An immersed boundary method is improved and applied to 2-D flow fields involving a single stationary/moving object. The present immersed boundary method employs a body force proportional to a solid volume fraction for coupling the solid and fluid motions at the interface. A hyperbolic-tangent function is newly introduced as a surface digitiser for computing the volume fractions at the interface cells, and this improvement is proved to be efficient for problems involving arbitrary shape object. The present method is applied to a uniform flow field around a circular cylinder and an interaction problem of fluid and free-falling object. The computational results are found to agree with the results by the different methods of the present authors and the results reported in the literatures. Also the computation time is considerably cut down compared to the other methods by the series of improvements to the immersed boundary method.
Omnidirectional reductions in drag and fluctuating forces were achieved for a circular cylinder subjected to cross-flow by attaching cylindrical rings along its span at an interval of several diameters. The Reynolds number based on the cylinder diameter, d, ranged from Red = 3000 to 38000 for the experiments. The aspect ratio of the cylinder, L/d, was approximately 20. The addition of the rings reduced the drag force for Red ≥ 20000, even though the projected area increased. This was attributed to the formation of separation bubbles on both sides of the ring, which led to pressure recovery on the rear of the cylinder. The optimum ring configuration for drag reduction was found to be D/d = 1.3, W/d = 1.0, and P/d = 6 at Red ≈ 30000, where D is the ring diameter, W is the spanwise width of the ring, and P is the spanwise pitch of the ring. This configuration reduced the drag force by 15%. In addition, the fluctuating lift, which was estimated from the fluctuating surface pressures, was weakened for all Reynolds numbers due to the suppression of vortex shedding.
A new primary standard for hydrocarbon flow measurements has been constructed at National Metrology Institute of Japan (NMIJ). The facility was designed for the calibration of hydrocarbon flowmeters in the flow rate range between 3 and 300 m3/h. The expanded uncertainty is estimated to be 0.03 % for volumetric flow rate and 0.02 % for mass flow rate (coverage factor: k = 2). The primary standard is based on a static and gravimetric method with a flying start and finish. The facility consists of two test rigs using kerosene and light oil as working fluids. The test lines for the flowmeters are 50, 100 and 150 mm in diameter and three servo positive displacement meters are used as working standards. To verify the calibration performance, a Coriolis flowmeter, a turbine meter and a positive displacement flowmeter have been calibrated at both test rigs. Furthermore, an international comparison with SP, Swedish National Testing Research Institute, was carried out. A screw-type positive displacement flowmeter was selected as the transfer standard and was calibrated at NMIJ and SP. The result shows that the two national standards at the two institutes agree within the quoted expanded uncertainties.
An improved finite difference lattice Boltzmann method (FDLBM) is developed to simulate incompressible flows. The basic idea is to decrease the density fluctuation which might lead to compressible error in the flows with high pressure gradient or high speed motion. The equation of state is modified by including the attractive and repulsive forces between particles to adjust the compressibility of fluid. This effect is incorporated by applying acceleration modification which is deduced based on macroscopic dynamics. Using this method the sound speed of the fluid can be also controlled easily. Numerical simulations of steady Poiseuille flow and unsteady Womersley flow were adopted to validate our model. The results show that the proposed model is much more efficient than the conventional LBGK method when solving the incompressible flows with high pressure gradient.
In order to atomize a liquid, the authors have investigated the behavior of air-water jets. In a series of experiments, we have discovered a strange phenomenon that the water jet accompanied with air suction from the free surface has made a periodic radial splash of water drop. The purpose of the present paper is to clear out the origin of this phenomenon and the behavior of water jet accompanied with air suction. The behavior of water jet has been photographed by a digital camera aided with a flashlight and high-speed video camera. Those experiments enable us to find the origin of a periodic radial splash due to a formation of single air bubble at the flow separation region inside the nozzle and due to explosive expansion of the bubble after injected in the free space. In order to analyze the radial splash of water, we have conducted the equation of spherical liquid membrane. The numerical results obtained have been compared with the experimental results and good agreement has been obtained in radial expansion velocity.
The purpose of this study is to develop a swimming human simulation model considering rigid body dynamics and unsteady fluid force for the whole body, which will be utilized to analyze various dynamical problems in human swimming. First, the modeling methods and their formulations for the human body and the fluid force are respectively described. Second, experiments to identify the coefficients of the normal drag and the added mass are conducted by use of an experimental setup, in which a limb model rotates in the water, and its rotating angle and the bending moment at the root are measured. As the result of the identification, the present model for the fluid force was found to have satisfactory performance in order to represent the unsteady fluctuations of the experimental data, although it has 10% error. Third, a simulation for the gliding position is conducted in order to identify the tangential drag coefficient. Finally, a simulation example of standard six beat front crawl swimming is shown. The swimming speed of the simulation became a reasonable value, indicating the validity of the present simulation model, although it is 7.5% lower than the actual swimming.
