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

在这项研究中，激光多普勒风速仪和超声波床测量表明，由于水流和砂床之间的相互作用，在宽阔、平坦的河流明渠中产生了细胞次级流和砂带。研究结果表明：(1)平均速度的高速带和低速带沿纵向平行，沿展向并排分布。低速区为上流，高速区为下流。因此，细胞次级流由流向的、反向旋转的、成对地并排躺在床上的螺旋涡组成。(2)高速带的床层剪应力大于低速带。（回想一下，高速区对应下行（V@O），低速区对应上行（V>O）。）高速带的层质输运速率较大，因此，较大的床层剪切应力对床层的冲刷作用更大。这样就形成了一个槽。另一方面，低速区床荷载较小，因此形成脊。反过来，这些沙带在附近产生了新的细胞次级电流。最终，在水流和河床的相互作用下，整个河道将形成细胞次级流和砂带。(3)湍流强度u‘、v’和雷诺应力-uv在脊上比在槽上大。这可以用这样的事实来解释：靠近床层的高湍流被上升气流从床层向上输送，而靠近自由表面的低湍流被下降气流向下输送。然而，由于高动量的向下传递，床层附近的雷诺应力在槽上迅速增加。

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.