水谷 夏樹

The authors applied an ultra-high-speed video camera to visualize crack propagation in brittle bodies, such as mortar specimens, under the impact splitting test. Strain of the brittle bodies in impact splitting tests was analyzed by means of PIV (Particle Image Velocimtery), which is usually used for measurements of flow fields with tracer particles. The results show that, when the applied impulse on the mortar specimens is increased, the crack propagation velocity reaches an upper bound. The upper bound of the crack propagation velocity was 2.6 km/sec. The horizontal tensile strain around the crack tip was estimated to be 370 mu by PIV measurement with the ultra-high-speed camera, and 270 375 mu by strain gages, respectively. Those results showed a good agreement with each other.

This study investigated about the continuity of momentum flux through an air-water interface. We conducted to measure air-flow and water-flow very close to a water surface in a wind wave tank using PIV technique. To evaluate of the continuity of momentum flux calculating from the friction velocity was very difficult, because we could not detect exact a mean water level to fit a log-law profile for the experimental data. The viscous stress affected not only the viscous sub layer but also the turbulent boundary layer. The ratio of the viscous stress in water-flow is from several percent to about ten percent. We deduced that the continuity of momentum flux calculating from the total stress was strongly affected by three-dimensional turbulent water-flow.

The characteristic of roll-wave flow, which is a flow with high Froude number which appears on the steep slope, is hardly known at present. In laboratory experiment, it was difficult to measure the flow profile which is a thin sheet flow having the characteristic of a wave and a flow.<BR>In this study we measured the velocity field of the roll-wave flow using PIV technique. We also investigated the behaviors of the velocity field in each region separated into five parts of one wave length. As a result, the relationship between velocity field and the water depth in each part was clarified. It also appeared that the velocity profile is grater than a log-law profile at every region.

The purpose of this research is to explore the mechanism required for the generation of earthquake vibration on the propagation velocity of the crack in fault plane. In this research, the impact splitting tensile tests of mortar and granite were carried out, and the propagation velocity of the crack was visualized and obtained by image analysis using ultra high-speed video camera. A relationship between the impulse which is impact loading and propagation velocity of the crack was observed. There appears to be a critical state in which propagation velocity does not increase, even if impulse is increased over it. It was proven to converge in 2.6 km/sec. Next, the mechanism of the development of the dynamics of the crack by the impulse was examined from displacement vector and Particle Image Velocimetry (PIV) analysis.

In order to clarify the mechanisms of wind waves, the air and water flows close to the water surface were simultaneously measured by using the Image Velocimetry, the IV. In this study, a Particle Tracking Velocimetry, so-called PTV, was used as IV, which can measure the velocity vectors in the vicinity of the water surface. The experiments were carried out in the range where whitecaps cannot appear. From the measured planar velocity distributions, the instantaneous vorticity distributions in air and water were evaluated.

Air flow over wind waves generated in a wind wave frame was visualized by water vapor, and instantaneous air flow fields over white-caps were obtained by a PIV system. Three air flow separation patterns were detected, which were decided by the situation of white-capping progress. The air flow separated from wind wave crest is accompanied by inverse flow at the forward face of wave crest. It was found that the frequency of air flow separation with inverse flow was about 6%. Also, the characteristics of the velocity distribution of the air flow field were compared with the mean wind velocity using conventional wind profiler.