The present paper treats the backflow vortex structure observed at the turbomachinery inlet at reduced flow rate. It is caused by the roll-up of the shear layer between swirling backflow and axial main flow. In order to verify this, a simple model test was carried out in which the effect of impeller was represented by an axisymmetric swirling backflow. In the present paper, the flow field of a simplified model test is simulated by using LES calculations to investigate detailed flow structure. The computed results are compared with experimental results.
In a solid-liquid two-phase flow, the separation of particles or a reduction of particle concentration is often desired. Utilization of ultrasound has been considered as one technique for the separation of particles. Particles are known to aggregate due to the radiation pressure of ultrasound. However, the effect of ultrasound including cavitation on particle behavior is not well understood. Thus, we horizontally irradiated water with aluminum particles having a density of 2720 kg/m3 and diameters of 50 to 150 μm or smaller aluminum powder (flakes) in a rectangular vessel by ultrasound at frequencies of 23 kHz or 100 kHz. In this way, a standing wave was formed. The following results were obtained. For ultrasound of 100 kHz, the aluminum powder aggregates in vertical bands. When acoustic cavitation existed at the frequency of 23 kHz, we noticed that the aluminum particles aggregate as clumps near antinodes of the sound pressure profile because of the flow induced by acoustic cavitation. When the particles are continuously provided in a relatively high concentration, particle clumps form and remain. Then, the particle clumps become larger and suddenly fall faster than the surrounding small particles. Such phenomena repeat themselves periodically. At relatively low concentrations, particle clumps do not become large and remain stationary at the same positions.
The characteristic intensity distribution in the N2 second positive bands observed in high-temperature air plasmas generated by air micro-plasmajets was reconstructed by the theoretical spectra with the effects of predissociation and non-Boltzmann rotational population distribution. In order to construct the precise theoretical spectra, collisional transition rate coefficient that was applicable at high temperatures was proposed based upon a simple collision theory. In addition, the effect of the radiative transition from C 3Πu to B 3Πg state was included in the master equation. The theoretical spectra with non-Boltzmann rotational population distribution calculated with such factors agreed very well with the experimental ones.
In the present paper, parametric numerical analyses of the separated transonic flows in overexpanded rocket nozzles are conducted. Several types of nozzles are examined numerically to clarify the detailed mechanisms of the occurrence of restricted shock separation (RSS) in compressed truncated perfect (CTP) nozzles. The effects of nozzle length and geometric compression factor are examined. In CTP nozzles with large length and small compression factor, a positive pressure gradient appears downstream of the Mach disk before the occurrence of RSS. Under the positive pressure gradient, the flow, which passed through the Mach disk, rolls up and forms a large vortex ring around the nozzle exit. The vortex ring pushes the separated jet towards the nozzle wall and results in RSS.
The effects of stable and unstable thermal stratifications on the cross spectra between passive scalar (dye concentration) fluctuation c and vertical velocity fluctuation υ in shear-free grid turbulence are investigated using previously published data. The results show that cospectra in the neutral stratification have a slope close to -7/3, as predicted by Lumley (1964). The coherences of υ and c in the neutral stratification have a slope close to -4/3, which implies the absence of a phase shift between υ and c. In the weakly stable stratification, both the cospectrum and the coherence decay due to stable buoyancy force. In the moderately stable stratification, the scaling laws of the cospectrum and coherence collapse in the downstream region. However, in the strongly stable stratification, the negative cospectrum in the countergradient region reconstructs a small but distinct region close to -7/3 after the collapse. In the unstable stratification, despite strong convective motions due to the unstable buoyancy force, the cospectra and the coherences exhibit -7/3 and -4/3 power decay regions, respectively.
The flow inside the two-dimensional semi-open-type nozzle for ship propulsion equipment, directly driven by high-pressure gas was investigated experimentally. The flow was unsteady and the gas and water phases clearly separated. We found that these waves appear on the interface for continuous gas ejection. It was clarified that waves play an important role in the pressure distribution. Intermittent gas ejection was also tried. The thrust itself decreases compared with continuous gas ejection, but propulsion efficiency, considering the gas ejection duration is increased. The flow patterns for intermittent gas ejection were also clarified.
