Numerical Simulations of the Urban Microclimate

As global urbanization is accelerating and the majority of the world\u27s population continues to reside in cities, sustainable urban development is becoming increasingly crucial. Evaluation of the urban microclimate is a vital aspect of planning sustainable cities, as it can significantly impact on the health and comfort of urban residents. Computational Fluid Dynamics is a cost-effective and flexible tool to predict microclimate conditions, although often not utilized in the urban planning process until the final stages of a project due to complex pre-processing. The current practice of urban planning also often involves simulating different physical phenomena in separate tools, making it difficult to understand the interaction. This thesis presents the potential of the numerical immersed boundary framework IBOFLow as a tool for urban planners to evaluate the urban microclimate at the early stages of the design processes. The complex and time-consuming pre-processing of urban regions is eliminated using automatically generated Cartesian octree grid meshes where the complex geometries are represented by the immersed boundary methodology. The framework is validated for wind using wind tunnel experiments and compared to a commercially used software to show the importance of including the complex local terrain to generate realistic results. Finally, initial results of the heat simulations are covered to visualize the idea of IBOFlow as a means to simulate the urban microclimate at large, including all necessary physics

Validation of an immersed boundary framework for urban flows

Urban heat islands, or the phenomena of locally increased temperatures of urban areas compared to their rural surroundings, are becoming increasingly problematic with global warming and the rise of urbanization. Therefore, new areas must be planned considering appropriate ventilation to mitigate these high-temperature regions and cooling strategies, such as green infrastructures, must be considered. Typically, these critical environmental issues are assessed in the final stages of urban planning when further strategic interventions are no longer possible. Here, a numerical framework is tested, that urban planners can use as a future tool to analyze complex fluid dynamics and heat transfer in the early stages of urban planning. The framework solves the RANS equations using an immersed boundary approach to discretize the complex urban topography in a cartesian octree grid. The grid is automatically generated, eliminating the complex pre-processing of urban topographies and making the framework accessible for all users. The results are validated against experimental data from wind tunnel measurements of wind-driven ventilation in street canyons. This work present similarities and differences between experiments and simulations using three different turbulence models. Finally, guidelines will be provided on the choice of minimum grid sizes required to capture the relevant flow structures inside a canyon accurately

Addressing wind comfort in an urban area using an immersed boundary framework

Considering wind, air and heat comfort in designing new urban areas is still a challenge for city planners. Urban heat islands, or the phenomena of locally increased temperatures in urban areas compared to their rural surroundings, are becoming increasingly problematic with global warming and the rise of urbanization. Therefore, new areas must be planned considering appropriate ventilation to mitigate these high-temperature regions and cooling strategies, such as green infrastructures, must be considered. Typically, most of the comfort criteria are evaluated and assessed in the final stages of urban planning when further strategic interventions are no longer possible. Here, a numerical framework is tested that urban planners can use as a future tool to analyze complex fluid dynamics and heat transfer in the early stages of urban planning. The framework solves the RANS equations using an immersed boundary approach to discretize the complex urban topography in a cartesian octree grid. The grid is automatically generated, eliminating the complex pre-processing of urban topographies and making the framework accessible to all users. The results are validated against experimental data from wind tunnel measurements of wind-driven ventilation in street canyons. After validation, we will apply the numerical framework to estimate the wind comfort in an idealized urban area. Finally, guidelines will be provided on the choice of minimum grid sizes required to capture the relevant flow structures inside a canyon accurately