Pollution in urban environments is increasingly becoming a major concern for our societies which, together with the increasingly powerful capabilities provided by computational fluid dynamics (CFD), has made pollution dispersion a common topic of research during the last two decades. Several factors are relevant to the dispersion of pollutants in an urban environment, such as the variable roughness of the urban canopy and the thermal stratification conditions. However, the effects of these two factors are only partially understood, especially when it comes to their combined effects.

The present research work has been motivated by an unintuitive strengthening of the streamwise-elongated coherent structures of the atmospheric boundary layer (ABL), observed by Jayaraman & Brasseur (2021), for a certain range of weakly unstable thermal stratification conditions. Such observation evinced the existence of a sweet-spot where the large-scale streamwise-oriented counter-rotating vortical structures, characteristic of any turbulent boundary layer, were able to strengthen themselves by collecting small-scale thermal plumes, eventually creating the so-called large-scale atmospheric rolls. The importance of these findings lies in the fact that large-scale atmospheric rolls are known to be one of the most important structures when it comes to vertical transport of momentum and scalars (i.e. pollutants) in the ABL.

Following from this observation, we hypothesize that the counter-rotating roll structures behind a high-rise building could potentially exhibit a similar behaviour and strengthen themselves under weakly unstable thermal stratification conditions by collecting small-scale thermal plumes. If this was the case, it would enhance large-scale vertical mixing which could be beneficial for street canyon ventilation, reducing urban pollution levels. Therefore, the importance of studying the evolution of the roll structures behind a high-rise building under different thermal stratification conditions becomes evident.

In this regard, the present work has focused on the study of the roll structures behind a high-rise building under different thermal stratification conditions, by means of wall-resolved Large-Eddy Simulations (LES) using the Nek5000 Spectral Element Method (SEM) code. The present work has succeeded in providing a qualitative proof of concept of the presented hypothesis, proving that the rolls behind a surface-mounted cube (model for a high-rise building) under weakly unstable thermal stratification conditions do strengthen themselves in a similar manner as observed by Jayaraman & Brasseur (2021) for the ABL. Moreover, a reliable LES numerical model for simulating flow past a surface-mounted cube (high-rise building) has been built and thoroughly validated against DNS data, tuning the optimal mesh, and simulation and filtering settings.