Development of a Large-Eddy Simulation model for flows over urban areas with application to the TU Delft campus

Abstract

Urban microclimate significantly affects people’s experiences and activities in urban environments by a series of phenomena, among which urban flow is an important factor to be considered. Computational Fluid Dynamics (CFD) method has become a popular tool for studying urban airflow because of its low cost compared with experiment methods. However, flows over urban areas exhibit turbulence nature of being three-dimensional, unsteady, and multi-scale. Additionally, the large computational domain that should be covered and the inherent inhomogeneity of the urban structures make it challenging to do full-scale modelings. Large Eddy Simulation (LES), with the development of computing power, becomes a promising tool to study such flows.

In the campus of Delft University of Technology (TU Delft), a crossroad near the EWI building (the main building of Faculty of Electrical Engineering, Mathematics and Computer Science) is constantly complained for its strong wind. This research tackles such problem using LES, and takes TU Delft campus area itself as case study. The development of this research is composed of three stages.

In the first stage, Vreman eddy viscosity model is implemented into Canonical Navier-Stokes (CaNS), a massively-parallel Navier Stokes solver developed by Costa, (2018). Based on a structured three dimensional Cartesian grid, the subgrid scale (SGS) eddy viscosity model is inserted into the Navier Stokes equation by adding an extra diffusion term. The inserted diffusion term is discretized with second-order difference scheme along with interpolation of the velocity field due to the staggered grid arrangement. The implementation is validated with a turbulent channel flow with friction Reynolds number 𝑅𝑒𝜏 = 360. The good agreement is found and the discrepancy is small.

In the second stage, the solver employs a direct-forcing Immersed Boundary Method (IBM) and is further validated with the flow over periodic cube arrays. Signed-Distance Field (SDF), as a convenient tool, is generated and functions as read-in data for IBM. The IBM processes the effect of the boundary as an added force on the fluid points at the interface. The stair-step approach approximates the structure boundary with the cuboid cells faces. The results match well with the wind tunnel test data from Castro et al., (2006) and a previous LES study by Tomas et al., (2016).

In the last stage, the validated solver is applied to a scaled-down TU Delft campus model. The simulation setup is designed by considering the achievability of a possible future wind tunnel measurement. Three grids are used for a grid convergence analysis by comparing the total IBM force, mean velocity, and Reynolds stress at certain locations. The flow converges with the finest grid with grid number 𝑁𝑥 × 𝑁𝑦 × 𝑁𝑧 = 960 × 880 × 240. Around the EWI building, a high-speed region is found at the cross road location. Behind the building, a wake area is observed, and a clear shear layer is on the top of the building.