The multi-blade fan, which has been widely used as a blower for air-conditioning systems of vehicles, is one of the well-established fluid machinery. However, many factors must be considered in its practical design because the flow generated in the fan is quite complicated with three-dimensionality and unsteadiness. The fundamental fan performance is primarily determined by the impeller of the fan, and is also affected by the scroll casing. However, the theoretical estimation of the effect of the casing on the performance has not been well established. In order to estimate the casing effect on fan performance, detailed three-dimensional (3D) flow analysis in the casing is necessary. Stereoscopic PIV (SPIV) is one of the useful techniques for experimental analysis of 3D flow fields. There are some difficulties in practical application of SPIV for flow analysis in fluid machinery with complicated geometry, but the results obtained provide useful information for understanding the 3D flow field. In this report, experimental investigation of the flow in the scroll casing has been carried out using PIV and SPIV under the premise of downsizing automobile air conditioner fans.
This study employs a digital Particle Image Velocimetry (PIV) to obtain detailed velocity distributions and flow visualization for an inclined jet through a forward expanded hole into mainstream over a concave surface at three different blowing ratios (M) of 0.5, 1.0, and 1.5. In this study, measurements are made in three different cases to identify the interaction between the ejected flow and the mainstream. These three cases are particles seeded in the ejected flow only, particles seeded in the mainstream only, and particles seeded in both the ejected flow and the mainstream. Measured results show that the blowing ratio can significantly affect the flow field of an ejected flow into the mainstream. From the 2D velocity measurement on the central plane of the injection hole at M=1.0, measured results show that a strong interaction occurs between the ejected flow and the mainstream at the leeward side of the ejected jet. In this region, the mainstream is entrained into the center plane that produces an outward radial velocity to lift the ejected flow away from the concave wall. In this study, there is no obvious radial velocity of the jet flow to up-lift itself on the central plane. The decay of the lift-off velocity from the mainstream along the streamwise direction is a cause for reattachment of a jet flow on the concave surface.
The mixing performance of optical micro-rotors, which were invented as an active micromixer, is studied by the aid of CFD simulations. Our focus in this paper is on the effects of paddle number (3,4 and 5) of the rotors. Fluid torque acting on the rotors is obtained by solving the 3-dimensional Navier-Stokes equations. Also optical torque produced on the rotors by a laser beam is calculated by considering changes in momentum of the laser beam at the times of refraction and reflection. To quantify the mixing effect of the rotor, the mixing rate is calculated. By balancing the fluid torque and the optical torque at various rotation speeds of the rotors and inlet speeds of the liquid reagents, the mixing performance is analyzed. It was found that the 5-paddle rotor has the best mixing performance in a large space but the 3-paddle rotor is the best in a Y-shaped channel. Further, similarity is observed between the mixing rate, rotation speed and inlet velocity.
A practical and efficient optimal design procedure is presented for three-dimensional micro-propeller. To manage many design related variables and operating conditions efficiently, the design procedure consists of two steps for optimization of operating conditions and blade geometries. First, operating condition points are extracted from the design-of-experiments, and provided as the input data of the geometry optimization step. Next, in the geometry optimization step, the 2-D airfoil shapes are optimized to provide the maximum lift-to-drag ratio along the radial blade section by using the XFOIL code, and the 3-D blade shapes are determined at the each operating condition by using the minimum energy loss method. Then, the performances of the optimized blade are calculated, and a Response Surface Model is constructed to decide the operating condition for the maximum propeller efficiency. To find the blade shape with better performance than the optimum shape in the initial design space, the design space is modified to a highly feasible design space by using the probability approach. Finally, the performance of the optimized propeller is compared with that of the Black Widow MAV propeller. The comparison showed that the optimized propeller had somewhat better performance. The present optimal design procedure is reliable and can be used as a practical design tool for micro propeller development.