Efficiency of a centrifugal pump is known to drop rapidly with a decrease of specific speed ns in the range of ns ≤ 100 [m,m3/min,min-1]. However, below ns = 60, the pump efficiency is not yet clear, and the spiral angle of a volute casing becomes too small to manufacture. To solve this problem, a circular casing is considered appropriate in the very low ns range. The present study is aimed to reveal the relation between pump efficiency and a specific speed in the range of ns ≤ 60, when a circular casing is used. The results show that a circular casing gives higher efficiency than a spiral casing, and that radial thrust is considerably small in both casings compared with that of ordinary specific speed pump.
We verify the accuracy and the truncation errors of approximation to the first derivatives and to Laplacian operator in the lattice Boltzmann method. The truncation errors are calculated by the Taylor series expansion, and the influences are analytically and numerically examined in the simulation of two-phase flow. We propose the 4th-order accurate approximations to derivatives that utilize the property of the tensor unlike the finite difference. The numerical solutions with various approximations agree well with the theoretical ones within the error of 7% in the test of the Laplace's law. Application of the 4th-order accurate approximation scheme to two-phase flow simulation reduces the spurious current around the interface of a stationary droplet about to one-half of the results with the 2nd-order accuracy. The small spurious velocity in the vicinity of the interface of the 4th-order approximation increases the speed of a moving droplet, and distorts the shape of the droplet little. In the simulation of phase separation, the growth rate of the typical domain size calculated with FFT is strongly affected by the truncation error in surface-tension driven regime for short time. It is seen that the 4th-order approximation saturates at 2nd-order accuracy in simulation of a decaying Taylor vortex flow, because the convergence rate of the LBM is 2nd-order in space. It is shown that the approximation method affects the important physical values, such as velocity, moving speed, or domain size in numerical simulations.
This paper shows the aerodynamic characteristics at the mid-length of a rotor blade of a 10-m-diameter wind turbine exposed to wind shear. A sonic wind speed meter and six cup-anemometers were installed one diameter upwind of the turbine in order to measure wind profiles. The anemometers at the top, middle and bottom levels were installed at heights of 18.3, 13.3 and 8.3 meters, respectively, which correspond to the heights of the tip of the blade at the blade top position, the hub height, and the tip of the blade at the blade bottom position, respectively. Our measurements suggest that the normal force coefficients in strong wind shear conditions are lower than those in weak wind shear condition. Even if the local angle of attack is almost the same, the normal force coefficient shows differences due to the hysteresis effect. In particular, the influence of shear is large not only when there is strong wind shear in a vertical direction, but also when there is strong wind shear in a horizontal direction. A remarkable difference appears in the pressure distribution under these conditions.
Performance of a fish caudal fin is brought out from many factors, such as the shape, the movement and the elasticity. The present study treats all of these factors simultaneously and attempts to visualize the complex design space using Kriging and SOM. As a result, the present study succeeded in visualizing the complex structure of the design space of the oscillating wing (caudal fin), and the combined effects of the design variables are shown. This data will become extremely useful for practical design of fish robots and other nautical machines.
Helical compression mechanism is the newly developed one that can compress gas by using helical components. This mechanism was designed in 1987, and the helical compressors were manufactured for refrigerators first in the world in 2000. This mechanism consists of three main parts, helical blade, roller, and cylinder. The roller with helical blade on its surface is positioned in the cylinder, and several compression chambers are formed being surrounded by those three parts. As the helical shape has uneven pitch, the volume of each chamber becomes smaller along the rotation, and the gas is transferred and compressed continuously. Uneven pitch shape is very important to realize high performance of helical compressor. This study has the object to see the best shape of helical structure to maximize the performance, and make sure how to decide the helical shape for each compressor specification as its capacity, compression ratio, and so on.
Shapes of single drops rising through infinite stagnant liquids under terminal conditions were measured using a high-speed camera for a wide range of fluid properties. Aspect ratios E and distortion factors γ of drops were evaluated from drop images. Effects of initial shape deformation, surfactants and viscosity ratio κ on E and γ were investigated. As a result, the following conclusions were obtained. (1) Initial shape deformation does not affect E and γ of single drops. (2) Addition of surfactants decreases E and breaks the fore-and-aft symmetry of a drop shape especially in systems of low Morton numbers M. (3) The Tadaki number Ta is the best dimensionless number among the Eötvös number Eo, the Weber number We and Ta for correlating E of drops, and an empirical correlation of E in terms of Ta was proposed for clean drops under the conditions of -11.6 ≤ log M ≤ -0.9, 1.5 × 10-2 ≤ Re ≤ 8.5 × 102, 1.7 × 10-2 ≤ Eo ≤ 9.3, 7.4 × 10-3 ≤ Ta ≤ 3.6 and 0.1 ≤ κ ≤ 100. (4) The viscosity ratio κ does not affect E so much but influences γ. For κ > 1, γ decreases with increasing the drop diameter, that is, the front part of a drop is flatter than the rear part. On the contrary, the rear part is flatter than the front part when κ < 1. The fore-and-aft symmetry always holds for drops with κ ∼ 1.0. (5) Various drop shapes were presented as a database utilized for verification of numerical methods such as an interface tracking method and a boundary element method.
