Large eddy simulation of the urban morphology impact on wind fields

Typhoon-induced wind hazards within urban neighborhoods are characterized with uncertain and extreme characteristics. The neighbourhood-scale wind fields in typhoon Mangkhut (2018) and Mui-fa(2022) were simulated using the Weather Research and Forecasting (WRF) model coupled with the Parallelized Large-Eddy Simulation Model (PALM). The pedestrian distress indicators (Gust Factor, GF and Speeding-up Efficacy, SE) were calculated for describing local risks. Positive correlations showed between λp (building area fraction) and pedestrian distress indicators. The probability density functions (PDFs) of GF and SE to λp can establish the connections between the extreme at specific probabilities and the average. With the increase of λp, these PDFs exhibit a slow rightward shift of the peak and a rapid broadening of the distributions. Positive correlations were also found between λf (frontal area density) and the vertical expanding coherent vortices. At the windward side of the high-rise, clear positive correlations between vertical momentum downward and the vortices with vertical geometry arise at the mid-high levels, corresponding momentum transfer efficiency and extreme gust exhibits more pronounced inside the vortices, but weak outside the vortices. This indicates vortices with vertical geometry around the high-rise contribute significantly to stabilizing momentum transfer and strengthening the extreme wind speed, revealing the prominent role of vortices on mid-high levels, rather than pedestrian levels and top levels, improving the understanding of the mechanism of wind hazards within urban blocks.