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CONTENTS
Volume 26, Number 3, March 2018
 

Abstract
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Key Words
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Address
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Abstract
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.

Key Words
Tornado-like vortex; large-eddy simulation; vortex wander; rotating downdraft

Address
Shuyang Cao and Jinxin Cao: State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China
Mengen Wang: College of Civil Engineering, Tongji University, Shanghai 200092, China
Jinwei Zhu: hanghai Urban Construction Design & Research Institute Shanghai, China
Tetsuro Tamura: Tokyo Institute of Technology, Japan
Qingshan Yang: Chongqing University, China

Abstract
Extreme wind speed analysis has been carried out conventionally by assuming the extreme series data is stationary. However, time-varying trends of the extreme wind speed series could be detected at many surface meteorological stations in China. Two main reasons, exposure change and climate change, were provided to explain the temporal trends of daily maximum wind speed and annual maximum wind speed series data, recorded at Hangzhou (China) meteorological station. After making a correction on wind speed series for time varying exposure, it is necessary to perform non-stationary statistical modeling on the corrected extreme wind speed data series in addition to the classical extreme value analysis. The generalized extreme value (GEV) distribution with time-dependent location and scale parameters was selected as a non-stationary model to describe the corrected extreme wind speed series. The obtained non-stationary extreme value models were then used to estimate the non-stationary extreme wind speed quantiles with various mean recurrence intervals (MRIs) considering changing climate, and compared to the corresponding stationary ones with various MRIs for the Hangzhou area in China. The results indicate that the non-stationary property or dependence of extreme wind speed data should be carefully evaluated and reflected in the determination of design wind speeds.

Key Words
extreme wind speed; exposure adjustment; non-stationary; statistical modeling; generalized maximum likelihood approach

Address
Mingfeng Huang, Qiang Li, Haiwei Xu and Wenjuan Lou: Institute of Structural Engineering, College of Civil Engineering & Architecture, Zhejiang University, Hangzhou 310058, China
Ning Lin: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA

Abstract
For wind engineering applications downbursts are, presently, almost exclusively modeled, both experimentally and numerically, as transient impinging momentum jets (IJ), even though that model contains none of the physics of real events. As a result, there is no connection between the IJ-simulated downburst wind fields and the conditions of formation of the event. The cooling source (CS) model offers a significant improvement since it incorporates the negative buoyancy forcing and baroclinic vorticity generation that occurs in nature. The present work aims at using large-scale numerical simulation of downburst-producing thunderstorms to develop a simpler model that replicates some of the key physics whilst maintaining the relative simplicity of the IJ model. Using an example of such a simulated event it is found that the non-linear scaling of the velocity field, based on the peak potential temperature (and, hence, density) perturbation forcing immediately beneath the storm cloud, produces results for the radial location of the peak radial outflow wind speeds near the ground, the magnitude of that peak and the time at which the peak occurs that match well (typically within 5%) of those produced from a simple axi-symmetric constant-density dense source simulation. The evolution of the downdraft column within the simulated thunderstorm is significantly more complex than in any axi-symmetric model, with a sequence of downdraft winds that strengthen then weaken within a much longer period (>17 minutes) of consistently downwards winds over almost all heights up to at least 2,500 m.

Key Words
downburst; CM1 cloud model; thunderstorm; numerical modeling

Address
Christopher Oreskovic and Eric Savory: Department of Mechanical and Materials Engineering, University of Western Ontario, Canada
Juliette Porto: Ecole Nationale Supérieure de l

Abstract
Tornadoes are one of the world\'s deadliest natural phenomena. They are characterized by short life span and danger. It has been observed through post-damage surveys that localities with large numbers of buildings suffer major damage during a tornado attack resulting in huge loss of life and property. Thus,it is important to study interfering buildings exposed to tornado-like vortices. The present study focuses on external and internal pressures developed on building models exposed to translating tornado-like vortices in the presence of an interfering building model. The effects of translating speed and swirl ratio of a tornado-like vortex on external and internal pressures for a principal building in the vicinity of an interfering building are investigated. Results indicate that external and internal pressures are enhanced or reduced depending on the location of the interfering building with respect to the principal building.

Key Words
tornado-like flow; interference effects; external pressures; internal pressure; translating speed

Address
G.R. Sabareesh: BITS Pilani Hyderabad Campus, Hyderabad, India
Shuyang Cao and Jin Wang: State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, China
Masahiro Matsui: Tokyo Polytechnic University, Atsugi, Kanagawa, Japan
Yukio Tamura: Chongqing University, China, and Tokyo Polytechnic University, Japan


Abstract
Translation of tornadoes is an important feature in replicating the near-ground tornado flow field which has been simulated in previous studies based on Ward-type tornado simulators using relative motion of the ground plane. In this laboratory investigation, effects of translation on the near-ground tornado flow field were studied using the ISU Tornado Simulator that can physically translate over a ground plane. Two translation speeds, 0.15 m/s and 0.50 m/s, that scale up to those corresponding to slowly-moving tornadoes in the field were selected for this study. Compared with the flow field of a stationary tornado, the simulated tornado with translation had an influence on the spatial distribution and magnitude of the horizontal velocities, early reversal of the radial inflow, and expansion of the core radius. Maximum horizontal velocities were observed to occur behind the center of the translating tornado and on the right side of its mean path. An increase in translation speed, resulted in reduction of maximum horizontal velocities at all heights. Comparison of the results with previous studies that used relative motion of the ground plane for simulating translating tornadoes, showed that translation has similar effects on the flow field at smaller radial distances (~2 core radius), but different effects at larger radial distances (~4 core radius). Further, it showed that the effect of translation on velocity profiles is noticeable at and above an elevation of ~0.6 core radius, unlike those in studies based on the relative motion of the ground plane.

Key Words
tornado simulation; ISU tornado simulator; translating tornado; Tornado-like vortex; tornado wind

Address
Alireza Razavi and Partha P. Sarkar: Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA


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