Large-eddy simulation study of turbulent flows over idealized urban roughness within a convective boundary layer

Abstract

Convective boundary layer over urban area is rarely studied in the literature due to the technical difficulty of field observation and laboratory experiments. Whereas, the importance of its understanding to many applications is undeniable. Therefore, large-eddy simulation (LES) is employed to study the turbulent flow over idealized urban roughness under a range of convective conditions. Unlike many studies of buoyant instability with urban structures, this LES study simulates explicitly both the motions at roughness and boundary-layer scales, facilitating the analysis on the multi-scale processes that are important under convective condition. The near-wall region is divided into inertial sublayer (ISL) and roughness sublayer (RSL). This study focuses on the Monin-Obukhov similarity theory (MOST), which states that the flow in ISL is scaled by local height rather than boundary layer depth, and the flow properties in RSL under convective condition. It is found that MOST describes the LES results approximately well, which means the applicability of MOST over idealized urban roughness is basically supported by this study. However, unsatisfactory results at slightly unstable state and near ISL upper limit, and the sensitivity test of the MOST statistics on the domain horizontal dimensions, suggests that boundary layer eddies can modulate the ISL flow statistics. Therefore, MOST is not strictly valid and this explains why most MOST data of field studies are usually unsatisfactory. LES result shows that the flow statistics in RSL deviates from those predicted by MOST, but they are found to be different from those of vegetative roughness. It is found that mesh effect is important in the vicinity of roughness, but it does not influence the flow in ISL and above since the turbulent processes are ‘local’ under neutral condition and ‘top-down’ 2 under convective condition. The horizontal inhomogeneity of flow statistics in RSL are enhanced by buoyant instability, especially the flux statistics. Functional forms of mean velocity and temperature profiles are derived analytically from the proposed MOST functions and compared well with LES results. The calculated aerodynamic and temperature roughness lengths show an increasing trend with instability at strong convective condition, which is found to be due to the enhanced turbulence at roughness scale. Therefore, the convectional view that roughness lengths are only functions of roughness morphology is not valid. The turbulence kinetic energy (TKE) budget analysis shows that the local buoyant production is much weaker than the shear production and the transport terms are enhanced in convective condition. The wake production term explicitly calculated to be negative by LES, rather than positive documented in some field studies. The convective coherent structures are found to be multi-scaled and formed collectively near surface and the two-point correlation of vertical velocity fluctuation is non-local. Similar to vegetative roughness, the sweep-ejection pair eddy is also found above urban roughness. The results imply the enhanced turbulence near wall in convective condition is governed by a ‘top-down’ mechanism rather than a ‘local’ one, which invalidates the Townsend’s hypothesis, while momentum and heat are actually transported by different organized motions.