三神 史彦

The flow around a sphere falling through viscoelastic wormlike micelle solutions is investigated by using a particle image velocimetry (PIV) when the falling velocity of the sphere is high compared to the shear wave speed of the fluid. The speed of the shear waves generated by the impulsive excitation of a plate is measured with a PIV. Mach waves due to the passage of the sphere are observed at viscoelastic Mach numbers greater than 1. In relatively dilute cases, Mach waves are accompanied by small vortices generated by the sudden drop of falling velocity of the sphere.

PIV measurements were conducted for the development of the withdrawal current of viscoelastic fluid induced by a line sink between parallel walls. The experiments were performed in a rectangular reservoir 700 mm long, 50 mm wide, and filled with a worm-like micellar fluid to a depth of 100 mm. The line sink was located at mid-depth. It is observed that the startup of discharge gives birth to a counter rotating vortex pair moving out from the sink. These results suggest the propagation of columnar disturbance modes responsible for the development of flow concentration in the withdrawal region.

Particle image velocimetry (PFV) based on an optical flow equation requires numerical evaluation of temporal differential terms between two particle images, which has an unmanageable source of noises. In this research, by using a generally available camera with a polarization imaging sensor, particle images are taken with gain variation within an exposure tune. Then, the weighted time integrals of the optical flow equation are tried to be used for optical flow estimation from single frame particle image. Images with sinusoidal gain variations during an exposure tune are used in this study by setting a linear polarization plate rotating at a constant angular velocity in front of the camera. Optical flow velocity fields are obtained by the proposed technique and the average velocity profiles show a good agreement with the imposed velocity profiles.

In this study, we focus on substance transportation of counter-rotating vortex-pair in the uniform flow, and its interaction with a flat plate is visualized. A water tunnel and aluminum wings (NACA0012) are used to generate a wing-tip vortex pair. Visualization dyes used are fluorescein sodium salt and rhodamine B. The vortex-plate interactions with periodic entrainment are observed when the distance between a plate and vortex-pair are relatively close each other. Experiment and an inviscid vortex dynamics calculation with an Oseen vortex are different when vortex and plate are close, where the effect of a secondary vortex induced by viscosity is important.

The motion of a sphere sedimenting through a viscoelastic worm-like micelle solution (cetyltrimethylammonium bromide and sodium salicylate in deionized water) in a square duct is experimentally investigated using a PIV (Particle Image Velocimetry). Abrupt changes in sedimenting velocity of a sphere are attributed to the onsets of strong downward flow behind a sphere which generates shear waves in the fluid. Subsequent oscillations in sphere velocity have the same frequency of the standing waves propagating along the square duct which forms a waveguide.

An experimental investigation is presented for the flow of a viscoelastic wormlike micelle solution (cetyltrimethylammonium bromide and sodium salicylate in deionized water) around an impulsively rotating cylinder. Flow fields generated by a rotating cylinder within a duct of circular and square cross section are obtained by a particle image velocimetry (PIV) in the plane of the light sheet perpendicular to the axis of rotation. In the early stage after the start of rotation, PIV measurements show that the shear wave is generated on the surface of the cylinder and is propagated by being repeatedly reflected by the duct wall and the surface of the cylinder. At higher rotation speeds, an abrupt change in flow is observed, where the time until the onset of the event is influenced by the effects of shear waves reflected from a duct wall. A core of fluid rotating like a solid body is found around the vicinity of the rotating cylinder. After sufficiently long time, the flow in the plane eventually becomes silent, but annular flow cells are generated wrapping around the rotating cylinder.

A surfactant, cetyltrimethylammonium bromide (CTAB), forms worm-like micelles in aqueous solutions with additive of sodium salicylate (NaSal). In this study, entry flow instability in worm-like micelle solutions through abrupt contraction microchannels is experimentally investigated. To obtain flow induced birefringence images, a polarization microscope is utilized. Test fluids used were CTAB/NaSal solutions. PDMS channel with 960μm height was made by the replica molding of a master which is fabricated by electrical discharge machining. Circularly-polarized crossed-Nicol image sequences show that burst flow occurs intermittently and its power spectrum variation is continuous and proportional to 1/f. Phase-contrast microscopy images show higher-order flow-induced structures (FISs), which are hardly observed with bright field images, are generated upstream of abrupt contraction. Based on the flow visualization results, Strouhal number increases with growth of entry flow and reaches its peak value, and then decreases.

