Cellular Secondary Currents and 3-D Sediment Transport in Open-Channel Flows

利用激光多普勒测速仪和超声波床测量结果表明，在宽阔、平坦的明渠水流中，由于水流与沙床的相互作用，产生了胞状二次流和沙带。主要研究结果如下：(1)平均速度的高速区和低速区在纵向上平行，在跨向上并列分布。上行流动发生在低速区，而下行流动发生在高速区。因此，胞状二次流是由沿流向、对向旋转的螺旋涡成对地排列在床面上。(2)高速区域的床面切应力大于低速区域。(回想一下，高速区对应于下行(V@O)，低速区对应于上行(V&gt；O)。)在高速区，推移质输沙率较大，因此，较大的床面剪应力对河床的冲刷作用较大。因此形成了一个低谷。另一方面，低速区的推移质较小，形成了沙脊。反过来，这些沙带在附近创造了新的细胞次级电流。在水流和河床的相互作用下，最终在整个航道上产生胞状二次流和沙带。(3)槽面上的湍流强度u‘、v’和雷诺应力-UV均大于槽面。这主要是由于床面附近的高湍流度通过上行气流从床面向上输送，而近自由面附近的低湍流度则由下水流向下输送。然而，由于高动量的向下输送，非常靠近床层的雷诺应力在槽内迅速增加。

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.