The flow of electrorheological(ER)fluids has been analyzed theoretically based on the constitutive equation for Bingham fluids. However, it is necessary to take account of internal rotation in ER fluids, because they are suspensions of particles. The equation of motion of an ER fluid is derived based on not only the constitutive equation of stress for a Bingham fluid but also on the theory of micropolar electrically conducting fluids, assuming the equilibrium equation of angular momentum. With respect to the yield stress, which is a characteristic of ER fluids, we consider that the interaction of particles, which are polarized by an applied electric field, is based on a two-body problem at the cutting surface of a cluster of particles. As a result, a new equation is derived based on the theory of dipole-dipole interaction. Yield stress in this study is not dynamic yield stress but static yield stress.
The efficiency of the Variable Soft Sphere(VSS)and Variable Hard Sphere(VHS)molecular models is investigated by estimating the transport coefficients, and simulating a free jet in a binary gas mixture, and a normal shock wave structure in a binary gas mixture, using the Lennard-Jones(6-12)(LJ)potential and LJ potential type VSS and VHS models.It is confirmed that the transport coefficients using the VSS model are consistent with those of the LJ potential, and calculated results using the VSS model for free jet and normal shock wave structures are in good agreement with those of the LJ potential. There are, however, some discrepancies in transport coefficients and calculated results using the VHS model and the LJ potential.
An attempt to control the three-dimensionalization process of T-S waves by utilizing the receptivity of a three-dimensional roughness element on a flat-plate boundary layer to outer acoustic disturbances is carried out by wind-tunnel experiments. The term "receptivity"(1)describes the process through which external distur-bances are transformed into instability waves in a boundary layer. It is known that several types of oblique waves can be found in the three-dimensionalization process in the later stages of the boundary-layer transition. In the present experiment, we artificially introduce oblique waves in the boundary layer in order to control the boundary-layer transition. It is shown that an oblique wave can be generated in a flat-plate boundary layer with a roughness element attached to the plate inclined and an acoustic forcing. An attempt to induce three-dimensionalization modes that are observed in the later stages of boundary-layer transition is also carried out. However, the natural three-dimensional mode flow structures could not be generated by our method.
The recovery of the turbulent boundary layer on a flat plate distorted by the reflection on the oblique shock wave, and the interaction between the reflected oblique shock system and the separation shock ahead of the forward-facing step through the distorted boundary layer were investigated both experimentally and numerically. As the increase in the shock strength, the interaction becomes severe, causing the bound-ary layer separation. The shape factor of the turbulent boundary layer distorted by the oblique shock reflection recovers exponentially in the downstream direction. The interaction is roughly classified into three typical cases ; In the first case, the separation shock is far downstream of the reflected oblique shock system, and they are almost independent each other. In the second case, the separation shock is blurred to the compression wavelets due to the weakened boundary layer, and the reflected shock system remains almost unchanged. In the third case, the separation shock occurs ahead of the reflected shock system, and merges into the front leg of the reflected shock system.
The characteristics of low-speed streaks in the turbulent channel flow of a drag-reducing surfactant solution are studied by employing the hydrogen-bubble flow visualization method. The mean velocity profiles and the distributions of streamwise turbulence intensity are also obtained. It is shown that, at a large drag reduction rate DR, the profile of the streamwise turbulence intensity has a plateau region around its peak. In the drag-reducing flow, the mean spanwise streak spacing reaches minimum at some distance from the wall. In the drag-reducing flow of high-density(1000 ppm)surfactant solution, the low-speed streaks appear intermittently at the Reynolds number smaller than those for the 'hump' of the f-Re curve(f:friction factor). The mean spanwise streak spacing very near the wall increases as DR becomes large. It is speculated that the nondimensional wall-normal distance y+ of the center of streamwise vortices is larger for the flow of large DR compared with that for the Newtonian fluid flow.
