Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150958

150958 - A New Stereo Vision Technique to Measure Three-Dimensional Trajectory of Flying Windborne Debris 

Windborne debris in complex turbulent wind fields can cause significant façade and interior damage to the neighboring buildings. To enhance the resilience of urban environments to this type of hazard, it is essential to study windborne debris flight patterns in turbulent winds. However, there is a notable lack of experimental data on the trajectories of windborne debris in highly turbulent winds. Thus, there is a need to conduct wind tunnel experiments to determine the trajectory of debris during flight. In such experiments, three-dimensional (3D) trajectory of flying debris needs to be measured. To this end, this study aims at measure debris trajectory in wind tunnel tests using one pair of high-speed cameras. 

The experiments of debris flight were conducted in the NSF NHERI Boundary Layer Wind Tunnel at the University of Florida. As this was the first step in studying debris behavior in a highly turbulent wind field, the experiments focused solely on spherical debris. This choice of debris simplified the tracking method, as it eliminated the need for six degrees of freedom (6DOFs including both rotational and translational DOFs) tracking, instead focusing only on the translation of the debris center. To ensure the results could be extrapolated to full-scale scenarios, the tests were nondimensionalized using Froude number similarity and selecting appropriate length scale and debris density to replicate full-scale conditions.

The proper wind tunnel scaling for both debris and building models resulted in the use of a debris model with a diameter of 1.5 mm. Measuring the 3D trajectories of these small, high-speed debris particles in a wind tunnel presents significant challenges. This high debris speed leads to large motions even within a short shutter speed of 4 microseconds of the cameras. Consequently, the shape of debris to appear elongated rather than perfectly spherical in the images. Further, the changing debris speed during its flight resulted in inconsistent debris shapes in the images. Additionally, high debris speed leads to substantial displacement of the debris between frames. These issues have rendered conventional object tracking methods like optical flow or digital image correlation inapplicable. 

To address these challenges, a novel stereo-vision technique was developed to track debris flight trajectories in a 3D turbulent wind field. This method combines frame differencing, intensity gradient calculation, nearest neighborhood analysis, and image classification to determine pixel locations and subsequently the displacement time history. The experiments utilized two synchronized cameras, which were calibrated. The performance of the tracking algorithm was evaluated, proving the method to be effective.

In summary, this study proposes a novel tracking method that effectively addresses the challenges posed by high-speed, large-motion debris. The method has proven to be reliable for tracking debris that exhibits varying size of elongated shape in images with large motions between each frame, even with low-contrast background behind the moving object. Additionally, by using frame subtraction, this method can be employed for measurement purposes where high pixel accuracy is needed. Another advantage of the proposed algorithm is its ability to resume the tracking the object even after it has been occluded and gets lost in some frames. The algorithm also accounts for sudden changes in intensity gradient due to irrelevant sources, such as the introduction of external objects, making it robust to significant changes in background and light sources. This method enables obtaining valuable experimental data on windborne debris trajectories in turbulent wind fields, ultimately contributing to improving building resilience against windborne debris hazards.

Presenting Author: Kimia Yousefi Anarak Colorado State University

Presenting Author Biography: I am a Ph.D. student at Colorado State University and my research focus is on the debris behavior in highly turbulent wind fields to improve building resilience against windborne debris hazards.

Authors:

Kimia Yousefi Anarak Colorado State University
Yanlin Guo Colorado State University
Kurtis Gurley University of Florida
John W Van De Lindt Colorado State University