浅水层流射流大尺度涡流场特征数值研究

Numerical investigation on the flow characteristics of the large vortex induced by laminar jet in shallow water

在密度均匀的浅水流体中,由于流体的上下边界作用,会抑制动量源流动的垂向传播发展,形成类似分层流体中偶极子涡的准二维大尺度涡结构。该文通过数值模拟手段,以圆形喷管射流模拟动量源,对层流动量源在浅水条件下生成准二维大尺度涡结构的流场特性进行了研究。数值研究中,为考虑浅水流体上边界自由面的影响,设置了空气层;流体的下边界层设置为固壁条件;同时还考虑了不同的雷诺数影响。研究结果针对流场中射流主流轴向速度分布、过射流轴线的水平断面和自由表面速度场和涡量场进行了分析。结果表明,由于上下边界的存在,主流轴向速度分布关于中心轴非对称,而仅在水平断面上保持轴对称,这与无界射流的情况相差较大。同时,过射流轴线的水平断面和自由表面流场具有较高的相关性,这表明大尺度涡的流动显示出很强的二维流动特性。进一步对上述两个平面的涡量场进行分析,发现流场的平均涡量与时间幂次成正比关系,而且三个不同雷诺数下平均涡量的增长和衰减规律是基本不变的。

The flow induced by a momentum source will be vertically suppressed by the top and bottom boundaries in a shallow homogeneous fluid. Such a dynamic will lead to the form ation of quasi-two-dimensional(Q2D) large vortex structures which are similar to the dipolar vortices in stratified fluids. To investigate the flow field characteristics of them, numerical simulation is employed and the momentum source is simplified as a laminar jet. An air-layer is designed at the top of the computation domain to treat the free surface while the bottom bound ary is considered a solid wall. Different Reynolds numbers are also investigated in the work. The focus of the analysis is the axial velocity distribution for the main stream induced by the jet, the velocity and vorticity fields on the central and surfacial horizontal plane. The results show that the axial velocity distribution is no longer central symmetry to the axis, but only axial symmetry on the horizontal planes, remarkably different from the circumstance of a jet in an infinity domain. Meanwhile, very strong correlation is shown between the velocity fields on the central and surfacial horizontal plane, indicating the two-dimensionality of the flow. The time variation of the average vorticity of the two planes is obtained and follows power-laws with different powers. It is also fo und that the power-laws of average vorticity are similar when Reynolds number is changed.