It is intended to investigate the mechanism of drag reduction of a turbulent flow in a channel by wall oscillation normal to main flow, in this report. DNS is carried out in which one wall of a two-dimensional channel is stationary but the other is in sinusoidal oscillation normal to the mean flow. The most effective oscillation period is adopted and about 35% reduction in drag is confirmed. For this case, generation terms of quasi-streamwise vorticity are examined from various points of view. It is shown that quasi-streamwise vortices which play a key role in sustaining wall turbulence are most strongly affected in the buffer layer. Vorticity production rate fluctuates with the phase of wall oscillation and is enhanced at some definite favorable phase but suppressed as a whole, in one period of oscillation. The stretching effect as well as tilting effect is important. However, the latter affects mainly weak chaotic vortices and hence the role of the former which modulates coherent quasi-streamwise vortices is more substantial. The suppression of vorticity generation by stretching is due to the decrease of inclination angle of a vortex to a wall. It is demonstrated that the fall of production rate of quasi-streamwise vorticity due to stretching effect is substantial for the phenomenon. The periodical fluctuation of the attitude angle, i.e., the inclination angle to the wall and the slant angle of a vortex to mean flow, aggravates the production. This mechanism is a straightforward extension of the one commonly found in the usual near wall turbulence.
To investigate the eddy structure of turbulent shear flow in both physical-space and Fourier-space, a new combination of conventional techniques of signal processing, called wavelet auto-correlation analysis, has been developed based on wavelet transform. The wavelet auto-correlation analysis provides the unique capability for describing the auto-correlation characteristics of arbitrary signals in terms of period and time delay. The wavelet auto-correlation technique was used to analyze the fluctuating velocities at various positions in a plane turbulent jet. It was found that the peaks of WR(a, τ)in the high period region correspond to the large peaks of R(τ), and the peaks of WR(a, τ)in the low period region are equivalent to the small peaks of R(τ). The branching structures in WR(a, τ)reveal that a periodic large eddy motion contains the periodic small eddy motions. In the shear layer at χ / d=10, the periodic eddy motions with a=15, 40, 75, 100 and 150 ms are clearly observed.
To establish a model for predicting the thermal and radiative properties of open-cellular porous materials, the potentialities of a Dul'nev cubic unit cell model are examined. First, the accuracy of Dul'nev's formula for the conductive effective thermal conductivity is quantitatively evaluated and then, on the basis of Dul'nev's model, analytical expressions for the radiative properties, such as the extinction coefficient, albedo and asymmetry factor of a phase function, are derived, and their validities are confirmed. Finally, the feasibility of using dul'nev's unit cell model to predict the heat transfer characteristics of a conducting-radiating, open-cellular porous ZrO2 layer is indicated.
The mixing phenomenon in fluid flow fields has two major components : the advective mixing process driven by the large scale fluid motions and the diffusive mixing process driven by small scale molecular interactions. It is well accepted and amply demonstrated that, in many flows of interest to engineering the overall mixing can be strongly enhanced by an efficient advective mixing, i.e.by bringing the different species involved in close proximity to each other for molecular diffusion to take place over a short period of time. Over the past several years, we have developed an experimental 3-D fluid measurement technique, namely 3-D particle tracking velocimetry(PTV), in which the 3-D fluid velocities are determined by establishing(by photographic means)the 3-D trajectories of flow tracer particles. This paper describes a method for extracting 3-D Lagrangian dispersion rates from such 3-D PTV measurements. Specifically, a quantitative measure of the advective mixing is obtained by evaluating the normal dispersion rate of instantaneous nearby physical trajectories(in direction normal to the instantaneous local velocity vector). Then the magnitude of the root mean square(rms)of the instantaneous local dispersion rate is computed(in the ensemble averaged sense)over many realizations throughout the entire volume of the fluid being studied. This technique is applied to an in-cylinder flow field in an internal combustion(IC)engine in order to promote a better under-standing of mixing characteristics by comparing the spatial distribution of the normal dispersion to the 3-D flow patterns.
We describe an experimental study on the simultaneous reduction of NOx and soot in diesel engines achieved using a new mixture preparation method. To produce a lean uniform mixture, a new type of injection equipment was used with various injection timings. Between the late and very early injection timings, there was an injection timing range at which the amount of NOx produced was extremely low and soot was not emitted. NOx production was suppressed to a level one hundredth that of conventional DI diesel engines. However, with increasing load, NOx production increased sharply and strong diesel knocking occurred. This indicated that NOx production was strongly influenced by the process of mixture formation.
