Particle Image Velocimetry Measurements of Turbulent Flow Within Outdoor and Indoor Urban Scale Models and Flushing Motions in Urban Canopy Layers
Particle Image Velocimetry Measurements of Turbulent Flow Within Outdoor and Indoor Urban Scale Models and Flushing Motions in Urban Canopy Layers
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DOI：
10.1007/s10546-011-9612-6
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发表时间：
2011-04
影响因子：
4.3
通讯作者：
H. Takimoto;A. Sato;J. Barlow;R. Moriwaki;A. Inagaki;S. Onomura;M. Kanda
中科院分区：
文献类型：
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作者：
H. Takimoto;A. Sato;J. Barlow;R. Moriwaki;A. Inagaki;S. Onomura;M. Kanda
利用粒子图像测速技术（PIV）研究了城市冠层气流的空间特性。研究地点是日本城市气候的综合室外尺度模型（COSMO）实验。PIV系统以30 Hz的采样频率连续一小时捕获冠层内的二维流场，从而提供可靠的室外湍流统计数据。在风洞设施中使用类似的粗糙度的几何形状，但较低的采样频率为4赫兹的PIV测量，也进行了比较。当使用摩擦速度缩放时，COSMO和风洞的湍流动量通量显示出相似的值和分布。室外和室内流场的一些不同特征主要是由于大气湍流导致风向波动较大所致。分析的重点是各种瞬时湍流结构。一个显著的流动结构被称为“冲刷”，即在建筑物间隙的整个垂直横截面上占主导地位的大规模向上运动。这是间歇性地观察到的，由此示踪粒子从冠层垂直冲出。在既没有热分层也没有外层湍流的风洞中也观察到冲刷现象。据建议，冲洗现象与冠层上方的大尺度低动量区的通过。
We investigate the spatial characteristics of urban-like canopy flow by applying particle image velocimetry (PIV) to atmospheric turbulence. The study site was a Comprehensive Outdoor Scale MOdel (COSMO) experiment for urban climate in Japan. The PIV system captured the two-dimensional flow field within the canopy layer continuously for an hour with a sampling frequency of 30 Hz, thereby providing reliable outdoor turbulence statistics. PIV measurements in a wind-tunnel facility using similar roughness geometry, but with a lower sampling frequency of 4 Hz, were also done for comparison. The turbulent momentum flux from COSMO, and the wind tunnel showed similar values and distributions when scaled using friction velocity. Some different characteristics between outdoor and indoor flow fields were mainly caused by the larger fluctuations in wind direction for the atmospheric turbulence. The focus of the analysis is on a variety of instantaneous turbulent flow structures. One remarkable flow structure is termed ‘flushing’, that is, a large-scale upward motion prevailing across the whole vertical cross-section of a building gap. This is observed intermittently, whereby tracer particles are flushed vertically out from the canopy layer. Flushing phenomena are also observed in the wind tunnel where there is neither thermal stratification nor outer-layer turbulence. It is suggested that flushing phenomena are correlated with the passing of large-scale low-momentum regions above the canopy.