The motion of a solid sphere sedimenting at its terminal velocity through worm-like micelle solutions is experimentally investigated. A rotational motion of a sphere is detected by tracking marker points on the surface of the sphere. Abrupt acceleration in sedimenting velocity of a sphere is found to be accompanied by its sudden rotation followed by sudden deceleration in sedimenting velocity and cessation of rotational motion. We suggest that the onset of this motion is caused by the partial detachment and adhesion of flow induced structured fluid on the interface of the sphere and unbalanced tensile forces lead to the rotational motions.

A surfactant, cetyltrimethylammonium bromide (CTAB) forms wormlike micelles in aqueous solutions with additive of sodium salicylate (NaSal). Wormlike micelles can self-organize under flow to form higher-order flow-induced structures (FISs). In this study, flow patterns and a FIS formation are investigated experimentally in the microchannel flow past obstructions. Test fluids used were entangled worm-like micelle solutions of CTAB and NaSal. Steady laminar flow patterns were observed in a water flow, where the flow can be considered as the Hele-Shaw flow. The Hele-Shaw flow pattern was altered drastically in a worm-like micelle solution (3mM, molar ratio 1:1), where the flow was no longer steady. The flow pattern change was accompanied by a formation of FIS in the worm-like micelle solution, which we observed directly with a phase-contrast microscopy. We observed sticky, viscous, transparent streaks of FIS in the fluid flowing down from the obstructions, where the apparent property of the fluid was locally changed. The streaks are having a refractive index different from that of surrounding fluid, but they were hardly observed with bright field images. The streaks were mostly observed in the stagnation flow behind each obstructions as well as the entry flow into the gap between obstructions. The flow with the streaks was heterogeneous and highly unsteady. The fluctuations of the streaks were observed downstream of the obstructions, where the difference in flow velocity across the streak plays a crucial role in the deformation of the streak.

A catiomc surfactant,cetyltrimethylammonium bromide,CTAB,forms wormlike micells in aqueous solution with sodium salicylate,NaSal.Wormlike micelle solutions have viscoelastic characteristics originated from network of micells.Aqueous solutions of CTAB/NaSal systems are used as test fluid.To obtain full-field flow birefringence image of transient flows around a sphere moving linearly at a constant speed in aqueous surfactant solutions,a polarization-imaging camera is utilized.Left circularly polarized light is used as incident light and the degree of linear polarization(DOLP) and the angle of polarization(AOP) of transmitted light are measured.The measurements show the periodic process of growth and deterioration of high DOLP field behind a sphere at a Deborah number greater than a critical value.

Axisymmetric flow of surfactant solutions past a sphere moving at constant velocity is experimentally investigated. Test fluids used are entangled wormlike micelle solutions of 30mM cetyletrimethylammonium bromide and sodium salicylate at various molar ratios. To obtain full-field flow birefringence image of transient flows behind an impulsively started sphere, a division of focal plane type polarization image sensor is utilized. Left circularly polarized light is used as incident light and the degree of linear polarization (DOLP) and the angle of polarization (AOP) of transmitted light are measured, from which distributions of spatially averaged values of the retardation and the angle of fast axis of the fluid are obtained. Distributions of the retardation along the centerline of the sphere are investigated at various Deborah numbers. At a critical Deborah number, the retardation distributions become propagate at a constant speed away from the sphere. The measurements also show the abrupt change in the wake accompanied by the necking of the cross section when Deborah number exceeds a critical value.

