Large-scale circulations around a city are co-modulated by the urban heat island and by regional wind patterns. Depending on these variables, the circulations fall into different regimes ranging from advection-dominated (plume regime) to convection-driven (bubble regime). Using dimensional analysis and large-eddy simulations, this study investigates how these different circulations scale with urban and rural heat fluxes, as well as upstream wind speed. Two dimensionless parameters are shown to control the dynamics of the flow: (1) the ratio of rural to urban thermal convective velocities that contrasts their respective buoyancy fluxes and (2) the ratio of bulk inflow velocity to the convection velocity in the rural area. Finally, the vertical flow velocities transecting the rural to urban transitions are used to develop a criterion for categorizing different large-scale circulations into plume, bubble or transitional regimes. The findings have implications for city ventilation since bubble regimes are expected to trap pollutants, as well as for scaling analysis in canonical mixed-convection flows.

中文翻译：

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羽状还是泡沫？混合对流流态和城市尺度环流

城市周围的大规模环流受到城市热岛和区域风模式的共同调节。根据这些变量，环流分为不同的状态，从对流主导（羽流状态）到对流驱动（气泡状态）。本研究使用维度分析和大涡模拟，研究了这些不同的环流如何随城市和农村热通量以及上游风速而变化。显示了两个无量纲参数来控制流动的动力学：(1) 农村与城市热对流速度之比，与它们各自的浮力通量形成对比；(2) 农村地区的整体流入速度与对流速度之比。最后，横切农村到城市过渡的垂直流速用于制定标准，将不同的大规模环流分类为羽流、气泡或过渡状态。这些发现对城市通风有影响，因为气泡状态预计会捕获污染物，以及对典型混合对流的尺度分析。