An experimental study was carried out to investigate the effect of coaxial nozzle operating conditions on near-field jet plume development. The study was conducted in a low speed water tunnel as well as in a high-speed airflow nozzle test facility. Laser Doppler Anemometry (LDA) and Laser Induced Fluorescence (LIF) techniques were employed to identify the flow structure as well as the mean velocity and turbulence structure of a coaxial nozzle under low speed flow conditions. Schlieren flow visualization, LDA and nozzle wall static pressure measurement surveys were performed in high speed flows. The effect of a nozzle shroud on jet development was studied and found very effective on suppression of the shock cells and on reduction of turbulence levels within the core region. The effect of the outer and inner Nozzle Pressure Ratios on shock cell structure and the nozzle internal wall pressure field were documented. LDA measurements in the water tunnel confirmed that the flow pattern produced of the Reynolds numbers and velocity ratios selected for this study was typical of practically occurring developing jet flow fields. Sufficient measured profiles of velocities, turbulence quantities and nozzle wall static pressures as well as jet plume images have been captured to serve as benchmark validation data for time-averaged turbulence-model-based RANS CFD predictions.
This research deals with the switching mechanism of a flip-flop jet nozzle with a connecting tube, being based on the measurements of pressure in two chambers, velocity in the connecting tube and velocity distribution between two inside walls of the nozzle, i.e., reattachment walls. The authors particularly focus upon the details of switching flow field inside the nozzle, using a ultrasound-velocity-profile monitor (UVP monitor). As a result, two re-circulating flows, viz., two vortical structures, are shown on both side walls inside the nozzle. By means of the simultaneous observation of chamber pressures and connecting-tube velocity with UVP results, we show a coherent scenario of the jet-oscillation phenomenon in concern.
To develop a general-purpose program for predicting the molding flow of polymeric liquid crystals, we present a basic model and its computational procedure. The flow is modeled by the Transversely Isotropic Fluid theory, which is equivalent to the Leslie-Ericksen equations in the high viscosity limit. In the modeling, the Hele-Shaw approximation is applied to reduce computational power. A finite difference technique is used to solve the governing equations, except for the angular momentum equation, which is solved by a streamline integration method. Two molds with thin and simple shape cavities are selected to evaluate the model. The computational results for the locations of the flow front, and for the distributions of the temperature and the molecular orientation show that the model successfully predicts a smooth molding process and that the molecular orientation direction depends strongly on the position in the gap direction. Since alignment of molecules is disordered by the occurrence of tumbling behavior, which depends on the fluid temperature and shear strain, the mold wall temperature and the gate position are important for effective molding.
Textures called wide stripe textures (WSTs) emerge after cessation of a simple shear flow of liquid crystalline polymers (LCPs). The present study considered the WST in a 50 wt% aqueous solution of hydroxypropylcellulose and investigated the distance between stripes. A test fluid was sandwiched between two parallel discs and shear was imposed on the fluid by rotating one of the discs. The experiments were carried out with varying the temperature of fluid, the shear rate γ, and the gap height h. The distance tends to increase with increasing the gap width, slightly depends on γ for small h, and increases with increasing γ for large h. It was suggested that the emergence of WSTs related the relaxation process of relatively large structures such as defects in LCPs.
To reduce the molecular diffusion mixing time in a microreactor, the efficacy of a layered flow pattern generated by an alternate pumping system has been reported. In this paper, we propose an asterisk-shaped channel, which has two inlets and two diverging mixing channels, for the enhancement of mixing using the layered structure flow pattern. Using scaled-up models, the characteristics of the flow pattern in the asterisk-shaped channel was investigated in association with the pumping condition and the channel geometry. Because the gap between the inlets is small in the asterisk-shaped channel, a layered structure is generated under easy pumping condition, which could be practicable. The layer thickness in the diverging channel becomes smaller downstream, so the diverging channels reduce the molecular diffusion time that is proportional to the square of the mixing length, i.e. the layer thickness in this case. For instance, in the case that the width and length of the mixing channel are 5 times and 20 times of the inlet width, the mixing time in the asterisk channel with alternate pumping could be reduced to two orders of magnitude smaller than that in an ordinary microreactor.
