Appropriate boundary conditions for computational wind engineering: Still an issue after 25 years

Abstract

The problems associated with creating a Horizontally Homogeneous Atmospheric Boundary Layer (HHABL) model in Computational Wind Engineering (CWE) are discussed. In order to achieve this it is important that ALL the conservation equations are in equilibrium. Previous papers have often focussed more on the turbulence property conservation equations, whereas the most common mistake is in not balancing the streamwise momentum equation. This paper primarily focuses on the required balance between the shear stress and the pressure gradient in a HHABL and the consequential impact on the turbulence profiles. The complex nature of the balance of forces and accelerations in the real atmosphere is discussed and it is shown that the surface layer can be modelled as a planar flow, partially driven by the pressure gradient and partially by the shear stress. It is also noted that in the surface layer, where diffusion of Turbulence Kinetic Energy (TKE) is relatively weak, the TKE profile is strongly related to the shear stress profile. The errors associated with basing inlet conditions on a subset of the conservation equations is investigated by analysing one set of profiles which are derived from the TKE conservation equation in isolation. The use of experimental data as inlet conditions is discussed, particularly in relation to the higher levels of TKE in the atmosphere compared to that suggested by the standard turbulence model constants.