PIV法および直接数値計算法による流砂のマイクロメカニクスの解明

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Toward the objective of understanding the micromechanics of bedload sediment transport, the present project had three objectives : (1) Development and validation of a numerical method to simulate the motion of many solid particles in a turbulent flow, (2) Development and validation of a micro force-moment sensor, (3) Clarification of the microdynamics of bedload particle motion.Concerning objective (1) : To perform simulations of the type envisaged, it is necessary to develop an accurate model for the forces between particles in water, in particular the viscous "lubrication" force. To that end,we dropped glass beads vertically onto a stainless steel plate whose roughness was varied over two orders of magnitude, from 1 to 100 microns. Contrary to previous theoretical arguments, we found that roughness decreased the coefficient of restitution, and by an amount that was similar in air and water, which suggests that energy dissipation is dominated by plastic deformation within asperities of the solid material. In other work, we have proposed an important variation of Kajishima's method of simulating solid particles in a turbulent flow (ref. 1 below).Objective (2) : We have micromachined chips with a crossbar structure capable of measuring all 6 components of force and moment applied to the structure. A test particle can be mounted thereon, and installed at the bottom of a water channel. Tests with the particle half-emerging from the bed plate have achieved a fairly reproducible signal down to ReT of 300(ref. 6 below).Objective (3) : We have performed DNS of turbulent flow in a smooth-bed channel flow with a surface-mounted hemisphere. We found an extremely high correlation between the lift on the hemisphere, its drag, and the nearby streamwise velocity component (ref. 2 below). We also found very different skewness of forces on a hemisphere of diameter d+=12 vis-a-vis a diameter of d+=33. 10. KEY WORDS