Natural transition in natural convection boundary layers
Natural transition in natural convection boundary layers
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DOI：
10.1016/j.icheatmasstransfer.2016.06.004
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发表时间：
2016-08
影响因子：
7
通讯作者：
Yongling Zhao;C. Lei;J. Patterson
中科院分区：
文献类型：
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作者：
Yongling Zhao;C. Lei;J. Patterson
用三维直接数值模拟方法研究了垂直等温面附近自然对流边界层的“自然转折”。为了在数值上触发“自然过渡”，在上游边界层中引入了空间和时间上的随机扰动。观察到了随机扰动在流向上的传播。发现存在两个相互竞争的展向涡旋结构的波数，一个较大，一个较小。上游边界层以大波数为主，下游边界层以小波数为主。在垂直于受热面的平面上观测到的平均(时间平均)流向涡度的流向演化通常揭示了两层和三层纵向滚转结构。本文还用双相干方法分析了过渡自然对流边界层中的非线性过程。根据边界层温度时间序列的功率谱和双相干谱，讨论了边界层温度时间序列的转变途径和机制。在“自然过渡”到湍流的过程中还观察到了一个谱填充过程，这在性质上与在Blasius边界层中观察到的相似。
The ‘natural transition’ of a natural convection boundary layer adjacent to an isothermally heated vertical surface is investigated by means of three-dimensional direct numerical simulation (DNS). In order to trigger ‘natural transition’ numerically, spatially and temporally random perturbations are introduced into the upstream boundary layer. The propagation of the random perturbations in the streamwise direction is observed. It is found that there exist two competing wavenumbers of spanwise vortical structures, one large and the other small. The large wavenumber dominates in the upstream boundary layer, whereas the small wavenumber dominates in the downstream boundary layer. The streamwise evolution of the mean (time-averaged) streamwise vorticity observed at planes perpendicular to the heated surface in general reveals two- and three-layer longitudinal roll structures. Nonlinear processes in a transitioning natural convection boundary layer are also analysed using Bicoherence method. The transition route and mechanism are discussed based on the power spectra and Bicoherence spectra of the temperature time series obtained in the boundary layer. A spectrum filling process during the ‘natural transition’ to turbulence is also observed, which is qualitatively similar to that observed in Blasius boundary layers.