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.