Measurements of internal waves and turbulence in two-dimensional stratified shear flows
Measurements of internal waves and turbulence in two-dimensional stratified shear flows
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二维分层剪切流中内波和湍流的测量
DOI：
10.1007/bf02188318
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发表时间：
1973
影响因子：
4.3
通讯作者：
Y. Pao
中科院分区：
文献类型：
--
作者：
Y. Pao
紊流分层剪切流是通过拖曳栅格或圆柱穿过分层盐水槽而在拖曳槽内产生的。在这些流动中产生的内部波和湍流用阴影图可视化，并用石英涂层热膜探头（最多四个探头用于速度波动）和单电极电导率探头（最多四个探头用于盐度波动）测量，这些探头以与障碍物相同的速度拖拽。速度和盐度信号被记录在磁带上。这些信号的一部分由数字计算机直接在线处理。从这些阴影图和探头测量中，我们观察到(i)在湍流已经消退的障碍物下游，分层流体总是具有层状结构。这种层状结构持续了很长时间，是湍流混合层受平均气流对流的结果。这些结果表明，在稳定分层的大气和海洋中，湍流已经消退，人们经常可以发现分层结构。（ii）当层理足够强时，测得的速度和盐度自谱出现谱峰和谱谷。在某些条件下，这些谱峰倾向于抬升谱曲线，显示出实质性的- 5/3子范围，尽管湍流雷诺数太低，流无法具有可识别的惯性子范围。这个反常f - 5/3子范围表明了利用f - 5/3子范围内的频谱测量通过Kolmogorov假设来预测湍流能量耗散率的缺陷。（三）利用内波的相位特性，提出了一种区分内波和湍流的诊断方法。(a)两个垂直分离的速度探头的共谱和正交谱测量结果可以方便地检测相位特性；(b)两个垂直分离的密度探头；(c)是速度探针和密度探针。这种方法在实验室中被证明是有用的，可以直接应用于大气和海洋测量，以区分内部波和湍流。（iv）从相干性测量中发现，整个湍流分层尾迹实际上是以Brunt-Väisälä频率对应的频率上下摆动。这表明，大气和海洋中类似的分层切变流，如大气中的急流和海洋中的克伦威尔流，可能会发生垂直振荡，进而引起水平振荡和弯曲。
A turbulent stratified shear flow is generated in a towing tank by towing a grid or a circular cylinder through a tank of stratified salt water. The internal waves and turbulence generated in these flows are visualized with shadowgraphs and measured with quartz-coated hot-film probes (up to four probes for velocity fluctuations) and single-electrode conductivity probes (up to four probes for salinity fluctuations) which are towed at the same speed as the obstacle. The velocity and salinity signals are recorded on magnetic tapes. A portion of these signals is processed directly-on-line with a digital computer.From these shadowgraphs and probe measurements, we observe that(i)Far downstream of the obstacle where the turbulence has already subsided, the stratified fluid always has a layered structure. This layered structure persists for a long time, and is a result of the convection of turbulently mixed layers by the mean flow. These results indicate that in the regions of a stably stratified atmosphere and ocean where the turbulence has subsided, one could often find layered structure.(ii)There are spectral peaks and valleys in the measured velocity and salinity autospectra when the stratifications are sufficiently strong. Under certain conditions, these spectral peaks tend to lift up the spectral curves to show substantialf−5/3subranges, although the turbulence Reynolds numbers are too low for the flows to have recognizable inertial subranges. This anomalousf−5/3subrange demonstrates the pitfalls of using spectral measurements in thef−5/3subrange to predict the turbulent energy dissipation rate through the Kolmogorov hypothesis.(iii)A diagnostic method is developed for distinguishing internal waves from turbulence, utilizing their phase characteristics. The phase characteristics can be conveniently examined from the cospectra and quadrature spectra measurements of: (a), two vertically separated velocity probes; (b), two vertically separated density probes; and (c), a velocity probe and a density probe. This method is demonstrated to be useful in the laboratory and can be applied directly to atmospheric and oceanic measurements to distinguish internal waves from turbulence.(iv)From the coherency measurements, it is found that the entire turbulent stratified wake is actually whipping up and down at a frequency corresponding to the Brunt-Väisälä frequency. This indicates that similar stratified shear flows in the atmosphere and in the ocean, such as the jet streams in the atmosphere and the Cromwell current in the ocean, may oscillate vertically, which in turn can induce horizontal oscillation and meandering.