Fluid tunnel research for challenges of urban climate

Experimental investigations using wind and water tunnels have long been a staple in fluid mechanics research. These experiments often choose a specific physical process to be investigated, whereas studies involving multiscale and multiphysics processes are rare. In the era of climate change, there is increasing interest in innovative experimental studies in which fluid (wind and water) tunnels are used in the modeling of multiscale, multiphysics phenomena of the urban climate. Fluid tunnel measurements of urban-physics-related phenomena are also required to facilitate the development and validation of advanced multiphysics numerical models. As a repository of knowledge for modeling these urban processes, we cover the fundamentals, experimental design guidelines, recent advances, and outlook of eight selected research areas, i.e., (i) absorption of solar radiation, (ii) inhomogeneous thermal buoyancy effects, (iii) influence of thermal stratification on land-atmosphere interactions, (iv) indoor and outdoor natural ventilation, (v) aerodynamic effects of vegetation, (vi) dispersion of pollutants, (vii) outdoor wind thermal comfort, and (viii) wind flows over complex urban sites. Three main challenges are discussed, i.e., (i) the modeling of multiphysics, (ii) the modeling of anthropogenic processes, and (iii) the combined use of fluid tunnels and scaled outdoor and field measurements for urban climate studies.