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Wind and Structures
  Volume 33, Number 1, July 2021 , pages 13-27
DOI: https://doi.org/10.12989/was.2021.33.1.013
 


POD-based analysis of time-resolved tornado-like vortices
Mengen Wang, Shuyang Cao and Jinxin Cao

 
Abstract
    In this study, three representative configurations of tornado-like vortices, i.e., single vortex, vortex breakdown and multi-vortex, are numerically simulated using large-eddy simulation (LES). Proper orthogonal decomposition (POD) is firstly employed to decompose flow-field snapshots into a series of orthogonal flow patterns (POD modes) and time-dependent coefficients. Then, a conditional-average analysis is conducted to obtain the four kinds of conditionally-averaged flow fields, which are then compared with instantaneous and ensemble-averaged flow fields. Next, a quadruple POD analysis is performed to decompose the instantaneous flow field into mean, coherent, transition and noise components. Finally, a qualitative analysis is implemented for unsteady vortex motions in horizontal and vertical planes. Results show that the conditional average shows larger-scale coherent structures than the classical ensemble average, while it loses the small-scale turbulent fluctuations present in instantaneous flow. The tornado vortex structure is controlled by the mean component in the single-vortex stage. With increase in swirl ratio, the tornado vortex evolves from single-vortex, to vortex-breakdown to multi-vortex, companied by kinetic energy transference to coherent and transition components. The horizontal and vertical vortex motions are essentially the results of horizontal and vertical velocity perturbations.
 
Key Words
    tornado-like vortex; Computational Fluid Dynamic (CFD); Proper Orthogonal Decomposition (POD); conditional average; vortex motion
 
Address
Mengen Wang:College of Civil Engineering, Tongji University, Shanghai, 200092, China

Shuyang Cao:College of Civil Engineering, Tongji University, Shanghai, 200092, China/ State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, China

Jinxin Cao:College of Civil Engineering, Tongji University, Shanghai, 200092, China/ State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, China
 

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