Steady-State Large-Eddy Simulations of Convective and Stable Urban Boundary Layers

Abstract

A comprehensive investigation is carried out to establish best practice guidelines for the modelling of statistically steady-state non-neutral urban boundary layers (UBL) using largeeddy simulation (LES). These steady-state simulations enable targeted studies under realistic non-neutral conditions without the complications associated with the inherently transient nature of the UBL. An extensive set of simulations of convective and stable conditions is carried out to determine which simplifications, volumetric forcings, and boundary conditions can be applied to replicate the mean and turbulent (variance and covariance) statistics of this intrinsically transient problem most faithfully. In addition, a new method is introduced in which a transient simulation can be 'frozen' into a steady state. It is found that nonneutral simulations have different requirements to their neutral counterparts. In convective conditions, capping the boundary-layer height h with the top of the modelled domain to h/5 and h/10 (which is common practice in neutral simulations) reduces the turbulent kinetic energy by as much as 61% and 44%, respectively. Consistent with the literature, we find that domain heights l z ≥ 5|L| are necessary to reproduce the convective-boundary-layer dynamics, where L is the Obukhov length. In stably stratified situations, the use of a uniform momentum forcing systematically underestimates the mechanical generation of turbulence over the urban canopy layer, and therefore leads to misrepresentations of both the inner-and outer-layer dynamics. The new 'frozen-transient' method that is able to maintain a prescribed flow state (including entrainment at the boundary-layer top) is shown to work well in both stable and convective conditions. Guidelines are provided for future studies of the capped and uncapped convective and stable UBL.

Neighbourhood-scale flow regimes and pollution transport in cities

“…2b, c. We also attribute the smaller u values above the UCL in CCR06 and CDT07 to the reduced amount of resolved TKE, which decreases with decreasing ratios of H i ∕H (Grylls et al 2020). Our simulated vertical profiles of these quantities do eventually decrease with height, as would be expected, for z∕H > 5 (not shown).…”

Neighbourhood-Scale Flow Regimes and Pollution Transport in Cities

“…If the parameters F u and u are scaled vertically by ( H i − H ) rather than H (not shown), the shapes of the vertical profiles above the UCL differ less than in Fig. 2b, c. We also attribute the smaller u values above the UCL in CCR06 and CDT07 to the reduced amount of resolved TKE, which decreases with decreasing ratios of H i ∕H (Grylls et al 2020). Our simulated vertical profiles of these quantities do eventually decrease with height, as would be expected, for z∕H > 5 (not shown).…”

On the Experimental, Numerical and Data-Driven Methods to Study Urban Flows

“…They found energetically predominant structures throughout the entire boundary layer, probably driving the flow dynamics, but this information was insufficient to identify the leading mechanism, i.e., whether it was associated to the inner or the outer layer. Finally, Crylls et al [96] introduced a new method using a LES approach (with periodic boundary conditions) to model the statistically-steady state of non-neutral urban boundary layers. This method freezes transient simulations into a steady state, showing a good performance in both stable and convective conditions.…”