大宽深比变曲率弯道水动力结构大涡模拟研究

弯曲型河流是自然界常见的河流形态,弯曲引起主流、二次流与湍流紊动相互作用,产生复杂的水动力结构,影响河流演变和物质输运。目前相关数值模拟及试验研究多针对小宽深、常曲率弯道,与天然河流大宽深、变曲率形态存在较大差异,为了揭示与天然河流相近的弯道水流运动特性,本文针对大宽深比正弦派生曲线弯道三维水流运动进行大涡数值(LES)模拟研究,结果表明:零曲率断面中心区域二次流沿河宽方向分布最为均匀,回流区范围最大,在大宽深比弯道中,回流区范围最大可达断面面积的15.5%。大宽深比弯道中部流动受到弯曲边界约束较弱,主流区内顺流向流速分布较小宽深比工况更接近于顺直矩形明渠,大宽深比情况下,主流核心区集中在弯道中线附近0.5~1倍水深范围,随着宽深比增大,弯顶附近涡量绝对值增大,对下游的影响范围增大。大宽深比弯道中水流紊动能整体水平较低,紊动能等值线分布和顺流向时均流速等值线一样,均受二次流影响,主流区中心紊动能最小,主流和局部回流区间的剪切层紊动能最大。

Large eddy simulations of hydrodynamic structure in channel bends with large width-depth ratios and variable curvatures

The meandering river, one of the most common types in nature, features various bends where the secondary flow is induced by curvature and interacts with turbulences, and usually develops into a complicated flow structure. Secondary flows are crucial to river evolution and nutrition material transport. Most previous numerical simulations and experiments focus on the small width-depth ratios and constant curvature of channel bends, but natural rivers tend to develop into a large width-depth ratio and variable curvature. This study conducts large eddy simulations (LES) of the flows in continuous sine-generated bends, and examines the hydrodynamic structure of large width-depth ratio bends under large Reynolds number conditions. The results show that at the cross-sections of zero curvature in the transition of two bends, the recirculation zone is the largest, and over the core region of these cross sections the secondary flow is transversely distributed most uniform. In the case of large width-depth ratio bends, this region can expand up to 15.5% of the cross-sectional area. The mainstream in this core region, relative to the small width-depth ratio case, is less effected by the curved boundaries, and the cross-sectional distributions of its time-averaged streamwise velocity component is closer to that in a straight, rectangular open channel, with the velocity peak located vertically very close to free surface and transversely within a range of 0.5-1.0 times the flow depth around the channel centerline. With an increasing width-depth ratio, Z-vorticity near the bend apex is increased, and the influence of bend curvature is extended further downstream. The secondary flow also changes the characteristics of flow turbulences in a meandering river. A larger width-depth ratio leads to less turbulent energy, and turbulent kinetic energy is the lowest in the core of the mainstream while the highest in shear layers.