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POD-based analysis of time-resolved tornado-like vortices
Mengen Wang,Shuyang Cao,Jinxin Cao 한국풍공학회 2021 Wind and Structures, An International Journal (WAS Vol.33 No.1
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.
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Numerical investigation of effects of rotating downdraft on tornado-like-vortex characteristics
Shuyang Cao,Mengen Wang,Jinwei Zhu,Jinxin Cao,Tetsuro Tamura,Qingshan Yang 한국풍공학회 2018 Wind and Structures, An International Journal (WAS Vol.26 No.3
Appropriate modeling of a tornado-like vortex is a prerequisite when studying the near-ground wind characteristics of a tornado and tornado-induced wind loads on structures. Both Ward- and ISU-type tornado simulators employ guide vanes to induce angular momentum to converge flow in order to generate tornado-like vortices. But in the Ward-type simulator, the guide vanes are mounted near the ground while in the ISU-type they are located at a high position to allow vertical circulation of flow that creates a rotating downdraft to generate a tornado-like vortex. In this study, numerical simulations were performed to reproduce tornado-like vortices using both Ward-type and ISU-type tornado simulators, from which the effects of rotating downdraft on the vortex characteristics were clarified. Particular attention was devoted to the wander of tornado-like vortices, and their dependences on swirl ratio and fetch length were investigated. The present study showed that the dynamic vortex structure depends significantly on the vortex-generating mechanism, although the time- averaged structure remains similar. This feature should be taken into consideration when tornado-like-vortex simulators are utilized to investigate tornado-induced wind forces on structures.
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Numerical investigation of effects of rotating downdraft on tornado-like-vortex characteristics
Cao, Shuyang,Wang, Mengen,Zhu, Jinwei,Cao, Jinxin,Tamura, Tetsuro,Yang, Qingshan Techno-Press 2018 Wind and Structures, An International Journal (WAS Vol.26 No.3
Appropriate modeling of a tornado-like vortex is a prerequisite when studying the near-ground wind characteristics of a tornado and tornado-induced wind loads on structures. Both Ward- and ISU-type tornado simulators employ guide vanes to induce angular momentum to converge flow in order to generate tornado-like vortices. But in the Ward-type simulator, the guide vanes are mounted near the ground while in the ISU-type they are located at a high position to allow vertical circulation of flow that creates a rotating downdraft to generate a tornado-like vortex. In this study, numerical simulations were performed to reproduce tornado-like vortices using both Ward-type and ISU-type tornado simulators, from which the effects of rotating downdraft on the vortex characteristics were clarified. Particular attention was devoted to the wander of tornado-like vortices, and their dependences on swirl ratio and fetch length were investigated. The present study showed that the dynamic vortex structure depends significantly on the vortex-generating mechanism, although the time-averaged structure remains similar. This feature should be taken into consideration when tornado-like-vortex simulators are utilized to investigate tornado-induced wind forces on structures.
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Multi-level Cross-attention Siamese Network For Visual Object Tracking
Jianwei Zhang,Jingchao Wang,Huanlong Zhang,Mengen Miao,Zengyu Cai,Fuguo Chen 한국인터넷정보학회 2022 KSII Transactions on Internet and Information Syst Vol.16 No.12
Currently, cross-attention is widely used in Siamese trackers to replace traditional correlation operations for feature fusion between template and search region. The former can establish a similar relationship between the target and the search region better than the latter for robust visual object tracking. But existing trackers using cross-attention only focus on rich semantic information of high-level features, while ignoring the appearance information contained in low-level features, which makes trackers vulnerable to interference from similar objects. In this paper, we propose a Multi-level Cross-attention Siamese network(MCSiam) to aggregate the semantic information and appearance information at the same time. Specifically, a multi-level cross-attention module is designed to fuse the multi-layer features extracted from the backbone, which integrate different levels of the template and search region features, so that the rich appearance information and semantic information can be used to carry out the tracking task simultaneously. In addition, before cross-attention, a target-aware module is introduced to enhance the target feature and alleviate interference, which makes the multi-level cross-attention module more efficient to fuse the information of the target and the search region. We test the MCSiam on four tracking benchmarks and the result show that the proposed tracker achieves comparable performance to the state-of-the-art trackers.
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