Measurements of Instantaneous Bed Pressure in a Backward-facing Step Flow

摘要

A revetment block which is located just downstream of a weir is sometimes destroyed during the flood. Nakagawa et al. (1987) pointed out that the one of the reason why the revetment is destroyed is instantaneous pressure fluctuations near the bed. The drag and lift forces acting on the bed materials above the bed are measured by many researchers. Turbulent structure in open-channel flows behind a backward-facing step is also investigated by making use of a PIV and LDA. In contrast, pressure fluctuations on the bed have not been measured, due to its difficulty. In this study, simultaneously measurements of turbulent structure, pressure on the bed wall and water surface fluctuations in a backward-facing step flow were conducted with a PIV, PTV and a supersonic wave gage. It was found that the instantaneous pressure increases when the K-H instability vortex, which is generated behind the backward-facing step, is falling toward the bed wall. The instantaneous re-attachment point is moving to upward and downward, alternately. This instantaneous deviation value from the time averaged re-attachment length is about 40%. The instantaneous pressure on the bed is not so controlled by the water surface fluctuations, i. e., 3%-10%.