Accuracy of CFD simulations in urban aerodynamics and microclimate: Progress and challenges

This review outlines historical and recent research progress on the accuracy of computational fluid dynamics (CFD) simulations of urban aerodynamics and microclimates and clarifies future research directions and significant challenges for accuracy and reliability using CFD in this field. First, the development and accepted concepts of verification and validation (V&V) and uncertainty quantification (UQ) in general computer simulations and CFD are reviewed. Subsequently, progress made in V&V and UQ for urban aerodynamics and microclimate CFD simulations is described. The required or acceptable accuracy in this field has been discussed in several studies; however, challenges specific to applying CFD in this area should be recognized considering that the target phenomenon in urban aerodynamics and microclimates is complex. In addition, constructing a conceptual model is challenging and has many uncertainties. Furthermore, this field is characterized by significant spatial distributions of physical quantities, such as wind speed, temperature, and other scalar variables; thus, specific validation metrics and multilateral methods, including a conventional profile comparison, should be used. Therefore, the V&V and UQ processes should be implemented thoroughly, considering the characteristics of urban aerodynamics and microclimates. Discussions presented in this paper are of utmost importance and very timely in terms of simulation quality controls, especially during this new era of machine learning and artificial intelligence-based models that started being applied to urban aerodynamics and microclimate predictions.