A cationic surfactant, cetyltrimethylammonium bromide, CTAB, forms wormlike micelles in aqueous solution with sodium salicylate, NaSal Aqueous surfactant solutions which form wormlike micelles have viscoelastic characteristics originated from network of micelles Aqueous solutions of CTAB/NaSal system are used as test fluid Birefringence distributions around a sphere moving linearly at a constant speed in aqueous surfactant solutions are visualized by using a polarization imaging camera Birefringence distributions along the center line behind the sphere are divided into two categories by a critical Deborah number When Deborah number is under a critical value, the peak position of birefringence distribution from the sphere is independent of the displacement of the sphere On the other hand, when Deborah number is over the critical value, birefringence distribution propagates away from the sphere

In the present study, flow structure behind a sphere falling through aqueous solutions of CTAB/NaSal are examined using a dye visualization and a PIV. Visualized dye patterns indicate that aqueous solutions of CTAB/NaSal forms multi layered structure behind a falling sphere. At relatively low salt concentrations, it is observed that the dye detaches from a sphere with its end moving upward where the velocity of fluid in the wake is in the opposite direction to the sphere sedimentation. On the other hand, at relatively high salt concentrations, these phenomena are not observed and the velocity distribution forms wedge like shear layers. At the sudden acceleration of the sphere during sedimentation, it is observed rapid stretching of dye and rapid increasing of upward velocity inducing strong flow behind a sphere.

In the present study, surfactant solution systems of CTAB/NaSal were used to examine the motion of a solid sphere falling through the fluid At relatively low salt concentrations, the terminal settling velocity increases with the number of successive sphere drops, and the oscillations in the velocity of a falling sphere are observed. On the other hand, at relatively high salt concentrations, no discernible change in the settling velocity is observed, and the spheres reach a terminal velocity without oscillations. The sudden acceleration of the sphere during sedimentation is related to the formation and breaking of shear-induced-state at the trail of a settling sphere.

An imaging interferometer with low-numerical-aperture objectives and a CCD camera is proposed and studied for application to a particle image velocimetry (PIV) system. Sectional images are obtained based on the optical coherence tomography with the lateral resolution about 40μm. The motion of tracer particles within a coherence gate are detected from difference images between video frames and vector maps are obtained by using auto-correlation PIV. Two types of particle image acquisition conditions are found according to the fringe spacing, which affects the detected motion of tracer particles.

Experimental studies have been made on turbulent structures of water and drag-reducing turbulent flow in a straight pipe of rectangular cross-section (15mm×30mm). Velocity measurements were made using a Particle Image Velocimetry. An increase in the magnitude of the negative skewness in the streamwise velocity fluctuations is observed with polymer additives. The skewness profiles for the wall-normal velocity fluctuations have little differences between the water and polymer cases, while an increase in the flatness profiles in the wall-normal velocity fluctuations is observed with increasing polymer concentration. Maps of Galilean decomposed fluctuating vectors show footprints of the hairpin vortices which are larger in size for the polymer solution flow than those for the water flow.

PIV measurements of wake vortices behind an axisymmetric paraboloid were made in an air chamber with a towing model at high angles of attack. PIV recordings were obtained in a plane perpendicular to the body axis to study detailed structure of the vortices in the cross-sectional flow. The λ_2-definition was used to detect vortex cores from PIV data and contra-rotating vortices are observed inside the main vortex. It is found that the shear layer originated from the secondary separation line is taken into the outer edge of the main vortex, interacting with the shear layer from the primary separation line, accompanied by the formation of a vortex street. These contra-rotating small vortices are taken into the main vortex, which causes the segmentation and spreading of the main vortex core.

The phenomenon of three-dimensional separation of the flow over a slender body at high angles of attack is difficult to model and still challenging problem. The singularity of the flow is caused by mutual intervention of three-dimensional separation and separated vortices. In this study, the flow in the cross-flow separation region of an axisymmetric paraboloid at 30,40, and 50 deg angle of attack has been investigated experimentally by hot wire anemometer. The growth of turbulence in separated vortices is discussed by r.m.s. and probability density function of velocity fluctuation in addition to Reynolds stress.