Masataka Shirakashi

Shear-induced structure change and flow-instability in start-up Couette flow of aqueous, wormlike micelle solution

The occurrence of shear-induced structures (SIS) of a wormlike micellar solution is investigated using an aqueous solution of cethyltrimetylammonium bromide (CTAB)/aqueous sodium salicylate (NaSal) with composition CTAB (0.03mol∕L)∕NaSal (0.23mol∕L). A concentric cylinder Couette cell made of transparent glass is used to generate stepwise start-up shear flow. The shear stress and the brightness, which is an index of the transparency of the fliud, are measured simultaneously and the fluid motion is observed by adding tracer particles to the fluid. Measurements of flow birefringence and light intensity of a transmitted laser beam passing parallel to the vorticity axis are also carried out to monitor flow-induced microstructural changes. The transient behavior of the macroscopic properties, i.e., the shear stress and the brightness, are consistent with each other and with the optical microstructural measurements, confirming that the occurrence of the transparent-to-opaque change in the wormlike micelle solut...

Influence of cross-sectional configuration on Kármán vortex excitation

Three cylindrical bodies with different cross sectional configurations, i.e. a circular, semi-circular and triangular cylinder, are used as the test cylinders, in order to investigate the influence of movement of separation point on the Karman vortex excitation. The cylinders were supported elastically by plate springs. The synchronization of Karman vortex shedding occurs on all three cylinders over almost equal ranges of oscillation amplitude and frequency given by the mechanical oscillator. However, the Karman vortex excitation behavior differs drastically among the three cylinders in spite that the cylinders are supported elastically with virtually equal structure parameters.

Flow Patterns of Ice/Water Slurry in Horizontal Pipes

The authors have observed flow patterns of ice/water slurry flow through horizontal circular pipes, and measured pressure loss for small diameter pipes, using three classes of ice particles. The slurry flows are classified into three patterns, dispersed-particle flow, cluster flow, and column flow. They are presented on a plane of two dimensionless parameters, the ice fraction f and the ratio of mixing energy to cohesion energy, π mix . The pressure loss for small diameter pipes is estimated accurately by the formula proposed in previous studies by the present authors based on experimental data for larger pipes.

Ice/Water Slurry Blocking Phenomenon at a Tube Orifice

Abstract: The phenomenon of ice‐particle/water mixture blocking flow through a pipeline is a problem that needs to be solved before mixture flow can be applied for practical use in cold energy transportation in a district cooling system. In this work, the blocking mechanism of ice‐particle slurry at a tube orifice is investigated and a criterion for blocking is presented. The cohesive nature of ice particles is shown to cause compressed plug type blocking and the compressive yield stress of a particle cluster is presented as a measure for the cohesion strength of ice particles.

Coherent structure in the turbulent wake behind a circular cylinder

A wake behind a circular cylinder at Reynolds number 850–1700 was visualized by the smoke-wire method. The observations of the How together with the results of quantitative measurements, such as various velocity correlation coefficients, illustrated the formation process of spoon-shaped large eddies in the region 90 ≤ x/d ≤ 230 attained through the deformation and rearrangement of the regular Karman vortices. A spoon vortex was likely to pair with the counterpart on the opposite side of the wake. The large-scale bulges of the turbulent and non-turbulent interface of the wake were shown to correspond to these spoon vortices. These results indicate that some coherent structures are organized by rearrangement and deformation of initially regular vortices in turbulent flow.

