  Understanding flow structures in urban areas is widely recognized as a
challenging concern due to its effect on urban development, air quality, and
pollutant dispersion. In this study, state-of-the-art data-driven methods for
modal analysis of simplified urban flows are used to study the dominant flow
processes in these environments. Higher order dynamic mode decomposition
(HODMD), a highly-efficient method to analyze turbulent flows, is used together
with traditional techniques such as proper-orthogonal decomposition (POD) to
analyze high-fidelity simulation data of a simplified urban environment.
Furthermore, the spatio-temporal Koopman decomposition (STKD) will be applied
to the temporal modes obtained with HODMD to perform spatial analysis. The flow
interaction within the canopy influences the flow structures, particularly the
arch vortex. The latter is a vortical structure generally found downstream of
wall-mounted obstacles, which is generated as a consequence of flow separation.
Therefore, the main objective of the present study is to characterize the
mechanisms that promote these phenomena in urban areas with different
geometries. Remarkably, among all the vortical structures identified by the
HODMD algorithm, low- and high-frequency modes are classified according to
their relation with the arch vortex. They are referred to as vortex-generator
and vortex-breaker modes, respectively. This classification implies that one of
the processes driving the formation and destruction of major vortical
structures in between the buildings is the interaction between low- and
high-frequency structures. The high energy revealed by the POD for the
vortex-breaker modes points to this destruction process as the mechanism
driving the flow dynamics.
