Measurements of the streamwise vortical structures in a plane mixing layer
Measurements of the streamwise vortical structures in a plane mixing layer
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平面混合层中流向涡旋结构的测量
DOI：
10.1017/s0022112092004385
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发表时间：
1992
影响因子：
3.7
通讯作者：
R. Mehta
中科院分区：
文献类型：
--
作者：
J. Bell;R. Mehta
对平面双流混合层的三维结构进行了直接测量的实验研究。在过去的流动显示研究中，已发现在主展向涡之间存在二次流向涡结构。本研究的主要目的是定量地确定在较高雷诺数(Reδ∼2.9×10~4)下平面湍流混合层中流向涡结构的存在和作用。在初始边界层为层流的情况下，产生了一个速度比为U2/U1=0.6的双流混合层。平均流动和湍流测量是在穿过混合层的几个流向位置的精细跨平面网格上进行的，使用了一个可旋转的十字探头。结果表明，导致流向涡旋形成的不稳定性最初在第一次展向卷曲的下游被放大。流向涡旋首先出现在包含这两个符号的涡度的集群中。在更下游，涡旋重新排列形成相反的旋转对，尽管单个涡旋的规模和强度有相对较大的变化。流向涡间距以阶梯式方式增加，至少部分是通过类符号涡的合并。在所研究的流动条件下，与流向涡相关的波长随混合层涡度厚度的变化而变化，而其平均强度的衰减约为1/X1.5。在近场中，流向涡严重扭曲了混合层内的平均速度和湍流分布。特别是流向涡度在位置、强度和尺度上与二次切应力(上标为w素数)有很强的相关性。二次剪应力数据表明，流向结构一直持续到通常被认为是自相似的区域，尽管在这一点上它们非常弱，而且混合层在其他方面看起来是二维的。
An experimental study has been conducted to investigate the three-dimensional structure of a plane, two-stream mixing layer through direct measurements. A secondary streamwise vortex structure has been shown to ride among the primary spanwise vortices in past flow visualization investigations. The main objective of the present study was to establish quantitatively the presence and role of the streamwise vortex structure in the development of a plane turbulent mixing layer at relatively high Reynolds numbers (Reδ ∼ 2.9 × 104). A two-stream mixing layer with a velocity ratio, U2/U1 = 0.6 was generated with the initial boundary layers laminar and nominally two-dimensional. Mean flow and turbulence measurements were made on fine cross-plane grids across the mixing layer at several streamwise locations with a single rotatable cross-wire probe. The results indicate that the instability, leading to the formation of streamwise vortices, is initially amplified just downstream of the first spanwise roll-up. The streamwise vortices first appear in clusters containing vorticity of both signs. Further downstream, the vortices re-align to form counter-rotating pairs, although there is a relatively large variation in the scale and strengths of the individual vortices. The streamwise vortex spacing increases in a step-wise fashion, at least partially through the amalgamation of like-sign vortices. For the flow conditions investigated, the wavelength associated with the streamwise vortices scales with the mixing-layer vorticity thickness, while their mean strength decays as approximately 1/X1.5. In the near field, the streamwise vortices grossly distort the mean velocity and turbulence distributions within the mixing layer. In particular, the streamwise vorticity is found to be strongly correlated in position, strength and scale with the secondary shear stress ($\overline{u^{\prime}w^{\prime}}$). The secondary shear stress data suggest that the streamwise structures persist through to what would normally be considered the self-similar region, although they are very weak by this point and the mixing layer otherwise appears to be two-dimensional.