单侧滩地植被作用下的弯曲主槽二次流特性

Secondary Flow Characteristics in Meandering Compound Channelswith One-sided Floodplain Vegetation

为探讨单侧滩地植被对弯曲主槽二次流特性的影响,概化设计了无滩地植被与有单侧滩地植被的弯曲河道模型,利用声学多普勒流速仪(ADV)测得其主槽特征断面的瞬时流速。试验结果显示:在单式弯道中,主涡的形成完全由弯道离心力决定;但在试验复式弯道中,主涡的形成与滩槽水流交换密切相关。当相对水深为0.15时,与无滩地植被工况相比,在滩地植被横向宽度较小的弯顶附近和直线过渡段,二次流强度出现了较大衰减,但在滩地植被横向宽度较大的弯顶附近,二次流强度无明显差别。当相对水深增至0.35时,两种滩地条件下弯曲主槽各断面的二次流强度均有所增强,但单侧滩地植被条件下各断面的二次流强度均小于无滩地植被条件下的对应值。在有单侧滩地植被条件下,对于同一相对水深,上游滩地无植被的直线过渡段,二次流强度明显大于上游滩地覆有植被的直线过渡段的情况。在无滩地植被条件下,雷诺应力R vw等值线的“岛心”位置可以标示二次流主涡的旋转中心,而“岛心”处雷诺应力R vw的正负可以表征二次流主涡的旋转方向。在有单侧滩地植被条件下,断面中部的雷诺应力R vw等值线分布十分凌乱,并无上述规律,且其绝对值明显小于无滩地植被条件的对应值。

To investigate the impact of one-sided floodplain vegetation on secondary flow in meandering channel,we designed meandering compound channels with smooth floodplains and one-sided vegetated floodplains in a generalized manner,and measured the cross-sectional instantaneous velocities using an Acoustic Doppler Velocimeter(ADV).The experimental results revealed that the formation of main vortexes in a single bend is solely influenced by centrifugal force,whereas the formation of main vortexes in the experimental meandering compound channel is closely associated with flow exchange between the floodplains and the main channel.When the relative water depth is 0.15,compared to the situation under smooth floodplain conditions,the secondary flow intensities at the apex sections with the narrowest vegetated floodplain and in crossover areas exhibit larger attenuation,but there is no apparent difference for the secondary flow intensity at apex section with the widest vegetated floodplain.For the relative depth of 0.35,the secondary flow intensity at each cross section in the meandering channel is enhanced,with the secondary flow intensity under one-sided vegetated floodplain conditions being smaller than the corresponding value under smooth floodplain conditions.In the case of one-sided vegetated floodplain conditions and the same relative depth,the secondary flow intensities in the crossover areas with a smooth upstream floodplain are notably larger than the corresponding values in crossover areas with a vegetated upstream floodplain.Under the smooth floodplain conditions,the“island center”of the cross-sectional contours of Reynolds stress R vw can indicate the rotational center of the main vortex.The positive or negative sign of the Reynolds stress R vw at the“island center”can reflect the rotational direction of the main vortex.Conversely,under the one-sided vegetated floodplain conditions,the R vw contours in the cross-sectional middle parts appear disordered and do not demonstrate the aforementioned characteristics,with the absolute values of Reynolds stress R vw being noticeably smaller than the corresponding values under smooth floodplain conditions.