Turbulent penetration of a thermally stratified interfacial layer in a wind tunnel
Turbulent penetration of a thermally stratified interfacial layer in a wind tunnel
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风洞中热分层界面层的湍流穿透
DOI：
10.1017/s0022112094002673
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发表时间：
1994
影响因子：
3.7
通讯作者：
Z. Warhaft
中科院分区：
文献类型：
--
作者：
Z. Warhaft
在风洞中研究了一个稳定的分层界面，下面有强湍流，上面有静止的空气，目的是模拟大气边界层顶部的逆温帽的条件。界面层是通过复合网格生成的，上面的网格尺寸较小，下面的网格尺寸较大（Veeravalli & Warhaft 1989）。在风洞的压力通风系统中产生的温度阶跃位于层的中心。没有剪切，因此湍流相互作用，通常掩盖了在传统的混合层的湍流生产，突出显示。在网格附近，速度波动很强，浮力效应不明显，但随着湍流衰减，浮力效应占主导地位。整体Richardson数N2 B/（u2 2/L2 u），其中NB是穿过层的Brunt-Väisälä频率，u2 2和Lu分别是层下侧湍流的速度方差和积分长度尺度，其变化范围从网格附近的大约0到下游的80。分层抑制湍流渗透到层中，降低中性情况下的速度场的高偏度和峰度，高斯值。该层最初随着下游距离的增加而变厚，当浮力变得明显时，由于热通量的崩溃而变薄。显着地区的反梯度热通量，和逆转的三重矩运输条款的迹象中观察到的层的上部。对温度波动条件下的热通量值的分析表明，与弱湍流相关的大的温度波动首先受到层结的影响。共谱分析表明，这些波动与大尺度。我们还表明，虽然速度和温度波动之间的联合正常近似是健全的被动标量场，它变得不太好的分层的发病，完全失败时，分层是强的。
A stably stratified interface, with strong turbulence below and quiescent air above, is studied in a wind tunnel with the aim of simulating the conditions at the inversion cap at the top of the atmospheric boundary layer. The interfacial layer was generated by means of a composite grid, with small mesh size above and a large one below (Veeravalli & Warhaft 1989). A temperature step generated in the plenum of the wind tunnel, was located at the centre of the layer. There is no shear and thus turbulence interactions, usually masked by turbulent production in traditional mixing layers, are highlighted. Close to the grid where the velocity fluctuations are strong, buoyancy effects are insignificant, but as the turbulence decays they become dominant. The bulk Richardson number, N2B/(〈u2〉2/L2u), where NB is the Brunt—Väisälä frequency across the layer, and 〈u2〉2 and Lu are the velocity variance and integral lengthscale, respectively, of the turbulence on the lower side of the layer, varied from approximately zero close to the grid to 80 far downstream. The stratification inhibited the turbulent penetration into the layer, reducing the high skewness and kurtosis of the velocity field for the neutral case, to Gaussian values. The layer, which initially thickened with downstream distance, thinned when buoyancy became pronounced, owing to the collapse of the heat flux. Significant regions of countergradient heat flux, and reversals in sign of the triple moment transport terms were observed in the upper part of the layer. An analysis of the value of the heat flux conditioned on the temperature fluctuations, showed that the large temperature fluctuations associated with weak turbulence became affected by stratification first. Cospectral analysis shows that these fluctuations are associated with large scales. We also show that although the joint normal approximation between velocity and temperature fluctuations is sound for a passive scalar field, it becomes less good with the onset of stratification, failing completely when the stratification is strong.