Estimating Aerodynamic Parameters of Urban-Like Surfaces with Heterogeneous Building Heights

Abstract

There are many geometrical factors than can influence the aerodynamic parameters of urban surfaces and hence the vertical wind profiles found above. The knowledge of these parameters has applications in numerous fields, such as dispersion modelling, wind loading calculations, and estimating the wind energy resource at urban locations. Using quasiempirical modelling, we estimate the dependence of the aerodynamic roughness length and zero-plane displacement for idealized urban surfaces, on the two most significant geometrical characteristics; surface area density and building height variability. A validation of the spatially-averaged, logarithmic wind profiles predicted by the model is carried out, via comparisons with available wind-tunnel and numerical data for arrays of square based blocks of uniform and heterogeneous heights. The model predicts two important properties of the aerodynamic parameters of surfaces of heterogeneous heights that have been suggested by experiments. Firstly, the zero-plane displacement of a heterogeneous array can exceed the surface mean building height significantly. Secondly, the characteristic peak in roughness length with respect to surface area density becomes much softer for heterogeneous arrays compared to uniform arrays, since a variation in building height can prevent a skimming flow regime from occurring. Overall the simple model performs well against available experimental data and may offer more accurate estimates of surface aerodynamic parameters for complex urban surfaces compared to models that do not include height variability.

Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas

“…This suggests z d can be greater than the average roughness-element height (e.g. Jiang et al 2008;Xie et al 2008;Hagishima et al 2009;Zaki et al 2011;Millward-Hopkins et al 2011;Tanaka et al 2011;Kanda et al 2013), with a peak z 0 up to five times greater and displaced to higher λ f (Hagishima et al 2009;Zaki et al 2011). Roughness-element staggering, orientation and most importantly height heterogeneity therefore need to be considered in morphometric calculations; especially in complex city centres, such as the current study site (Sect.…”

“…However, the correction is not considered here as no basis is provided and z d is not addressed. Kastner-Klein and Rotach's (2004) empirically derived relationship using wind-tunnel results from a scaled physical model of Nantes, France, is also not considered because it does not incorporate λ f , a parameter that is regarded as important (Millward-Hopkins et al 2011;Mohammad et al 2015a). Two morphometric methods that directly incorporate roughness-element height variability are explored: the Mho (Millward-Hopkins et al 2011) and Kan (Kanda et al 2013) methods.…”

“…comm. ), therefore, a relation based on the more accessible standard deviation of roughness-element heights has been developed (Millward-Hopkins et al 2011),…”

Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

“…in Millward-Hopkins et al (2011) and Kanda et al (2013). The main limitation of these above-mentioned studies is the fact that the suggested correlations y 0 = y 0 (λ F , λ P ) are calculated over a collection of data sets from disparate sources with significantly different boundary conditions and characterised by geometrically different roughness elements.…”

“…The main limitation of these above-mentioned studies is the fact that the suggested correlations y 0 = y 0 (λ F , λ P ) are calculated over a collection of data sets from disparate sources with significantly different boundary conditions and characterised by geometrically different roughness elements. Regardless, amongst the morphometric bulk drag prediction methods, Macdonald's (1998) appears to be widely used as it incorporates a broader range of scenarios when compared to other methods (Grimmond &Oke 1999 andMillward-Hopkins et al 2011). Figure 2(a) shows its prediction for y 0 as a function of λ F and λ P .…”