LES of ABL flow in the built-environment using roughness modeled by fractal surfaces

The available methods to model ground roughness are not suitable for practical unsteady flow simulations in built-environment, or are limited to slightly rough terrains, or do not provide enough details on the flow structure close to the ground. In the current study, a robust model that simulates rough terrains and provides reasonable transient flow details close to the ground is proposed. The model can handle wide variety of rough exposure encountered in the built-environment. It is based on coupling two existing models in the literature; (i) surface gradient drag-based and (ii) canopy models. In this study the terrains are represented by equivalent fractal surfaces generated from random Fourier modes (RFM). Further, in a boundary layer wind tunnel testing, it is a common practice to use code-based prescribed aerodynamic roughness, z0, to define the terrain exposure type and to select the inflow mean velocity profiles and turbulent intensity levels. For a similar numerical application, there is no guidance on how to simulate turbulent flow corresponding to a specific prescribed z0. For such applications a new scaling technique is developed to scale arbitrary fractal surfaces in order to produce the prescribed z0. ABL flow, over three types of terrain exposures, is investigated using LES employing the new model. It is observed that the resulting ABL flow characteristics in terms of mean and fluctuating velocity profiles as well as velocity spectra match very well with the target engineering data, which validates the proposed method.