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Experiments in Fluids

, Volume 52, Issue 2, pp 479–493 | Cite as

Near-ground tornado-like vortex structure resolved by particle image velocimetry (PIV)

  • Wei ZhangEmail author
  • Partha P. Sarkar
Research Article

Abstract

The near-ground flow structure of tornadoes is of utmost interest because it determines how and to what extent civil structures could get damaged in tornado events. We simulated tornado-like vortex flow at the swirl ratios of S = 0.03–0.3 (vane angle θv = 15°–60°), using a laboratory tornado simulator and investigated the near-ground-vortex structure by particle imaging velocimetry. Complicated near-ground flow was measured in two orthogonal views: horizontal planes at various elevations (z = 11, 26 and 53 mm above the ground) and the meridian plane. We observed two distinct vortex structures: a single-celled vortex at the lowest swirl ratio (S = 0.03, θv = 15°) and multiple suction vortices rotating around the primary vortex (two-celled vortex) at higher swirl ratios (S = 0.1–0.3, θv = 30°–60°). We quantified the effects of vortex wandering on the mean flow and found that vortex wandering was important and should be taken into account in the low swirl ratio case. The tangential velocity, as the dominant velocity component, has the peak value about three times that of the maximum radial velocity regardless of the swirl ratio. The maximum velocity variance is about twice at the high swirl ratio (θv = 45°) that at the low swirl ratio (θv = 15°), which is contributed significantly by the multiple small-scale secondary vortices. Here, the results show that not only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow. The intensified mean flow and enhanced turbulence at the ground level, correlated with the ground-vortex interaction, may cause dramatic damage of the civil structures in tornadoes. This work provides detailed characterization of the tornado-like vortex structure, which has not been fully revealed in previous field studies and laboratory simulations. It would be helpful in improving the understanding of the interaction between the tornado-like vortex structure and the ground surface, ultimately leading to better predictions of tornado-induced wind loads on civil structures.

Keywords

Vortex Particle Imaging Velocimetry Vortex Core Particle Imaging Velocimetry Measurement Vortex Center 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

This research was supported by a grant from NOAA (Grant No. NA06OAR4600230). The authors would like to thank technician Bill Rickard and the Aerospace Engineering undergraduates for their assistance in preparation of the facility and experimental setup. The authors also acknowledge valuable comments provided by the anonymous reviewers, which definitely helped to improve the quality of this work.

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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  1. 1.Aerospace Engineering DepartmentIowa State UniversityAmesUSA
  2. 2.Saint Anthony Falls LaboratoryUniversity of MinnesotaMinneapolisUSA

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