This paper presents an investigation of the unsteady three-dimensional flow in an overexpanded rocket nozzle during a shutdown transient. Experimental and numerical investigations were carried out. It is known that, in some cases, an asymmetric flow induces a serious lateral force called “side load” during transient operations. The focus of the present study is to clarify characteristics of the asymmetric flow patterns. Results obtained by measurements and computations for the cases of the shutdown transient have shown qualitative agreement. The flow pattern becomes asymmetric during the transition of the separation pattern from restricted shock separation (RSS) to free shock separation (FSS). As a result, a large and impulsive side load was observed. Additionally, during the transition of the separation pattern, a three-dimensional vortex structure appeared on the nozzle wall in a separation bubble.
Secondary mean motions of Prandtl's second kind near an undulatory surface are explained in terms of the turbulent blocking effect and kinematic boundary conditions at the surface, and the order of magnitude is estimated. The isotropic turbulence is distorted by the undulatory surface with a low slope h/λ, where λ is the wavelength and h is the amplitude. The prime mechanism for generating the mean flow is that the far-field isotropic turbulence is distorted by the nonlocal blocking effect of the surface to become the anisotropic axisymmetric turbulence near the surface with a principal axis that is not aligned with the local curvature (slope) of the undulation. Then, the local analysis can be applied and the mechanism is similar to that of the mean flow generation by the impingement of homogeneous axisymmetric turbulence over a planar surface, i.e. the gradient of the Reynolds stress caused by the turbulent blocking effect generates the mean motion. The results from this simple analysis are consistent with the previous exact analysis, in which the effects of the curvature are taken into account. The results also qualitatively agree with the flow visualization over an undulatory surface in a mixing box.
Free surface wave arisen from the shear layer instability on a liquid jet is investigated experimentally and theoretically. The stability analysis is conducted for initially laminar free-surface shear layer with different levels of simplification of velocity profile (i.e., numerical shear-layer profile separated from nozzle exit and two kinds of simplified velocity profiles obtained through linear and piecewise-linear fitting of numerical profile). It is shown that the linear perturbation equation has temporally-neutral, spatially-unstable solutions by substituting all kinds of local velocity profiles. On the other hands, the periodic two-dimensional waves on liquid jet are measured by using a laser beam refraction technique. The frequency of the most-unstable solutions shows fair agreement with the measured dominant frequency of free surface waves. The linear stability analysis can be simplified drastically by introducing linear or piecewise linear approximation of the velocity profile. Such simplification does not result in large discrepancy from the detailed-model, or experimental data, except in the initial region of the jet where arbitrariness exists in simplification of velocity profiles.
A nanoscale laser induced fluorescence imaging was proposed by using fluorescent dye and the evanescent wave with total internal reflection of a laser beam. The present study focused on the two-dimensional measurement of zeta-potential at the microchannel wall, which is an electrostatic potential at the wall surface and a dominant parameter of electroosmotic flow. The evanescent wave, which decays exponentially from the wall, was used as an excitation light of the fluorescent dye. The fluorescent intensity detected by a CCD camera is closely related to the zeta-potential. Two kinds of fluorescent dye solution at different ionic concentrations were injected into a T-shaped microchannel, and formed a mixing flow field in the junction area. The two-dimensional distribution of zeta-potential at the microchannel wall in the pressure-driven flow field was measured. The obtained zeta-potential distribution has a transverse gradient toward the mixing flow field and was changed by the difference in the averaged velocity of pressure-driven flow. To understand the ion motion in the mixing flow field, the three-dimensional flow structure was analyzed by the velocity measurement using micron-resolution particle image velocimetry and the numerical simulation. It is concluded that the two-dimensional distribution of zeta-potential at the microchannel wall was dependent on the ion motion in the flow field, which was governed by the convection and molecular diffusion.
