The convection of large and intermediate scale fluctuations in a turbulent mixing layer
The convection of large and intermediate scale fluctuations in a turbulent mixing layer
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DOI：
10.1063/1.4837555
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发表时间：
2013-12
影响因子：
4.6
通讯作者：
O. Buxton;R. Kat;B. Ganapathisubramani
中科院分区：
文献类型：
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作者：
O. Buxton;R. Kat;B. Ganapathisubramani
通过粒子图像测速（PIV）实验，研究了二维平面混合层中大、中尺度速度脉动的对流速度及其与长度尺度的关系。一个“全球性”的对流速度，包含对流中存在的所有长度尺度，是通过检查速度波动之间的自相关函数在连续的PIV记录整个混合层。这个“全球”对流速度被发现是类似的平均流量，虽然平均对流速度大于平均值和波动的混合层的高速侧上的混合层的低速侧上的波动被观察到对流速度小于平均值。然后，通过检查连续PIV记录中流向速度波动迹线中特定于一个波数的谱内容之间的相位差，产生尺度特定对流速度剖面，偏移时间为τ。概率密度函数（PDF）产生的相位差，这是随后转换成对流位移，这些表明，对流的单一长度尺度波动表现出显着的变化，特别是这样的大规模波动和高速侧的混合层。对流速度分布从这些PDF使用的平均对流距离和模态对流距离。据观察，对流速度是相对不敏感的长度尺度的波动考虑，特别是当使用的平均对流距离。一个轻微的敏感性，然而，观察对流速度的基础上的模态对流距离。这种依赖性主要在混合层的高速侧观察到，其中较小的长度尺度波动比较大的长度尺度波动更快地对流。它还观察到，波动本身的大小影响对流速度与较大的幅度波动对流不太迅速比低幅度的最大长度尺度的调查与这种行为被逆转在更中间的长度尺度。
The convection velocity of large and intermediate scale velocity fluctuations in a nominally two-dimensional planar mixing layer, and its dependence upon the length scale, is explored by carrying out particle image velocimetry (PIV) experiments. A “global” convection velocity, containing the convection of all the length scales present in the flow, is produced by examining the autocorrelation functions between velocity fluctuations in successive PIV records across the mixing layer. This “global” convection velocity is found to be similar to the mean flow, although fluctuations on the low speed side of the mixing layer on average convect at speeds greater than the mean and fluctuations on the high speed side of the mixing layer are observed to convect at speeds less than the mean. Scale specific convection velocity profiles are then produced by examining the phase difference between the spectral content specific to one wavenumber in streamwise velocity fluctuation traces in successive PIV records, offset by time τ. Probability density functions (pdfs) are produced of this phase difference, which is subsequently converted into a convection displacement, and these show that the convection of single length scale fluctuations exhibits a significant variance, particularly so for larger scale fluctuations and in the high speed side of the mixing layer. Convection velocity profiles are produced from these pdfs using both the mean convection distance and the modal convection distance. It is observed that the convection velocity is relatively insensitive to the length scale of the fluctuations considered, particularly when the mean convection distance is used. A slight sensitivity to length scale is, however, observed for convection velocities based on the modal convection distance. This dependence is primarily observed in the high speed side of the mixing layer, in which smaller length-scale fluctuations convect more quickly than larger length-scale fluctuations. It is also observed that the magnitude of the fluctuation itself affects the convection velocity with larger magnitude fluctuations convecting less rapidly than lower magnitude ones at the largest length scales investigated with this behaviour being reversed at more intermediate length scales.