Flow over a hill covered with a plant canopy
Flow over a hill covered with a plant canopy
复制标题
DOI：
10.1256/qj.02.177
复制
发表时间：
2004-01-01
影响因子：
8.9
通讯作者：
Belcher, SE
中科院分区：
文献类型：
--
作者：
Finnigan, JJ;Belcher, SE
我们开发了一个分析模型，用于分析覆盖有植被冠层的山上的大气边界层流。假设山的坡度足够小，树冠上方的流动可以在亨特的线性框架内处理。对树冠内流动的扰动是由与山上流动相关的压力梯度驱动的。在上层冠层中，这种压力梯度通过向下的动量湍流传输和冠层阻力来平衡。那里的流量可以根据线性动力学计算得出，这表明最大的流向风是扰动压力最小的地方，即靠近山顶的地方。在树冠深处，与山上流动相关的压力梯度由树冠阻力平衡，这里是非线性树冠阻力。这种非线性平衡表明，在扰动压力梯度最大的地方，即在山的逆风坡上，顺流风是最大的。在山的背风处，该非线性解显示了压力梯度如何使树冠深处的风减速，从而在树冠足够深时导致与逆流区域的分离。冠层内部和上方的异相流之间的耦合意味着最大速度位于山顶的上风处，而不是崎岖山丘上的流动，而冠层引起的额外湍流混合显着降低了山上速度加速的幅度。最后，我们发现不存在正式的极限过程，其中带有天篷的解决方案可以产生众所周知的越过崎岖山丘的流动解决方案。这一发现对粗糙度长度在加速或减速湍流边界层中的使用提出了质疑。
We develop an analytical model for atmospheric boundary-layer flow over a hill that is covered with a vegetation canopy. The slope of the hill is assumed to be small enough that the flow above the canopy can be treated within the linear framework of Hunt. Perturbations to the flow within the canopy are driven by the pressure gradient associated with the flow over the hill. In the upper canopy this pressure gradient is balanced by downwards turbulent transport of momentum and the canopy drag. The flow there can be calculated from linearized dynamics, which show that the maximum streamwise winds are where the perturbation pressure is at a minimum, i.e. near the crest of the hill. Deep within the canopy the pressure gradient associated with the flow over the hill is balanced by the canopy drag, here the nonlinear canopy drag. This nonlinear balance shows how the streamwise winds are largest where the perturbation pressure gradient is largest, i.e. on the upwind slope of the hill. In the lee of the hill this nonlinear solution shows how the pressure gradient decelerates the wind deep within the canopy, leading to separation with a region of reversed flow when the canopy is sufficiently deep. Coupling between the out-of-phase flows within and above the canopy means that the maximum velocity is further upwind of the hill crest than in flow over a rough hill, while the extra turbulent mixing caused by the canopy significantly reduces the magnitude of the velocity speed-up over the hill. Finally, we find that there is no formal limit process where the solutions with a canopy yield the well-known solutions for flow over a rough hill. This finding calls into question the very use of a roughness length in accelerating or decelerating turbulent boundary layers.