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

在这项研究中，激光多普勒测速仪和超声波床测量显示，细胞二次流和沙带产生在宽，平坦的河流明渠流，由于流动和沙床之间的相互作用。主要研究结果如下：（1）平均速度的高速区和低速区平行于纵向，沿展向并排分布。上升流发生在低速区，而下降流发生在高速区。因此，细胞二次流包括流向，反向旋转，螺旋涡并排成对躺在床上。(2)高速区床面切应力大于低速区床面切应力。（回想一下，高速区对应于下流（V@O），低速区对应于上流（V> O）。）在高速区，推移质输沙率较大，因此，床面受到较大的床面剪应力的冲刷。从而形成槽。另一方面，在低速区，推移质小，因此形成了山脊。反过来，这些沙带在附近产生新的细胞次级电流。在水流与底形的相互作用下，最终在整个河道上产生胞状的次级流和沙带。(3)湍流强度u '，v'和雷诺应力-uv在脊上比在槽上大。这可以通过以下事实来解释，即床附近的高湍流通过上升流从床向上输送，而自由表面附近的低湍流通过下降流向下输送。然而，由于高动量的向下输送，非常靠近床的雷诺应力在槽上方迅速增加。

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.