LES case study on pedestrian level ventilation in two neighbourhoods in Hong Kong

Abstract

Hong Kong is one of the most densely built-up and populated cities in the world. An adequate air ventilation at pedestrian level would ease the thermal stress in its humid subtropical climate, but the high-density city severely reduces the natural ventilation. This case study investigates pedestrian level ventilation in two neighbourhoods in Kowloon, downtown Hong Kong using the parallelized large-eddy-simulation (LES) model PALM. The LES technique is chosen here for a city quarter scale pedestrian comfort study despite of its high computational cost. The aims of the paper are a) to get a comprehensive overview of pedestrian level ventilation and a better understanding of the ventilation processes in downtown Hong Kong, b) to test the LES technique on this urban scale compared to the wind tunnel and c) to investigate how numerical/physical parameters influence ventilation. This case study is restricted to neutral stratification in order to allow a direct comparison with the wind tunnel. A sensitivity study quantifies the dependence of site-averaged ventilation on numerical and physical parameters and determines an appropriate urban LES setup for two 1 km 2 neighbourhoods in Kowloon (Tsim Sha Tsui, Mong Kok) that are investigated for prevailing E and SW wind. The results reveal the critical dependence of ventilation on the urban morphology. Air paths, street orientations, ground coverage, sites fronting the water, inter connectivity of spaces, building podium size and building heights can all affect the pedestrian wind environment. Isolated tall buildings may have a pronounced impact on ventilation both locally and downstream.

Sensitivity of local air quality to the interplay between small- and large-scale circulations: a large-eddy simulation study

“…It should be noted that a 10 m resolution is clearly too coarse to resolve the circulation in street canyons (Letzel et al, 2008(Letzel et al, , 2012. It is also too coarse for the simulation of the stably stratified ABL over flat and homogeneous surfaces (Beare et al, 2006;Mason and Derbyshire, 1990).…”

PALM-USM v1.0: A new urban surface model integrated into the PALM large-eddy simulation model

“…Moreover, buildings and other urban components can locally decrease the ventilation (e.g. Letzel et al, 2012), thus adding to thermal discomfort. Chemical processes, and consequently air quality, are also affected by the urban environment.…”

Numerical framework for the computation of urban flux footprints employing large-eddy simulation and Lagrangian stochastic modeling

“…The influence of the structural details of the urban surface (balconies, chimneys, ventilation ducts, stationary cars, small-scale vegetation, etc.) not included in the urban topography model are taken into account by specifying a uniform roughness length z 0 = 0.05 m on the bottom boundary surfaces (Letzel et al, 2012). The precursor simulation generates a highly resolved ABL solution that will be utilized, first, in a recursive manner to initialize the entire urban LES flow field with turbulence and, second, to aid construction of appropriate inlet boundary conditions though a technique labeled turbulence recycling, which is based on the method by Lund et al (1998) with modifications by Kataoka and Mizuno (2002).…”