Flow visualization has been performed in the vicinity of a roughness element for boundary layers developing over rough surfaces with roughness pitch ratios p of 2 and 4 at low Reynolds number. The momentum thickness Reynolds numbers at streamwise distance x=865 mm for p=2 and 4 are 300 and 500, respectively. For the rough surface of p=2, typical eddy structures are classified into 4 patterns that are consistent with the results of Townes et al., whereas for the rough surface of p=4, three patterns are recognized. As compared with the rough surface of p=2, the rough surface of p=4 takes a larger value of occurrence frequency normalized with respect to the cavity width and the friction velocity. Occurrence frequency is equal to the sum of the frequency of each pattern, and is closely related with momentum exchange. This quantitative result can suggest that the pressure drag acting on a roughness element for the rough surface of p=4 is larger than that of the rough surface of p=2.
The local relative velocity model proposed by Tomiyama, et al. was applied to the estimation of solid volume fraction in solid-liquid two-phase pipe flows for a wide range of diameter ratio of particle diameter to pipe diameter. The diameter ratio was varied from 0.0226 to 0.818. No estimation methods proposed thus far can cover this diameter ratio range. The local relative velocity model takes into account the effects of the distributions of local volume fractions and local velocities in a cross-sectional area of a pipe, being similar to the drift-flux model. An empirical correlation of the terminal velocity of a single solid particle falling in a liquid-filled pipe was applied to the phase-averaged “local relative velocity”, and a simple empirical correlation of another parameter in the local relative velocity model was deduced from measured solid volume fractions and phase-averaged velocities. Substituting these two correlations into the local relative velocity model, we developed a new simple correlation of solid volume fraction. This correlation was confirmed to yield good estimations of solid volume fractions both qualitatively and quantitatively.
Numerical simulation is performed for the vortex breakdown flows induced in a cylindrical container with rotating top disk. Solutions are obtained for the parameter ranges where steady axisymmetric flows are stable with respect to asymmetric disturbances. By varying the number of grid points and applying the Richardson extrapolation, resolution independent solutions are obtained and flow state diagram is numerically constructed on (h, Re)-plane (Re is the Reynolds number and h is the cylinder aspect ratio). The value of critical Reynolds number for the creation of breakdown bubbles agrees on the average within 1% discrepancy with the previous experiments. Solutions obtained in the present study are expected to be useful as a benchmark solution vis-a-vis newly developed numerical methods are compared.
Taking into account the seriousness of the greenhouse effect, drastic measures must be taken to prevent carbon dioxide emissions from transportation systems. In Japan, over 20% of the carbon dioxide is emitted from transportation systems. Aero-Train is a new zero-emission high-speed vehicle proposed by the authors. Aero-Train levitates aerodynamically using the wing-in-ground effect (WIG). In order to investigate the stability of Aero-Train, wind tunnel experiments of the front body section of a test run model of Aero-Train (ART002) and a levitating test run of ART002 using an improved control program were performed. The results showed that the front section of the body is unstable or is weakly stable in the direction of yawing and rolling. In the levitating test run of ART002, levitated running without surface contact was achieved inside the guide way. The amplitude of the roll angle oscillation was reduced by more than 50% compared to that of the previous control program. However, the vibration of the rolling remained.
This study focuses on the mixing process of the driving jet and the induced flow in the throat to improve the efficiency of jet pumps used in Boiling Water Reactors. The effect of nozzle and throat shapes on the performance of jet pumps are examined experimentally under normal temperature and small-scale model conditions using three kinds of nozzles (i.e., circular, notched, and flower-shaped) and two kinds of throats (i.e., conventional straight throat and diverging throat with a very gentle gradient). A nozzle throat area ratio, which influences the flow rate ratio at peak efficiency, is kept constant to avoid significantly changing the flow rate ratio. The velocity profile and local skin-friction coefficient in the throat are investigated experimentally to evaluate the effect of nozzle and throat shapes on the mixing process. The following results are obtained. The diverging throat reduces skin-friction loss and improves efficiency for all three nozzles. The efficiency curve of the circular nozzle with eight notches slightly shifts to higher flow rate region by the reduction of the jet size compared with the circular nozzle. The flower-shaped nozzle decreases efficiency owing to the increase of the skin-friction loss caused by enhanced mixing. The circular nozzle with a diverging throat improves the peak efficiency about 2% without changing the nozzle throat area ratio.
