An experimental field campaign is designed to unveil mechanisms responsible for turbulent exchange processes when mechanical and thermal effects are entwined. The focus is an urban street canyon with a mean aspect ratio H / W of 1.65 in the business centre of a mid-size Italian city ( H is the mean building height and W is the mean canyon width). The exchange processes can be characterized by time scales and time-scale ratios specific to either mechanical or thermal process. Time scales describe the mixing caused by momentum and heat exchange within different canyon layers, while their rates are surrogates of their efficacy. Given that homogeneous mixing does not always occur within the canyon, several time scales are estimated at different levels, showing that mechanical and thermal processes may both contribute to enhance mixing. By computing mechanical time scales, it is found that the fastest mixing occurs at the canyon rooftop level for perpendicular or oblique wind directions, while slow mixing occurs for parallel directions. Thermal processes are faster than the mechanical ones and are particularly efficient for perpendicular wind directions. By calculating the time-scale ratios, exchange processes are found to facilitate mixing for most wind directions and to regulate the pollutant-concentration variability in the canyon. This variability can be associated with the local-circulation regime, demarcated as thermally driven or inertially driven using a buoyancy parameter, i.e., the ratio between thermal and inertial forcings. Using this approach, a generalization of the results is proposed, enabling the extension of the current investigation to different street-canyon aspect ratios.

中文翻译：

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真实城市冠层中湍流交换过程的特征尺度

一项实验性实地活动旨在揭示当机械和热效应交织在一起时负责湍流交换过程的机制。焦点是位于意大利中型城市商业中心的平均纵横比 H/W 为 1.65 的城市街道峡谷（H 是平均建筑高度，W 是平均峡谷宽度）。交换过程可以通过特定于机械或热过程的时间尺度和时间尺度比率来表征。时间尺度描述了由不同峡谷层内的动量和热交换引起的混合，而它们的速率则代表了它们的功效。鉴于峡谷内并不总是发生均匀混合，在不同水平上估计了几个时间尺度，表明机械和热过程可能都有助于增强混合。通过计算机械时间尺度，发现在垂直或倾斜风向的峡谷屋顶水平发生最快的混合，而在平行方向发生缓慢的混合。热过程比机械过程更快，并且对于垂直风向特别有效。通过计算时间尺度比率，发现交换过程有助于大多数风向的混合并调节峡谷中的污染物浓度变化。这种可变性可能与局部循环状态有关，使用浮力参数（即热力和惯性力之间的比率）划分为热驱动或惯性驱动。使用这种方法，建议对结果进行概括，