A Numerical Study of Turbulence Statistics and the Structure of a Spatially-Developing Boundary Layer Over a Realistic Urban Geometry

The applicability of outer-layer scaling is examined by numerical simulation of a developing neutral boundary layer over a realistic building geometry of Tokyo. Large-eddy simulations are carried out over a large computational domain (19.2km×4.8km×1km in the streamwise, spanwise, and vertical directions, respectively), with a fine grid spacing (2 m) using the lattice-Boltzmann method with massively parallel graphics processing units. The simulation produces a ratio of the boundary-layer height to the average building height of more than 50. Results from simulations show that outer-layer features are maintained for turbulence statistics in the upper part of the boundary layer, as well as the width of predominant streaky structures throughout the entire boundary layer, despite the very large roughness. This is caused by the existence of very large streaky structures extending throughout the entire boundary layer, which follow outer-layer scaling with a self-preserving development. We assume the top-down mechanism in the physical interpretation of results.