In order to develop the utilization technology in which unexploited energy is retrieved and transformed into power, we have proposed a heat engine incorporating shape memory alloy wires. In this work, the output power characteristics are investigated experimentally from the viewpoints of maximization of the output power, stabilization of the power during operation and the amount of work to failure. Experiments are conducted at various heating and cooling temperatures, velocities of heating and cooling water and wire diameters. As a result, it is clarified that the output power increases with increases in both the heating temperature and the velocity of heating water. The output power also increases with decreasing cooling temperature. However, the velocity of cooling water at which the output power is maximum varies with the cooling temperature. Furthermore, the amount of work to failure does not always increase with increasing fatigue life and becomes maximum under the operating conditions at which the output power becomes maximum.
This paper deals with the cold latent heat energy release characteristics of direct-contact heat exchange between solidified oil droplets[tetradecane, CH3(CH2)12CH3, melting point of 5.8°C]and hot air. The hot air was injected into a solidified oil droplets-water mixture layer from the distributor with a number of small circular nozzles. The solidified oil droplets-water mixture layer was fluidized by air bubbles ascending in the layer, and the air bubbles were cooled as they passed through the layer, by direct-contact heat exchange. The present paper describes the characteristics of temperature and humidity of the outlet air from the layer, and time history of the cold heat energy release. The experiments were performed with reference to several experimental parameters, i.e., air flow rate, air temperature and humidity, and the proportion of solidified oil droplets to water in the mixture layer. Finally, some correlation equations for the outlet air temperature and humidity, and the completion time of cold latent heat energy release were derived in terms of various nondimensional parameters.
A linear analysis model for a free piston Vuilleumier heat pump(FPVM)was constructed to derive closed from solutions for the operating frequency, phase angle and displacer strokes during self-excited operation and forced oscillation. Using the linear analysis, conditions for self-excited operation and the effects of connecting the hot and the cold displacers by a spring on the dynamic motion of the displacers and the FPVM performance were investigated. Energy flow in the FPVM was also pbtained. A simple 2nd order isothermal simulation combined with the linear analysis was developed which enabled calculation of the FPVM performance characteristics. It was found that the simulation results were qualitatively in good agreement with the experimental results during self-excited operation. Simulation was used to calculate the dependence of the dynamic motion of the displacers and the FPVM performance on the spring constants, displacer masses and damping coefficients.
Dynamic motion of displacers and basic performance characteristics of a free piston Vuilleumier heat pump(FPVM)during forced vibration were investigated experimentally and compared with 2nd order simulation results. One of a pair consisting of one hot and one cold displacer was driven by a linear motor. Sensitivity studies were conducted with respect to key operational and environmental parameters such as driving frequency, hot gas temperature and motor input power. The dependences of the phase angle and strokes of the displacers on the basic performance of the Vuilleumier heat pump were also examined by driving both displacers of the FPVM prototype machine in order to investigate the performance characteristics of the FPVM operated at various phase angles and strokes. The dependence of the phase angle on the driving frequency of the hot displacer was the reverse of that on the driving frequency of the cold displacer. It was found that the simulation results were qualitatively in good agreement with the results of the experiments.
A prototype of a free piston Vuilleumier heat pump(FPVM)was designed and fabricated in order to study the basic FPVM performance characteristics during self-excited vibration. The FPVM has a cooling capacity of 3.2kW and a cooling COP of 0.6 during self-excited vibration. Dependence of the dynamic motion of the displacers and the FPVM performance on the mean gas pressure, hot gas temperature, and hot and cold water temperatures was investigated experimentally and analyzed using a simple 2nd order isothermal simulation method combined with linear analysis. The performance characteristics of the FPVM when a hot and a cold displacer were, and were not, connected by a spring were examined and discussed. There was no qualitative difference between the FPVM's. It was found that the simulation results agreed well qualitatively with the experimental results and that the simulation method was suitable for analyzing the FPVM performance characteristics and designing a practical machine.