Large-eddy simulation of turbulent flow in a densely built-up urban area

Turbulent flow in a densely built-up area of Seoul, South Korea, is numerically investigated using the parallelized large-eddy simulation model (PALM). Based on the analysis of streamwise velocity and column-averaged vertical turbulent momentum flux, three areas of interest are selected: a downstream area of an apartment complex, an area behind high-rise buildings, and a park area. In the downstream area of the apartment complex, a large wake develops and a region of strong vertical turbulent momentum flux appears above the wake. At the height of maximum vertical turbulent momentum flux magnitude, all the four quadrant events occur in larger magnitude and contribute more to the vertical turbulent momentum transport than the averages in the main domain. In the area behind the high-rise buildings, fluctuating wakes and vortices are distinct flow structures around the top height of the tallest building and updrafts induced by the flow structures appear as strong ejections just behind the high-rise buildings or farther downstream. While strong ejections are dominant at building-top heights, downdrafts along the windward walls of high-rise buildings are distinct below building-top heights and they induce high turbulent kinetic energy and winding flow around the high-rise buildings near the ground surface, transporting momentum downward and intermittently into nearby streets. In the park area located downstream in the main domain, turbulent eddies exist quite above the ground surface, and the thickness of the interfacial region between low-speed air and high-speed air increases and complex turbulent flow appears in the interfacial region.