For realistic turbulent flow simulations, quantitative representation of turbulent flow dynamics is desired. In the present study, two-dimensional homogeneous isotropic turbulence is simulated by using a grid-free vortex method to focus on the viscous dissipation process. The results are compared with those of spectral DNS. Two viscous diffusion models, i.e., a core spreading model and Moving Particle Semi-implicit (MPS) Laplacian model, are compared. For the former model, merging and insertion of particles are incorporated to ensure uniform distribution of vortex elements. It is shown that the MPS Laplacian model is superior to the conventional core spreading model in terms of the decay rate of enstrophy and energy spectra. Furthermore, the computational time is remarkably reduced by using a Fast Multipole Method (FMM), while retaining accuracy.
As a fundamental research for developing new liquid crystalline devices, numerical simulations on flexoelectric polarization in liquid crystals under shear flow has been performed using the Leslie-Ericksen continuum theory. We have used two types of liquid crystals for computation; one is N-(p-methoxybenzylidene)-p'-butylaniline (MBBA), which is an aligning liquid crystal, and another is 4-n-Octyl-4'-cyanobiphenyl (8CB), which is a tumbling liquid crystal. These liquid crystals are placed between two parallel plates, and one plate is moved to its plane direction in order to impose shear flow on the liquid crystals. For simplification, the director is assumed to lie in the shear plane. As the boundary condition of the director, the director orientation on one plate is anchored strongly, and the director on the other plate is weakly anchored so that the director orientation depends on shear rate. The flexoelectric polarization for the aligning liquid crystal increases monotonically to reach a steady state value, while it shows peaks for the tumbling liquid crystal; the number of peaks depends on the Ericksen number (ratio of viscous to elastic stress), and the peak value is considerably larger than the steady state value for the aligning liquid crystal. Since the flexoelectric polarization is estimated from the difference of the director angle between both plates, it is found that the flexoelectric polarization can be controlled by the anchoring strength as well as the Ericksen number.
In this paper, a new sterilization method using cavitating flow is presented. Water with bacteria was pressurized up to 105 MPa and flushed out through two very small nozzles 0.1-0.31 mm in diameter, where a cavitating jet was generated containing bubbles that collapsed downstream. First, the effects of jet velocity and cavitation number on the sterilization rate of Escherichia coli JCM1649T(E. coli) were examined. The sterilization rate increased with jet velocity. The rate was proportional to the 3rd power of the velocity. All the E. coli cells were killed by three successive treatments at V=355.7 m/s and cavitation number σ=0.154. The sterilization rate has a peak depending on cavitation number at the low-jet-velocity region of less than 300 m/s. An experiment was also performed to compare two types of bacteria, E. coli, as typical Gram-negative bacteria and Bacillus subtilis JCM1465T(B. subtilis), as typical Gram-positive bacteria. Additional tests were performed using Pseudomonas putida JCM13063T, Gram-negative bacteria and Bacillus halodurans JCTM9153, Gram-positive bacteria. The sterilization rate of the Gram-positive bacteria was much lower than that of the Gram-negative bacteria under the same experimental conditions. Gram-positive bacteria have a thicker peptidoglycan layer than Gram-negative bacteria. This may be the reason why B. subtilis is more resistant to the mechanical stress caused by cavitating flow.
By using laser-induced thermal acoustics, we demonstrate a non-invasive and remote method to measure the speed of sound and temperature ranging from 278 K to 341 K in distilled water at atmospheric pressure. The accuracies of the measured speed of sound and temperature were found to be 3% and 4%, respectively. Single-shot precisions based on three standard deviations of 20 samples were within 4% for the speed of sound and the temperature. The time resolution for each measurement was 300 ns.
There are many dynamical problems in front crawl swimming which have not been fully investigated by analytical approaches. Therefore, in this paper, standard six beat front crawl swimming is analyzed by the swimming human simulation model SWUM, which has been developed by the authors. First, the outline of the simulation model, the joint motion for one stroke cycle, and the specifications of calculation are described respectively. Next, contribution of each fluid force component and of each body part to the thrust, effect of the flutter kick, estimation of the active drag, roll motion, and the propulsive efficiency are discussed respectively. The following results were theoretically obtained: The thrust is produced at the upper limb by the normal drag force component. The flutter kick plays a role in raising the lower half of the body. The active drag coefficient in the simulation becomes 0.082. Buoyancy determines the primal wave of the roll motion fluctuation. The propulsive efficiency in the simulation becomes 0.2.