Turbulence intermittency over idealized urban areas based on empirical mode decomposition

• EMD and CWT are used collectively to examine intermittent urban flows and turbulence. • Hilbert-Huang transform (HHT) unveils joint probability of flow strength and frequency. • Energy transfer over cities is intermittent, occurring mainly between adjacent scales. • Rougher surfaces possess larger intermittent energy than their smoother counterparts. • Small-scale eddies are more intermittent than their large-scale counterparts. Atmospheric turbulence, which is irregular, random, rotational, dissipative, and multi-scale, is the key driving force for ground-level ventilation and pollutant removal. This is particularly important to cities due to various factors such as building blockage, flow separation, and vortex shedding, to name just a few. Intermittency, as a fundamental characteristic of turbulence, not only facilitates intense momentum, mass, and heat transfer processes but also plays a crucial role in the vertical transport of aged/fresh air and the dilution/removal of air pollutants. In this connection, this study systematically examines the intermittent properties of coherent structures and energy transfer processes in the airflows over urban areas using wind tunnel experiments. Arrays of LEGO® blocks of different density/patterns are adopted to represent idealized urban morphology of a range of drag. In order to identify intermittent structures of all sizes and obtain the phase information in a multi-scale manner, a combination of empirical mode decomposition (EMD) and continuous wavelet transform (CWT) is adopted to analyze the velocity fluctuations measured by hot-wire anemometer (HWA). Distinct scale-dependent, transient characteristics are revealed. For instance, intermittency is more noticeable at smaller scales, peaks of local intermittency measure (LIM) occur simultaneously, and phase synchronizes exclusively between adjacent time scales. It is thus indicated that energy transfer processes are intermittent that tend to exchange between neighboring comparable scales. The findings unprecedentedly elucidate the transport processes in urban areas in response to a range of surface roughness and shed some light on the fundamental mechanism of urban airflows and pollutant dispersion.