On the flow structure and turbulence during sweep and ejection events in a wind-tunnel model canopy
On the flow structure and turbulence during sweep and ejection events in a wind-tunnel model canopy
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风洞模型冠层扫掠和喷射过程中的流动结构和湍流
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发表时间：
2007
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通讯作者：
J. Katz
中科院分区：
文献类型：
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作者：
Weihong Zhu;R. Hout;J. Katz
利用在冠层边界层风洞模型中获得的粒子图像测速（PIV）数据，研究了冠层边界层的平均流动和湍流特性。矢量间距在柯尔莫哥洛夫尺度的1.7到2.5倍之间变化。基于象限的条件采样，即基于速度波动的迹象，揭示了流动结构的根本差异，特别是在主导流动的扫射和喷射事件之间。在扫掠过程中，向下流动在冠层高度下方形成一个狭窄的、高度湍流的剪切层，其中包含多个小尺度涡旋。在抛射过程中，向上流动将该剪切层和相关的小尺度流动结构扩展到位于冠层上方的广阔区域。因此，在扫掠过程中，湍流动能（TKE）、雷诺应力以及产生和耗散率在冠层高度以下有明显的窄峰，而在喷射过程中，这些变量在冠层高度以上有较大的最大值。比较了三种估算耗散率的方法，包括谱拟合、不同尺度下的亚网尺度能量通量测量和略欠分解瞬时速度梯度的直接测量。在惯性范围内，SGS能量通量是基于梯度的（直接）滤波器尺寸估计值的40-60%，而在耗散范围内，如预期的那样，随规模减小。光谱拟合在直接估计值的5-30%范围内。光谱拟合在冠层高度附近超过直接估计值，但在冠层高度以上和低于冠层高度时则较低。冠层高度以下耗散率随速度大小的增大而增大，即在扫掠和象限1事件时耗散率最大，在弹射和象限3事件时耗散率显著降低。在冠层上方，抛射是最耗散的。扫掠过程中的湍流输运在冠层内狭窄剪切层下方起源作用，在冠层上方起汇作用。在抛射事件中，只有在冠层以上的地方才有运输的来源。TKE输运方程中的残差项主要代表压力-速度相关性的影响，仅在冠层内是重要的，并且由扫掠主导。
Particle image velocimetry (PIV) data obtained in a wind-tunnel model of a canopy boundary layer is used to examine the characteristics of mean flow and turbulence. The vector spacing varies between 1.7 and 2.5 times the Kolmogorov scales. Conditional sampling based on quadrants, i.e. based on the signs of velocity fluctuations, reveals fundamental differences in flow structure, especially between sweep and ejection events, which dominate the flow. During sweeps, the downward flow generates a narrow, highly turbulent, shear layer containing multiple small-scale vortices just below canopy height. During ejections, the upward flow expands this shear layer and the associated small-scale flow structures to a broad region located above the canopy. Consequently, during sweeps the turbulent kinetic energy (TKE), Reynolds stresses, as well as production and dissipation rates, have distinct narrow peaks just below canopy height, whereas during ejections these variables have broad maxima well above the canopy. Three methods to estimate the dissipation rate are compared, including spectral fits, measured subgrid-scale (SGS) energy fluxes at different scales, and direct measurements of slightly underresolved instantaneous velocity gradients. The SGS energy flux is 40–60% of the gradient-based (direct) estimates for filter sizes inside the inertial range, while decreasing with scale, as expected, within the dissipation range. The spectral fits are within 5–30% of the direct estimates. The spectral fits exceed the direct estimates near canopy height, but are lower well above and below canopy height. The dissipation rate below canopy height increases with velocity magnitude, i.e. it has the highest values during sweep and quadrant 1 events, and is significantly lower during ejection and quadrant 3 events. Well above the canopy, ejections are the most dissipative. Turbulent transport during sweep events acts as a source below the narrow shear layer within the canopy and as a sink above it. Transport during ejection events is a source only well above the canopy. The residual term in the TKE transport equation, representing mostly the effect of pressure–velocity correlations, is substantial only within the canopy, and is dominated by sweeps.