Projects and Solutions

Flight of flat plates in extreme winds

Windborne debris is a major cause of damage in the strong wind events such as hurricanes and tornadoes. Following wind-induced failures, building components (or portions of components) can fly through the air like missiles and penetrate adjacent, otherwise safe, structures increasing damage costs significantly. Debris has been categorized into three groups by shape and aerodynamic properties as compact (e.g., rocks), plate-like (e.g., plywood sheets or shingles), and rod-like (e.g., 2x4 lumber). ). Using this categorization, several researchers have discussed and applied model equations to predict trajectories and flight speeds in uniform, smooth flow. To do that, several models of the force coefficients have been presented to solve the two-dimensional equations of motion with the quasi-steady theory used for the model of aerodynamic force. Previous models have been modified by Kordi and Kopp (2008 a,b) to correct the rotational behaviour by incorporating recent research results from research in the fluid dynamics community. The new model can predict the existing experimental results more accurately. Figure 1 shows comparison between the Lin et al. (2006) experimental results with the numerical results using our proposed force model shown by solid line and the last two proposed force models shown by broken and dotted lines.

Comparison of the current computed trajectories with the TTU data

There have been relatively few studies on the effects of the initial conditions for debris flight. Moreover, wind borne debris in real storms is also in a turbulent boundary layer flow and the role of turbulence has not been explored. To examine the effect of initial conditions such as the position of debris on the roof, hold down force, and wind direction, neighbourhood housing along with the effect of turbulence on the trajectory of plate like object, series of tests were undertaken to investigate the trajectory of roof tiles and shingles by varying the wind angle, initial location on the roof, and surrounding buildings. It was observed the local velocity is critical to the flight of wind borne debris (Kordi and Kopp 2008 c,d), eg. (i): two different trajectory can be observed from the same nominal initial condition, Figure 2 (a,b), (ii): failures within large separation bubbles do not always lead to flight; in some locations the debris element moves upstream after failure and rests on the roof, Figure 2(c).

Strobe images taken from the high speed video of the initial flight patterns of the roof tiles

In the future, we will conduct a series of Particle Image Velocimetry tests to visualize the flow field at different locations on the roof. This will give us a comprehensive understanding of the effect of the initial location on the flight trajectory. At the end of this research, we hope to be able to predict flight path of wind borne debris originating from roof covers and sheathings of the low rise buildings in the turbulent wind flow which can be used to improve building code requirements for debris which will mitigate the amount of damage after extreme wind events.

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Updated June 22, 2010 by contactWE
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