Turbulent Intensities and Velocity Spectra for Bare and Forested Gentle Hills: Flume Experiments
Turbulent Intensities and Velocity Spectra for Bare and Forested Gentle Hills: Flume Experiments
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裸露和森林覆盖的平缓山丘的湍流强度和速度谱：水槽实验
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
G. Katul
中科院分区：
文献类型：
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作者：
D. Poggi;G. Katul
为了研究地形和林冠共同作用对流速变化和频谱的影响，在一系列地表覆盖度不同的平缓余弦丘陵上进行了两个水槽试验。第一个实验是在裸露的表面上进行的，而第二个实验是在密集排列的杆状树冠内和上面进行的。比较了这两个实验在中层、内层和近表层的速度变化和频谱。在中层，对于冠层，纵向和垂直速度差(sigma_u^2，sigma_w^2)与山丘诱导的空间平均速度摄动(Δu)在所谓的背景状态(这里取为给定高度的纵向平均)附近是同相的，这是由快速变形理论预测的。然而，对于裸露地表情况，$$和$$与Δu相差约L/2，其中L为山体的半长。在冠层，尾迹的产生是湍流能量的一个重要来源，它的作用是使这些层中的速度差与Δu重新对准，这一机制在裸露地表的情况下是完全不存在的。这种较低的‘边界条件’导致了冠层表面上的Δu几乎与Sigma_w^2}$$的经向变化同步。在内层和中层，对于裸露地表的背景状态，丘陵的光谱失真仍然很显著，而对于冠层的背景状态，山丘的光谱失真不明显。特别是，在裸露表面的内层和中层情况下，从当地测量的功率谱得到的有效指数与惯性子程尺度的预期−=5/3值有偏差。这些偏离在空间上与丘陵表面相关。然而，对于冠层表面的情况，光谱指数接近于冠层以上5/3的−，尽管与−的微小差异也与丘陵表面有关。在冠层内部，尾迹产生和能量短路导致显著偏离−至5/3。这些偏离−至5/3也似乎通过尾迹产生的贡献及其与Δu的排列与丘陵表面相关。此外，与冯·卡曼街道涡度相称的尺度很好地描述了冠层内的尾迹产生尺度，证实了平均流量在产生尾迹中的重要作用。山丘背风面冠层内的光谱表明，与回流区外的相应区域相比，那里的尾迹产生规模更大。在回流区内，与回流区外的区域相比，在高频下有明显更多的能量。
To investigate how velocity variances and spectra are modified by the simultaneous action of topography and canopy, two flume experiments were carried out on a train of gentle cosine hills differing in surface cover. The first experiment was conducted above a bare surface while the second experiment was conducted within and above a densely arrayed rod canopy. The velocity variances and spectra from these two experiments were compared in the middle, inner, and near-surface layers. In the middle layer, and for the canopy surface, longitudinal and vertical velocity variances ($${sigma_u^2,sigma_w^2}$$) were in phase with the hill-induced spatial mean velocity perturbation (Δu) around the so-called background state (taken here as the longitudinal mean at a given height) as predicted by rapid distortion theory (RDT). However, for the bare surface case, $${sigma_u^2 }$$ and $${sigma_w^2 }$$ remained out of phase with Δu by about L/2, where L is the hill half-length. In the canopy layer, wake production was a significant source of turbulent energy for $${sigma_w^2 }$$ , and its action was to re-align velocity variances with Δu in those layers, a mechanism completely absent for the bare surface case. Such a lower ‘boundary condition’ resulted in longitudinal variations of $${sigma_w^2}$$ to be nearly in phase with Δu above the canopy surface. In the inner and middle layers, the spectral distortions by the hill remained significant for the background state of the bare surface case but not for the canopy surface case. In particular, in the inner and middle layers of the bare surface case, the effective exponents derived from the locally measured power spectra diverged from their expected − 5/3 value for inertial subrange scales. These departures spatially correlated with the hill surface. However, for the canopy surface case, the spectral exponents were near − 5/3 above the canopy though the minor differences from − 5/3 were also correlated with the hill surface. Inside the canopy, wake production and energy short-circuiting resulted in significant departures from − 5/3. These departures from − 5/3 also appeared correlated with the hill surface through the wake production contribution and its alignment with Δu. Moreover, scales commensurate with Von Karman street vorticies well described wake production scales inside the canopy, confirming the important role of the mean flow in producing wakes. The spectra inside the canopy on the lee side of the hill, where a negative mean flow delineated a recirculation zone, suggested that the wake production scales there were ‘broader’ when compared to their counterpart outside the recirculation zone. Inside the recirculation zone, there was significantly more energy at higher frequencies when compared to regions outside the recirculation zone.