三神 史彦

Rheometers have been used extensively for evaluating dynamic viscoelasticity of viscoelastic fluids by measuring the stress response to the sinusoidal strain applied to the fluids. It was reported that dynamic viscoelasticity was also derived based on the propagation velocity and the damping ratio of the shear waves propagating in the fluids. In this study, a polarization imaging camera and a particle image velocimetly (PIV) are utilized to visualize shear waves generated by an oscillating plate in a viscoelastic wormlike micellar fluid in order to derive dynamic viscoelasticity. Entangled wormlike micellar fluid of 3.0×10^-3 mol/l cetyltrimethylammonium bromide (CTAB) water solution with additive of sodium salicylate (NaSal) as counter ions at a molar ratio of S =4 was used in this study for a test fluid. Propagation velocities were obtained from a sequence of visualized images of propagating waves, which show good agreement to the velocities calculated from the products of wave frequencies and wavelengths. Damping wave profiles were derived from the retardation distributions which were calculated from the captured birefringence images. The dynamic viscoelasticity data derived from the propagation velocities and the damping ratios were in close agreement to the measured data using a rheometer, which shows validity of this method at relatively high frequencies. When the ratio of the amplitude to the wavelength exceeds a critical value, onsets of equilibrium points rising are observed in the damping retardation profiles depending on the amplitude, which attributes to the orientation of micelles on the plate.

Visualization and PIV measurements were conducted in a towing air-chamber for wake vortices behind a paraboloidal-nose cylinder at high angles of attack. The evolution of the wake was then measured by a particle image velocimetry (PIV) using cross-sectional and longitudinal cuts which were produced by the sheet illumination of the Nd:YAG laser. To visualize the three-dimensional evolution of flow in the vicinity of wall, we employed a titanium tetrachloride (TiCl4). It was predicted by the PIV study that serial vortex loops with almost constant spacing appear around the main vortex.

Flow visualization was used to investigate experimentally the evolution process of the instability in the separated shear layer behind an axisymmetric paraboloidal-nose cylinder at high angles of attack. The appearance of instability is observed as uniformly spaced vortex loops showing wavy folds of a shear layer. The distance between the counter-rotating primary and secondary vortices varies with time and the vortex loops always appear first in the region between the primary and secondary vortex at the time and place where these vortices approach closest together. The second vortex loops appear in the downstream of the first ones delayed by the appearance of the first vortex loops. It is suggested that the short wavelength perturbations of the preceding vortex loops advect on the outer surface of the primary vortex along the fluid path accompanied by the dominating vortex loops in downstream region.

Flows in the separation region of an axisymmetric paraboloid at angle of attack alpha >= 40deg are investigated, first by a hot wire anemometer and second by a Particle Image Velocimetry. Reynolds number ranges from 7000 to 18000. The present Ply results show that series of vortex loops appear with almost constant spacing around and coaxially with the main vortex.

In this study, we numerically investigate the incompressible flow between a rotating disk and a stationary disk in a cylindrical enclosure. Considering the application to wafer coating, the effects of the injection of flow and particle size on the particle density distribution are discussed, together with the flow behaviors on the basis of results using a finite difference scheme.

A technique is presented by which 3-D particle images can be recorded with a CCD camera in situations where the illumination is the incoherent light. The experimental setup, based on integral photography, is described and experimental results are presented. Recording of particle image and numerical image reconstruction based on a geometrical optics are discussed. The particle locations in depth can be obtained from the intensity peaks of the reconstructed particle images.

Particle Tracking Velocimetry (PTV), in conjunction with the refractive index matching technique and laser induced fluorescent (LIF) tracer particles, was used to overcome the visualization problem in a particle suspension. A square test section was filled with the particle suspension and impulsively heated from the bottom wall while the two facing vertical walls were kept at a constant temperature. The two-dimensional velocity fields, particle distributions in a plane and the wall temperature fields were visualized simultaneously using three cameras. The results showed peculiar flow patterns such as the formation and vanishing of two-layer convection cells which were distinct from those in a clear, particle-free fluid. Sedimentation driven convection is thought to be the fundamental mechanism for the formation of these two layer convection cells. The overlying particle-free-layer convection was started by the release of the heated clear fluid at the interface between the particle-free layer and the suspension. (c) 2005 Elsevier Ltd. All rights reserved.

Hele-Shaw flows around micro obstacles are visualized by using micro streaklines generated by two-beam interference technique of UV laser, in combination with the photochromic method. This type of flow is a model of blood flow in the lung. Two types of flow channels are used: (1) the Hele-Shaw cell on the slide glass containing irregular shaped obstacles of about 300μm in size, (2) the microfluidic PDMS chip containing 200μm diameter cylinders. The flow in the both cells are visualized by streaklines with their interval in the order from 80μm to 90μm. It was found that the visualized image agrees with the potential streaklines calculated by a panel method.

レーザビームスキャニングは，連続発振レーザを用いたシート光によるパルス照明と比較してレーザ光のパワーを効率よく利用できることから，微小なトレーサ粒子からの弱い散乱を捉えるのに適している．二色のビームを同一平面内で位相をずらして走査すると，露光順序がわかるカラー二重露光画像が得られるが，本研究では，このカラー二重露光画像を利用して，迎え角40°における回転放物体背後の気流のPIV計測を行った．二重露光画像を色情報に基づいて時間分離した画像にはノイズが多く含まれるため，相関ピークを求める際のS/N比が悪くなり，過誤ベクトルが発生しやすいという問題が従来あった．本研究では，条件の悪い画像から直接S/N比の高い相関を得るための計算方法を提案し，その有効性を確認した．これより，物体対称軸に垂直な断面内では可視化結果と一致する主渦を含む速度ベクトル場が得られ，また，対称面内の速度場からは，はく離渦の発達を示す速度分布が得られた．

Three-dimensional separation of the flow over a slender body at high angles of attack is difficult to model and still a challenging problem. The singularity of the flow, such as large side forces and yawing moments, is caused by mutual interactions of three-dimensional separation and separated vortices. In this study, the flow in the cross-flow separation region of an axisymmetric paraboloid at 50 and 60 deg angles of attack has been investigated experimentally by hot wire anemometer. Reynolds numbers are 9.0×(10)^3 and 1.8×(10)^4 referred to the base diameter. It was suggested that the relations between separated vortex and the other flow regions, such as primary separated flow region, reattachment flow region and external flow region, are estimated calculating two-points velocity cross- correlation in various cross-sections.

Natural convection in a suspension was studied experimentally using both refractive index matching and laser induced fluorescence (LIF) techniques. A rectangular test section was tilled 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.

Experimental studies have been made on turbulent structures of water and drag-reducing turbulent flow in a straight duct of rectangular cross-section (15mm×30mm). Velocity measurements were made using a high-resolution CCD camera with a telecentric lens for Particle Image Velocimetry. Time averaged statistics of the turbulent flow show a reduction in both the wall-normal velocity fluctuations and the Reynolds stress through out the channel. Maps of fluctuating velocity vectors show dramatic suppression of vortical motions in drag-reducing polymer solution flow, where the formation of groups of the fluctuating velocity vectors orienting to the streamwise direction was observed near the wall.