Turbulent flows over dense canopies consisting of rigid filaments of small size are investigated using direct numerical simulation. The effect of the height and spacing of the canopy elements on the overlying turbulence is studied. In these dense canopies, the element spacing is the key parameter determining the penetration of turbulent fluctuations within the canopy and the height of the roughness sublayer. The flow within and above the canopy is found to become independent of the filament height at large element height-to-spacing ratios, $h/s \gtrsim 6$. We also study the effect of the canopy parameters on the Kelvin$-$Helmholtz-like instability typically associated with dense-canopy flows. We find that the instability is inhibited in canopies with small element height-to-spacing ratios, $h/s \sim 1$, due to the blocking effect of the wall at the base of the canopy, while a stronger signature of the instability can be observed as the height is increased. Canopies with very small element spacings, $s^+ \lesssim 10$, also inhibit the instability, owing to the large drag they exert on the fluctuations. The streamwise wavelength of the instability depends on the canopy shear-layer thickness, and we show that, for the present dense canopies, this thickness also scales with the element spacing. Some of the effects of the canopy parameters on the instability can be captured by a linear stability analysis.

中文翻译：

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湍流流过密集的长丝檐篷

使用直接数值模拟研究了由小尺寸刚性细丝组成的密集檐篷上的湍流。研究了顶篷元件的高度和间距对上覆湍流的影响。在这些密集的冠层中，单元间距是决定冠层内湍流波动渗透和粗糙度子层高度的关键参数。发现在大单元高度与间距比 $h/s \gtrsim 6$ 下，冠层内部和上方的流动与灯丝高度无关。我们还研究了冠层参数对通常与密集冠层流相关的 Kelvin$-$Helmholtz 类不稳定性的影响。我们发现不稳定性在具有小单元高间距比 $h/s \sim 1$ 的檐篷中得到抑制，由于顶篷底部墙壁的阻挡作用，而随着高度的增加，可以观察到更强烈的不稳定性特征。具有非常小的元素间距 $s^+ \lesssim 10$ 的檐篷也抑制了不稳定，因为它们对波动施加了很大的阻力。不稳定的流向波长取决于冠层剪切层厚度，我们表明，对于目前的密集冠层，该厚度也与单元间距成比例。冠层参数对不稳定性的一些影响可以通过线性稳定性分析得到。由于它们对波动施加的阻力很大。不稳定的流向波长取决于冠层剪切层厚度，我们表明，对于目前的密集冠层，该厚度也与单元间距成比例。冠层参数对不稳定性的一些影响可以通过线性稳定性分析得到。由于它们对波动施加的阻力很大。不稳定性的流向波长取决于冠层剪切层厚度，我们表明，对于目前的密集冠层，该厚度也与单元间距成比例。冠层参数对不稳定性的一些影响可以通过线性稳定性分析得到。