Background.

Severe windstorms, such as hurricanes and tornadoes, cause enormous damage, destruction, and failure of civil structures, which dominates the economic losses in billion-dollar weather events of the United States [1]. Post-disaster surveys have evidenced that failure of roofs and roof coverings account for the majority of the damage of low-rise buildings. Roof failure is often initiated at windward roof edges and corners, due to peak suction (negative pressures) induced by flow separation and rooftop vortices.  Various types of parapets (solid, partial or porous parapets) and rooftop spoilers were used to reduce high suction from the conical vortices in oblique winds. We have not reached consensus about their effectiveness in hurricane-type high winds [2].

On the other hand, Nature has long used a multi-scale fractal pattern to manipulate flows, such as the tree-like fractal pattern (Fig.  2).  To fully realize biomimicry potential, this IRES student project will: (1) design a set of tree-like porous parapets based on scaling parameters [3]; (2) examine the formation of rooftop vortices on a low building without and with parapets at high Reynolds numbers. This study will advance the current knowledge of flow control strategies by multi-scale fractal grids concerning wind hazard mitigation and provide valuable implications for a broad range of industrial flow-control applications.

Figure 1: Bio-inspired flow control to reduce roof suction: rooftop vortices formed on the edges and corners (left). Tree-like fractal parapets to suppress rooftop vortices (middle);  and tree-like fractal pattern (right).

IRES student involvement. 

Drs. Zhang and Lee will work with an IRES student on this project.  The IRES student will build a low-building model and measure the roof vortice structure over the roof, with and without the tree-like parapets, in an atmospheric boundary layer (ABL) wind tunnel at POSTECH South Korea. A planar two-dimensional Particle Imaging Velocimetry (PIV) will be used to quantify the vortex structure and determine the effects of the tree-like parapets on the formation of rooftop vortices.

References

1. National Oceanic and Atmospheric Administration National Climatic Data Center, “Billion-Dollar Weather and Climate Disasters: Overview" 2018
2. D. Surry and J. X. Lin,  “The effect of surroundings and roof corner geometric modifications on roof pressures on low-rise buildings,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 58, pp. 113–138, 1995.
3. D. Hurst and C. J. Vassilicos, “Dissipation and decay of fractal-generated turbulence,” Physics of Fluids, vol. 19, no. 10, pp. 1–31, 2007.
4. J. P. Lee, E. J. Lee, and S. J. Lee, “Shelter effect of a fir tree with different porosities,” Journal of Mechanical Science and Technology, vol. 28, no. 2, pp. 565–572, 2014.
5. S. McClure, J. J. Kim,  S.  J.  Lee,  and  W.  Zhang,  “Shelter effects of porous multi-scale fractal fences,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 163, pp. 6–14, 2017.