The influence of the upper wall on the die entry flow of viscoelastic fluids

The influence of the upper wall on the die entry flow of viscoeleastic fluids was investigated experimentally. Aqueous solutions of Separan AP-30 were pushed out by a compressed gas pressure through a capillary attached to the bottom of a reservoir having a height-adjustable upper wall. The driving gas pressure, the flow rate and the pressure at the center of the upper wall were measured and the flow patterns in the entry region above the inlet section of the capillary were observed.Flow rate measurements under the condition of constant driving pressure reveal that the maximum height of the upper wall required to reduce the flow rate is much larger for viscoelastic fluids than for Newtonians, and that this effect of the upper wall becomes more conspicuous with increasing driving pressure. What is curious is that under some conditions the flow rate is larger for a moderate upper wall height than for an infinite. These phenomena are shown to be attributed to the increasing and the reducing effects of the entry pressure loss by the upper wall. The latter effect may be called a new type of pressure-loss reduction phenomenon of viscoelastic fluids.The observation of the flow and the measurement of the pressure at the upper wall center show that typical flow patterns of the die entry flow of viscoelastic fluids are responsible for the far greater pressure loss than for Newtonians, and that the entry pressure loss for a spiralling flow is a little smaller than for the other two types even at an equal driving pressure.

Effect of Elasticity of a Polymer Solution on the Hele-Shaw Flow

In this study, the Hele-Shaw cell is used to examine the effect offluid elasticity on the flow patterns of two-dimensional potential flow. Flows around a circular cylinder, a square cylinder and flows through abruptly converging-diverging channels (slits) with different throat lengths are tested for water and 0.2 wt % polyacrylamide aqueous solution (PAA-solution). The viscosity of the latter is well modeled by the power law, and the first normal stress difference in the steady shear flow is around ten times higher than the shear stress. Although the PAA-solution is highly shear-thinning, the flows of PAA-solution well reproduce the two-dimensional potential flow patterns that correspond to the respective flow configurations when the flow rate is very low. The potential flow patterns ofPAA solution are disturbed in the opposite way of inertia effect observed for water. The streamlines near the upstream stagnation point of cylinders are shifted upstream separating from the cylinder surface when the flow rate is higher, while streamlines in the wake approach closer to the downstream stagnation point. Streamlines offlow through the slit at flow rates higher than the potential flow region show that a pair ofvortices is formed upstream the slit entrance, while the streamlines remain attached to the downstream wall after passing the slit.

LVD measurement of the flow field in a constant-extensional-rate channel

Abstract LDV measurements were carried out for flows of a Newtonian fluid (water) and a shear-thinning elastic fluid (a PAA-solution) in a converging channel which was designed to have a constant rate of extension for extensional viscometry. Measurements were made at several stations along the channel and at various flow-rates, yielding Reynolds numbers ranging from 20 to 1500. For both fluids and for Re higher than 102, the data showed that centerline velocity increased linearly in the flow direction, confirming that the channel creates constant rates of extension in the core flow. Predictions of velocity profiles from a quasi-similarity solution agreed well with the water profiles. The same theory, modified for power-law fluids, was much less successful in predicting the profiles for the PAA solution. One cause of the discrepancy was shown to be early development of the velocity field, generated by fluid elasticity.

Turbulence structure in a periodically diverging-converging channel flow

Abstract Organized structure in a periodically diverging-converging channel flow is investigated experimentally. The mean frequency and duration time of bursting are obtained using the VITA method. The contributions to uv in the four quadrants of the uv-plane are examined by means of the quadrant analysis technique. The bursting frequency and the contribution of the ejection to Reynolds shear stress decrease in the converging section and increase in the diverging section. These results are consistent with the finding that the Reynolds shear stress decreases due to a favourable pressure gradient and increases due to an adverse pressure gradient.

Measurement of planar elongation stress and viscosity in step planar elongation flow for mobile viscoelastic fluids

We propose a new technique which can treat low viscous viscoelastic fluids and which can measure the start-up and relaxation behavior of elongational stress in step elongation rate flow. This new device consists of two parts: the squeeze cell and the optical analyzer. The squeeze cell generates the planar squeeze flow and causes the step planar elongational flow at the stagnation point. The optical system measures the transient behavior of both birefringence and orientation angle in the start-up and relaxation region at the stagnation point. In this paper, we demonstrate the two-dimensionality of the flow in the planar squeeze cell. Next, we evaluated the planar elongational stress and viscosity from optical results for Maxwell fluid. These results show good agreement with Maxwell model. Finally, we clarify the measurement accuracy from the measurement results in channels of various sizes.