| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1987: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 1986: ¥1,200,000 (Direct Cost: ¥1,200,000)
|
|---|
| Research Abstract |
In this study, the Laser Doppler anemometer and ultrasonic bed measurements were used to show that cellular secondary currents and sand ribbons were generated in wide, flat fluvial open-channel flows, due to the interaction between the flow and sand beds. The main findings obtained from the present study are as follows:
(1) The high-speed zones and low-speed zones of mean velocity are parallel to the longitudinal direction, lying side by side in the spanwise direction. The upflow occurs in the low-speed zones, while the downflow occurs in the high-speed zones. Therefore, cellular secondary currents consist of streamwise, counterrotating, spiral vortices lying side by side in pairs on the bed.
(2) The bed shear stress is greater in the high-speed zones than in the low-speed zones. (Recall that high-speed zones correspond to downflow (V@O) and low-speed zones to upflow (V>O).) The bed load transport rate is larger in high-speed zones and, thus, the bed is more scoured by the larger bed shear stress. and consequently a trough is formed. On the other hand, the bed load is small in low-speed zones, and consequently the ridges are formed. In turn, these sand ribbons create new cellular secondary currents in the neighborhood. Eventually,cellular secondary currents and sand ribbons will be produced across the entirechannel by the mutual interaction of the flow and the bedform.
(3) The turbulence intensities u', v' and the Reynolds stress -uv is larger over the ridges than over the troughs. This can be explained by the fact that high turbulence near the bed is transported upwards form the bed by the upflow, while low turbulence near the free surface is transported downwards by the downflow. However, the Reynolds stress very near the bed increases rapidly over the trough due to the downward transport of high momentum.
|
