Large eddy simulation of tip and Karman vortices around a square prism: dynamic characteristics and their impact on lift force

Abstract

As a crucial component in the wind resistance design framework, flow-around analysis plays an essential role in assessing wind effects on structures. In this study, the dynamic behavior of tip- and Karman vortex around a ground-mounted finite length square prism (with an aspect ratio H/B = 9) subjected to atmospheric boundary layer flow representative of suburban terrain, and their relationship to lift force are investigated using large eddy simulation (LES). The Reynolds number based on the square prism width and incoming velocity is $\text{Re}=3.0\times {10}^4$. By combining with multiple-point synchronous pressure measurements and particle image velocimetry (PIV) wind tunnel tests, as well as other existing research, the wind loads characteristics are thoroughly examined from the perspective of mean and fluctuating wind pressure distribution on the model surface, local aerodynamic force distribution and overturning moments. Through time- and frequency domain analysis, two distinct lift fluctuation patterns are identified. The three-dimensional vortex structure analysis reveals that the Low-Amplitude Fluctuation (LAF) and High-Amplitude Fluctuation (HAF) of the lift force correspond to symmetrical and alternating vortex-shedding patterns, respectively. Symmetrical vortex shedding is primarily influenced by the downwash of tip vortex, while alternating vortex shedding pattern is driven by the Karman vortex. The notable differences in lift force fluctuation characteristics at various spanwise positions are associated with the range of influence exerted by the tip vortex downwash near the top of the square prism. The identification of the two lift force fluctuation patterns has practical implications for wind load assessment and structural design.