The turbulent boundary layer over urban-like roughness

Abstract

The sheer scale and nature of urban boundary layers present a number of practical challenges that make remarkably difficult to experimentally determine the aerodynamic characteristics of the surface roughness, or to obtain a complete description of the flow field within the canopy layer, including static-pressure measurements. Consequently, there is a lack of quality data sets against which to validate urban-canopy models for weather forecasting or pollutant dispersion. And the few that exist almost exclusively comprise velocity statistics, so the mechanisms responsible for drag generation that are essentially pressure-based still remain to be investigated. These points are addressed in this work by exploring alternative measurement techniques and analysis methods. A novel floating element (FE) balance is developed to overcome the weaknesses of traditional single-pivot balances, which are sensitive to streamwise pressure gradients and buoyancy effects. Measurements are conducted for two staggered arrays of cuboids with different height distributions, and a detailed uncertainty analysis is carried out. The skin-friction coefficient for both surfaces can be estimated to within 2%. Snapshots of the flow field are taken using planar particle image velocimetry (PIV) at multiple spanwise locations. Mean and instantaneous maps of the overlying static-pressure field are estimated via 2D-RANS and 2D-TH, by neglecting the contribution of the out-of plane components of velocity and acceleration in the momentum transport equation. Special care is exercised to resolve the inner canopy region, so the surface pressure can be extrapolated from the nearest point, and estimates of the form drag and the zero-plane displacement height can be inferred. Comparisons with FE data and other experimental studies demonstrate the viability of this approach for urban-like roughness, when direct measurement techniques are not available, outperforming an ill-conditioned three-parameter fit of the mean velocity distribution. Horizontally-averaged profiles of the streamwise velocity and Reynolds shear stress above and below the canopy top are analysed, drawing attention to the most limiting assumptions of urban canopy models. Specifically, the current results are not consistent with a constant mixing-length distribution or sectional-drag coefficient, and the axial velocity profile across the canopy height does not follow an exponential function. In the interest of improving current formulations, an alternative parameterisation of the sectional-drag coefficient is considered, which explores the self-similar behaviour of the axial pressure difference across individual roughness elements. Finally, coupled statistics of the forces acting on a target roughness element of a staggered-cube array, in combination with velocity-pressure correlations, are used to elucidate the mechanisms responsible for drag generation and how they relate with the turbulent structure in the roughness sublayer. Evidence suggests that although large-scale structures are not as significant as the small and intermediate scales for the drag-force fluctuations, they still play an important role in modulating the small-scale pressure signal in the near-wall region.

More information

Identifiers

ORCID for Manuel Aguiar Ferreira: orcid.org/0000-0002-2428-0284

ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

Catalogue record

Export record