In the present investigation, detailed observation on the pressure drop characteristics affected by the wall wettability in a curved mini-channel were carried out. The test channel is the serpentine channel with six straight sections and curved sections. The test channel has a square cross section that has 2.0mm on each side, and the radius of curvature was 1.0mm. The test channel was made of the transparent acrylic resin for flow visualization. In order to investigate the influence of the wall wettability, water-repellent and attracting treatments were applied to the acrylic channel, respectively. In this investigation, in relation to the analysis method of the pressure drop, we evaluated the time averaged pressure drop and the pressure fluctuation, respectively. In the case of the water-repellent and attracting channel, the time averaged pressure drop was lower than the normal channel. In addition, in the straight section without the water repellent, the fluctuation of the pressure drop was considered to be correlated only by the Lockhart-Martinelli's parameter. On the other hand, in the cases the effects of channel curvature and water repellent are outstanding, the fluctuation of the pressured drop was widely scattered and hard to be correlated by the Lockhart-Martinelli's parameter.
The flow induced energy-loss of a rotating disk in enclosed fluids bring out significant challenges in the engineering domain of high speed grinding, turbo machinery, circular saws, hard disk, and so on. Since the fluid is supplied continuously to the neighborhood of the disk surface, much of disk's rotational energy is considered to change into kinetic energy of fluid motion. In this study, a control circular cylinder was placed near the disk with its axis of symmetry coincide with that of disk to reduce the supply of flow to the disk. As a result, the fluid torque acting on the disk was found to be reduced significantly when the distance between the disk and the cylinder was intimately small. Furthermore, to measure the fluid torque directly, a new measurement method using load cell was developed in this study.
To investigate the mechanism of noise generation by a train-car gap, which is one of a major source of noise in Shinkansen trains, experiments were carried out in a wind tunnel using a 1/5-scale model train. We measured velocity profiles of the boundary layer that approaches the gap and confirmed that the boundary layer is turbulent. We also measured the power spectrum of noise and surface pressure fluctuations around the train-car gap. Peak noise and broadband noise were observed. It is found that strong peak noise is generated when the vortex shedding frequency corresponds to the acoustic resonance frequency determined by the geometrical shape of the gap, and that broadband noise is generated at the downstream edge of the gap where vortexes collide. It is estimated that the convection velocity of the vortices in the gap is approximately 45% of the uniform flow velocity.
The mechanism of secondary flow was discussed based on a result of LES of turbulent flow in a subchannel. The LES was carried out on the Cartesian grid system using an improved immersed boundary method developed for accurate simulation of complex wall boundary of industrial interest. The method was verified by comparing the LES with a DNS of turbulent flow in a pipe. The LES for the subchannel reasonably reproduced the secondary flow of the second kind. To analyze the mechanism of the secondary flow, a theoretical method was proposed. The result of the analysis indicated that main source of the secondary flow was a non-conservative component of Reynolds stress as was the case for a square duct. The theory suggested that the non-conservative force corresponds to an imbalance between pressure along the wall and Reynolds stress decreasing rapidly near the wall. The present theory will unify an understanding between secondary flows of the first and second kind. The present LES technique and theory are useful for investigating physics in turbulent flow bounded by complex walls.
The number of computational cells assigned to a small bubble is liable to be small in a volume tracking simulation of a poly-dispersed bubbly flow. The volume tracking method is, therefore, expected to at least give qualitatively reasonable predictions of bubble motion without assigning many cells. In this study, bubble motions in stagnant water and shear flows are simulated with low spatial resolutions using a volume tracking method proposed in our previous study. As a result, we confirm that (1) the method can yield correct characteristics of rising velocity and lateral motion of bubbles, and (2) the bubble volume and interface sharpness are kept well even after a bubble had experienced a large deformation in a strong shear flow.
Numerical methods for predicting poly-dispersed bubbly flows in bubble columns are indispensable in the design of bubble column reactors. The objectives of the present study are (1) to experimentally investigate the effects of a bubble size distribution on poly-dispersed bubbly flows in an open vessel and (2) to examine the applicability of an (N+2)-field model (NP2) to poly-dispersed bubbly flows. Distributions of void fraction and liquid velocity in air-water bubble plumes in the vessel are measured using an experimental setup with a bubble injection device by which the ratio of the volume flow rate of large bubbles to that of small bubbles is controlled to a desired value. The main conclusions obtained are as follows: (1) experimental data on the effects of a bubble size distribution on air-water bubble plumes are obtained, and (2) NP2 gives good predictions for poly-dispersed bubble plumes.