Dispersion in neutral boundary layer over a step change in surface roughness—I. Mean flow and turbulence structure

Abstract

In this study we have compared two quite different simulated boundary layers over homogeneous surfaces with an internal boundary layer (IBL) developed over a step change in surface roughness. The two simulated boundary layers are representative of rural and urban type environments under neutral conditions (Counihan, 1975), which are characterized by different power-law profiles, ratios of shear stress to freestream velocity and ratios of boundary-layer height to roughness parameter. For the change-in-roughness case, the need to incorporate the displacement height in evaluation of the IBL height ( h i ) is identified. The growth of h i with distance x from the location of roughness change is well described by Elliott's formula h i/Z 02 = a(x/Z 02) p , with p = 0.8 and a ≅ 0.35, and agrees with that observed in the field by Bradley (1968). The shear-stress measurements show considerable overshooting at the location of the roughness change before slowly settling down to their new surface equilibrium values further downstream. The overshooting of near-surface values is also present in σ u, σ v and σ w measurements. Ignoring the initial phase of overshooting of the surface stress measurements, the lowest portion ( ∼ 1 3 ) of the IBL can be identified as an equilibrium layer, in which flow properties are essentially in equilibrium with the underlying surface. The variations of the non-dimensional wind shear and the ratio of shear stress to turbulent kinetic energy in the IBL are also studied.