reduces momentum of the coolant and promotes the lateral spreading. The better cooling performance is obtained with shaped hole #1 due to the uniform diffusion of coolant in the film cooling hole.
In this study, the authors evaluated an effective temperature scale, which indicates the thermal comfort level under urban heat island formation in Tokyo for both the present and in ca. 2030. Standard New Effective Temperature(SET) is adopted as an effective temperature scale. The authors call the effective temperature scale for the urban outdoor environment as the USET (Urban SET) in the present paper. The USET maps in the Tokyo metropolitan area are presented and its validity discussed in this study. In order to evaluate the USET, radiation was incorporated through two-dimensional radiation model for urban street canyons. From the present USET maps, it will be shown that the future urban environment in Tokyo (ca. 2030) is no longer tolerable for inhabitants since the USET value exceeds 48°C, which is by far larger than the critical value ever experienced.
In our previous works, the preferential diffusion effect was found to play an important role in premixed turbulent combustion characteristics, where nitrogen was added as an inert gas to several fuel/oxygen mixtures. In this study, the inert gas is changed to Ar, He and CO2, and the relationship between the turbulent burning velocity and the diffusion coefficient of each inert gas in the multi-component mixture is examined experimentally. As a result, Ar and CO2 added mixtures show slightly larger turbulent burning velocities than that of nitrogen added mixtures. On the contrary, He added mixtures show very smaller turbulent burning velocities at the same equivalence ratio. These characteristics are discussed in connection with each diffusion coefficient of fuel, oxygen and inert gas in multi-component system.
Experimental investigation was conducted on two droplet-array combustion of methanol and methanol/dodecanol mixture fuels in microgravity. For methanol, effects of ambient pressure and droplet spacing were examined. Results show that the droplet lifetime decreases with increasing spacing at relatively low pressure and the droplet lifetime becomes independent of spacing at higher-subcritical and supercritical pressures. For methanol/dodecanol mixture, effects of pressure, fuel composition were investigated in terms of occurrence of disruption. Disruption of droplet during combustion was demonstrated both for single droplet and droplet pairs.
Shredded tire, considered as a single compound in defining theoretical air/fuel ratio, was heated in a quartz reactor placed in an electric furnace under conditions of various excess air ratios, temperatures and heating rates; temperature of up to 1050°C, heating rate of 20, 50, and 80°C/min, and FR of 0.2-1.0. Mass of solid char decreased as the final reaction temperature was raised up to about 600°C around which it maintained a relatively constant value before beginning to increase again at the final reaction temperature of about 800°C. The two-phased decrease of solid char mass with a transition seems to be caused by different modes of combustion. The effects of heating rate and FR (Feed Ratio) were relatively smaller than that of temperature on the mass reduction of solid char, but were significant on the emission of CO and its oxidation to CO2. The fraction of smaller pores in solid char grew bigger as the final reaction temperature went up, resulting in higher surface area. Surface area also increased with decreasing FR from 0.8 to 0.4. Water injection proved so effective in increasing the surface area that the BET value almost doubled when proper amount of water was injected during combustion.
We have developed five kinds of high- and low-temperature differential Stirling engines and their engine performance was investigated experimentally. In order to determine the parameters that affect engine performance, experimental results were discussed and compared with results calculated using analytical methods. We show an arranging method for the experimental results, and consider the performance of general Stirling engines. After using the arranging method with nondimensional numbers obtained by a dimensional analysis, a prediction method, which is used at the early design stage, is formulated. One of the nondimensional numbers in this prediction method is calculated based on engine specifications, including the properties of the working gas. The prediction method can predict engine speed, output power, the effect of working gas and operating conditions.
The multidimensional engine simulation code, FREC-3D(CI), has been used to elucidate the effects of injection rate and split injection on diesel combustion, NO, and soot emissions. The combustion submodel has been updated, including the ignition submodel previously based on a one-step global mechanism. In-cylinder NO and soot formations were predicted by a Zeldovich mechanism with a partial equilibrium assumption and Morel’s soot formation with an oxidation submodel, respectively. In result, computations give good agreement between measured and predicted trends of in-cylinder pressure, and rate of heat release, and a trade-off relationship between NO and soot emissions at pilot injection with high pressure injection. Computations also show that a high turbulence kinetic energy caused by a higher initial combustion is retained at the late combustion stage after fuel injection, and promotes the soot oxidation process. Predictions made with split injection suggest that a combination of high pressure injection in conjunction with a short period in second pulse is effective to reduce soot emission.
A fast response piezo-fluidic gaseous fuel injector system designed for natural gas fuelled internal combustion (IC) engines is described in this paper. The system consists mainly of no moving part fluidic gas injector and piezo controlling interface. It can be arranged as a multi-point injection (MPI) system for IC engine fuel control. Both steady state and dynamic characteristics were investigated on a laboratory test rig. A comprehensive jet attachment and switching simulation model was also developed and reported. The agreement between predicted and experimental results is shown to be good.
This paper describes a study of the exhaust aerosols produced by a diesel engine. A combination of techniques for collecting and measure particulate matter in a diluted exhaust gases are presented. Three techniques have been used: a Micro Orifice Uniform Deposit Impactor (MOUDI), a Low Pressure Impactor (LPI) and a Scanning Mobility Particle Sizer (SMPS). A direct injection naturally aspirated diesel engine was used in the study at three different equivalance ratios: 0.3, 0.45, and 0.6 at an engine speed of 1400 rpm which is rated torque speed. Mass concentration measurements made with the MOUDI were in qualitative, but not quantitative, agreement with those calculated from the aerosol volume concentrations measured by the SMPS. The particulate matter obtained from the LPI was analyzed using transmission electron microscope and was found to be comprised of individual spherical particles ranging from 10 nm to 50 nm with a mean size of approximately 25 nm. Some conclusions about the size distribution measurement possibilities can be drawn.